Author Topic: Never Trust Other People's Equipment!  (Read 2493 times)

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Offline IDEngineer

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Never Trust Other People's Equipment!
« on: October 11, 2020, 06:23:14 pm »
Sharing a event last week that reminded me to not be lazy and always check other people's equipment.

A few weeks ago we received an opportunity to make a modified version of an existing product for a potential new customer. It's a version we'd thought about for a couple of years but didn't have a ready-made customer for it, so with being busy doing lots of other things we never bothered to do it. However, that meant we had lots of the basic details already worked out so hand-building some breadboards for R&D, and then some prototypes for demonstration at the customer, went pretty quickly. We were on the plane last Sunday and in their building Monday morning.

This is a marine application - devices meant for installation on watercraft. As such it's an "automotive" environment, 12VDC lead acid cells backed by one or more alternators on an engine, and a CAN network at 250KHz. This is stuff we know very well with many years of experience, multiple products shipping, and many thousands of products in the field.

Since they're in the same environment, they have a nice test bench set up with everything our device should connect to. Power comes from a big benchtop analog power supply. At first look I was jealous of this beast... sometimes the loads we control have inrush currents in the many tens of amps and all of our supplies crowbar, and the array of lead acid cells we keep around for such tests are annoying to use and maintain. This box was beautiful in its simplicity: A power switch and two terminals on the front, a power cord and fuse holder in the back. That's it. No adjustments, no selectors. The white silkscreen on the front could not be misunderstood: Just the manufacturer's name and "13.8 VDC 50A" under the terminals. I could imagine the boat anchor power transformer lurking inside. This was a supply tailor-made for handling large automotive loads.

We only needed two of our devices to demonstrate, but we had hand-built five "just in case". Each one took many hours of careful assembly, using our standard version with a daughterboard of perfboard glued to the PCB.

We hooked up the first two and turned on the power supply. We enabled the first device (they operate alternately, in pairs) and everything worked briefly. Then there was a very audible POP. I heard it, my companion heard it, the customer's staff heard it. Everything stopped working. There was no covering it up.

I was thunderstruck. Baffled. Absolutely mystified. I had personally tested all five of these devices, put them through rigorous abuse. We were spending thousands of dollars on a trip to the customer, and I wanted to be darned certain the demo went perfectly. AND that we had backups in case anything went wrong. This is not my first rodeo. I like to be prepared and look good in front of the customer.

Our device was dead. Reapplying power did nothing. It didn't smoke, but I may have been able to smell something (though that could have been residual odors from the assembly process).

I did a bunch of hand-waving and distracting, as did my companion, while I swapped in a replacement device.

Click. POP. Same result. WTF?!?

OK, maybe possibly perhaps one of the devices got "damaged" on the trip (though I personally carried all of them in individual antistatic bags in my padded backpack which was in my possession the entire time). But the odds were zero of two getting "damaged" in the same way, so as to fail in exactly the same manner. And now we're down to "just" three devices. We can "afford" to test, and lose, only one more before the trip is a failure. And our reputation is falling in real time.

On trips like these I tend to bring a mini-development environment so I'm prepared for most things. I didn't bring the portable scope, but I did bring a Fluke DMM. On a whim, running out of excuses and distracting magic tricks and starting to sweat a little, I whipped out the DMM  :-DMM  and measured the voltage coming out of their supply.

29.1 VDC.

|O :scared:   :wtf:   :rant:

Keep in mind this was a simple analog supply. Not a battery charger. Not a jump starter. It had typical threaded banana jacks on its front panel. It was meant for a bench environment. And it was labeled 13.8VDC right on the front panel, with no adjustments anywhere on its six outside surfaces.

Hey, we know how to design for nasty automotive electrical systems. We spend extra money and PCB real estate on power protection components and aggressively test our designs. We expect reversed power connections. We expect spikes of both polarities, spikes amplitudes in the hundreds of volts, alternator field collapse, etc. But we do NOT expect sustained DC voltage well over 2X nominal backed by 50A of sustained current capacity.

Having identified the culprit, I whipped out the little switcher that I built a while back to do hotel-based development. It confirmed that our remaining three devices worked fine, but that little supply didn't have the grunt to power the loads in question. At this point I knew for certain we could give a great demo and save the day - we just needed a reliable source of 13.8VDC.

When scheduling the trip I had asked the customer if they had lead acid cells and they had assured me they had a full array of power sources. On the spot, it turned out this 29V power supply was pretty much it. I asked again if they had batteries - nope.

Desperate, realized we had brought a solution with us. There was going to be a demo today one way or another. They had a full machine shop, so I asked for a wrench so I could go remove the battery from our rental car. While they were finding that, the general manager said "Hey, we have forklifts...."

And so they drove one of their propane-fueled forklifts over to the test bench. We opened the engine compartment and while my companion handled our devices, I held two wires against the battery terminals of the forklift. It all worked perfectly, just as we knew it would. We demonstrated multiple modes of operation, different configurations, everything.

They were very impressed. They apologized for their supply. And we came away with a great new working relationship that is already yielding benefits to both parties.

Back home, I personally did the post-mortem on the two blown devices. I believe the sequence of events was as follows:

1) TVS's have this "spread" of voltage thresholds and specifying the best part for the job is a study in compromises. Too low a voltage and the TVS is conducting too often; too high a voltage and the downstream components aren't protected. The unidirectional TVS we have settled on has an initial threshold of 16V and a clamping voltage of 26V, and has been working great in thousands of devices in the field for many years so we have a lot of confidence in its behavior in a normal (awful) automotive electrical environment. But in THIS environment, the TVS saw a sustained voltage of 29V - well above its 26V clamping voltage where the TVS turns fully on in an attempt to sink enough current to bring the voltage down. "Sink enough current" in this context is 50 amperes. The max forward voltage of the TVS is spec'd at ~3V, so the TVS was trying to conduct a sustained ~50A and dissipate a sustained ~150W. That there was a delay of any length between power on and the POP is a miracle.

2) With ~50A flowing, the forward biased polarity protection diode (rated for 1000V 1A) failed short. This didn't really affect the outcome, just added insult to injury by raising the voltage experienced by the downstream TVS by whatever the diode drop had been previously.

3) Next the TVS couldn't take it anymore and failed short. This is the normal mode of failure for a TVS. At this point we had the full voltage and full current of the supply running through the connector, the diode, and TVS, and the PCB traces that connect all of them.

4) Finally, one of the PCB traces acted as a fuse and blew, opening the circuit and ending the nightmare. The same trace blew on both devices so we know which of the involved traces is the weakest! :o

Moral of the story: Just because a nice looking fixed voltage bench power supply reads "13.8V" on its front panel does not mean you can trust it. You might think you can... it has no adjustments so it cannot be misadjusted... a motor connected to it moments before worked and didn't fry, giving you a false sense of confidence... but don't be fooled. Bring a meter that you trust and confirm. It might save you a trip.
« Last Edit: October 11, 2020, 06:34:34 pm by IDEngineer »
 
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Offline genghisnico13

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Re: Never Trust Other People's Equipment!
« Reply #1 on: October 11, 2020, 06:35:13 pm »
I didn't read anything about a PolySwitch, in that case (with a high enough voltage rating) should protect the device.
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #2 on: October 11, 2020, 06:55:21 pm »
I didn't read anything about a PolySwitch, in that case (with a high enough voltage rating) should protect the device.
Perhaps. But polyswitches say "speed of response for a PTC is similar to the time delay of a slo-blo fuse", so in the environment in question the TVS would still have to sustain ~50A and ~150W for, what, several hundred milliseconds? TVS's can be damaged and still be semi-functional so this raises the threat of having a marginal device rather than one which fails hard, immediately revealing that there is a problem.

You have a great point but polyswitches aren't a panacea. I'm take another look at their specs but I'm not sure they're a good fit. And besides, I haven't yet seen the 12V automotive battery that accidentally generates twice the expected voltage from its chemistry.  :)  We're not in a rush to redesign for this condition as we consider it highly unlikely to occur in an actual watercraft environment.
 

Offline radiolistener

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Re: Never Trust Other People's Equipment!
« Reply #3 on: October 11, 2020, 08:54:47 pm »
Here is protection circuit which allows to protect equipment from issues like overvoltage and reverse polarity for high current.

It should help in your case.
 

Offline Mr. Scram

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Re: Never Trust Other People's Equipment!
« Reply #4 on: October 11, 2020, 09:03:49 pm »
Don’t trust your own, for that matter. And definitely don’t trust yourself.
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #5 on: October 11, 2020, 09:13:15 pm »
Here is protection circuit which allows to protect equipment from issues like overvoltage and reverse polarity for high current.
Yep, if you have more room you can implement more elaborate solutions. The PCB in question is 2x2 inches and nothing but the connector (which consumes 25% of that real estate) can exceed 0.25in in height.

As I said, with many thousands of devices in the field, our existing solution is proven in the nasty environment of an automotive electrical system. It also takes up much less room than (for example) a relay. I was sharing the experience in case I'm not the only one who forgets to check other people's equipment. {grin}
 

Offline thinkfat

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Re: Never Trust Other People's Equipment!
« Reply #6 on: October 11, 2020, 09:24:01 pm »
If you want to use a TVS diode for overvoltage protection, you must combine it with a fast acting fuse. A polyfuse is waaay too slow. A crowbar circuit with an SCR would work better with a polyfuse (tried and tested). The T in TVS gives away what you would use it for.
Everybody likes gadgets. Until they try to make them.
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #7 on: October 11, 2020, 09:41:53 pm »
I prefer the term "tranzorb" though most people think "TVS" so that's what I typed.

Our tranzorb's turnon time is, according to the spec sheet, 1ps typical (yep, picosecond). No (poly)fuse is going to come within many orders of magnitude. Remember, we aren't expecting the tranzorb to deal with steady-state overvoltage conditions. Its job is to absorb transients, hence the name.

Since this is the second comment involving a fuse: This is a potted module which gets certified to SAE J1171 for ignition-proof operation in engine compartments. All components are embedded in epoxy. Replacement of anything, including a fuse, is impossible. So far we've had zero returns from the field after several years and thousands of installed units. We're comfortable continuing production without adding a fuse.
 
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Offline nctnico

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Re: Never Trust Other People's Equipment!
« Reply #8 on: October 11, 2020, 09:55:14 pm »
I didn't read anything about a PolySwitch, in that case (with a high enough voltage rating) should protect the device.
Polyswitches are not the best option. In this case I'd use a fuse in the design (internal or external). That would have prevented the device from being blown up. Since it appearantly is automotive I'd expect the device to be behind a fuse anyway so why no add that to a demo setup just in case?
« Last Edit: October 11, 2020, 09:56:46 pm by nctnico »
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Offline thinkfat

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Re: Never Trust Other People's Equipment!
« Reply #9 on: October 11, 2020, 10:05:19 pm »
I prefer the term "tranzorb" though most people think "TVS" so that's what I typed.

Our tranzorb's turnon time is, according to the spec sheet, 1ps typical (yep, picosecond). No (poly)fuse is going to come within many orders of magnitude. Remember, we aren't expecting the tranzorb to deal with steady-state overvoltage conditions. Its job is to absorb transients, hence the name.

Since this is the second comment involving a fuse: This is a potted module which gets certified to SAE J1171 for ignition-proof operation in engine compartments. All components are embedded in epoxy. Replacement of anything, including a fuse, is impossible. So far we've had zero returns from the field after several years and thousands of installed units. We're comfortable continuing production without adding a fuse.

Well no problem with that of course. If it's outside of the requirements to have a protection against steady-state over-voltage conditions, that's fine, and obviously it's sufficient for your uses.

My remark was more towards the suggestion to use a polyfuse. These are really difficult to dimension correctly, especially when they don't operate under free air. A potted polyfuse may never blow, or very late, depending on how thermally conductive the potting material is.

Zener/SCR crowbars cope better with continuous over-voltage (you'll still need a fuse but it can be slow), but they are much slower than a TVS and not a replacement for transient absorption.
Everybody likes gadgets. Until they try to make them.
 

Online KE5FX

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Re: Never Trust Other People's Equipment!
« Reply #10 on: October 11, 2020, 10:11:43 pm »
Great story.  It always sucks to have to haul a ton of extra 'just in case' devices and equipment/tools back from a trip where they weren't needed, but I'm sure you didn't regret it this time.  Good thinking on the rental car battery as well, I don't know if I'd have come up with that idea.  In your case I'd be tempted to implement radiolistener's approach with one (or maybe two) power FETs instead of the relay and SCR.   It should be possible to come up with a circuit topology that safeguards against both overvoltage and reverse polarity.

I've heard that forklift batteries are an underrated (reputation-wise) source of home energy storage.  Massive power capacity, deep-cycle compatibility, relatively low cost, and the companies that sell them are equipped to bring them to you and take the old ones away.
 

Offline NANDBlog

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Re: Never Trust Other People's Equipment!
« Reply #11 on: October 11, 2020, 10:15:26 pm »
The word "calibration" comes to my mind.
There are two kind of equipment in a lab, calibrated ones and paperweight. Even if you dont cablibrate it with the same tests, that the manufacturer envisioned. Or with the same specification, that the original one.
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #12 on: October 11, 2020, 10:17:41 pm »
Great story.  It always sucks to have to haul a ton of extra 'just in case' devices and equipment/tools back from a trip where they weren't needed, but I'm sure you didn't regret it this time.
Indeed. I have a dedicated backpack for it now. All the mission-critical stuff goes into that and it stays with me as a carryon "personal item". If they lose my luggage I might show up in yesterday's clothes and smell funny, but the technical aspects of the meeting will proceed!

The one bugaboo used to be hand tools. TSA really hates those. I guess they're fearful you might disassemble the plane starting in the bathroom or something. But they specifically allow tools shorter than seven inches so I carry a few critical ones like tweezers, needle nose pliers, cutters, etc. in a six-inch-long ziplok. Sadly, that doesn't allow a large enough wrench to pull a battery from a rental car but nor did I think we'd need to do that!

Quote
I've heard that forklift batteries are an underrated (reputation-wise) source of home energy storage.  Massive power capacity, deep-cycle compatibility, relatively low cost, and the companies that sell them are equipped to bring them to you and take the old ones away.
That's true for electric forklifts, and when they first suggested using their forklift my first question was "what voltage are the batteries?". Most electric forklifts run packs in series/parallel and operate at some multiple of 12V. I hoped they exposed the individual packs so we could get to 12V. Fortunately, as mentioned above these were propane fueled forklifts so they had a traditional Group 24-style 12V starting battery, perfect for our needs. It was even conveniently mounted on a wheel transport so we didn't have to carry it across the factory! {grin}

EDIT, since you added something after my response:
Quote
It should be possible to come up with a circuit topology that safeguards against both overvoltage and reverse polarity.
Reverse polarity is covered nicely by a small series 1000V 1A Schottky diode. Nothing complex needed, we can afford the voltage drop, and sub-500nS response is plenty fast. Transients we have covered too. That leaves long-term overvoltage, and frankly it's not an issue in the real world. As I said, I've yet to see the lead-acid battery that accidentally generates 2X is chemistry's rated voltage. And our spec sheets are very clear about the max input voltage.

EDIT #2: I just finished characterizing our standard protection circuit for sustained high voltage conditions. We can safely tolerate a sustained overvoltage of ~18VDC before the tranzorb's conduction climbs enough to matter (above 10's of milliamps), and then the limiting factor is only internal heating of the tranzorb (proper device operation continues). That's fully 50% over the nominal rating of 12VDC, and 30% above the typical alternator voltage of 13.8VDC. If someone's on-vehicle regulator is permitting a sustained output voltage 30% higher than alternator nominal, ours won't be the only device that fails. In production there's always a limit to how much you can spend on protection against extreme cases while remaining commercially viable. I think we're OK where we are with 30-50% margin.
« Last Edit: October 11, 2020, 10:29:41 pm by IDEngineer »
 
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Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #13 on: October 11, 2020, 10:34:52 pm »
The word "calibration" comes to my mind. There are two kind of equipment in a lab, calibrated ones and paperweight.
For Engineering T&M power supplies I would have the same opinion. But this wasn't anything remotely like an Engineering T&M power supply. This was a grunty high current fixed voltage power source. No adjustments were made available. I didn't crack the box but I'd bet there aren't any internal adjustments either - it's likely designed with fixed components and no "calibration" is required or even possible.

Next time I'm there, I'll ask if I can open it up. I suspect they'll be OK with that. I'll take photos of what's inside and post them here.
 

Offline radiolistener

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Re: Never Trust Other People's Equipment!
« Reply #14 on: October 11, 2020, 10:43:00 pm »
Reverse polarity is covered nicely by a small series 1000V 1A Schottky diode. Nothing complex needed

Schottky diode has voltage drop, for high current like 50-100 Amps it will dissipate a lot of power. Schottky diode is useful for a low power devices, with current consumption below 1 Amp.
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #15 on: October 11, 2020, 11:02:48 pm »
Schottky diode has voltage drop, for high current like 50-100 Amps it will dissipate a lot of power. Schottky diode is useful for a low power devices, with current consumption below 1 Amp.
The small signal circuitry this is protecting consumes <50mA typical. The 1A rating comes from our desire for the 1000V rating, given the magnitude of spikes that can occur in automotive systems due to field collapse. From a current perspective the diode is substantially overrated but a nice side effect is that at our 50mA it doesn't even get warm (as proven by our thermal studies).
 

Online mawyatt

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Re: Never Trust Other People's Equipment!
« Reply #16 on: October 11, 2020, 11:09:38 pm »
I prefer the term "tranzorb" though most people think "TVS" so that's what I typed.

Our tranzorb's turnon time is, according to the spec sheet, 1ps typical (yep, picosecond). No (poly)fuse is going to come within many orders of magnitude. Remember, we aren't expecting the tranzorb to deal with steady-state overvoltage conditions. Its job is to absorb transients, hence the name.


Nice story, glad it worked out well in the end.

Transorbs bring back memories of the 70s when we were designing some equipment for controlling (synchronizing) the massive power generators in power stations. This equipment had to survive an almost direct lightning strike to the power grid, we used the Transorb to capture the leading edge of the wavefront, then an array of MOVs for the middle of the wavefront and followed by a ionization tube to dissipate most of the wavefront energy.

I recall long ago the Transorbs were originally developed by General Semiconductor under a USG contract. A story behind this development (can't say it's true) was a direct lightning strike hit an ICBM silo and caused multiple "Fail Safe" circuits to "Fail Unsafe" :o  Evidently back then the usual over voltage protection wasn't quick enough nor strong enough to deal with the lightning wavefront, and better protection was required.

Best,
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Offline NANDBlog

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Re: Never Trust Other People's Equipment!
« Reply #17 on: October 11, 2020, 11:26:29 pm »
The word "calibration" comes to my mind. There are two kind of equipment in a lab, calibrated ones and paperweight.
For Engineering T&M power supplies I would have the same opinion. But this wasn't anything remotely like an Engineering T&M power supply. This was a grunty high current fixed voltage power source. No adjustments were made available. I didn't crack the box but I'd bet there aren't any internal adjustments either - it's likely designed with fixed components and no "calibration" is required or even possible.

Next time I'm there, I'll ask if I can open it up. I suspect they'll be OK with that. I'll take photos of what's inside and post them here.
When you measure the output voltage, that is technically calibration. Int he original sense of the word.
"In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy."
You eventually did this, just in the wrong order.
 

Offline Bud

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Re: Never Trust Other People's Equipment!
« Reply #18 on: October 11, 2020, 11:27:42 pm »
Quote
so I asked for a wrench so I could go remove the battery from our rental car

Bet you could end up with a non-working airbag at a minimum, and had to deal with the car rental company to fix things.
Facebook-free life and Rigol-free shack.
 
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Re: Never Trust Other People's Equipment!
« Reply #19 on: October 11, 2020, 11:40:20 pm »
Great story, IDE. I had my share of faulty environmental issues caused by bad customer-furnished hardware, but nothing as dramatic. In my case it was malfunction that threw us in a loop until we sourced the root cause to things like ground loops coming from bad plugpacks, faulty USB or serial cables, misconfigured hardware, etc.
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Offline radiolistener

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Re: Never Trust Other People's Equipment!
« Reply #20 on: October 11, 2020, 11:53:53 pm »
Bet you could end up with a non-working airbag at a minimum

why? I'm switch off the battery when the car is not used for a long time, it prevents discharge.

At standby mode it consume about 50 mA.
24 * 0.05 = 1.2 Ah every day.
30 * 1.2 = 36 Ah for month.

36 Ah is a 60% for 60 Ah battery.
« Last Edit: October 11, 2020, 11:57:59 pm by radiolistener »
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #21 on: October 12, 2020, 03:13:54 am »
When you measure the output voltage, that is technically calibration. Int he original sense of the word.  "In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy."You eventually did this, just in the wrong order.
To me, "calibration" implies adjusting to spec - not simply measurement (unless the DUT is already in spec). If I measured this power supply and thus knew it was emitting 29V, yet left it in that condition despite its front panel clearly stating "13.8VDC", I would never describe what I had done as "calibrating it". Likewise, if I told someone I had "calibrated" that power supply and it was still emitting 29V, they would have a legitimate argument with me. One that might involve fisticuffs! :box:

 
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Offline Golds

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Re: Never Trust Other People's Equipment!
« Reply #22 on: October 12, 2020, 07:12:51 am »
You are right!  I support your point of view. :-DD

Offline paulbt

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Re: Never Trust Other People's Equipment!
« Reply #23 on: October 12, 2020, 11:56:51 am »
I whipped out the DMM  :-DMM  and measured the voltage coming out of their supply.

29.1 VDC.

|O :scared:   :wtf:   :rant:

Keep in mind this was a simple analog supply. Not a battery charger. Not a jump starter. It had typical threaded banana jacks on its front panel. It was meant for a bench environment. And it was labeled 13.8VDC right on the front panel, with no adjustments anywhere on its six outside surfaces.

Hi, IDEngineer !

Interesting story you shared to us, thank you! It is definitely a lesson worth to be learned or at least remembered from time to time. Good thing you brought with you a trusty dmm  :-DMM , it saved you from total embarassment.  :-+
About that power supply, if it was linear and beefy as you said, I think the pass transistor was shorted and the full voltage from the bridge rectifier was delivered to the output terminals (assuming it was a very simple design w/o crowbar or other safety circuitry).

Have a nice day! Cheers !

"Simplicity is the ultimate sophistication."  Leonardo Da Vinci
 

Offline Berni

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Re: Never Trust Other People's Equipment!
« Reply #24 on: October 12, 2020, 12:09:15 pm »
Nice adventure there. Love the MacGyvered forklift solution too.

Pretty sure i would have done the same mistake, if it looks like a fixed output PSU and it has the output written right there on the front id believe it and hook up to it. Surely they would have found out beforehand if it was faulty or something...right? Guess not. :palm:
 

Offline AndyC_772

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Re: Never Trust Other People's Equipment!
« Reply #25 on: October 12, 2020, 12:26:51 pm »
To me, "calibration" implies adjusting to spec - not simply measurement (unless the DUT is already in spec).

From the UKAS accredited laboratory we use:

"Calibration" = measuring the output of an instrument under known conditions and recording its output.
"Adjustment" = making changes to an instrument in order to make its output match the conditions it's actually measuring.

A reliable instrument can be calibrated every year without ever changing how it reads, and that's important. It means measurements you take with it today are directly comparable with ones you made last year, and the year before that, and the year before that etc.

If an instrument is adjusted, then its readings today are not directly comparable with those taken before adjustment was carried out. It may, however, be more accurate than it was previously, in absolute terms.

Which you need depends on your application. Normally when you send away (say) a DMM, it will come back calibrated - but only adjusted if it was actually out of spec to begin with. Even then, some labs wil ask first.

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Re: Never Trust Other People's Equipment!
« Reply #26 on: October 12, 2020, 07:11:09 pm »
To me, "calibration" implies adjusting to spec - not simply measurement (unless the DUT is already in spec).

From the UKAS accredited laboratory we use:

"Calibration" = measuring the output of an instrument under known conditions and recording its output.
"Adjustment" = making changes to an instrument in order to make its output match the conditions it's actually measuring.

A reliable instrument can be calibrated every year without ever changing how it reads, and that's important. It means measurements you take with it today are directly comparable with ones you made last year, and the year before that, and the year before that etc.

If an instrument is adjusted, then its readings today are not directly comparable with those taken before adjustment was carried out. It may, however, be more accurate than it was previously, in absolute terms.

Which you need depends on your application. Normally when you send away (say) a DMM, it will come back calibrated - but only adjusted if it was actually out of spec to begin with. Even then, some labs wil ask first.
Yes well, Cal technicians I know here in NZ look at instruments a little different.

First its performance is verified and if meets manufacturers specs a Cal cert is issued.
If performance/accuracy doesn't meet manufacturers specs adjustment is made until it does and then a Cal cert is issued.
Yes....it's all in the wording and not some magic voodoo process but a simple check and then adjustment if necessary.
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Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #27 on: October 12, 2020, 07:38:07 pm »
Right. Definitely splitting hairs on word definitions, but I doubt any self-respecting Calibration Tech would issue a "Calibration Certificate" for something that was merely measured and left out of calibration.

Maybe I'm just old school, but "calibrated" to me means the device is in spec - whether it was already there or had to be adjusted to get there.
 

Offline Fretec

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Re: Never Trust Other People's Equipment!
« Reply #28 on: October 12, 2020, 09:30:43 pm »
Maybe I'm just old school, but "calibrated" to me means the device is in spec - whether it was already there or had to be adjusted to get there.

This is the only meaning of "calibrated" I ever knew - it was checked to comply with the specs and it does, if it had to be adjusted it will be mentioned.
 

Offline fourfathom

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Re: Never Trust Other People's Equipment!
« Reply #29 on: October 12, 2020, 11:28:32 pm »
frankly it's not an issue in the real world. As I said, I've yet to see the lead-acid battery that accidentally generates 2X is chemistry's rated voltage. And our spec sheets are very clear about the max input voltage.

EDIT #2: I just finished characterizing our standard protection circuit for sustained high voltage conditions. We can safely tolerate a sustained overvoltage of ~18VDC before the tranzorb's conduction climbs enough to matter (above 10's of milliamps), and then the limiting factor is only internal heating of the tranzorb (proper device operation continues). That's fully 50% over the nominal rating of 12VDC, and 30% above the typical alternator voltage of 13.8VDC. If someone's on-vehicle regulator is permitting a sustained output voltage 30% higher than alternator nominal, ours won't be the only device that fails. In production there's always a limit to how much you can spend on protection against extreme cases while remaining commercially viable. I think we're OK where we are with 30-50% margin.

FWIW, marine batteries often see higher charging voltages than you find in automotive applications.  You may see over 15V sustained during equalization, and over 14V at normal high charge rates.  Your 18V seems safe enough, but cars and boats do usually have different battery environments.  Of course, some boats have 24V, 36V, or 48V systems as well. 12V is by far the most common though.
 

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Re: Never Trust Other People's Equipment!
« Reply #30 on: October 13, 2020, 01:00:08 am »
Right. Definitely splitting hairs on word definitions, but I doubt any self-respecting Calibration Tech would issue a "Calibration Certificate" for something that was merely measured and left out of calibration.

Maybe I'm just old school, but "calibrated" to me means the device is in spec - whether it was already there or had to be adjusted to get there.
Perhaps the Power supply had one of those NCR (No Calibration Required) stickers? I see several of them on power supplies around the lab in my company.
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Offline srb1954

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Re: Never Trust Other People's Equipment!
« Reply #31 on: October 13, 2020, 01:09:45 am »
Right. Definitely splitting hairs on word definitions, but I doubt any self-respecting Calibration Tech would issue a "Calibration Certificate" for something that was merely measured and left out of calibration.

Maybe I'm just old school, but "calibrated" to me means the device is in spec - whether it was already there or had to be adjusted to get there.
Perhaps the Power supply had one of those NCR (No Calibration Required) stickers? I see several of them on power supplies around the lab in my company.
I think that whoever thought up the idea of a No Calibration Required sticker was a penny-pinching accountant. What they really mean is that "this device should be checked or calibrated but we can't be bothered doing it".

Most of the instruments I have seen with these stickers really do require some form of calibration but it is left up to the user to somehow verify for themselves that the equipment performance is within spec. This is sometimes difficult to do when every piece of test equipment available to you has an NCR sticker on it.

 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #32 on: October 13, 2020, 01:17:09 am »
Perhaps the Power supply had one of those NCR (No Calibration Required) stickers? I see several of them on power supplies around the lab in my company.
I didn't see one on any of the six outside surfaces of its enclosure, and I definitely looked. While I was holding it I was tempted to throw it across the room but it would have probably gotten a final laugh by straining my muscles - as you might imagine, it had some heft.
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #33 on: October 13, 2020, 01:20:43 am »
FWIW, marine batteries often see higher charging voltages than you find in automotive applications.  You may see over 15V sustained during equalization, and over 14V at normal high charge rates.  Your 18V seems safe enough, but cars and boats do usually have different battery environments.  Of course, some boats have 24V, 36V, or 48V systems as well. 12V is by far the most common though.
Noted, thanks. Although the battery charging blocks (take in the alternator and emit to one or more batteries) seem to stay below 14V in this industry. Most of these watercraft have a helm-mounted digital voltage display and I've never seen one hit 14V, always 13.X with the engine running. And those displays track our own voltage measurements with independent meters. I suspect they are simple diode-isolated designs so both our numbers may be accurate and simply separated by a ~1V diode drop.
 

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Re: Never Trust Other People's Equipment!
« Reply #34 on: October 13, 2020, 01:41:22 am »
To me, "calibration" implies adjusting to spec - not simply measurement (unless the DUT is already in spec).

Definitely not a given.  Not all devices are adjustable, so calibration often involves recording the measured value. Others may have an adjustment or calibration constant procedure, but rarely or never need adjustment unless certain parts are replaced.

I  think the missing links here are the terms 'broken' and 'diagnosis'.  If your first notion after finding a device out of calibration is to 'adjust' it, even if it has a nice assortment of fiddly trimpots, then you should have your fingers slapped with a ruler by an angry nun.  You need to at least think through why it would be out of spec.

One thing that comes to mind in the case of your device-eating power supply is that by either a defective part or defective design, it may have excessively high voltage until some minimum load is put on it.  Imagine a simple system, designed only with large loads in mind, with a rectified and filtered supply voltage of 29 volts at open circuit, followed by a linear regulator at 13.8 volts, but with a 100R resistor bypassing the regulator.  Under any significant load, you have 13.8 volts, but your DMM shows 29 and your devices 'felt' about 29 volts.  Suppose there was a calibration procedure and it called for measuring the voltage and ripple at 1A and 50A, and nothing more.  It would pass 'calibration' and might never have any issues when used only with circuits with substantial draws--like simulating a boat electrical system or charging large batteries.  And then you come along and....zap!
« Last Edit: October 13, 2020, 01:45:06 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

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Re: Never Trust Other People's Equipment!
« Reply #35 on: October 13, 2020, 01:44:45 am »
I think that whoever thought up the idea of a No Calibration Required sticker was a penny-pinching accountant. What they really mean is that "this device should be checked or calibrated but we can't be bothered doing it".

It is a talisman to ward off evil ISO9000 auditors.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 
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Re: Never Trust Other People's Equipment!
« Reply #36 on: October 13, 2020, 02:15:30 am »
Right. Definitely splitting hairs on word definitions, but I doubt any self-respecting Calibration Tech would issue a "Calibration Certificate" for something that was merely measured and left out of calibration.

Maybe I'm just old school, but "calibrated" to me means the device is in spec - whether it was already there or had to be adjusted to get there.
Perhaps the Power supply had one of those NCR (No Calibration Required) stickers? I see several of them on power supplies around the lab in my company.
I think that whoever thought up the idea of a No Calibration Required sticker was a penny-pinching accountant. What they really mean is that "this device should be checked or calibrated but we can't be bothered doing it".
Well, to be perfectly honest, it could be penny pinching or engineers just like me that want to really leave a simple variable PS alone and not bother me with compliance questions, procedure and red tape... But that's just me. ::)
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Offline fourfathom

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Re: Never Trust Other People's Equipment!
« Reply #37 on: October 13, 2020, 04:09:51 am »
FWIW, marine batteries often see higher charging voltages than you find in automotive applications.  You may see over 15V sustained during equalization, and over 14V at normal high charge rates.  Your 18V seems safe enough, but cars and boats do usually have different battery environments.  Of course, some boats have 24V, 36V, or 48V systems as well. 12V is by far the most common though.
Noted, thanks. Although the battery charging blocks (take in the alternator and emit to one or more batteries) seem to stay below 14V in this industry. Most of these watercraft have a helm-mounted digital voltage display and I've never seen one hit 14V, always 13.X with the engine running.
I'm more familiar with sailboats where we may have a large "house" battery bank charged by high-current alternators and a multi-stage "smart" charger.  With this setup we try to get as much juice into the battery as we can, running the engine as seldom as we can.  We will occasionally run a higher-voltage equalization cycle.  There will often be a separate starter battery, with some sort of charge-sharing mechanism, but most of the charging current goes to the house bank.  The charging profile is much different than you might see on a small powerboat, which usually has a smaller alternator and a regulator similar to that in an automobile.

The above is for lead-acid batteries (flooded, gel, AGM, or foamed carbon).  Obviously Li batteries are coming into more general use and these will have still different voltage profiles.

Regardless, in a 12V system I think 15 or 16V should be a safe maximum sustained voltage to design for.
 

Offline srb1954

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Re: Never Trust Other People's Equipment!
« Reply #38 on: October 13, 2020, 04:32:10 am »
I think that whoever thought up the idea of a No Calibration Required sticker was a penny-pinching accountant. What they really mean is that "this device should be checked or calibrated but we can't be bothered doing it".

It is a talisman to ward off evil ISO9000 auditors.
You could well be right on that.
 

Offline Fretec

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Re: Never Trust Other People's Equipment!
« Reply #39 on: October 13, 2020, 01:31:40 pm »
I think that whoever thought up the idea of a No Calibration Required sticker was a penny-pinching accountant. What they really mean is that "this device should be checked or calibrated but we can't be bothered doing it".

It is a talisman to ward off evil ISO9000 auditors.
You could well be right on that.

He is, otherwise the not-calibrated unit would present an issue.
 

Offline NANDBlog

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Re: Never Trust Other People's Equipment!
« Reply #40 on: October 13, 2020, 07:14:12 pm »
Right. Definitely splitting hairs on word definitions, but I doubt any self-respecting Calibration Tech would issue a "Calibration Certificate" for something that was merely measured and left out of calibration.

Maybe I'm just old school, but "calibrated" to me means the device is in spec - whether it was already there or had to be adjusted to get there.
Perhaps the Power supply had one of those NCR (No Calibration Required) stickers? I see several of them on power supplies around the lab in my company.
I think that whoever thought up the idea of a No Calibration Required sticker was a penny-pinching accountant. What they really mean is that "this device should be checked or calibrated but we can't be bothered doing it".

Most of the instruments I have seen with these stickers really do require some form of calibration but it is left up to the user to somehow verify for themselves that the equipment performance is within spec. This is sometimes difficult to do when every piece of test equipment available to you has an NCR sticker on it.
I had PSUs calibrated, and the only thing I can say: Maybe I should've bought a second power supply and stick a "not calibrated" sticker on the first one.
Now we have 1 PSU which is calibrated externally, and that's what we use for traceavle measurement, for everything else, I wrote a stupid 1 page calibration manual. You only need a DC load and an Oscilloscope BTW for that. And the original manual asked for a 6.5 digit DMM, for good measure. Pun intended.
 

Offline Mr. Scram

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Re: Never Trust Other People's Equipment!
« Reply #41 on: October 18, 2020, 11:55:56 pm »
Right. Definitely splitting hairs on word definitions, but I doubt any self-respecting Calibration Tech would issue a "Calibration Certificate" for something that was merely measured and left out of calibration.

Maybe I'm just old school, but "calibrated" to me means the device is in spec - whether it was already there or had to be adjusted to get there.
I'm pretty sure you'll get into a fist fight with the owner of something like a 3458A sooner or later if you start adjusting equipment without them asking for it. You only adjust equipment if and when asked.
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #42 on: October 19, 2020, 12:13:51 am »
Not trying to fight with anyone. But if I wanted a cal lab to just report to me a device's present status, I'd ask for something like an "evaluation". If I sent it in for "calibration" I'd expect it to be in spec when it came back. But I accept that mine may just be the commonly accepted definition and not the strict definition of the word in the context of metrology. That said, I bet the majority of people - even in Engineering - would be startled if "calibration" didn't yield a device that met its specifications.

Here's an honest question: When a device has a Calibration sticker with a date on it, what exactly is being communicated? I've never seen a sticker that said anything like "Measured to be out of specification by 0.1% on the following date". It's just "Last calibrated on {date}". If all they did was determine that the device was out of spec, how are you supposed to know that from the sticker and its date? On the other hand, if they only give it such a sticker when it's within spec, doesn't that imply that "calibrated" means "in spec"?
 

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Re: Never Trust Other People's Equipment!
« Reply #43 on: October 19, 2020, 01:27:59 am »
In the lab that I work, which I suspect reflects others, an equipment with a valid calibration means it is still within the manufacturer's accuracy specifications. If the manufacturer says their product operates with an accuracy of +/-0.005% or +/-5% or +/-50% of a standard reference, that is what you shouldexpectt when using it.

Many products are taken out of the lab and go to maintenance or are decomissioned in case they are "out of spec".
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Offline 0culus

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Re: Never Trust Other People's Equipment!
« Reply #44 on: October 19, 2020, 01:38:50 am »
Not trying to fight with anyone. But if I wanted a cal lab to just report to me a device's present status, I'd ask for something like an "evaluation". If I sent it in for "calibration" I'd expect it to be in spec when it came back. But I accept that mine may just be the commonly accepted definition and not the strict definition of the word in the context of metrology. That said, I bet the majority of people - even in Engineering - would be startled if "calibration" didn't yield a device that met its specifications.

Here's an honest question: When a device has a Calibration sticker with a date on it, what exactly is being communicated? I've never seen a sticker that said anything like "Measured to be out of specification by 0.1% on the following date". It's just "Last calibrated on {date}". If all they did was determine that the device was out of spec, how are you supposed to know that from the sticker and its date? On the other hand, if they only give it such a sticker when it's within spec, doesn't that imply that "calibrated" means "in spec"?

I think it's more of a traceability thing. If some question about a measurement comes up, you can conclusively prove that the instrumentation was in spec. Doing it super regularly is probably more of a CYA thing from legal (for both the vendor suggesting you do it regularly, and you at the company having it done regularly). 
 

Offline IDEngineer

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Re: Never Trust Other People's Equipment!
« Reply #45 on: October 19, 2020, 02:15:45 am »
 It's been asserted that "calibration" is merely measuring what the device does - not adjusting it to spec. So if you find an in-date calibration sticker, could that mean the device was merely measured on that date with no guarantee of being in spec? After all, that's all a "calibration" sticker says based on this thread's proposed definition.

If that's true, a sticker is meaningless. I've never seen one that states how far OUT of spec the "calibrated" device was measured to be. So all it tells you is that someone measured it on a given date. You know nothing about what was actually discovered.
 

Online bdunham7

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Re: Never Trust Other People's Equipment!
« Reply #46 on: October 19, 2020, 02:34:57 am »
It's been asserted that "calibration" is merely measuring what the device does - not adjusting it to spec. So if you find an in-date calibration sticker, could that mean the device was merely measured on that date with no guarantee of being in spec? After all, that's all a "calibration" sticker says based on this thread's proposed definition.

If that's true, a sticker is meaningless. I've never seen one that states how far OUT of spec the "calibrated" device was measured to be. So all it tells you is that someone measured it on a given date. You know nothing about what was actually discovered.

A calibration sticker, without the actual calibration certificate, is technically pretty worthless.  If you were audited, for example, you might have to go back to the calibration lab for a copy if you didn't have it.

That said, for most common things you would think of having calibrated, like a typical DMM, the usual calibration service would indeed be verifying that it is within the manufacturers specifications and adjusting it where possible.  This is a pretty standard service and for most customers and most labs with most service-bench DMMs, a unit that is out of spec and cannot be adjusted for one reason or another will get a 'REJECT' sticker.  So if you see a Fluke or HPAK bench DMM with a calibration sticker from Fluke or Keysight that is less than a year old and the meter appears undamaged and operating properly, you can be pretty sure it is within the published specs.  Well, actually not quite even that because the calibration procedure doesn't necessarily cover the performance limits.  So  you'd need to study both the specs and the calibration procedure itself to be as sure as possible.

The problems occur when you try to apply this principle too widely without actual, specific knowledge about what happened, which means reading the actual certificate.  And, in some cases, deciding whether to believe it. 

https://www.eevblog.com/forum/metrology/can-we-believe-a-calibration-certificate/msg3141840/#msg3141840

EDIT:  Also wanted to mention, 'calibration' and 'calibration sticker' are not synonymous.  Devices like precision standards, Weston cells and the like may have labels where data is recorded or may say 'see certificate' or 'see log'.  I would not expect an ordinary sort of bench service device with a plain calibration sticker with no info except a date and expiration to not be calibrated to manufactures specs if done by an outside lab.  However, I'm sure that there are exceptions to even that expectation--I just can't recall any right now.



« Last Edit: October 19, 2020, 03:13:58 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Offline 0culus

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Re: Never Trust Other People's Equipment!
« Reply #47 on: October 19, 2020, 02:36:44 am »
Presumably there is paperwork on file. Case in point, my HP/Agilent 8664A sig gen. It was seen by Agilent Calibration in 2013, and the sticker expired 3 years later. I called Keysight's cal folks up out of curiousity, and they dug up the full paperwork on it and sent it to me. It was exactly what you'd expect...detailed the uncertainties of various things and whether or not it met the original manufacturer specifications.

Any other definition of "calibration" is pure :bullshit:, IMO

 

Offline 0culus

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Re: Never Trust Other People's Equipment!
« Reply #48 on: October 19, 2020, 02:40:48 am »
I will add another curiousity to the pile though...the HP 53310A modulation domain analyzer. These instruments are often found on the used market with cal stickers and seals...which is really much ado about nothing as anyone with a reasonably high quality 5V reference source can do the calibration. That is the ONLY external signal needed!
 

Offline Berni

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Re: Never Trust Other People's Equipment!
« Reply #49 on: October 19, 2020, 04:59:58 am »
The purpose of the calibration sticker is to be able to quickly identify if the equipment you are using is still within its calibration period, that is IT.

The calibration lab that put the sticker on has the full report on file, so if you are there costumer you can call them up and have them dig it up for you according to the equipment serial number.

So what use is that piece of paper that goes with the sticker? It was a pretty damn expensive piece of paper after all! Well if shit hits the fan and someone threatens you that your products are out of spec you can show them a calibration report that your equipment is in spec. Especially if you have two reports, one before the use of it and one after. This definitely proves that the gear was in spec at the time the measurement was taken since its extremely unlikely it would have drifted off and then drifted back in time for the calibration. But if say you are a multimeter manufacturer and need to calibrate your own meters, in that case you might even use the numbers off the calibration sheet of your expensive transfer standard multimeter to compensate for its error. Who cares if it shows 4.9999871 V when you give it 5V, its within spec and if it shows 4.9999876V on the next calibration you can be sure you can trust it down to almost the last digit. You know when its showing 4.999987x that is bang on 5V according to the cal labs fancy pants NIST traceable standard.

A calibration sticker does NOT automatically mean it is in spec. It just usually means it is in spec, but does sort of grantee its not way off. When the cal lab notices something being out of spec they will notify the costumer for further action. If it slowly drifted slightly out of spec they might calibrate it anyway and just keep that in mind to avoid loosing cal traceability. But if something shows way out of spec then they know there is something wrong with the equipment so they need to repair it or decommission it and it will not get a calibration sticker (but likely instead a big red fail sticker). If you don't need past traceability and its slightly out of spec then you can ask the call lab to also adjust it (this is never done without your permission).

So to sum it up. A calibration sticker on its own mostly means the equipment was in good working order on that day. People using the equipment will very rarely test the equipment for proper functionality or drift, but equipment carrying calibration stickers is checked out regularly by trained professionals to make sure it is performing as it should.
« Last Edit: October 19, 2020, 05:01:59 am by Berni »
 
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Offline uski

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Re: Never Trust Other People's Equipment!
« Reply #50 on: October 20, 2020, 04:51:32 am »
Last automotive device I designed could handle ~100V sustained at its input for any period of time. In both polarities. And higher for pulses (TVS).
Shipped hundreds of thousands, no noticeable failure rate in the power supply. But it did cost a few extra parts.

Check out IEC 7637-2 for some scary overload voltage waveforms ("load dump") especially for 24V systems (I designed for 12V)
 

Offline uski

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Re: Never Trust Other People's Equipment!
« Reply #51 on: October 20, 2020, 04:54:15 am »
I would like to add that you should really add a fuse in your power supply. You decide if you want to put it before or after the TVS (I would put it before, but yes it will blow up if there is a big spike that could otherwise be absorbed by the TVS).

There are failure modes which can lead to a high current consumption and many, many, many people installing devices in cars add high gauge wires like 24AWG, with no additional fuse, on a 10A or 20A line, which can be a fire hazard.
An internal fuse will at least protect the wiring if there is an internal fault such as a MLCC cap shorting or similar. Just my opinion.

Cheapest and simplest is a 0805 or similar non replacable fuse which is what I would install. You could also go with a polyfuse, but I don't really like them since they get really hot in case of failure and also do not turn off the current completely. Or go with a replaceable fuse but most of the time it is not appropriate, you don't want people to start replacing fuses in an automotive PCB.
« Last Edit: October 20, 2020, 04:55:49 am by uski »
 


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