Handheld meter robustness testing

[bdunham7]

Protecting a meter would need something much faster.   

Why does it need to be particularly fast?  You're really only protecting the shunt or in most cases, the shunt bypass diodes.  I think the one in line G from this datasheet would work as long as the 1R shunt is rated for 1W or more with a good surge rating.  The only catch with polyfuses is that their trip characteristics are highly dependent on ambient temperature, their initial state and probably some luck.  It might all work fine at room temperature but not so well in hot or cold environments.

One other issue I see is the max fault current seems low, so this might mean there is really no point in using a higher voltage PPTC because it will fail anyway just when you need it.  If I'm interpreting that spec correctly,  in effect their "interrupt rating" is so low that it renders them useless for this purpose.

[joeqsmith]

Protecting a meter would need something much faster.   

Why does it need to be particularly fast?  You're really only protecting the shunt or in most cases, the shunt bypass diodes.  I think the one in line G from this datasheet would work as long as the 1R shunt is rated for 1W or more with a good surge rating.  The only catch with polyfuses is that their trip characteristics are highly dependent on ambient temperature, their initial state and probably some luck.  It might all work fine at room temperature but not so well in hot or cold environments.

One other issue I see is the max fault current seems low, so this might mean there is really no point in using a higher voltage PPTC because it will fail anyway just when you need it.  If I'm interpreting that spec correctly,  in effect their "interrupt rating" is so low that it renders them useless for this purpose.

As far as cost, I'd think that a PTC along with maybe one step up in shunt size (power) would allow them to eliminate the protection diodes as well.

I think these two posts go hand-in-hand.   When I tested using that 4007 diode, I wanted to get a feel for if it would be damaged from that longer trip time.  4007 seems to be a common choice in many of the meters I have looked at.   They are only rated for 1A, but can hand handle much higher currents for short duration's.  The cold trip of the PTCs could be several amps.   

Without a way to clamp it,  the fuse may never trip and you could potentially expose the meter to higher voltages than it was designed for.  Looks like the UT61E has a 100k in series with the current input.  Maybe that is enough to allow the IC to survive, but I doubt it. 

[bdunham7]

I think these two posts go hand-in-hand.   When I tested using that 4007 diode, I wanted to get a feel for if it would be damaged from that longer trip time.  4007 seems to be a common choice in many of the meters I have looked at.   They are only rated for 1A, but can hand handle much higher currents for short duration's.  The cold trip of the PTCs could be several amps.

I saw that earlier and was going to mention that the 1N4007 is actually tougher than that, but didn't see how it was relevant.  Now that the relevance is apparent, I'll point out that a properly installed 1N4007 can withstand a continuous current of greater than 2 amps and a surge current of >30A for quite a long time (1/2-cycle).  That 1A rating is for average rectified current at an ambient temp of 75C.  Several amps for a few seconds to trip a PTC shouldn't be an issue.

[joeqsmith]

I think these two posts go hand-in-hand.   When I tested using that 4007 diode, I wanted to get a feel for if it would be damaged from that longer trip time.  4007 seems to be a common choice in many of the meters I have looked at.   They are only rated for 1A, but can hand handle much higher currents for short duration's.  The cold trip of the PTCs could be several amps.

I saw that earlier and was going to mention that the 1N4007 is actually tougher than that, but didn't see how it was relevant.  Now that the relevance is apparent, I'll point out that a properly installed 1N4007 can withstand a continuous current of greater than 2 amps and a surge current of >30A for quite a long time (1/2-cycle).  That 1A rating is for average rectified current at an ambient temp of 75C.  Several amps for a few seconds to trip a PTC shouldn't be an issue.

As I mentioned, it was a "Very crude test".  Voltages were no were near 60V, and the parts were not cooled.  As far as how long that part I cobbled up would take to clear and if a 4007 would survive it under these conditions, hard to say.   60 - say 2V diode drop and cables....  PTC may take 10s of ms to clear.   

Let's just say I am not going to stress that Fluke meter running these tests with that cobbled part I show. 

[rsjsouza]

Joe, I think one that has been making the rounds is the ....  and also sold cheaper as AstroAI M6KOR - mind you, these are better built than the absolute bottom of the barrel I mentioned in the other thread.  ...

Sorry, a bit late to the party... Excellent video! I could tell this little guy was quite above the average when I did my review and tests.
Seeing the switch was still in excellent shape, indeed the puny PTC seems to be the one that cascaded down to the other parts of the DMM.

[joeqsmith]

Sorry, a bit late to the party... Excellent video! I could tell this little guy was quite above the average when I did my review and tests.
Seeing the switch was still in excellent shape, indeed the puny PTC seems to be the one that cascaded down to the other parts of the DMM.

Glad you enjoyed it.   It was doing well, until it wasn't...     I was surprise that we really didn't see much happen.  Sure a few breakdowns, a couple of transistors, and a smoked trace,  but not at all what I was looking for with a copper plated board.     Someone had asked about if the fuses were filled and how they performed.   Filled it easy enough to check but I am not sure how I would test them.   In the past, I have compared fuses against a datasheet to try and determine if they were counterfeit.  With these low end fuses, I am not sure what I would use for a criteria.   

[bdunham7]

As I mentioned, it was a "Very crude test".  Voltages were no were near 60V, and the parts were not cooled.  As far as how long that part I cobbled up would take to clear and if a 4007 would survive it under these conditions, hard to say.   60 - say 2V diode drop and cables....  PTC may take 10s of ms to clear.   

IDK the exact part number of PTC you have, but they seem to have max surge current ratings similar to the diode.  I also don't know how badly that spec can be abused or what happens if you exceed it.  I know they use parts in automotive applications that would appear to have a 100A fault current rating when obviously fault currents could be higher.  Perhaps they're gambling that typical fault current won't be that high or perhaps that spec doesn't mean what I think.  Or maybe they always fail open with surge currents so there's no safety or fire issue.

In any case, as you increase the surge current the instantaneous power dissipation of the PTC goes up as the square of the current and then more as it warms up, while the diode power only goes up slightly more than linearly and then less as it warms up, at least to a point.  I tried an axial-leaded 1N4004 I had out and put it across a PSU.  After 3A for 30 seconds V_f was ~0.9V and the case temperature was 148C according to my thermal camera.  So at the low end I'm pretty sure the PTC would trip before the diode melted.  As for the high end, it would be an interesting race I guess.  I don't have a source for that unless I start stringing car batteries together or something, but I did take that same 1N4004 and put it across a transconductance amp (current calibrator) and gave it 20A pulses of increasing duration.  It was fine at 20A for 200ms every 10 seconds, but as you can see in the picture when I extended that out it failed just after the 500ms mark.  I'd guess that race would be a close call at 60V (and the PTC might fail anyway) but up to 20-30V it will work out.  That makes me wonder how that Mastech meter with the PTC would actually perform with 240VAC from your service panel.

[joeqsmith]

  After 3A for 30 seconds Vf was ~0.9V and the case temperature was 148C according to my thermal camera.  .....  but I did take that same 1N4004 and put it across a transconductance amp (current calibrator) and gave it 20A pulses of increasing duration.  It was fine at 20A for 200ms every 10 seconds, but as you can see in the picture when I extended that out it failed just after the 500ms mark.

I am surprised it would take that much abuse.  Then again, the generator I use to test multi-meters pushes many of the components well over their ratings.  The transients it generates are so short,  its not a problem.   Still, there is nothing in the datasheets for the regions I run them so a bit of trial by fire.  Literally.   

That makes me wonder how that Mastech meter with the PTC would actually perform with 240VAC from your service panel.

  That's not happening any time soon.     Maybe Dave will get one to demonstrate it. 

[rsjsouza]

Sorry, a bit late to the party... Excellent video! I could tell this little guy was quite above the average when I did my review and tests.
Seeing the switch was still in excellent shape, indeed the puny PTC seems to be the one that cascaded down to the other parts of the DMM.

Glad you enjoyed it.   It was doing well, until it wasn't...     I was surprise that we really didn't see much happen.  Sure a few breakdowns, a couple of transistors, and a smoked trace,  but not at all what I was looking for with a copper plated board.     Someone had asked about if the fuses were filled and how they performed.   Filled it easy enough to check but I am not sure how I would test them.   In the past, I have compared fuses against a datasheet to try and determine if they were counterfeit.  With these low end fuses, I am not sure what I would use for a criteria.   

I was wondering about if the grease gave an edge of protection to the switch contacts... Just above the protection devices. Oh well... It survived quite the jolt, albeit not to your max standards.

[joeqsmith]

I was wondering about if the grease gave an edge of protection to the switch contacts... Just above the protection devices.

The grease may indeed help increase the breakdown, in that area.  Of course, the fused link also helped to minimize the damage.   

***
https://youtu.be/e_YzwO62feQ?t=2370
***

Oh well... It survived quite the jolt, albeit not to your max standards.

That is a true statement when looking at all of the meters I have tested in general.  A high percentage of them did not even survive to the 2.5kV test this meter was exposed to prior to being damaged.  That said, the bar is set pretty low as most of the meters I have looked at are on the bottom end of the spectrum. 

Maybe the Klein will hold up better.

[ispoki]

Hi. Have problem with Uni-T UT139E DMM, I think that it took some voltage on Resistance mode. Unfortunately, BJT transistor clamp protection didn't help, they are fine, checked. Obviously the blame is to DTA0660L chip. It is w well known in cheap meters 6000 ADC chip, but, I can not find any datasheet for it. Is it the same as DTM0660, or some other chip? I also can not find this chip anywhere, not as for DTM0660, which are a lot in marketplaces. May someone help with it?
In my meter this chip DTA0660L is used with an MCU HY11P14. As I understand from PCB, the last one is responsible for LCD indication, and, probably for RMS measurement. DTA0660L, as I see is used for Resistance, diodes, caps, etc..

The question is, may I replace DTA0660L by DTM0660L? Or, may be, someone has any information about this chip?

[joeqsmith]

Brymen latest DMM from the BM78x series complete with BlueTooth.  My plan it to write some software to interface with it and then like always, put it through it's paces.  Stay tuned.

[EEVblog]

Brymen latest DMM from the BM78x series complete with BlueTooth.  My plan it to write some software to interface with it and then like always, put it through it's paces.  Stay tuned.

Well then, cats out of the bag. You got one before me. My BM787BT is still on the truck somewhere.
Brymen originally promised this back before the BM780 was released, but they found development was too hard, so put it on the backburner.
There is only an android app at present.

[bdunham7]

Does the BT add that much to the price? 

[EEVblog]

Does the BT add that much to the price?

It does actually, the factory FOB cost is now more than my 121GW. So it will be the most expensive meter I sell.

[tautech]

Does the BT add that much to the price?

It does actually, the factory FOB cost is now more than my 121GW. So it will be the most expensive meter I sell.

Yup, add BT to anything and the cost jumps due to licensing. 

Currently looking for a exterior IP68 rated BT security keypad with phone APP capability right now and the WiFi capable ones are 1/2 the price.   

[bdunham7]

Yup, add BT to anything and the cost jumps due to licensing. 

But the 121GW has BT...

I understand that the BT-SIG overlords need their cut, but unless I miscalculate this is adding ~$80 USD to the price?

[EEVblog]

The v14 firmware indicates that the firmware could be identical on the BM780 and BM780BT, the only difference being the checking on the firmware to enable the <<D>> annunciator and data output.
The original BM786/9 had the <<D>> LCD annunciator and the bluetooth module header connector, as it was original supposed to be released at the same time or not long after the BM780.
It even has a BT production marker on the PCB.
I'd be very surprised if the BT PCB is any different at all to the 780 series except the BT module added to the existing header pins.
Mine is due for delivery tomorrow.

[EEVblog]

I understand that the BT-SIG overlords need their cut, but unless I miscalculate this is adding ~$80 USD to the price?

Where did you get the pricing from?

[bdunham7]

Where did you get the pricing from?

Your 786 is $139 USD in the EEVBlog store, the 121GW is $205 USD.  You said the new meter would be more than the 121GW and there's $66 between them.  So maybe I should have said "at least $67"? 

[EEVblog]

Your 786 is $139 USD in the EEVBlog store, the 121GW is $205 USD.  You said the new meter would be more than the 121GW and there's $66 between them.  So maybe I should have said "at least $67"?

I'm terrible at setting and updating margins and prices, and prices vary based on the exchange rate at the time I last purchased stock (Shopify forces me to price all products on the store in AUD, not USD that I purchase at)
I can say that the BM787BT factory FOB cost to me will be more than the 121GW factory FOB cost, so in theory, if the margin are the same, the BM787BT should cost more.

[GuidoK]

I can't wait to see the next video's about the BM788BT. Your reviews are on another level! 
The absence of logging always seemed strange to me in the BM78x series, especially as the pcb inside had a type checkbox called "BT".
And of course it was by my knowledge the only modern/recent high count multimeter on the market that didn't have logging.

[joeqsmith]

Brymen latest DMM from the BM78x series complete with BlueTooth.  My plan it to write some software to interface with it and then like always, put it through it's paces.  Stay tuned.

Well then, cats out of the bag. You got one before me. My BM787BT is still on the truck somewhere.
Brymen originally promised this back before the BM780 was released, but they found development was too hard, so put it on the backburner.
There is only an android app at present.

Having looked at so many Brymen products,  I'm not too concerned about running my potentially destructive tests on it.  I plan to start by writing a Windows application for it.  Once I have a way to log data, I'll run some tests with it.

The only testing I have done was what was shown in that intro.  I didn't notice any problems and assume that with the 789 being a mature product, most of the major ones have been resolved.  Still, I'll look forward to seeing your review on it. 

[joeqsmith]

I can't wait to see the next video's about the BM788BT. Your reviews are on another level! 
The absence of logging always seemed strange to me in the BM78x series, especially as the pcb inside had a type checkbox called "BT".
And of course it was by my knowledge the only modern/recent high count multimeter on the market that didn't have logging.

I agree with your comments about the data logging.   Even that cheap OWON I looked at supports BLE, although I never wrote any software for it.  That early prototype Brymen provided has no radio nor any markings, but it was on their radar back then.   

[bdunham7]

I'm terrible at setting and updating margins and prices, and prices vary based on the exchange rate at the time I last purchased stock (Shopify forces me to price all products on the store in AUD, not USD that I purchase at)

I think your prices are right, the 786 does seem like a bargain for what it is but other sellers have the 789 for not too much more.  The prices all show as USD to me now, I've seen them in AUD previously. 

I can say that the BM787BT factory FOB cost to me will be more than the 121GW factory FOB cost, so in theory, if the margin are the same, the BM787BT should cost more.

Well I guess we'll have to see what price the BM788BT shows up at.  Any clue how much battery life will be affected?  Is the BT always on?