Power multimeter from wall

[stj]

So I bought a 25F 2.7v super capacitor and built a small circuit using LT1307 to boost the 1v .. 2.7v to 5.5v to keep the multimeter happy.

To charge the supercapacitor, I use a simple 1117 linear regulator which charges the supercapacitor from USB (5v) with 2.65v (to keep the voltage below the supercapacitor's maximum 2.7v and the linear regulator has an internal current limit of around 1.2A so the USB ports aren't overloaded. 

It takes about 1 minute, maybe a bit more to charge the supercapacitor and then I can disconnect the multimeter from usb and it will run just fine for more than 30 minutes.

you may want to read the datasheet for that "super"cap,
all the ones i'v seen state the lifespan in charge/discharge cycles at something like 500 - no better than a LiPo battery except the battery will last more than 30min's per cycle!

[retiredcaps]

But anyway stay away from 9v meters, they discourage use.

Why such a blanket statement?  Some models of the Fluke 70 series I, II and III can run 2,000 (two thousand) hours on a 9V battery.  The popular Fluke 87V can run 400 hours on a 9V battery.

Some advantages of a 9V battery are potentially more voltage on diode test to light up a led and any possible leakage might be contained inside the 9V case.

Having said that, I prefer AA/AAA batteries in my meters only because I have a large stash of Eneloops.

[retiredcaps]

Well I went and replace the battery and hmm something seems off.  The "dead" battery shows 9.43v so when it shutoff and would not come back on I thought it was a bad battery but it was something else.

Time to measure how much current your meter consumes if you can't find a manual and the above is happening.

My guess is that normal current consumption should be 5mA or less.  Assuming a 500mAh 9V battery, that gives you about 100 hours.

[mariush]

So I bought a 25F 2.7v super capacitor and built a small circuit using LT1307 to boost the 1v .. 2.7v to 5.5v to keep the multimeter happy.
[..]

you may want to read the datasheet for that "super"cap,
all the ones i'v seen state the lifespan in charge/discharge cycles at something like 500 - no better than a LiPo battery except the battery will last more than 30min's per cycle!

It was a Cooper Bussman HV series supercapacitor, 25F 2.7v : http://www.cooperindustries.com/content/dam/public/bussmann/Electronics/Resources/product-datasheets/Bus_Elx_DS_4376_HV_Series.pdf

The only reference to lifespan in datasheets is where it says  Cycle Life :  <=30% capacitance change after 500k charge-discharge cycles , where cycle is "cycling between rated voltage and half voltage, 3 seconds rest at +20 °C"

A number that low (at 500) would really surprise me. Eve something like 10k would surprise me. I think that value was more or less something about how the supercapacitor would tolerate instant peak discharge, like shorting it's leads on something - i could understand the internals could be damaged with repeated abuse like that.

[Brumby]

If it was me, I'd consider getting an SLA and put a linear voltage regulator on it.  Have it connected to the meter and charging circuit via a DPDT switch and you will have full isolation - plus zero ripple to the meter.

Yes, the losses in the regulator won't be ideal - but it's a bench setup, so no biggie.

[joseph nicholas]

My suggestion would be to build a capacitor dropper.  You could put this tiny circuit inside the multimeter and you would then have just a small wire coming out to a mains plug.  Just measure the current draw of the meter and use the correct zener voltage.  Simple and cheap, plus you would essentially have a linear power supply.

Sorry Joseph but that's the most dangerous idea ever. You would have no galvanic isolation between the mains nd your meter probes. Don't even think about it! 

I've built on of these already to power my cheap function generator.  It works great.  I put in a case and tested it for hours.  It gives just enough power to make the thing work.  It is such a cheap, fast and easy way to get dc from mains I plan to use it for all my micro controller projects.  I will use a rewound MOT if I need more amps in my projects.  Give it a try, you wont be disappointed.

nico

[Jay_Diddy_B]

Hi group,

My advice to the OP would be find a used Fluke 8010A (3.5 Digits), 8012A(3.5 Digits with low resistance ranges) or 8050A (4.5 digits). Preferably one without the rechargeable battery options. The NiCad batteries in these are a PITA. These are quite cheap.

Use this when on the bench and save the Protek for portable use.

Fluke had wall warts for there 8020 and 8060 series hand held DMMs, but these were special wall adapters with high isolation.

I would not modify a DMM. The battery is probably connected directly to the common input terminal and could create a safety hazard.

Regards,

Jay_Diddy_B

[eventhorizon]

First I wish to that you all for the replies.  Very helpful and I went down more than one rabbit hole on this thread reading and learning so I got my money's worth out of this crazy idea that I had.  Unfortunately I think what ever happened to my meter to make it shutdown did something worse to it because now I can't measure amps with it.  When this shutdown happened I did I was measuring about 1.2 amps on it using the 15a fuse setting so I don't think I popped a fuse and looking at the fuse it looked fine (only visual checks at this time).  Now when hitting the same load I am seeing it spike up to 3amps then drop to 600ma then go to 2.3a so its all over the place on the reading and it should show a steady 1.2a.  If only for the learning I am going to troubleshoot this and see if I can find out what happened but even if I can fix this I am still going to be looking for a new one.

[Brumby]

If you can spring for $125 start with this one (a lot of happy users) :

https://www.amazon.com/EEVblog-BM235-Brymen-Multimeter/dp/B01JZ1ADCO/ref=sr_1_1?s=industrial&ie=UTF8&qid=1474659132&sr=1-1&keywords=eevblog

I - and many others here - will vouch for that.

[BMack]

First I wish to that you all for the replies.  Very helpful and I went down more than one rabbit hole on this thread reading and learning so I got my money's worth out of this crazy idea that I had.  Unfortunately I think what ever happened to my meter to make it shutdown did something worse to it because now I can't measure amps with it.  When this shutdown happened I did I was measuring about 1.2 amps on it using the 15a fuse setting so I don't think I popped a fuse and looking at the fuse it looked fine (only visual checks at this time).  Now when hitting the same load I am seeing it spike up to 3amps then drop to 600ma then go to 2.3a so its all over the place on the reading and it should show a steady 1.2a.  If only for the learning I am going to troubleshoot this and see if I can find out what happened but even if I can fix this I am still going to be looking for a new one.

Good choice to start looking for a new one. It's worth it to have a really good meter if you're going to use it a lot. I'm a huge fan of Fluke and every single one of ours at the shop lasts a LONG time with a 9V battery. Then you can fix your current meter and have a backup but remember your backup is only good if you can trust it to work EVERY TIME, it needs to work when your good meter doesn't. This is why I have two Fluke 87Vs, can't really explain why I have an 87III, a 115 and a 12 but I can count on every one of them working every time I grab it. The non-Fluke DMMs I have at one point have given me a false reading, all name brand $50-130 retail(none were Brymen or Agilent/Keysight, which are highly regarded).

The Fluke continuity beeper is great too. It's the only brand I've tried that I can put one probe somewhere in a circuit and zip across one side of a chip and quickly hear if I have a connection or not, this saves a lot of time.

[Kleinstein]

Well I went and replace the battery and hmm something seems off.  The "dead" battery shows 9.43v so when it shutoff and would not come back on I thought it was a bad battery but it was something else.  Its running now with its new battery and I have not tried to put the old one back in but it does make me wonder what happened to it.

If an "empty" battery still ready 9.x V this indicates a cheap old style "dry" cell (not alkaline). It is normal for these cells to have nearly full voltage till the end, but the internal resistance goes up. Alkaline cells are different: the open voltage goes down but internal resistance stays relatively low until below 1 V per cell.

I would look for better alkaline cells. They don't leak as often ( anyway only once per cell), leakage is not that corrosive and have considerably higher capacity (e.g. 2-4 times). So maybe all that happened was that you got a few of those bad dry cell batteries.

[joeqsmith]

Doesn't your multimeter have some smarts in it so it turns itself off if it is not used for a few minutes? My Fluke multimeter does and its Name-Brand 9V alkaline battery lasts for many years.
My multimeter has a low current LCD display with no high current backlight. I hope your multimeter does not have a current-hungry LED display or backlight?

Ya no fancy like that.  No backlight or auto-off but it does give me a "don't forget to turn off" beep if I do forget so at least it has that.   I wish I could find the manual for this and the googles have failed me so I don't know what the draw is on it or average battery life. 

If you forget to turn off after a period of time it will automatically shut down.

https://translate.google.com/translate?hl=en&sl=zh-CN&u=http://www.0211718.cpooo.com/product/3193123.html&prev=search

I have a small generic wall power supply that you can select the voltage and it has several ends on the output.  It is a linear type.  I am not sure what the isolation would be.  Personally, I would never consider going this route.  If there is a problem with the meter, have it repaired or replace it. 

It's an interesting meter with the tachometer feature.  How did you like (before it became damaged)?   Did you use the tach?  Curious how well it worked with various engines (wasted spark, coil over plug....)?

[Gyro]

I would not modify a DMM. The battery is probably connected directly to the common input terminal and could create a safety hazard.

Actually it is probably slightly worse than that, many meters using 7106 type ICs generate their own common input reference based on a weak regulator / reference at some fixed offset from one of the battery terminals. Injecting noise and spikes between the input common terminal and an external 'battery' supply would throw off the reference or kill the IC even before it became a user safety hazard.

As Davd Hess said a while back, handheld meters simply aren't designed to have their battery terminals exposed when they are in operation, it's just not safe.

[David Hess]

As Davd Hess said a while back, handheld meters simply aren't designed to have their battery terminals exposed when they are in operation, it's just not safe.

I have occasionally read stories where someone used a handheld meter to measure its own battery voltage.  This is safe to attempt *if* the common lead is not connected to anything but if common is connected even to the negative side of the battery, bad things™ may occur like destruction of the meter depending on the design.

Bench meters have galvanically isolated input circuits for safety and performance reasons so they can get away with measuring their own internal voltages *except* for those of the galvanically isolated input circuits themselves.  Tektronix designed the 7D13 DMM plug-in for their 7000 series oscilloscopes with galvanically isolated inputs so the 7D13 DMM can be used for troubleshooting and calibrating the oscilloscope that is is plugged into assuming that the CRT readout works; as I recall, some of their service manuals even do this but normally you would have some other meter available.

Some bench power supplies have floating outputs allowing either the positive or negative output to be tied or not tied to ground or any other reasonable potential as required; a Tektronix PS503 for instance allows up to 350 volts between ground and the floating outputs.  One of these could be used to power a handheld meter but performance would likely suffer and safety certainly would.  They sometimes do not support more than a few tens of volts of isolation so powering a handheld meter and then connecting the ground lead of the handheld meter to the 370 volt DC bus of an off-line switching power supply could be momentarily exciting.  The same issue comes up if you remove the chassis ground connection between an oscilloscope and power outlet allowing the oscilloscope to make floating measurements.

Somewhere I have a "small" modular high isolation power supply which uses an air gapped transformer for 1000s of volts of AC line to regulated DC output isolation (and like 30pF of capacitive coupling) but it is not much smaller than most handheld meters and larger than some.

An issue which will matter in some applications is the common mode capacitive coupling.  With a handheld meter in your hand, this will be about 200pF between common and earth ground.  The high isolation power supply I mention above adds 30pF to this.  A Tektronix PS503 which supports 350 volts of isolation adds 2000pF which will affect measurements of some circuits.

[eventhorizon]

I did manage to open it up and have a look see.  I could not find any thing obvious that was wrong.  Actually for a old multimeter it looked really good that has been covered in oil and sitting in a tool box for years.

It's an interesting meter with the tachometer feature.  How did you like (before it became damaged)?   Did you use the tach?  Curious how well it worked with various engines (wasted spark, coil over plug....)?

Thanks for the link man that does give me more information about this little guy than I ever had before.  I never did get to use the tach or really anything but volt, amp and ohm functions.  I always wanted to just never had the opportunity or the instructions/special probes if needed to do it.  Years ago this was given to me by my stepfather when I said I needed to measure something and since then I have always had it with me but he no longer had the temp or any other probes, manuals, case etc so he got a new one and this was his spare.

[joeqsmith]

I like the test lead connectors.  Crimped and soldered.    Brass standoffs.  Then there is that thermocouple connector.  Looks really well made for it's age.   Being your first meter and a gift at that, may be worth repairing just to have it.  Then again, I'm a bit of a packrat.   

[grifftech]

a pack of 6 D cells would last.

why not F cells from a lantern battery?