Can a DVM be Powered by a Wall Wart - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: October 15, 2023, 12:04:38 am
content: I have an Extech DVM that was tossed by my company due to the batteries leaking and eating away the plastic; including the tech who tried disassembling it and cracking it further.I was curious if using a wall wart would induce too much noise and ruin measurements.I thought a wall wart would be great because I can fit it into the unit, but then realized, wall warts are noisy.Maybe I can use a voltage regulator, but those are a bit noisy too compared to a DC battery.

author: pqass
author_rank: Super Contributor
timestamp: October 15, 2023, 12:18:12 am
content: And  screw-up isolation. Kaboom!

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 15, 2023, 12:19:32 am
content: Quote from: bostonman on October 15, 2023, 12:04:38 amAn external battery module is another option ?Or, allow both, find a low noise wall wart and pair it with a low noise regulator.It may never be perfect with a wall wart, but could still be useful.

author: bostonman
author_rank: Super Contributor
timestamp: October 15, 2023, 12:22:59 am
content: External battery module is a good idea.I have a box of wall warts and sometimes I run to them too quickly. They also have horrible voltage regulation when unloaded, so I always get nervous resorting to them.I may go with the external battery module.

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2023, 12:51:08 am
content: QuoteNO.1. BATT(-) is not input COM(-) and getting that wrong will blow up the DMM IC.2. Wallwart Y-cap will present high stray voltage of ~60VAC which will make some readings noisy and possibly damage some semiconductors that you are probing.3. You would depend on the wallwart's isolation to PE or neutral when measuring things, and who knows what quality WW you are going to use.I would fix the original battery holder, or put an external battery holder on it.

author: bostonman
author_rank: Super Contributor
timestamp: October 15, 2023, 01:02:42 am
content: Good call, I didn’t consider isolation.

author: Kim Christensen
author_rank: Super Contributor
timestamp: October 15, 2023, 01:11:09 am
content: Quote from: bostonman on October 15, 2023, 12:04:38 amI'd avoid the wallwart retrofit.Must be some strange/powerful acid to eat away the plastic.What type of batteries were they? Obviously not 9V because that would be easy. AAs? You can buy new contacts that might just fit the existing plastic holder, as long as the "acid tech" didn't damage it too much.Even places likedigikey stock those.

author: bostonman
author_rank: Super Contributor
timestamp: October 15, 2023, 01:23:13 am
content: It takes six AA’s, and it was quite eaten away. Most screw mounting posts were broken off, and then I had the (bright) idea of cutting away more plastic thinking I can fit a 9v behind the cover with a 3D printed holder that would hold everything in place.Looking online, these aren’t expensive meters, so I’m probably putting in more effort than it’s worth; especially since I have four or five Fluke units.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 15, 2023, 01:23:56 am
content: you can if you use real good transformer based ac adpators, you have to be sure .... not the one based on buck boost buck down electronics ... not crappy oneshad no problems of isolations like this .... sill i would avoid over 240vac or 240vdc voltages measurementsstill even on scopes with isolation 1:1 xformers even if they are based on real or smps supply never had problems if you use line and neutral but not the earth

author: wasedadoc
author_rank: Super Contributor
timestamp: October 15, 2023, 05:42:46 am
content: An external battery case also raises isolation issues. Potentially exposes you to the voltages being measured.

author: mariush
author_rank: Super Contributor
timestamp: October 15, 2023, 06:30:23 am
content: Make your own wallwart with a classic transformer - you can get 2-10VA pcb mounted transformers for under $10 - add a bridge rectifier, a capacitor and a linear regulator and you have a power supply.There are also digital power isolators you could use but usually they're quite inefficient (30-40%) and expensive.One other thing you could do is to have a super capacitor or two inside, and use a QI charger : get 5v from qi receiver, convert it to <2.5v if you're using 2.5v rated super capacitors and then use a step-up regulator to boost the voltage to the voltage needed by the multimeter.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 15, 2023, 07:19:47 am
content: There are 2 problem for using a wall wart with a DMM:1) Coupling of hum or other interference from a SMPS to the measurement. Those ready made small DCDC converters often produce quite some common mode interference.2) A saftey problem due the level of isolation: The cable to the wall wart would be somewhat connected to the measured circuit and this cable is usually only specified and made for low voltage, not withstand mains or possibly more. Depending on what is measured one may also get more than the normal mains votlage (e.g. from a 2nd phase) at the isolation of the transformer. Wall warts are anyway a product under cost pressure and it is not so rare to find dubious isolation quality.Because of the cable even an external battery pack is not ideal, but still way better than a wall wart.It is not impossible to use a wall wart, but it needs extra care, like a classic transformer based wall wart to start with an than an additional low EMI DCDC converter with good isolation inside the meter.For the usual low power handheld meters battery power is the way easier way and better isolation / less EMI than even the best DCDC converters.

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2023, 08:27:03 am
content: I'm saying NO due to the many issues, especially zapping the DMM IC as there is no protection at all between BATT(-) and input COM(-) so many people blow the IC. And the isolation (safety) issue as well. And the CM noise issue as well.Of course it can be done... this might be called a bench multimeter lol.I've an old mains-powered bench multimeter, with power transformer. I discovered it has a 1,000pF 3kV+47Ω 1/2W resistor to earth ground for the measurement side. Why?To control HF common-mode noise and prevent bad readings or crashing. If a multimeter is well shielded- some have metal foil wrappers inside (but most do not), then there is little problem measuring something very noisy like SMPS or VFD motor drive etc.But a handheld... they are untested for this level of CM.OP maybe a rechargeable lithium battery could fit? Like EBL 9V. But never USB charge it while taking readings, that's when you have no isolation at all.

author: bostonman
author_rank: Super Contributor
timestamp: October 15, 2023, 01:55:25 pm
content: QuoteI considered this too.Honestly.... I got this free and it's really not an expensive meter. I may just go with the enclosed 9v battery case, a connector, and call it a day. I don't see myself using this meter often unless it's to take measurements where I could jeopardize the meter.The plastic is damaged so badly (not by me) that I can't even screw it back together due to standoffs broken off.

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2023, 07:33:33 pm
content: I have a cheap dollar store multimeter that uses 12V A23/23A- battery which is just terrible due to either low quality or high price.So I glued a 9V battery holder on the back lol. It's a toolbox/car DMM so no hurt feelings about it.

author: bostonman
author_rank: Super Contributor
timestamp: October 15, 2023, 07:50:10 pm
content: That’s what I considered too

author: bostonman
author_rank: Super Contributor
timestamp: October 18, 2023, 02:19:21 am
content: I made an error, this meter is a bit more expensive than I initially thought. I have the Extech MG300https://www.agriculturesolutions.com/extech-cat-iv-insulation-tester-multimeter-pc-interface-mg300?gclid=EAIaIQobChMImaPL0MP-gQMVicvICh2mXAbfEAQYAiABEgL8ZPD_BwE

author: Kleinstein
author_rank: Super Contributor
timestamp: October 18, 2023, 07:50:57 am
content: A big part of the costs is likely due to the CAT 4 rating. With the modified battery box the CAT rating would no longer be valid, though the protection is likely still good.In this case it makes some sense that the company can no longer use the meter with a damaged battery compartment - chances are they need a meter with good protection and a no longer valid safty rating is a no go for commercial use.

author: Gyro
author_rank: Super Contributor
timestamp: October 18, 2023, 11:43:51 am
content: You might want to keep an eye out for a blown parts (well, case) mule on ebay. Of course a Cat IV meter is less likely to blow.

author: floobydust
author_rank: Super Contributor
timestamp: October 18, 2023, 08:13:27 pm
content: Nothing to lose by contacting ExTech and seeing if a replacement piece is available. They get dropped, scratched etc so they might sell bits.Don't ask the dealers - go to the manufacturer.

author: BrokenYugo
author_rank: Super Contributor
timestamp: October 18, 2023, 08:44:12 pm
content: Smashed up DMMs can be handy if you need a fairly high precision meter built into something, where the isolation won't matter as much.If you drip CA glue on fibrous material (cloth, cotton fluff, paper towel, anything wicking really) it kicks off fast (and hot, add dropwise and go slow) and quickly forms a pretty nice fiber filled plastic material, useful trick for building up destroyed screw posts, reinforcing cracks, etc.