Power multimeter from wall

[eventhorizon]

I have a Protek d-488 DMM that I got years ago and has always been my go to meter for what I do.  I until recently never did much with hobby electronics so usually I used it to test power supplies, computer connectors and car batteries and what not but never did much more than volts.  Now as I am measuring ohms and amps as much as I am doing volts I just spent my 3rd and last battery 9v battery that I have without making a trip to the store for more in less than a month.  While these batteries are not pricy really ~2.5$ per battery I am just tired of having to take them out and replacing them left and right and to make it more of a pain its one that I have to take the back cover off to get to the battery replacement.

So my thought was that I have lots of things that generate and manipulate power around me.  I have bucks, lm317s a box full of power supplies and also lots of protected 18650s.  So if I say use a buck converter with a 12v and have the buck set to 9v then I can work this as long as I want but what I really wonder is if the ripple in the converter will have an effect on the accuracy of the meter.

[Simon]

Not sure. You could heavily filter it with lots of LC low pass filters

[rstofer]

You can get the batteries for about $1.20 from Amazon:
https://www.amazon.com/9V-Batteries/b?ie=UTF8&node=389576011

I doubt very much that your meter will be satisfactory after bashing it up to replace the need for a battery.

Buy the handy 8 pack (I would buy a couple of them) and call it good.   Free 2 day shipping, I could have them on Sunday (given that today is Friday).

[eventhorizon]

You can get the batteries for about $1.20 from Amazon:
https://www.amazon.com/9V-Batteries/b?ie=UTF8&node=389576011

I doubt very much that your meter will be satisfactory after bashing it up to replace the need for a battery.

Buy the handy 8 pack (I would buy a couple of them) and call it good.   Free 2 day shipping, I could have them on Sunday (given that today is Friday).

I wasnt planning on drilling a large hole onto it and putting a connector on it or making it look nasty or anything.  Something small enough for to wires put a circuit board where the 9v would go and connecting it to the same connection as the 9v battery would use so when the time came and it needed to me a portable one again I just remove the circuit and pop in a battery and off I go.

[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.

[stj]

a pack of 6 D cells would last.

[Kleinstein]

Adding an external DC adapter usually has the adapter connected to the measured circuit. This can be a safety issue when measuring more than 50 V. Also the adapter could add capacitive coupled hum of RF noise. So usually this is not a good idea, unless you know what you ar doing.

One might go for a cheaper source of batteries or maybe rechargeable ones. However the  rechargeable ones are not without problems as NiMH can have poor performance and the voltage is often lower (e.g. 8.4 or even 7.2 V nominal).

[IanB]

Hand held meters are powered by batteries so they are floating and isolated from the device under test. Powering them from the mains is a potential safety issue that is best avoided. Try to find a cheaper source of batteries, perhaps by buying online in bulk. If the meter really eats batteries, maybe you could consider a new meter?

[Gyro]

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! 

[retiredcaps]

A quick search for a manual for your meter yields nothing.  What does your manual say in terms of expected battery life?

If it is nowhere close to the expected battery life, you could have excessive current draw despite your meter working.

[ez24]

When you get tired of farting around, look at the Multimeter spreadsheet for meters that use AA or AAA batteries (you can use rechargeable ones).  My only 9v meters are cheap Harborfreight meters that I use only to measure current (and will not feel bad if I blow it up).

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

But anyway stay away from 9v meters, they discourage use.  (if you are a Costco member - 9v are cheaper there)

[Audioguru]

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?

[Seekonk]

I have one that runs on a supercap and recharges from 120 AC or up to 24V DC.  You could always recharge it in a minute, but it seemed it always needed to be recharged.  Supercap died.  Waiting for a free one to show up.

[David Hess]

This is neither a good nor safe idea at all.

Bench meters implement high voltage galvanic isolation internally so *all* meter inputs are isolated from the power line including any common inputs.  Handheld meters rely on isolated battery power and good insulation.

[eventhorizon]

Man you guys are fast.

Adding an external DC adapter usually has the adapter connected to the measured circuit. This can be a safety issue when measuring more than 50 V. Also the adapter could add capacitive coupled hum of RF noise. So usually this is not a good idea, unless you know what you ar doing.

One might go for a cheaper source of batteries or maybe rechargeable ones. However the  rechargeable ones are not without problems as NiMH can have poor performance and the voltage is often lower (e.g. 8.4 or even 7.2 V nominal).

Hand held meters are powered by batteries so they are floating and isolated from the device under test. Powering them from the mains is a potential safety issue that is best avoided. Try to find a cheaper source of batteries, perhaps by buying online in bulk. If the meter really eats batteries, maybe you could consider a new meter?

Thats a good point that did pop into my head last night when I started thinking about the "I wonder if I could ..."  I feel myself leaning more towards rechargeable 18650 if I do end up doing the modification to her to move away from 9v batteries.  I think I have a ton of them laying about and depending on the drop out voltage of the meter (I will see when I test the bad battery) I may be able to get away with 2 in series (8.4v to 6v or so).  A new good quality meter is not really feasible or justifiable at the moment.  I would rather get the best use out of the stuff that I have and put my money for this hobby into other things.  If this means that I get batteries in bulk or what not then thats fine too.

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. 

This is neither a good nor safe idea at all.

Bench meters implement high voltage galvanic isolation internally so *all* meter inputs are isolated from the power line including any common inputs.  Handheld meters rely on isolated battery power and good insulation.

Thank you.  I was wondering after the replies how the big boys do it I am going to look into that for learning and such. 

[ez24]

Here are some things you can do with your 9v battery

over 10 million views !



For more help on hooking up AC, look at this guy's videos for ideas

[gnavigator1007]

Wasn't joeqsmith joking on the forum about doing something like this with the bm235 just the other day?

[eventhorizon]

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.

[Mechatrommer]

Here are some things you can do with your 9v battery
over 10 million views !

yeah that is what the youtube for, go back to stone age, dont use a lighter. my heat glue gun is missing again, my heat glue stick are dissapearing fast because the kids were watching youtube and before i know it, all the effort made is gone to the trash, duh...

[eventhorizon]

Here are some things you can do with your 9v battery
over 10 million views !

yeah that is what the youtube for, go back to stone age, dont use a lighter. my heat glue gun is missing again, my heat glue stick are dissapearing fast because the kids were watching youtube and before i know it, all the effort made is gone to the trash, duh...

You can also save a lot of cost and time by just touching the battery to a bit of steel wool.  Did that all the time as a kid to start fires.

[mariush]

I have an UT61E multimeter, which uses 9v batteries but works perfectly fine with as little as 3v, though it shows low voltage at around 5v.
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.

This could work for you but with my solution, you'd have to remember to always disconnect the meter from usb when you want to do measurements with mains voltages (as the ground of the meter would be connected to the ground of the usb). There are power isolator chips like Adum5000 for example which would fully isolate the meter but add to the price.

[David Hess]

This is neither a good nor safe idea at all.

Bench meters implement high voltage galvanic isolation internally so *all* meter inputs are isolated from the power line including any common inputs.  Handheld meters rely on isolated battery power and good insulation.

Thank you.  I was wondering after the replies how the big boys do it I am going to look into that for learning and such.

You can learn a lot from older meters by studying their full documentation.  Units like the Tektronix DM501 and DM502 have internal high isolation inverters which supply a floating power supply for the front end circuits despite having a pair of floating power supply windings available from a 60 Hz power transformer.  It is not difficult to design and build a high isolation DC to DC inverter using transformers designed for this specific application or pulse transformers intended for driving transistors across an isolation barrier like in an off-line switching power supply but I still would not recommend this course of action for a handheld meter.  It would be better to invest in some low discharge NiMH batteries and a charger.

[ez24]

You can also save a lot of cost and time by just touching the battery to a bit of steel wool.  Did that all the time as a kid to start fires.

Yeah, I remember doing that - make a video - maybe you will get 10 million views.  That is what is hard to understand, I just cannot imagine how much money they are making off the video.

[bitseeker]

Why not just use a rechargeable 9V-style NiMH battery? No need to go through modifications or having bulky battery packs hanging off the meter, unless you want to do so as a hobby project.

Although the nominal voltage for NiMH batteries is 7.2V (6-cell) or 8.4V (7-cell), they start off at 9-10.5V and drop very slowly until they're depleted at 6-7.5V. So, they work just fine for things like DMMs.

Be sure to get low self-discharge batteries. These are most easily identified by purchasing "pre-charged" batteries.

[David Hess]

They also make rechargeable 9 volt lithium chemistry batteries.