Modifying a multimeter to take external power instead of battery

[MrQuan]

Hello!

I'm using a multimeter (old Protek 608) to do some data logging of power circuits. I have some software on a Raspberry Pi that reads the RS232 output of the DMM and writes hourly CSV log files (~2Hz data resolution from DMM). This is great, but I need to monitor this for potentially days/weeks and have had to swap out the 9V battery.

This got me thinking about modifying the DMM to take external power so I could run it continuously from an external power source (larger battery or wall-wart). I'm considering the mod route because I don't want to spend a lot of cash on a bench top multimeter, and need this to be relatively mobile.

I would like to make this safe, so I was considering some sort of isolated supply, perhaps an isolated DC-DC converter? Has anyone done something like this? Is there anything else I should be mindful of?

A battery is obviously a very clean supply. I have concerns that a switching supply (i.e. DC-DC) might introduce noise to my measurements. Is that something I need to be concerned about?

[retiredcaps]

I don't know how safe this would be for obvious reasons, but one possible idea is to use something like this??  You would probably have to put electrical tape around the exposed outside contact areas to minimize any potential shorts, etc.

http://www.ebay.com/itm/181167620918

6 x AA Size Cell Battery Holder Box 9V Case With Snap Connector Wire Lead

You can use 6 AA Eneloops (1900 mAH typical) or 6 AA alkaline (say 2500 mAH typical).

The Protek 608 uses 7mA according to manual.  With a standard 9V alkaline (say 500 mAH), you would get roughly 71 hours of usage.

With the above 6 AA adapter, you would get around 1900/7 = 271 hours (or 11 days) with Eneloops or 2500/7 = 357 hours (or 15 days) with alkaline.  Using lithium AA cells would yield longer run time, but perhaps not the best bang for your buck?

PS. I use something like this at home for powering some 9V projects.

[retiredcaps]

Just found this as well.

http://www.ebay.com/itm/251644737454

C Size 9V DC 6 Cell Battery Power Supply Holder Holds Case Box with Wire

C alkaline cells are rated up to 8000 mAH according to wiki.

http://en.wikipedia.org/wiki/C_battery

8000/7 = 1142 hours (47 days).

[Lightages]

There are reasons that there aren't too many handheld meters with external power. It is difficult to make an external power connection and still have it remain isolated. It is also very expensive. Remember it is not the power supply that needs to be isolated, it is the connection at the meter.

[retiredcaps]

An alternative to your Protek DMM and external power question.

The Tekpower 4000ZC consumes under 5mA when data logging, but uses 2 AA cells.  Assuming a constant 5mA discharge and a cutoff voltage of 1.15V per cell, you will get 500 hours as per Duracell datasheet.

http://ww2.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1500_US_CT.pdf

Martin did a review and in part 2 of the video around 17:47, he shows the low battery warning coming on around 2.34V while readings were still accurate.  A bit later on he shows current consumption while data logging and I think the peak current consumption was around 4.5mA.  If we round up to 5mA, then we can use the Duracell datasheet to get a pretty accurate idea of the battery life.



So 500 hours/24 = 20.833 days.  Thus, you are likely to get 3 continuous weeks of data logging using the internal power 2 AA cells.

I don't know if the Tekpower meets your requirements or not, but the manual for the Tekpower is at

http://www.multimeterwarehouse.com/TP4000ZC/Data%20logger%20%20Manual.pdf

It comes with data logging software, but if you want to see the protocol that is documented as well.

http://www.multimeterwarehouse.com/TP4000ZC/TP4000ZC_serial_protocol.pdf

It is sold on Amazon for $36.99 USD.  If you are going to buy this meter, consider supporting Martin via his Amazon affiliate link.  He probably makes a whopping $1.85 USD in commission. 

At $37 USD, you can get several for different projects and won't cry too much if they get stolen.  I can't think of any other battery powered multimeter that has data logging with more than 500 hour continuous run time.

PS. My name is mentioned in Martin's video, but I have no financial benefit or affiliation with your purchase on a Tekpower 4000ZC.  I brought this meter to Martin's attention because of his expensive Metrix data logging multimeter and he asked for alternatives.

[retiredcaps]

I just noticed that UltraDMM also supports the Tekpower 4000ZC.

http://www.ultradmm.com/History.aspx

Version 1.0.5.0 - December 17, 2014
+ Added Support for Digitek DT-4000ZC
+ Added Support for Tekpower TP4000ZC

Written by a fellow Canadian who is also a member here ...
https://www.eevblog.com/forum/projects/ultradmm-software-project/

http://www.ultradmm.com/About.aspx

UltraDMM was originally created to replace the DMM software that ships with Uni-Trend UT series meters. It quickly grew from that point with many advanced features.

Features:
Support for multiple simultaneous meters
Advanced Triggers can alert via audio or visual feedback on meter value change
Analog/Digital Gauges can help represent the data more accurately
Gauges can combine values from multiple meters via various algorithms
Virtual meters that can aid in simulations
Save workspaces for common scenarios and sharing among group of engineers
Export data for more later analysis in Excel

[bitwelder]

What about basically having two 9V battery packs in parallel, each one with a switch:
- pack#1 is connected, pack#2 disconnected
- when pack#1 is running out of juice, you flip the switch for pack#2
- at this point you should be able to disconnect pack#1 and replace its batteries
- repeat the sequence in reverse when pack#2 is getting exhausted

[rs20]

Why is nobody suggestion hooking up a standard isolated lab power supply set to 9V? Be appropriately careful with the isolation rating of the power supply, and the insulation on the wires leading to the DMM (and by appropriately careful, I mean exactly as careful as you would be with the leads on the front of the DMM). Am I missing something?

[Seekonk]

Many years ago I had a meter that had no battery, this was when 1F capacitors were new.  You just stuck the leads onto some source 12-300V for a minute.  At times inconvenient, but you never were stuck without a battery.  Can't remember whatever happened to that meter.  I have a remote solar site and have thought about using one of those free HF meters with a DPDT switch and a couple thousand uF cap.   When I need to make a reading, switch to the cap and charge for a few seconds.  Then switch back and read the voltage.  Should last for about a minute.

[Wytnucls]

I'm using a 5V USB wallwart (Logitech 1A) to power a Gossen 30M through the original DMM power receptacle. I don't see any difference in accuracy measurements when switching between battery power and external power.

[Ian.M]

I see no particular electrical problems if you were to run it off a DC-DC converter with at least 3KV isolation, probably followed by a linear regulator to get a stable, low noise 9.0V, but the problem is doing so while maintaining the integrity of the meter case which is vital for safety. 

If you have access to a 3D printer or a scrap identical meter you can rob the battery compartment lid from it becomes much easier, otherwise, unless you are careful, this mod is not going to be easily reversible.   It may be possible to get a flat-flex cable out of the battery compartment and still be able to close the lid properly.   

The DC-DC converter *must* be inside the existing battery compartment and its input side must have adequate safety clearance from the meter's circuitry.

I should also point out that this mod would only be appropriate for private use.  If its going to be used in a business or public environment, get a logger that is rated for the voltages you need to monitor, don't hack a multimeter!

[MadTux]

What is the problem with using a larger transformer based wall wart with well isolated cable directly to such an clip:
https://www.led-tech.de/images/products/resized/LT-362-1264685968.jpg
Put clip into battery compartment, make hole for cable and you're done.

Every normal bench meter uses transformers for powering inguard and outguard section with a separate winding. If you use a wall wart with such a transformer:
http://www.mpja.com/images/7838.jpg
 that has good separation between primary and secondary winding, you'll get at least 1000V isolation, probably more. So more than enough for everyday work, unless you do HV experiments and like floating the DMM

[tszaboo]

Why dont you do yourself a favor and buy a bench DMM?

[Lightages]

What is the problem with using a larger transformer based wall wart with well isolated cable directly to such an clip:
https://www.led-tech.de/images/products/resized/LT-362-1264685968.jpg
Put clip into battery compartment, make hole for cable and you're done.

The problem with this is that the cable running out of the back of the meter all the way to the transformer, and the secondary of the transformer, are not isolated. They will be at the potential of the device under test. The output of a wall wart is not rated for any kind of safety at all. You can also be sure that the insulation on the wire will not be rated for 600V. The OP said he wanted to do this safely and that idea is not safe.

As suggested, probably the safest thing to do, if not buying a proper meter with a proper isolated power input jack, is to make an isolated DC-DC converter for inside the battery compartment and then connect that to a wall wart. This does not guarantee that the meter still meets its CAT ratings but at least it is isolated if done properly.

I have also wondered about doing something like this and thought that perhaps a Qi wireless charging system could be adapted. The problem is the noise generated by the charging system at 60kHz to 400kHz and at fairly high levels very close to the front end of the meter.

[Wh1sper]

Do not forget LiON 9V Battery.
simple swap and 4-5 times the capacity as AlkaliMn
4 time expensive as well :-(

[SeanB]

Old NE2000 series DC DC converters with 5V input and 9V output and 2kV isolation are cheap modules. Then the wall wart can be an old 5V phone charger only.

[Lightages]

OK, do you have a link to information on these parts? A quick search gives me nothing.

[SeanB]

OK, do you have a link to information on these parts? A quick search gives me nothing.

Here you go.

https://www.meanwell-web.com/product_info.php/products_id/SRS-0509

Mouser actually has stock... $6.88 each.

http://za.mouser.com/ProductDetail/RECOM-Power/R1S-0509-P/?qs=sGAEpiMZZMvGsmoEFRKS8PNQK9Wr5ol%2fskuRstlyVrs%3d

Otherwise look on eBay for old ISA network cards with 10M data capability, the old NE2000 series boards all have one on them. Valor branded pack in the images.

http://en.wikipedia.org/wiki/NE2000

[Lightages]

Ah OK, I was wondering why you were referring to an NE2000. I searched for that as a specific part.I didn't realize you were referring to a part on the card. Too bad I threw a bunch out when I moved to Chile.

I have found one with better isolation specs, but it is $20
http://www.digikey.com/product-detail/en/NMS0509C/811-1576-5-ND/1927137

[SeanB]

Ah OK, I was wondering why you were referring to an NE2000. I searched for that as a specific part.I didn't realize you were referring to a part on the card. Too bad I threw a bunch out when I moved to Chile.

I have found one with better isolation specs, but it is $20
http://www.digikey.com/product-detail/en/NMS0509C/811-1576-5-ND/1927137

Cheap on fleabay though, i saw them there for under $5 shipped.

[Lightages]

Well now that this is getting worked out, it is obvious to me what the OP should do. Buy one of the converters mentioned and use a wall wart, or buy a bench top. I think it would be possible to make a wall wart/dc converter battery eliminator for under $50.  In the long run it would save that much money if you are using non-rechargeable batteries. The problem is violating the battery compartment with a hole for the cable to pass through. Other than that, a shielded dc convertor potted in a shape of a 9V battery probably could take car of much of the converter noise.

Hmm, another project to do that won't get done......

[MrQuan]

Thanks for the excellent feedback!

The 9V alkaline battery died Sunday, so I've just replaced it again. I measured the old battery at 5.5V!! So it well and truly used it up.  I needed to continue data logging ASAP, so I opted for a f#% expensive 9V lithium primary ($16 from Altronics!!)  ...I was desperate. Anyway, that should keep it running for at least another week. So I have time to implement something better.

To answer a few questions. A bench-top unit may be an option, but I'll be bigger/bulkier, and cost me a bit. I'm happy with the Protek 608 performance so a cheap mod will be ideal.

The battery compartment has it's own door and compartment, so a DC-DC module to fit in there looks like the ticket. I would like to have a DC jack on the door so everything is nicely sealed. Then I could plug in a 6xAA battery pack or wall-wart.

I'm halfway through building a 3D printer, so soon I could even make a custom door and not have to drill a hole in the original door. I will connect the DC-DC converter to the DMM with it's existing 9V battery snap. This way I can remove my mods, and nothing is permanent.

I've attached a couple of photos of the battery bay.