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I've recently got hold of a second hand Thandar TM355 bench multimeter. It can be supplied by either 6 "C" cells or there's a 2.5mm barrel plug for power. Unfortunately, the service manual doesn't give any more information than that one must use an "approved AC adaptor".

Looking inside the box, it seems that the batteries are just in parallel with the power input making me think that I just need to supply it with 9V DC (6 times 1.5V), but I thought I should check here whether anyone either (a) knows what the TM355 expects or (b) has experience with this sort of thing and can tell me whether my guess is likely to be right.

I presume that the "AC adaptor" that the manual refers to implies some sort of power brick: there's no obvious bridge rectifier or the like anywhere near the input so I assume that it's expecting DC.

So, the question: Is a 9V DC plug likely to be OK, or will it blow things up?

(PS: I hope this is on topic: Although it's related to a specific device, the question is one that I'd expect some electronics professionals to be able to answer, through having used one themselves)

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    \$\begingroup\$ Wired directly accross the batteries? That sounds unlikely since it could easily damage the batteries. Does the power plug perhaps have a switch built in so that the batteries are disconnected by the mechanical presence of the plug? If so, yes, you just need a DC power supply that mimics the battery voltage. \$\endgroup\$ – Olin Lathrop Jun 8 '12 at 11:54
  • \$\begingroup\$ ... and make sure you use a regulated adapter (an unregulated one could easily output 12V under low-load conditions). \$\endgroup\$ – MikeJ-UK Jun 8 '12 at 11:59
  • \$\begingroup\$ Olin: Ah, looking again at the barrel socket (which conveniently has one side open), that's what's going on. And, MikeJ, thanks for the pointer about the regulated adapter. \$\endgroup\$ – Rupert Swarbrick Jun 8 '12 at 12:16
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So, the question: Is a 9V DC plug likely to be OK, or will it blow things up?

This is much harder than is appreciated by most.
Death is a potential outcome of doing it wrong.

- If you use the meter to measure mains or high voltage

- the external supply will often also be at high voltage.

  • If the external power supply fails when mains or HV DC is applied to the meter then the user may experience electric shock and/or a breakdown path to "outside the meter" may occur. Depending on fusing or protection, outcomes ranging from a blowing fuse to a fatal arc over might occur.

  • People have died due to meter arc-over and will occasionally die in future. Using an external supply that does not properly resist applied voltages is one way of increasing the chance of this happening. Proper high rupture capacity fusing will usually prevent this happening, but this introduces another factor into the mix.

  • The problems expressed here may seen unlikely to occur and the approach may seem pedantic. They are and it is. This is of no comfort in situations where Murphy wins.

See below.

6 x C Alkaline can supply up to 6 x 1.65V = 9.8V, say 10 VDC when new.
You should not use a voltage that high BUT that shows what it should withstand.

When almost exhausted 6 x C cells may produce 6V BUT some meters stop working before batteries are exhausted.

The nominal value of a C cell is 1.5V (as you note) so 6 x 1.5V = 9V is safe. I'd expect anything from say 8V to 9V to be OK.

So ...

  • A regulated 9V DC power supply of correct polarity should work properly. Note that the supply should be isolated from ground and that

  • Battery supply internally is usually connected by low impedance to one test probe and effectively there is often a low impedance path to both.
    So if you use the meter to measure mains or high voltage then the external supply will often also be at high voltage.*

*If the manufacturer has taken the most exceptional care in design it is possible for the supply and meter terminals to be mutually isolated, but it is almost certain that this has not been done.

This does not mean that you should not use an external supply BUT you need to know that you can eg apply AC mains to either input pin INDEPENDENTLY with no damage or problem at all. In a properly isolated supply this will often be the case.

If you only ever use the meter on extra low voltage then the above may not apply but the meter should have a large label attached and you should post a 1 million dollar/euro bond and provide full contact details for when somebody else ignores this and kills themselves or somebody else.

An internal DC DC converter with isolation suitable for peak meter voltage would allow the constraints on the external supply to be relaxed.

Who would have thought it could be so hard :-).


Service message: If you had asked this question 24 hours ago you could, worst case, be dead soon thereafter as I was ritually barred from answering for reasons which I consider improper. Don't expect this message to be left intact.

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  • \$\begingroup\$ He hasn't been seen for 3 hours ... :( \$\endgroup\$ – MikeJ-UK Jun 8 '12 at 15:57
  • \$\begingroup\$ @MikeJ-UK - It's OK - I'm on watch :-) \$\endgroup\$ – Russell McMahon Jun 8 '12 at 16:25
  • \$\begingroup\$ Thank you for the help. In fact, I've not had the opportunity to kill myself, since it turns out I'd measured the barrel plug wrong and will have to get a replacement plug and/or socket. grump Anyway, thanks for pointing out something I hadn't thought about (as well as the 6V - 9V range) \$\endgroup\$ – Rupert Swarbrick Jun 12 '12 at 16:24
  • \$\begingroup\$ @RupertSwarbrick For interest. That's a seriously old piece of equipment and literally a collectible. Did you buy it as a collector's item or just to use. There's nothing wrong with using something like that if it still goes OK, but its 'cute' value is probably as great as its utility value. \$\endgroup\$ – Russell McMahon Jun 12 '12 at 19:53
  • \$\begingroup\$ @Russell Mostly for use: In fact I was given it by someone disposing of a friend's estate (I think). It seems dead at the moment, but when I get some time I intend to see if I can bring it back to life... \$\endgroup\$ – Rupert Swarbrick Jun 22 '12 at 13:20

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