Power supply requirements for TM355
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 :-).



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