Any problem leaving my DMM on for long periods?

[BatteryCharger]

A while ago I bought a few of those $3 DMM's from Harbor Freight. I want to use one when I'm charging up my SLA batteries to show me the current the whole time. (~14 hours) I am going to wire up a 9v wall wart in place of the 9v battery. Is there any reason why I can't leave it hooked up and measuring current for that long?

 

[PEU]

not as long you use a screen saver /ubbthreads/images/graemlins/smile.gif

seriuosly, I don't see any problem.

 

[Doug S]

No harm but it would be easier to just use the 10A jack and just turn on the meter when you need a reading. Using the 10A jack the meter will pass current even when turned off.

 

[evan9162]

Probably should be fine - though for added safety you could use an external shunt resistor, and use the meter to measure the voltage across that, so you don't have current passing through your meter all the time (prevent your meter from melting down if something catastrophic happened - i.e a short). This would be something to do if you wanted to be extra cautious.

 

[CNC Dan]

Re: Any problem leaving my DMM on for long periods

I have to agree with evan9162 about using a external resistor.
If you look at the fine print, you may see that you are only supposed to read max amps for a few seconds. That is because the internal resistor is getting hot inside the case.

 

[brickbat]

Re: Any problem leaving my DMM on for long periods

I don't see any problem either. But, keep in mind that on some DMMs, the input terminals are not isolated from the battery power. So, if you go and measure 120VAC, the 9v battery input leads could be hot.

 

[Doug Owen]

Re: Any problem leaving my DMM on for long periods

No reason at all not to. I have one of mine set up with a NiMH battery for just this use.

FWIW, the current draw must be very low on these meters, I've left them on for days on the cheap battery it comes with.

Go for it.

Doug Owen

 

[Jack_Crow]

Re: Any problem leaving my DMM on for long periods

BC,
The post about current limiting makes sense. A fuse in the line never hurts.

I have never had a three dollar DMM.

Picked up on EBAY for 90 dollars once a fully functional HP 3456A. I was calibrating instruments when these were new. You can find many a fine example of these for sale.

It's a 6.5 digit DMM. So you can watch the microvolts when measuring 1.5 volts. With it you can get a feel for the self discharge rate in cells. It has an IEEE 488 port for computer controled measurments. Filters, and averaging. Also a high/low decision maker.

Four wire ohms and a killer AC True RMS converter.

If I wanted to use one to measure current, I would need a shunt resistor.

Yeah it's over kill for hobby electronics, but it impresses all my ham radio friends.

At my last job I left behind a Fluke 8500A. It had everything but the ohms converter. That was a 5 and one half digit unit. Very slick for it's day.

If your serious about your projects there are cheap 3 1/2 digit DMM's that hook up to most PC's with an rs 232. At the last job we used them to get a feel for battery charge and discharge times for Li packs in portable radios and base stations. They were more than good enough for casual measurments. Graphing functions are nice though.

Later dude
Jack Crow in Iraq