Multiple DMMs on a single power source

[valterra]

I'm using the cheap HF DMMs to measure incoming current from a few re sources.  When I construct a solid case to mount everything, I was thinking about disassembling them slightly (eg remove the circuit boards from the plastic cases) and mounting them inside where the lcd display can "peek" through holes in the case.

My quandry is about how to power them.  They run on 9V batteries.  Just as an experiment, I tried powering one of them with the 9VDC output jack of my HF solar charger thingie.  (I only have that hooked up to function as a dc-dc converter, it is not hooked up to the solar panels)  It was giving me weird readings.  

I surmised that it can't do a good job of measuring voltage on a circuit if it is also being powered somewhere along the way to that same circuit bus.

Unless people throw up a bunch of red flags, I will try again to power one from the HF controller, but this time for amperage readings. I know that the way a meter reads voltage and amperage are different from each other.  I'm hoping that having the meter powered by the same bus that the current is flowing to won't create a short.  

My hope is to run several of these meters from a single source - thereby eliminating the frequent changing of several 9v ($$) batteries.

Will keep you posted on the results.

[kurt]

most meters do not like to be powered from the same source that they are measuring unless you use an isolated power supply. they could quite possibly go pop and never work again. i kinda doubt that that cheap harbor freight controller is an isolated supply.

[tecker]

The meter is using the battery for comparison voltage and to run the lcd it needs the isolation. A few rechargables for the meteres in question should be a good compromise.

[DamonHD]

As an aside...

The e-Flite meter that I use is happy to be powered from what it is measuring, but it uses a little too much power for me to leave it collected permanently.

Rgds

Damon

[DaveW]

  Two years ago I was in need of a battery charger for when the wind won't blow.  I wound a transformer for my 48 volt battery sets using 10 ga. square wire on the secondary, and everything worked as planned until I hooked up the meters for voltage and current.  Blew them both with the flip of a switch.  Plan B, I scrounged two small transformers from the junk box, LM317s, resistors, small caps, and board mount trim pots.  The new meters are now driven from isolated supplies about the size of the last joint of a thumb. I would suggest that if you are not certain that your meters are isolated, do not hook them up without protection.  Smoke is expensive, and hard to put back in the case.

[valterra]

I know!  They want an arm an a leg for Silicon Smoke Injectors nowdays.

[valterra]

That's kinda what I was thinking.  Rechargeable pack is the way I'll go, since I can easily recharge some NiCd batteries on my RE system.

[claude]

see this, it worked great for me. It still does. It can put out up to 200mA, plenty of DMM to power up.

http://www.thebackshed.com/Windmill/articles/PanelMeter.asp

Claude

[ghurd]

I think you got lucky!  The yellow HF meters do that smokey popping thing.

At the shop, the battery meters are powered by a very faulty 12V 7AH, that won't go over 10V, but rarely drops below 8V.  It is direct connected to a $10/12 red HF 1.5W PV.

For a while it was a decent 7AH and a 7809 voltage regulator.

Would be just as good with 10 AA ni-cd pack and 7809.

Could add a mom push button to turn the meter on to check volts.

I'm a bit concerned about the meters reading amps through the micro-wires they came with.

G-

[valterra]

Concerned about 4A?  I've never felt the wires even get warm.  

I want to measure total in / total out, and hf has a -20/20 ampere meter (analog) that I was thinking about using.

If the actual wires are a concern, of course ANY wire can be substituted when they're  taken apart.

[joestue]

The real issue is the resistance wire inside the meter, @ .1 ohms

Its measuring voltage anyway so you can of course build your own shunt,

[Bruce S]

Valterra;

 I just put an update on my post about the newer PDUs from HF. Using a real 9V battery in the meter, you might want to check the output of that 9V power tap too.

See if it is actually putting out 9V.

I got away for doing this by adding a Watts up meter to my circuit. It shows incoming voltage level and accumilated Ahrs. Can also be wired backwards for usage instead or buy another one and install too.

When we swapped out the SLAs in the company velomobile for NiCd packs, the power meter in it we knew would not read correctly so we installed one of these so we could get a real time correct level of useage.

Hope this helps;

Bruce S

[valterra]

I got ya.  That HF box doesn't put out a solid voltage.  "9v" may actually be 10, etc.

But I think mine just died anyway.  I flipped the switch to check my line voltage and nothing happened.  It's just flat out dead.  And I wasn't using it as a controller - figured the dc-dc part would still work well even if the controller part was crappy.

OH WELL!  At least I've read here 100 times that the unit is a POS - otherwise I'd feel really bad right now that it doesn't work!!!