Customizing voltmeter

[ferincr]

Hi, I have one of those digital panel voltmeters and I'd like to customize it to be able to measure my Rx battery packs with some sort of charge added to it.
Can any one tell me how to add some sort of device (I guess resistors would do but I don't know which values) to be able to do this.
I'd like to put some sort of 3 position switch to be able to change from .5 to 1.0 to 5.0 amps.
Not sure if this sounds clear.
Any help??? Thanks.

[FredericG]

So you would like to put something in parallel to the voltmeter that draws a constant current form the battery?

Frederic

[ZAGNUT]

here's a little tool i've been using for years. much better than just using resistors as you get the same load no matter what voltage you're testing.

can't remember if the LM350 is good for three or five amps but the data sheet will tell you. there for sure is a similar device rated for five amps. data sheet will also tell you how to select resistors to get the loads you want. even though it's only for momentary use it's still a good idea to bolt it to some kind of heat sink, even just a small piece of aluminium sheet. heat sink is not ground.

much better in my opinion to use pushbuttons instead of a switch. makes it easy to compare loaded and unloaded voltages and how quickly the pack bounces back.


dave

[ferincr]

much better in my opinion to use pushbuttons instead of a switch. makes it easy to compare loaded and unloaded voltages and how quickly the pack bounces back.dave
That's a good idea I didn't think of.
If I add a third one of 0.6 ohms with a third push button would that work for 0.5 amp???
Fernando

[ZAGNUT]

no, .6 ohm would be for a 2 amp load. the two paralleled 1.2ohm resistors i show are the same thing but easier to find and cheaper than a two watt .6 ohm.

for .5 amp you would need a 2.4 ohm resistor or a pair of 1.2 ohms in series.

the similar part that's rated for 5 amps is the LM338


dave

[ferincr]

no, .6 ohm would be for a 2 amp load. the two paralleled 1.2ohm resistors i show are the same thing but easier to find and cheaper than a two watt .6 ohm.

for .5 amp you would need a 2.4 ohm resistor or a pair of 1.2 ohms in series.

the similar part that's rated for 5 amps is the LM338


dave
One last question (I think...)
With the LM338 should I keep the same value resistors???

[ferincr]

I went today to buy the components to do the mod and the LM 338 that they had is the flat metal case with two conectors on the bottom.
I have a conector for V in another for adj. and the case is output.
Which would be which in your drawing???
Can I assume that the one going to the pos. is Vin, the one in the middle Adj and the one on the left Output. Or the one in the middle is Adj???
I just don't want to burn it since is not cheap here.
Thanks,
Fernando

[ZAGNUT]

from the left the pins are adjust, out, in

this data sheet shows how the metal case compares to the one in my drawing: http://cache.national.com/ds/LM/LM138.pdf

and just remember that the metal case or any heatsink you put on it should not be grounded.

resistor values stay the same.


dave

[ferincr]

I got it working, I'm going to make it look nicer now
Thanks Dave.

[ferincr]

Dave,
Thanks again it works like a charm.
Here are a couple of pics of the final
The 4 buttons gives me 0.5A, 1A, 2A and the last one since I didn't have any other value resistors I made it 2A again so pressing the last two I get 4A
Fernando

[Carlos_M]

Fernando,

Are you sure the resistors you used are of 1W?, they look like to me of 0.5W as much, if this is the case you can burn up the resistors fairly easily.