DIY Loaded Voltmeter Help
 

Hi All,

Im looking to make a loaded voltmeter.

I have got the voltmetr but want to put a 500 mah load on it to simulate servo load i have been told to put a pr13 bulb on it for this load is this correct? 500mah.

Is there any component i can put inline to take away 500mah load instead of a bulb if so what would i need to take 500mah away

Many Thanks,

Jonny

RE: DIY Loaded Voltmeter Help
 

With regard to the second part of your question. you could just wire up a shorted bulb base and put that in place of the "load" bulb when you want no load. . Rich

RE: DIY Loaded Voltmeter Help
 

Just place a 10 ohm 10 watt resistor in parallel with the meter probes for a 4 or 5 cell battery check, use a 22 ohm 10 watt resistor for the 8 cell battery. No need for a special loaded voltmeter as this does the exact same thing yet leaves your voltmeter available for other measurements.

RE: DIY Loaded Voltmeter Help
 

what load does this put on the battery

RE: DIY Loaded Voltmeter Help
 

It would be nice to have a constant current load so you could pick 500ma, for example, and no matter what voltage is across it, only 500ma is drawn. That would be nice, but is more complicated than what most people want to get into so they tell you to use a resistor. The current load will be the voltage (under load) divided by the resistance. So if you have a battery that under load gives you 7.4 volts and you put a 10 ohm resistor in across it while the voltmeter is connected, you get 0.74amp load or 740ma. So if you wanted a 500ma load then you want a resistor that is 7.4/0.5 that comes out to be 14.8ohm. Did that help? The problem with using a light bulb is the resistance changes as it heats up so a resistor is generally better. Make sure that the resistor can handle some power.....there is an easy way to calculate that as well....sorry if I told you something you already kenw.

RE: DIY Loaded Voltmeter Help
 

Jonny,
I am assuming this is a receiver battery since you reference servos.
Ohms law states that the resistance = voltage / current or R=E/I
So with 4.8 volts divided by .5 amps (500mAh) you need a 9.6 ohm resistor.
Power = volts x amps so 4.5 x .5 = 2.25 watts.
Your 10 ohm resistor needs to be about three watts.
The easiest way to get there is to use 4 47 ohm resistors in parallel which will give you about 10.2 ohms.
If they are half watt resistors you will have 2 watts of capacity.
I would use one watt resitors or don't leave it plugged in but a few seconds.
I hope this helps.
I think you are going down the right path doing this rather than buying an exspanded scale voltmeter.
This is much more flexible.
Good Luck,
KW_Counter

RE: DIY Loaded Voltmeter Help
 

Reading through this interesting thread...it would seem that one could make up a number of "loads" that could be interchanged as the -to-be-measured-battery is selected. i.e. Blue one for receiver batteries; Red one for larger voltage TX batteries, etc. Is that right?

Just put the blue one on when you want to check the receiver; the red one on when you want to check the TX; etc. And leave the plug-in/plug-out "loads" off when you want to use the VOM for other purposes. ??? Rich

RE: DIY Loaded Voltmeter Help
 

or a little box with three or four resistors and a rotary switch...then lable each position for the loads at specific nominal voltage drops....could easily fit it into a altoids box....then you have all the bases covered for nearly no money

RE: DIY Loaded Voltmeter Help
 

If your voltmeter has the standard 3/4" jacks for the test leads, just get a dual banana plug (stackable) as in the attached picture. Then place your load resistors across the banana plug, tighten the set screws, plug it into your voltmeter and plug your test leads into the jacks on the plug. Voila, a loaded voltmeter. Actually the load is on the battery and your voltmeter is just reading the voltage across the load...........RJ

RE: DIY Loaded Voltmeter Help
 

You can easily get high wattage resistors at most electronic supply stores such as Radio Shack. Do get 5 or 10 watt resistors as they will last longer, are more robust and will not get as hot under load (you will burn your fingers if the wattage value is to small as well as possibly burn out the resistor). Standard values are 3.3, 4.7, 6.8, 10, 22 ohms. On the large sandbars (the slang name for the large square power resistors), the 10 and 22 ohm values are usually more available. Your load need not be an exact 500ma, 1 amp etc.; just as long as a reasonable load is on the NiCad or NiMh (LiPo and A123 do not require a load as open circuit voltage is a good representative of the charge level) to give you a reliable state of condition.

RE: DIY Loaded Voltmeter Help
 

Rich,
You are correct. I do exactly what RJ is reommending.
I make a requirement before I buy any meter that these dual banana plugs fit.
You may have some trouble getting the resistors you need at Radio Shack.
Hopefully you have a real electronics store in your area.
Rodney is also correct, use the higher wattage resistors if you can.
They will last longer and don't get as hot.
Good Luck,
KW_Counter