changing my VMeter from 0.25A to 25A

Discussion in 'Electronic Basics' started by Hadry LittleWood, Sep 8, 2004.

hi

I have a volt meter which can reads DC current up to 0.250A
How do I make it able to read up to 25A

thanks

jif

2. GeneGuest

Use a shunt resistor in series with the circuit, then measure the voltage
across the shunt and by applying Ohm's law you have the current. (for
example 0.01 ohm shunt would give you 0.25V @ 25A)
Gene

5. CFoley1064Guest

Subject: changing my VMeter from 0.25A to 25A
Hi, Jif. Most multimeters have a shunt resistor built-in, and use the internal
voltmeter to read voltage across that shunt resistor. Technically, if you know
that resistor value, all you would have to do is find a resistor 1/99th that
value, and place it in parallel with the ammeter. Then 99% of the current will
0.25A = 25A.

Practically speaking, there's a bit of a problem, though. It's very hard to
get precision oddball value low ohm resistors at reasonable price/delivery.
Also, depending on the value, your R shunt value may be high enough to result
in an uncomfortable amount of power dissipation.

As a practical matter, it's a lot easier to just buy a 50A shunt and read the
voltage with any standard voltmeter. The shunt has a 1 milliohm resistor,
which results in 1mV per Amp up to 50A. This harkens back to the time when
analog meters ruled the earth, and they had 50mV full scale deflection burden.
Easy and straightforward.

And, lo and behold, All Electronics, one of the better US electronics surplus
outfits, has a 50A shunt available for \$12 USD in stock. They claim 0.5%
accuracy:

http://www.allelectronics.com/cgi-bin/category.cgi?category=385&item=SNT-5
0&type=store

Remember to use the big screw terminals for the current, and the small ones for
your voltmeter, if you need that accuracy.