Improvised ammeter usage question...

[sloppycoder]

Hello there, and hi from me! I've just joined to find out about this particular issue but hope to also be an active participant in this forum!

 

 

I've been measuring current in a circuit with an improvised ammeter. This consists of a known good Digital Volt meter connected across a 10 Ohm 7 watt resistor. This (shunt) resistor is then placed in series with the circuit (IE circuit is broken until resistor is connected)

 

Two questions really;

 

1) Is that the correct physical layout for this type of test?

2) Is my maths right.......

 

Measured 0.0052v across resistor. By Ohms law i should devide this by the resistor value to get the current so 0.0052/10 = 0.00052 amps.

 

Many thanks, all help appreciated.

 

Alf

[Danijel Gorupec]

Hi,

 

1. Yes, it is correct.

 

2. Your math is right.

 

Notes:

 

- in some cases adding a 10 ohm resistor to the circuit might change current significantly. However, this is probably not the case in your case - I guess this because current is very small so I suppose that overal resistance of the circuit is much larger than 10 ohm.

- You don't need 7 watt resistor for such small current

- If there is already any other resistor (of known value) in the circuit that caries all the current, you could simply measure voltage over it (this way you will be sure that you didn't disturb the circuit by adding 10 ohm resistor to it)

- Most important: Because voltage measured is very very smal, I doubt that your measurement is very precise (unles you have a very very good instrument and you care about many things). Try with 1000ohm resistor - it should develop cca 0.5V. If it develops significantly less, then obvously 1000ohm resistor disturbs the circuit too much.

 

 

 

 

[ewmon]

If it's a simple linear circuit (ie, flashlight, etc), you can calculate/eliminate the effects of the added resistor and determine the current without the resistor; however, I agree with Danijel that you might need a larger resistor for more accurate values.

[sloppycoder]

Thanks very much for the replies That has put my mind at ease!

 

What i was attempting to measure was the idle drain current on the battery installed in my van. I tried measuring the resistance between the battery terminals (with one disconnected from the battery) and that was incredibly high something like 10,000m Ohm? Battery voltage is ~ 13.5v

 

I might try again with a higher value resistor for a more accurate reading but i think i can probably safely rule out a battery drain as the source of my flat battery problems.

 

Thanks again, will return with more conundrums another time.

 

Alf