Checking currents

[fsleeman]

I was working on a simple project with transistors and kept getting strange currents so I decided to test something really, really simple. Unfortunately, I still cannot make any sense out of what I am seeing. For my simple test, I connected a battery to a resistor and read the resistance, voltage drop, and the current. The resistance and voltage was exactly what I expected but the current was not. With a 1.6V battery connected to a 1k resistor I get a current around 630 mA. If this circuit is following Ohm's law, I would expect the current around 1.6/1000 (1.6mA).

Am I doing something really dumb? Is there a funny thing with battery sources? I am more of a software/micro controller these guy and am trying to relearn all of that analog stuff I learned back in school, so I am a bit confused.

[GeekGirl]

Hi,

Couple of points :

#1 are you sure the resistor is 1K ? (IE have you measured it ?)
#2 are you putting the meters probes in SERIES with the resistor and battery ?
#3 are you sure the meter is accurate, or is it a cheap one ?

 

[rossmoffett]

I agree, I think you may just want to read up on taking measurements with a multimeter.  Measuring resistance in circuit, and especially in a live circuit, isn't very useful most of the time.

[fsleeman]

#1 are you sure the resistor is 1K ? (IE have you measured it ?)
#2 are you putting the meters probes in SERIES with the resistor and battery ?
#3 are you sure the meter is accurate, or is it a cheap one ?

1) Yes
2) No
3) Yes

So I guess its number 2, and since I have never measured currents before this did not occur to me (although now it makes a lot of sense). While this method works with a breadboard or loose parts, how would you measure current in a permanent circuit?

[EEVblog]

So I guess its number 2, and since I have never measured currents before this did not occur to me (although now it makes a lot of sense). While this method works with a breadboard or loose parts, how would you measure current in a permanent circuit?

You usually have to break the circuit, buy lifting a component leg or cutting a PCB track etc. Current measurement can be annoying!

BTW, by putting the meter in parallel in current mode you were shorting out the battery in this case, hence the high current you measured. The current range fuse in the meter often blows in this case.

Dave.

[rossmoffett]

How permanent?  One answer is that you don't, you simply measure voltage across a known resistance and use Ohm's law.  That's what I used to be forced to do because I didn't have a uA multimeter.

The other is that you make it non-permanent while you make your measurement.

A third answer is that mA clamp meters are now available, I've used one and they work great, but you won't like the price tag!  A not-so-accurate clamp meter with DC ability is available from dealextreme.com, but I cannot attest to its quality.

[fsleeman]

Yeah, I think I got ahead of myself thinking about how a final circuit would be tested rather than what I actually had in front of me. I guess checking currents in soldered circuits is pain after all! Deriving currents might not be so bad. Do active elements (LEDs, transistors, etc) cause problems detecting the current, or do you need to find the current using KCL using non-active parts?