Re: How do I measure amps?

"Wayne Lundberg" <Waynelund@xxxxxxxxxxxxxxxx> wrote:
"Gordon McComb" <NOSPAMgort@xxxxxxxxxxxxxxxxxx> wrote in message

Thanks Gordon. My clamp on meter is a Gen-Tech and shows ACA and External
Unit reading Ohms in the range of 20M to 2,000M

That's a standard clamp-on AC amp meter. It's just a transformer. That
only works with AC because only a changing magnetic field will induce a
current in a wire. A constant magnetic field will not. That's why it's
not as easy to to detect DC currents with a clamp on device.

The DC current probes take advantage of the hall effect. The probe has a
current flowing though it, and when that is placed in a constant magnetic
field (created by DC current flow), it causes the electrons to curve as
they move though the probe sense wire which causes them to pile up on one
side of the sense wire. That piling up effect causes a slight voltage to
develop from one side to the other in the probe. That can be used to
determine the strength of the magnetic field and in turn, the DC current.
These devices have in the past tended to be very expensive (I don't know
why for sure), but based on the comments I guess they are getting cheaper.
I've never owned one. But you can't find them at Home Depot or Radio Shack
(I don't think). You have to go to an electronics store.

---- my other VOM is a
A.W. Sperry DM-4300A with a plug for up to 10 Amps, and a switch for
either AC or DC... but I am hesitant to put the full load in line because
I'm not sure if this is the right way to do it. I lost the instructions
years ago.

Yes, that's how you use a normal amp meter. You have to put it in line (in
series) with the circuit. But I don't know if 10 Amps is going to be high
enough for your application. If your motor draws too much current, it
could burn out your amp meter.

The amp meter function has a very low internal resistance (think of it as a
wire). Don't connect it direct across a battery or it will fry in an
instant! Only connect it in series with a normal load.

I've got a Fluke digital meter with a 10A range option (labeled as
unfused). I just checked the manual (which I still have only because it
fits in the case that the meter is in). It says the meter can read up to
20A for up to 30 seconds, but that anything over 30 seconds or 20A is
likely to damage the meter (aka melt stuff inside that can't be easily
replaced).

So your 10A meter might work similar to that. If your load is under 10A
most the time and only shoots over it by a little bit for only a short
period. I'd suggest you look on the web for the manual however - you can
probably find it and then you will know for sure what the limit it.

If your meter has a needle, then a large over-range condition can actually
bend the needle as it whacks up against the high range stop - so that's
another danger to watch out for.

The ideal solution to my problem would be to simply connect the Sperry
inline from battery to motor and see what happens under different load
conditions. But I need some reassurance here.... Before making the smoky
genie come out of the device.

Again, the load is a windshield wiper gearmotor for 12V automotive.

I have no experience with those so I don't know what they would be likely
to draw. But I know my car uses fuses from 10A to 30A or so for various
circuits like that.

Wayne

I think you said the idea of a shunt was magic to you in another post?
Here's a quick course in basic electricity that might help you (if you
don't already know this)....

All you do is hook a very low value resistor in series with your load.
This extra resistance will reduce the voltage to your load slightly
(depending on how low the resistor value is and the load your motor is
creating). But because of Ohm's law, the voltage across this resistor,
will be directly proportional to the current flowing through it. And the
current flowing though it, will be identical to the current flowing though
the motor, because they two are hooked in series.

So, if you connect a resistor like that, and then hook a normal volt meter
across the resistor, and measure the voltage across the resistor, you can
calculate the current flowing though it.

If you use a .1 ohm resistor, and you measure a voltage across it of 1
volt, then the current is:

I = E / R = 1 V / .1 Ohm = 10 Amps.

The amount of power turned to heat in the resistor is E * I, or E^2 / R
watts. In the example above, for 10 Amps of current, that would 1^2 / .1
or 10 Watts. So you would need a 10 Watt power resistor in the above case
to be sure you didn't burn up your resistor.

So, if you dealing with currents higher than your amp meter can deal with,
and you don't own a DC current probe, you can just put a low value but high
wattage resistor in series with your circuit, and use your voltmeter to
measure the voltage across the resistor.

If you are dealing with a starter motor that draws 150 amps, then you
better use something with a lot less resistance to keep it from melting -
which is what the "high current shunt" was about - just a short and heavy
duty wire with a known but very small resistance.

All wire has resistance and acts like a resistor so the above formulas work
to calculate voltage across any length of wire with current running though
it. You can just connect a volt meter at two points on a wire and measure
the resistance when it's out of the circuit, then measure the voltage when
it's in the circuit for the same connection, and calculate the current
flowing though it. But unless you have a very sensitive ohm meter and volt
meter, that won't work very well for most wires because the resistance, and
the voltage, will be too small to measure accurately.

--
Curt Welch http://CurtWelch.Com/
curt@xxxxxxxx http://NewsReader.Com/
.

Relevant Pages

  • Re: identifying a panel meter
    ... I think you are talking about an audio volume meter, ... So it can be seen both as a current meter and a voltage meter. ... If you want to use it as a voltmeter for higher voltages you need to put ... a resistor in series with it. ...
    (sci.electronics.basics)
  • Re: Photodiode Dark Current Measurement
    ... Voltage accross Photodiode: 0.82V ... 1.39V drop across the 1M resistor, ... the resistor and deduce the 10 meg shunt your meter is adding to it. ... repeat the measurement with a smaller ...
    (sci.electronics.basics)
  • Re: a strange problem
    ... :DMM set to measure resistance of a circuit with a capacitor ... :on the meter, that is *not* the meter's fault. ... :of the resistor, and that, if it doesn't, it's wrong. ... What you are suggesting is that a resistance reading taken with a digital ...
    (sci.electronics.design)
  • Re: Measuring mA (debate solution needed)
    ... He's calculating the current in a circuit my measuring the voltage ... read the meter. ... voltage drop across the resistor is more accurate. ... Understanding measurement errors can get quite complicated, ...
    (sci.electronics.design)
  • Re: How does an amp meter work?
    ... How does the amp meter work in my Fluke 189? ... A high precision "shunt resistor" with a known value is across the ... and that voltage is expressed ... on the display as a directly converted amperage reading. ...
    (sci.electronics.design)