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Rich Grise
Guest
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 Posted:
Sat Feb 19, 2005 3:22 am Post subject:
Re: Amp Meter |
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On Fri, 18 Feb 2005 20:38:46 +0000, Roger Johansson wrote:
| Quote: | Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:
that looks to be a 150 uA meter at full scale.
if memory serves,.i think you said the meter was a 0.050 mv?
to get a 6 amp scale.
6.00 - .000150 = 5.999850
Shunt = 0.050 / 5.999850 = 0.0083 ohms
i think that is correct,i normally measure the
coil R and use the I (current) to do all of this
how ever, this should still work.
this is just off the top of my head, i am sure some
one will correct me.
I think you are right.
In practical terms: Take a piece of copper wire, diameter 1-2mm.
Send 1 Amp through it. Put the wires from the meter together on that
wire. Pull them slowly apart until the meter reads 1 Amp. Mark these
measuring points and solder the instrument wires to them. Cut off the
thick wire outside the measuring marks and attach the current connections
to the ends of the wire.
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I've also seen a circuit where a microammeter is turned into a voltmeter -
in this case, 10.4K gives 1.5V full-scale. A shunt resistor that drops
1.5V at 60A would be .025 ohm, which is much more manageable of a value
than 0.0083 ohm, and the calibration isn't anywhere near as persnickety.
Good Luck!
Rich
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Rich Grise
Guest
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| Posted:
Sat Feb 19, 2005 3:29 am Post subject:
Re: Amp Meter |
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On Fri, 18 Feb 2005 09:24:23 -0500, Nikki wrote:
| Quote: | Thanks guys
If I have a circuit lets say 1.5 volt bat and a 1.5 volt lamp and I want to
measure the current I would put the meter parallel to a shunt resistor in
the circuit right. But does the shunt resistor not cut down on the voltage
to the lamp
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Yes, by 0.05 volts. :-)
But don't just slap a shunt in there and expect to get accurate readings.
Either give yourself a way of trimming it while live, or actually
determine the internal resistance of the meter. Take your setup with
the batt. and series R, adjust it for full-scale, as you've done, and
then take another pot, put it in parallel with the meter (so the meter
and new pot are now in parallel with each other, and that combination
is in series with the B and R), and adjust it until the meter reads
half-scale.
But this will only work if the meter actually _has_ a Zero indication.
Did you meant to say that your meter face has "1" at the very leftmost
spot? If so, you might have to do some more experiments to find out
exactly what this meter is expecting - usually, when there's a meter
that doesn't start at 0, it's used with some kind of custom circuit to
get that offset.
Of course, the other thing (depending on how much headroom you have)
is to turn the meter into a voltmeter, by just adding a series resistor,
and measuring the voltage across the shunt. Doing that gets your
calibration up out of the mud. :-)
Good Luck!
Rich |
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Roger Johansson
Guest
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| Posted:
Sat Feb 19, 2005 3:37 am Post subject:
Re: Amp Meter |
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Rich Grise <richgrise@example.net> wrote:
| Quote: | I've also seen a circuit where a microammeter is turned into a
voltmeter - in this case, 10.4K gives 1.5V full-scale. A shunt resistor
that drops 1.5V at 60A would be .025 ohm, which is much more manageable
of a value than 0.0083 ohm, and the calibration isn't anywhere near as
persnickety.
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In this case you are wasting a major part of the signal. That is not a
good idea, seen from an engineering point of view.
We want a shunt which gives as low voltage drop as possible. Your
idea would make the voltage drop many times bigger, from 50mV to 1500mV.
The 0.0083 Ohm shunt is easy to construct from heavy gauge copper wire.
Or a thinner wire laid double, in quadruple, etc..
Adjustment can be made by moving the probe measuring point a little, or
by connecting an adjustment pot in parallell with the shunt. A wirewound
pot will be suitable for this.
--
Roger J.
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Jamie
Guest
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| Posted:
Sat Feb 19, 2005 4:16 am Post subject:
Re: Amp Meter |
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Nikki wrote:
| Quote: | Guys thanks for all your replies
I took the suggestion of Rich and found that with a 1.5volt battery and a
resistance of 10400 ohms I can get the meter to read at its last increment.
The meter is marked in increments of 10 up to 60 how would I get a 2 amp
draw to read 20 where a 6 amp draw would be the last increment on the meter
Thanks
Nikki
that looks to be a 150 uA meter at full scale. |
if memory serves,.i think you said the meter was a 0.050 mv?
to get a 6 amp scale.
6.00 - .000150 = 5.999850
Shunt = 0.050 / 5.999850 = 0.0083 ohms
i think that is correct,i normally measure the
coil R and use the I (current) to do all of this
how ever, this should still work.
this is just off the top of my head, i am sure some
one will correct me. |
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Rich Grise
Guest
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| Posted:
Sat Feb 19, 2005 6:10 am Post subject:
Re: Amp Meter |
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On Fri, 18 Feb 2005 21:37:55 +0000, Roger Johansson wrote:
| Quote: | Rich Grise <richgrise@example.net> wrote:
I've also seen a circuit where a microammeter is turned into a
voltmeter - in this case, 10.4K gives 1.5V full-scale. A shunt resistor
that drops 1.5V at 60A would be .025 ohm, which is much more manageable
of a value than 0.0083 ohm, and the calibration isn't anywhere near as
persnickety.
In this case you are wasting a major part of the signal. That is not a
good idea, seen from an engineering point of view.
We want a shunt which gives as low voltage drop as possible. Your
idea would make the voltage drop many times bigger, from 50mV to 1500mV.
The 0.0083 Ohm shunt is easy to construct from heavy gauge copper wire.
Or a thinner wire laid double, in quadruple, etc..
Adjustment can be made by moving the probe measuring point a little, or
by connecting an adjustment pot in parallell with the shunt. A wirewound
pot will be suitable for this.
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Well, in my defense, in a different post, I did mention that this
technique depends on the headroom of the supply. And you can certainly
decrease the ranging resistors, to decrease the shunt drop. It's just
a seat-of-the-pants sort of thing - you decrease the pickiness of the
shunt by adding _some_ resistance in series with the meter.
I've calibrated ammeter shunts, and it's a PITA. They wouldn't let
me grind on the brass strip with the Dremel while power was on. ;-)
I've also seen ammeters that have no leads at all - they have a clip
on the back that clips over the cable, and the meter itself is nothing
but a spring-loaded compass. You calibrate them by moving the cable
around until the meter reads right, then hot-glue it.
Cheers!
Rich |
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