We used to call this the 'voltmeter method' of measuring current. You put a resistor in
series with a milliammeter to convert it into a voltmeter -- and then use it to measure
the voltage drop across another resistor (the shunt) in series with the load.
If you choose the resistors so that the voltage across the shunt when the meter reads
full-scale is just below the forward voltage of the diodes, then the meter can't absorb
more current than a little more than the full-scale amount.
Use fixed-width font to see this picture:
Series R
----vvvv--------------->
| | | |
| < | |
**** < *** V Diodes
* MA * Shunt R < ^ ***
**** < | |
| | | |
|---------------------->
So if you want to measure 1 amp full scale you might make the shunt resistor 0.45 ohms.
That would produce 0.45 volt at full scale. You could use diodes with a forward voltage
drop of 0.51 V, like the 1N5408.
The series resistor would be R = (0.45 / Im) - Ri, where Im is the full-scale current of
the meter movement and Ri is the internal resistance of the meter.
On 10/10/2013 8:34 AM, Bill Turner wrote:
ORIGINAL MESSAGE: (may be snipped)
On Thu, 10 Oct 2013 07:57:20 -0700, Hardy wrote:
Bill,
I don't get the meter connection. Do you mean a diode in series with a
resistor and then that series combination across the meter?
N7RT
REPLY:
No. The meter and resistor are in series. The diode is placed across the
both of them. In other words, the diode goes from the side of the meter
which is not connected to the resistor to the side of the resistor which is
not connected to the meter. The point where the resistor and meter are
connected is not connected to anything else.
I always use two diodes, back to back just in case of reverse current.
73, Bill W6WRT
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