On Mon, 18 Dec 2006 04:59:13 GMT, "west" wrote:

I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a shunt
resistor or something to take all that current. Any ideas? Thank you.

In many cases you should already have a suitable shunt resistor
installed. In power supplies built around the 723 voltage regulator
IC, there are is usually a low resistance (but high power handling)
resistor for the short circuit protection (usually between the power
transistor emitter and power supply output). When the voltage drop
across this resistor exceeds about 0.7 V, the short circuit protection
will kick in, limiting the current.

By measuring the voltage drop across this resistor, the output current
can be determined. By using a moving coil instrument and a suitable
series resistor, the meter can be easily calibrated.

Since you have a 40 A power supply, there are most likely several
power transistors in parallel, each having a low resistance resistor
at the emitter to balance unequal currents through the power
transistors.

The voltage drop across one of these resistors could be used to give
an estimate of the total current. This is just a rough estimate, since
the currents through the transistors varies e.g. with temperature (the
transistors at the end of the array may have a lower temperature than
those in the middle of the heatsink), the currents are not exactly
equal, so multiplying the measurement with the number of transistors
is not very accurate, but may be sufficient in most cases.

Paul OH3LWR