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Problem with DC power supplies...

An interesting problem came up at work the other day. We were installing
a 3-wire pressure sensor (wires are COM, DC+ 24v, and SIGNAL (0-5vDC).
The transfo feeding the controller is 120v-24v AC, one side grounded on
the secondary. The DC power supply for the sensor is fed from the same
transfo; it has as outputs + and - stamped on the circuit card. It is
set up as a half-wave rectifier.

The COM and DC + of the sensor were wired into the DC outputs of the
power supply; the SIGNAL went to an input on the controller (exactly as
per the instructions).

However, we were getting very erratic readings on the input. Checked the
voltage from COM to DC+, 24v. However there was a difference of 9v DC
between the controller common and the DC power supply common. Changing
the AC wire positions feeding the power supply made no difference. This
seemed to be the source of the problem…

We swapped the power supply out for a different one; this one had as
outputs DC+ and COM stamped on the circuit card. Now there was no
difference between the DC COM output and the common on the controller.
However we now measured 24v AC on the DC COM! We swapped the AC inputs
on the power supply and bingo, no more problems. Sensor is now reading

Now my questions are dealing with the internal circuitry of the power
supplies. First, why would the first PS have as outputs POS and NEG,
and the second POS and COM? Is there an internal difference between the
two? I thought all of these linear power supplies were basically the

And how did we have 24v AC on the common? And why did swapping the AC
inputs on the power supply correct it? (I have noted that some power
supplies have the AC waveform symbol next to the inputs, and others say
AC and COM, or HOT and COM. Is there a difference in the internal

If anyone can shed some light on this I would appreciate it…

Ross McGregor
[email protected]