dx593
Mechanical
- Sep 27, 2003
- 3
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.
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. Figured that was one problem solved.
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 fine.
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
same.
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
wiring?).
If anyone can shed some light on this I would appreciate it…
Ross McGregor
dx593@freenet.carleton.ca