Should I put a relay switch before or after a 120v-24v transformer
I am running a whole bunch of small AC motors off of a 120v-24v 250VAC transformer. I am using a programmable relay switch to turn this circuit on and off every 30 seconds for weeks at a time. Do I have to worry about any back emf or inductance in the transformer depending on whether I place the relay before the transformer or after?
Edit: Thanks for the answers. I will be switching from the load side.
Posted Feb 5, 2019, 3:50 PM UTC · edited
13
You will have the leakage induction of the transformer trying to keep current flowing through the relay regardless of which side of the transformer it is on. Generally, voltage stress will wear out a relay faster than current stress. So that might argue switching the load side. However, that also depends on the specific characteristics of the relays you're looking at, and depending upon how reliable you need this to be your how long it needs to run, you probably want to look for a realize that have detailed enough data sheets to show what the switching life is for your actual operation.
But a better idea would be to use a solid state relay which basically won't wear out, and then you can put it in either location, and I would recommend in that case putting on the line side to avoid power losses in the transformer when it's not running.
You could take it one step further, and use a switching power supply instead of the transformer, at least if your loads can be run on DC, and then you'll get better efficiency, and there might be a enable pin on your power supply that allows you to use the switches that are built into the power supply to turn it on and off without needing a separate relay.