Project files: A smaller mains controller…

What is it?
As mentioned a few weeks ago I’ve recently built another control board for switching a mains transformer with a low-voltage (latching) switch. This in a slightly different form factor so that if your application requires it, the board can be stacked with a matching standby-PSU and mains splitter and/or my passive softstart board. It is possible (just) to stack all three boards on top of each other in a 2U/80mm high enclosure or just two boards in a 50mm tall enclosure.

You can decide which standby voltage should be used by choosing the right relay in resistor values and in addition to using a latching switch for engaging the relay, you can also use a DC-voltage between app. 3-30V as the trigger. This input is isolated via an optocoupler and the trigger circuit only requires app. 15mA from the triggering device.

The matching standby PSU board uses the (by now) well-known IRM AC-DC power modules from Mean Well. There are two versions, one for the 3W module which is 100% outline-compatible with the control board and a version for the 5-10W modules where some of the connectors had to be shifted but the mounting holes still fit. The PSU board also provides a splitter-function to give two mains outputs.

How big are the boards?
All the boards are 2” x 2” (app. 51 x 51 mm) – the original theslowdiyer industry standard ™ 😀

What is the status of the boards?
These boards are v1.0 and they all work as expected.

Does it use any special/expensive/hard-to-find parts?

Anything else I need to know?

  • The switch must be a latching type (meaning it stays in either on or off positions) and to turn the relay on you connect the switch so that the + voltage is connected to the switch pin. This turns on a transistor which switches the relay on.
  • The relay is rated for 5A inductive loads, so should be good for transformers up to around 1000VA at 230VAC (to give a bit of safety margin).
  • The optocoupler on the trigger input is fed from a constant-current source (CCS) made from an LM317L voltage regulator. If I was designing a commercial product this would probably be a sacking offence because it’s much more expensive than the alternatives, but for our purposes it works quite well 🙂
  • There are two LEDs on the control board, one to indicate the board is powered and one to indicate the relay is on.
  • The “ext” output is intended for us if you want to feed the unswitched standby voltage to some other circuit. There’s space for a bigger resistor here if you need to drop voltage for e.g. LEDs, but you can also jumper the resistor to just get the raw voltage (or leave the output if you don’t need it).
  • The mains connectors on the standby-PSU are marked as inputs and outputs, but in reality it doesn’t matter what you use as inputs and outputs.

Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.

Remember that these boards use mains voltage. Be careful when mounting and handling them!