Output Stage

The Output Stage

The output stage is fairly simple.

For three wire output, power is always applied to the output via the common  wire and the control wires are grounded out when they are triggered. This allows multiple outputs to be safely wired together if needed. In most cases though, simply all three wires will be wired from the output to the switch. The resistors R21 & R24 used by two wire switches are optional, but can safely be installed if the circuit might be used both ways.

For two wire output, the output resistors R21 & R24 are required and also help limit how much power is available in two wire mode. The two switched lines are used to connect to the switch, the common is not used. Power is left applied via the resistors and depending on which side is grounded will run the switch as needed. Negative voltage is never actually applied, but voltage reversal is achieved by switching moving the ground.

Hight Current vs. Low Current

The circuit is designed to use either high current TO-220 MOSFET's or Transistors, or low current TO-92 MOSFET's or Transistors. On my circuit design, holes for both are available, but you only populate one of them. The high current parts are required for running relays but can handle the low current switches as well. The low current parts should only be used if only low current is required and you need to save money. If you have both parts sitting around, or the board might be used for different purposes, it's recommended only the high current parts are installed.

Care needs to be taken when installing the MOSFET's and even Transisyots. Some devices are Reversed for the pinout versus the case. Because of this, I did take the time in my design to mark the Gate/Drain/Source on the TO-92 pins since those are the ones most likely to be reversed.

For really high current or constant output usage, space had been provided for some small heatsinks on the TO-220 cases. I designed the circuit board layout for 6A continuous current and 10A surge. If higher current requirements are needed, you need to reinforce the power and ground runs from the power connector(s) to the the high current MOSFET's and optionally the outputs. The heatsinks and air flow over them will start becoming important at this time.

Under normal usage with switches, even high current 12V ones, heatsinks will not be required, even if running in a hump switch yard. Tests have been run with this circuit switching a heavy duty ConCor style switch a full cycle every 7 seconds for 10 minuts and the relay gets hotter then the 20A MOSFETS. The same test burns out an Atlas Snap Switch within several minutes.

MOSFET's vs. Transistors

While the circuit is designed for with, it is recommended that MOSFET's be used for high current systems and either can be used for low current. In my tests, I was very satisfied with using 20A or greater MOSFET's for high current, they ran cooler and no heatsink were required even under high usage. You're more likely to to damage the relays before they they will have problems.

If transistors are used with relay type switches D1 & D3 are required. With MOSFET's they are used only if really large coils are involved or if high usage is, they are optional.

Diodes D2 & D4 are required for any relay or coil device to prevent damage. IF you have any doubts, just put them in. The only time you won't need these is only tortoise type switches are being used or it you are running non-coil or motor type loads, like lights.

Output Voltage and Current

By default, my circuit uses the same power to run all sections. It is designed that only two runs need to be cut to isolate the power of the output stage from the rest. That is the only time you need to add the power connector that is part of the output stage unless an extra connector is desired.


Plenty of capacitors can be added. Please make sure that they are rated for more then the voltage applied is. If a relay type switch with a good high current MOSFET or Transistor won't switch, that usually means the the power supply can't handle it. Adding more capacitors helps offset weak power supplies.

During testing a 2A 12VDC supply was used to run 3A relays simply by adding several 1000uF capacitors to help handle the surge required by the relay. Several different hole spacings were even supplied to allow for a variety of capacitors that a hobbyist may have on hand. Just watch the Voltage and Polarity when installing them!

Output schematic

Updated ...
Doing even more testing with additional devices and scoping my circuit, I've decided that when MOSFET's are used for the output stage, the resistor to the gate works better with a value as low as 100 ohms. The original 470 ohm value had been selected because of the transistor option and that value was already used. The 100 ohm value for the MOSFET driver helps get everything moving when you have sub-optimal situations, which can happen easily.