GCA120 Heavy load relay


By Peter Giling

The prototype


The GCA50 or CAN-GC2 are only able to switch 25mA by 5 Volt on each output.

This is fine to switch a relay like Omron GV5-1 but then there is only 1 Amp maximum allowance.

Even that 1 Amp is too much, if the relay has to switch the full load many times.

That is my experience.

To make it possible to switch up to 10 Amp, a bigger relay is needed, and also some way of controlling that relay from LocoNet.

One option is to switch a relay like Omron G2R by use of GCA77.

But if you need to setup a GCA77, just for the relay only, then this unit provides all in one.

One side is connected to an output of GCA50, and to a power adapter 12-16V dc or transformer with max 18V ac.

The other side has a connector which provides a Normal open and normal close contact. One or both can be used.

I constructed this because most railroad will have (at least I have) a transformer for feeding illumination and also for the power to GCA76, GCA77 or GCA136.

In order to be able to use that transformer for powering this relay, an on-board opto-coupler separates the supply from LocoNet GCA50.

Anything up to 10Amp can be switched now.

The MGV120 is developed to switch a max of 42 Volt.

The relay will be able to handle 230V, but CE regulations demand an open space of 5mm between tracks on the pc-board and connections.

Both connector J2 and the tracks to the relay do not comply with that.

Also, an correct isolation is not taken care for.

So 42 V is the maximum allowance.

Depending of the used supply, please consider that the relay must be adapted to that. See bill of materials.


- Opto isolated input for 5-12Vdc (like from GCA50 or CAN-GC2).

- 42V - 10Amp power switching.

- supply 12 Vdc or 16 Vac (look in bill of materials for differences).

Bill off materials

Qty Ref. description supply 12 dc description supply 16-18V ac
2 R1,R3 2K2 2K2
1 R2 10K 10K
1 R4 330E 330E
1 C1 ELCO 220 uF 25V ELCO 220 uF 35V
1 Lg1 LED 3mm green LED 3mm green
1 D1 DIODE 1N4002 DIODE 1N4002
1 D2 DIODE 1N4148 DIODE 1N4148
1 T1 BC517 BC517
1 K1 OMRON G2R 12V
alt. FIN 40.31.9 12V
alt. FIN 40.31.9 24V
alt. Reichelt
1 J1 AKL012/04 AKL012/04
1 J2 AKL012/03 AKL012/03

The design

The schematics
The pc-board + parts placement
N.B. Self made pc-boards are not supported!

All connections are signed at pcb drawing.

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