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GCA76 High current interface


By Peter Giling

**Very important issue**

Peter Giling developed a large number of useful pc-boards, all made for self assembling.
This self assembling will include the fact that the assembler / installer of these units must confirm CE regulations.
As long as this installer is the only user of the assembled units, there is hardly no problem,
but as soon as other people are involved, one should be aware of that.
Allmost all designs stay below 42 Volt and max 3 Amp, which makes CE regulations pretty low level.
But his unit is made to supply more then 3 Amp, so be very carefull with that.
The unit, as delivered standard as kit, is not able of supplying more than ca 2 Amp total.
If you need more, you will have to change the fuse F1, and changing that is your responsibility.
Peter Giling cannot be hold responsible for any mistake in design or installation.
If you are not sure how to comply to CE recommandations and/or regulations, please refrain from changing anything in this unitand you will be perfectly safe.
In case you need more advice, contact Peter.


Some turnout drives need far more power than a standard interface like GCA77 can supply.
One example is the Peco double coil drive, and another are the Märklin drives 74490 and 7549.
Refer to list below for various turnouts.
With this interface, the internal switch of these turnout drives can be bridged. This will improve the function of it.
Thanks to the options of LocoNet or MERG-CBUS, all you have to do is program the GCA50 / CAN-GC2 / GCA_PI02 with a hardware pulse output. ( see at the bottom of this page )
This will take care that the coil in the drive will only be powered for ca 0.2 seconds.
The power darlington transistors T1..T8 are 10 Amp types.
With P-FET transisotr it is going further than that.
The unit is electrically isolated from GCA50 / CAN-GC2 / GCA_PI02 by opto-couplers.
The board needs addional power of 16Vac minimum 24 VA.
Not all turnout drives really need that high voltage. Always refer to propriate manual.
An ac power (transfromer) used to supply this interface will rresult in a higher dc voltage to the turnouts.\\
Here is an approx calculation for that:
dc on board = (1,4 x ac_of_trafo) - 1,3
with a 16V ac connected, the dc on board will be 1,4 x 16 - 1,3 = 21,1 Volt.
Please consider that in practice this will be slightly higher when transformer is without load.

Safety precaution

A short circuit at one of the outputs the fuse F1 will trip .
It resets itself after problem is solved, or ac power has been shut off.
With complete kits, a 1,8 Amp fuse is included, which will normally trip when more than 3 Amp is absorbed.
This fuse can be changed by higher values, but never more than 3 Amp.
Doing so, will be for your own responsibility.

The final result

Hardware GCA76 (no longer in stock)

Hardware GCA76v3 (FET version)

The latest version with P-Fet transistors.

The board changed slightly, and partslist is different for using P-Fet power transistors.
A higher current with lower heat production on the PCB is the result.
The changes are:

  • Rp3,RP4,Rp5,RP6 are changed to 10 KOhm.
  • D1..D8 are removed.(built in Fet)
  • T1 .. T8 changed to IRF 9Z34N.
  • extra led L10 for 5 V indication on J1.
  • a separate common terminal connection for each turnout.
  • standard kit is delivered with high active pulse inputs.
  • choice of all inputs low active or high active (selectable with jumper) This option only with LTV844 Optocouplers.

previous MGV76 board files

Cable to GCA50 / CAN-GC2 / GCA_PI02 / GCA_pi03

Known current of switch coils

For detailed info about different makes of turnout solenoids please refer to this page

Instructions for controlling GCA76 (e.g. for turnout coils)

The GCA76 must be activated with a positive pulse.
That means that there will be 5 volt feed into the input, wich will be shut off shortly after that.
On GCA76 itself , there is no electronic device that will take care for this shut off.
Standard turnout decoders usually (there are exceptions) do not have this facility also.
Shut off is done by software in central units or programs, which will activate the coil, and then inactivate after approx 200 miliSecs.
With an GCA76 connected to GCA50 /CAN-GC2 this can be done more safely by GCA50 / CAN-GC2 hardware itself.
Examples below show how settings could be done.

:!: *It is essential to do the settings of GCA50 / CAN-GC2 / GCA_PI02/03 first, before connecting switches!
:!: *Always repower GCA50 / CAN-GC2 after programming !

The best way to check if programming is done in the right way, and pulses are working, is using a small 12 volt lamp (from automobile ) to check each output.
Each ouput should only shortly light up on activation.
The latest version of GCA76, as deliered bow, does have Leds on board, connected to the output.
This makes the check easy, before connecting coils.

Example of GCA50 settings:
Adjust GCA50(0)?600 GCA76 is connected to J5 on GCA50
J6 on GCA50 is used for connection
of GCA93 current detector.

Example of CAN-GC2 settings:
The first 8 ports are set to control 4 turnouts (8 coils)
Adresses of Turnouts in Rocrail to be set to 25,27,29,31
Ports 9-16 are set to be connected
with GCA93 current detector (Adress 1-8)

GCA76 is connected to J4 on CAN-GC2
GCA93 is connected to J3 on CAN-GC2

Using the test-button
will change position of turnout on any toggle.

Example of RocNet GCA_PI02 or GCA_PI03 settings for GCA76/77

Setting example for Rocrail

Output 6 is used to command this turnout.
Please realise that adress 7 is used for the the second coil.

Using GCA76 for permanent switching applications (e.g. motors, lighting with leds/bulbs)

GCA76 not only can be used for pulse switching coils of turnouts (with short pulses, see above). Also permanent switching on and off functions, e.g. leds, bulbs, or motors is possible.
For those applications GCA50 / CAN-GC2 / GCA_PI02 have to be programmed with option switch (not pulse).
:!: Please note that max load of the printed circuit can not exceed 2 Amp.
If more than 300 mA is to supplied by the circuit, the Fuse F1 has to be adapted.

Example of CGA50 settings – easy setup menü
GCA76 is connected to J5 of a GCA50.
Ports 1-8 are programmed for
permanent on/off
of functions like lights
or motors of function models.

J6 of the GCA50, ports 9-16,
is used to connect a
GCA93 current detector.

:!: Note that when programming in the easy setup menu of the GCA50, the switching outputs are on by default.

:!: If it is desired that the outputs are off when the GCA50 is turned on, the I/O menu must be used:

Example of the CGA50 settings - I/O menu
GCA76 is connected to J5 of a GCA50.
Setting of each port,
here for permanent switching,
where the outputs are off
when the GCA50 is switched on
(Setup - Output setup - Low at startup).

For more details see LocoIO Dialog Tab 'I/O'

Example of CAN-GC2 settings
Port 1-8 are set to control
of 8 switching outputs of the GCA76
(addresses 25-32),
where the switching state
of ports 5-8 is inverted.

Port 9-16 are set for connection
of GCA93 current detector
(address 1-8).
GCA76 is connected to J4
of CAN-GC2.
GCA93 is connected to J3 of CAN-GC2.

For more details see CAN-GC2 Universal 16-port I/O node for CANBUS

Example of RocNet GCA_PI02 or GCA_PI03 settings for GCA76
Port 1-8 are set to control
8 switching outputs of GCA76.

For more details see GCA_PI02 32 I/O I2C board for Raspberry Pi. - Address selection

mgv76-en.txt · Last modified: 2023/04/21 16:24 by campus