User Tools

Site Tools


GCA79 Contactless ac/dc switching for turnout coil drives with internal timer and feed-back

coming up after pcb production
KICAD 3-D The real thing


By Peter Giling


The previous GCA78 unit provides a nice solution to activate coil switch drives very solidly.
But the mix-upo of configuration with two ports, and both being puls output, give some problens for many
railroad hobbyist.
This board GCA79, based on the same principle has some nice extra features.
ON board is a processor which selects the right output and has also timing on board, to switch off after pulse has been set.
Timing can be selected with on board dip switch in 4 different timings

Each combination of two coils for 1 turnout has 1 command input.
This command is given by any port from GCA50(0) , GCA185 , CAN-GC2(a) , GCA_PI02 or most of WIO units (if applicable).
Depending ON or OFF the GCA79 will activate one of the two coils with the selected timing.
After the set time is counted to zero, the GCA79 sends a logical zero back to the commanding unit.
This commanding and feed-back will be possible via I/O1 connector in the same way as used for GCA136

The firmware in this chip makes it impossible to activate coils during power up.
This will initiate a very quiet startup, no high demands for the power supply for the turnout coils.
After this initial starttime, the GCA79 considers the actual value of the command inputs as the last used position,
and set all feed-backs to 'ready'.
After this quiet power-up, you will still have the option 'start of day' to be sure that all tunouts are in the right position.

Why is this a better solution, compared to relays?

A switching relay always has the disadvantage of 'contact dender'.
This causes the initial power to the switch motor to be reduced at first switch on,
causing a sometimes weak respond of the switch motor, and that just at the moment that the most power is needed to start the move.
GCA78 does not have that problem, either dc or ac , the internal Fets in the Opto-coulper give maximum power right from the beginning.
Together with the extremely low internal resistance makes this a better solution, compared to the use of a relay.
Practice has proofed, even with dc, that an old fashioned switch motor works more reliable in all situations.

Another important issue with coil switches.

Each coil where a current is going through (coil is activated), produces a high reverse voltage when switched off.
This will cause many times a distorsion pulse in electronic systems.
A simple and most always way to supress this, is to connect a diode parallel to the coil (kathode to +).
But many turnout coil drives have a built in switch which then also separates the diode from the coil.
That makes the diode totally useless.
For AC switching, a diode is not possible.
The GCA78/79 are equipped with special VDR resistors, so ac or dc, the high reverse voltage is always suppressed.
But the same issue stands, the VDR will be separated from the coil at the moment it is needed.
Another matter is the fact that this built-in switch of a turnout coil drive makes the start of movement of the turnout sincerely more difficult,
and unfortunately that is exact the moment that the supplied power causes the lowest energie due to way it is constructed.
So for both situations as described above, it is most important to remove the switch and short circuit the contacts.
Specially for DCC or Motorola turnout decoders (except for Digital-Bahns 'Weich-Ei'), this is unacceptable.
Sometimes the switch off command is missing, and the coil will burn out.
There might be many more of turnout coil drives, provided with the vdr or diode option, of which I am not aware of, or has no knowledge of it.
Most certain is that diodes will not be used, or the specification for that particular drive is that it is to be used for dc only.
It is mostly not a mentioned detail in user manuals so you will have to check that by yourself.

GCA79 has its own timing, so it is best to use the feed-back to Rocrail.
Then Rocrail will wait on this feed-back before the turnout is given free
To be sure that a rather small power supply ac or dc is needed for the turnout coils, the GCA79 can only activate 1 turnout at the same time.

It is also possible to use manual switches with the GCA79.
Each turnout needs only 1 double task switch , switching any of the four inputs to either 5V or 0V.
To safe some wiring only: connect a resistor (10 KOhm) between +5V and the input, and let the switch connect input to zero or left open.
Also there, the timing will be very convenient.

Dip switch settings for timing

dip switch 1 2
0.25 sec OFF OFF
0.5 sec ON OFF
1 sec OFF ON
2 sec ON ON

I/O declaration

I/O1 pt value turnout# straight turnout# round turnout# feed-back supply
1 +5V
2 0V
3 1 1 -
3 0 - 1
4 1 2 -
4 0 - 2
5 1 3 -
5 0 - 3
6 1 4 -
6 0 - 4
7 1/0 1
8 1/0 2
9 1/0 3
10 1/0 4

power supply

The necessary 5V for this board comes via I/O1 connector.


The GCA79 is developed with KICAD
Schematic drawing
PCB with parts postions
Bill of materials coming up


The firmware of IC1 is developed with PicSimulator IDE
will be published after pcb production and final test.
gca79-en.txt · Last modified: 2022/08/06 17:27 by phg