Switch decoder external power supply

  • Thread starter Thread starter AVR
  • Start date Start date
AVR

AVR

Registered
Hi all,

Can I connect an external power supply negative to the central solder pad of one of the contact inputs so as to drive LED signals from the external positive and not the decoder positive. I want to reduce the load on the central station
AVR
 
Moved
 
Last edited by a moderator:
John S said:
Only if you want to destroy the switch decoder! Refer to answer on other thread..................
Click to expand...
High John S
I had seen the disclaimer from Massoth
I just mused that perhaps if the outputs are low side switches connecting to an internally generated ground derived from the DCC signal and the switch inputs are shorted to this generated ground then I could actually drive my LEDs from a separate 12v supply and take the load off the massoth central station.
I’m sure it’s not conventional but thought someone may have tried it.
Bill
 
Bill, you have said it yourself...

..connecting to an internally generated ground derived from the DCC signal..

You don't have a reference between your external PSU and the 'derived ground'.. I suppose you *might* get away with it if you tied decoder -ve. to your external supply -ve. .. But I would not recommend it.
 
PhilP said:
Bill, you have said it yourself...

..connecting to an internally generated ground derived from the DCC signal..

You don't have a reference between your external PSU and the 'derived ground'.. I suppose you *might* get away with it if you tied decoder -ve. to your external supply -ve. .. But I would not recommend it.
Click to expand...
Sort of hoped the switch common might just be the internal negative.

I’ll have to poke around with a meter ir a scope and see what there is.

I have asked Massoth but so far they have inly sent a reply saying they are busy
 
Moved
 
Last edited by a moderator:
If your using LEDs the power requirement is tiny. May I respectfully suggest - dont reinvent the wheel.
 
John S said:
The key fact being "they are inputs" for that matter they are actually " zero potential" i.e. "NO VOLTAGE"

They are switched internally, but only to the point that the on-board microcontroller sees a resistance value across which ever input port is connected to the pads, and then acts upon the instruction set, which in this case being the values as stored in the CV's.

In the case of a momentary switch or a reed switch, when connected and activated, makes a complete circuit to GND at "zero potential" which triggers a pulse (resistance) to the relevant input port on the microcontroller.

There is no input voltage protection to these ports!

By placing a voltage through these inputs, voltage will travel directly to the micro controller ports, and DESTROY IT!

Yes it is indeed possible to connect an "external voltage source" to drive the inputs at zero potential, but it's of no use for this particular request, as they are inputs only, they cannot be configured to operate as an output.
Click to expand...

I am not suggesting using the actual inputs since, as yousay, they are merely connected to the micro ports. I was surmising that the common pin was actually connected to the internal notional zero volts and could thus be used to connect the negative of a separate power supply for the leds.
Visual inspection of the pc suggests this point is the power ground and so this may be so but caution and cheque book have so far stopped me experimenting.

I can see if I connect this notional ground to other notional grounds on other similar switch decoders I may have issues if the voltage decays on switching off the main power supply and the led power supply are different and the led supplies stays on longer thereby possibly reverse biassing the low side drivers.

Ikll see what Massoth say when they respond.
 
idlemarvel said:
Wouldn't it be better to use relays rather than risk blowing your decoder?
Click to expand...

That was what I used to do but I had a major disaster wherein nearly all of my point mounted switches failed due to what appears to be uncorrectable corrosion.
Could be the consequence of living in farm land not too far from the sea.
 
stockers said:
If your using LEDs the power requirement is tiny. May I respectfully suggest - dont reinvent the wheel.
Click to expand...

I agree, but it may be the final straw that limits how many locos I can run simultaneously.

I also enjoy playing and trying to do that bit more with stuff.

If the idea works then I can control lights in buildings and my fairground etc.

Bill
 
AVR said:
I agree, but it may be the final straw that limits how many locos I can run simultaneously.

I also enjoy playing and trying to do that bit more with stuff.

If the idea works then I can control lights in buildings and my fairground etc.

Bill
Click to expand...

But you can use the OUTPUTS to control lights anyway??
 
You must log in or register to reply here.
Back
Top