Digitrax function only- keep alive ain’t keepin’ alive.

[craigolio1]

Hey all.

I’m using a Digitrax 4 function,TF4 function only decoder to power some lights on a piece of rolling stock and I’d like to add keep alive.

It’s got two 1/2 wave rectifiers and the blue possitive is on the one side where the red track wire comes in. With my multimeter I found the negative on the side where the black wire comes in. I soldered a wire to it and with everything powered up I get 12v on those leads.

I soldered a 470uF tantalum cap to the leads. Blue positive to the line on the cap.

The decoder is wired to an ESU decoder tester. When I wiggle one track wire into the tester all the lights flicker. So I wired a second cap in parallel. Same result.

It appears that the keep alive doesn’t work which to means I’ve wired it wrong.

Can anyone offer any advice?

Thanks.

Craig.

[craigolio1]

Really it’s only the headlight I don’t want to flicker. I could just put a cap/resistor on that for constant lighting, but keep alive for the whole decoder wood be preferable.

Craig.

[peteski]

I used that decoder on one of my camera equipped locos for the headlight functionality and when  video from the camerawas really noisy, troubleshooting the problem I realized that the TF4 was actually the source of the noise!  I tossed it in the trash and replaced it with a ESU LokPilotFX decoder.

I know that this is not really answering the question, but that decoder is not something I'll ever use again.

I don't have that decoder anymore but from looking at the photo it looks like you did find the proper connections to the raw rectified voltage coming from the two diode pairs.

470 or even 940µF is really not a lot of energy storage but the decoder's circuitry and couple of LEDs (no motor) don't need a lot of current, so the caps should provide some keep-alive functionality.

There few things you can do to verify the caps themselves are working.

1. Measure the voltage across the cap while the decoder is powered up.  Should be around 12V (dependent of  what the DCC voltage is).
2. If you have an old analog multimeter see how fast the cap voltage decays when the track voltage is cut.
3. if you don't have an analog multimeter, to visually check the voltage decay rate connect a white LED with a 10k or even 20k series resistor across the cap and see how long it takes for the LED to gradually dim.

This should give you a clue showing if the caps actually work properly. If you see that the LED stays illuminated for some time then that shows that the decoder should stay active for some time after track power is cut.
Maybe the decoder is designed to shut down the functions when the DCC packets coming from the track are lost.

[craigolio1]

Thanks for the tips @peteski !

With a white LED and 10k resistor across the parallel caps, the LED takes about 6 seconds to use up the charge.

What I ended up doing was experimenting with the actual LED set up I’ll run on this car. It’s going to have 4 head lights. I ended up with two 470uF caps and a 3.6k resistor. This gave me the brightness I want and a suitable anti flicker. I applied this directly to the function output instead to the rectifier.

The headlight is the only one that I care about anti flicker as the other lights will only be operating when the car is stationary.

Thanks. Craig.

[peteski]

Good deal Craig!
Seems that the decoder is not really designed to work with a keep-alive of any sort, but as long as you solved this another way, all is good. 

[Sdynamo]

If memory serves, when you hack a KA onto a DT decoder, you need to disable analog DC mode to get it to work.

[craigolio1]

If memory serves, when you hack a KA onto a DT decoder, you need to disable analog DC mode to get it to work.

Now THATs interesting. I never thought of programming being required to get a hardwired passive device to act differently.

Craig

[peteski]

Now THATs interesting. I never thought of programming being required to get a hardwired passive device to act differently.

Craig

If that's what causing the KA not to work, it is a firmware problem, or actually it is likely working as designed (by Digitrax programmers).  It would be interesting to find out if Craig is willing to try wiring the caps back to the rectifier and disable the analog mode..

[craigolio1]

If that's what causing the KA not to work, it is a firmware problem, or actually it is likely working as designed (by Digitrax programmers).  It would be interesting to find out if Craig is willing to try wiring the caps back to the rectifier and disable the analog mode..

I’ve decided that a decoder is too much a pain for this car. I have two lighting circuits. I’m going to use the simple constant lighting circuit you posted here years ago and the. On the bottom of the car make a couple of safety pin like switches to make contact. It’s a car that’s really just for display purposes.

I can still do the test with the decoder though as it will arm us with info we can use on future projects.

Craig

[peteski]

I’ve decided that a decoder is too much a pain for this car. I have two lighting circuits. I’m going to use the simple constant lighting circuit you posted here years ago and the. On the bottom of the car make a couple of safety pin like switches to make contact. It’s a car that’s really just for display purposes.

I can still do the test with the decoder though as it will arm us with info we can use on future projects.

Craig

Hey, whatever works for you.
If you are willing to test the decoder with a KA installed and DC disabled to see if the KA works, that would be very helpful to know.