Capacitor Install ? ? ?

[C855B]

OK, with those clarifications, I disagree. If you think adding a full keep-alive may interfere with programming, then why do it? It makes everything a lot more complicated than it needs to be. All Carl wants is to keep the LED(s) from flickering. A cap on the LED leads solves that... done.

[peteski]

OK, with those clarifications, I disagree. If you think adding a full keep-alive may interfere with programming, then why do it? It makes everything a lot more complicated than it needs to be. All Carl wants is to keep the LED(s) from flickering. A cap on the LED leads solves that... done.

Quoting my earlier post:
As mentioned in above post, a capacitor (with a series-connected low resistance current limiting resistor) hoked up between the function output and blue positive wire should keep the LED lit for a very brief time, but it will not keep the decoder circuitry (the microcrontroller) running during the  power dropout.  So, depending on the length of the power dropout, it can cause the LED to flicker (when the microcontroller reinitializes after the power is restored).

Plus I'm worried that the cap in the function circuit might back feed the rest of the decoder through the function output  transistor (probably not a problem if that transistor is a MOSFET, but could be problematic with a bipolar transistor).

I guess Carl has multiple options . . .

[carlso]

Wow guys, you are all talking above my grade level.

I put some white paint on the items and rubbed it off leaving it in the stamped #'s. Here is that image, I hope.



You can see that the two items on the left side top and bottom are the same component. The large lower right is only a 3 legged component. The small item next to the yellow one has either an E1 or 13, can't read it that well.

Just a little more info,
Carl

[peteski]

Wow guys, you are all talking above my grade level.

I already identified and explained the circuit in my earlier post, giving you a Keep Alive hookup diagram (after checking if my assumptions are correct).  it is all in the write up in that post with the diagram.

See https://www.therailwire.net/forum/index.php?topic=47620.msg628019#msg628019
If something is still unclear, let me know and I'll try to explain it further.

[Point353]

If that is the case then you can go ahead and connect the Keep Alive circuit (shown in yellow) to the decoder.  That should eliminate any flickering or decoder dropouts.  If 100 uF is not enough, add more capacitance. 1000 uF was chosen arbitrarily (I don't expect you will need  more than few hundred micro Farads).

Do you think it's likely that sufficient capacitance can be added to keep the decoder alive and still have the decoder with the extra capacitors fit into a caboose cupola as the OP wants to do?

[peteski]

Do you think it's likely that sufficient capacitance can be added to keep the decoder alive and still have the decoder with the extra capacitors fit into a caboose cupola as the OP wants to do?

Carl mentioned in initial post that he might have room for couple of added caps. Maybe single 100uF cap will be enough, but more is better.  The  decoder itself uses very small amount of current, with only 2 LEDs as a load the total load will likely be less than 20-30mA (just guessing).  Whether a 100uF caps will be sufficient depends on how long the power interruptions are.

[carlso]

Pete,

I thank you for your help and yes I understood your post. I tested the 3 points and found continuity as you suggested, so I am going to connect a circuit to decoder as you suggested. I have room in the cupola to place 2 100uF, 20v caps. I also will add a micro plug to one of the cap leads so I can unplug for programming, if necessary.

You suggest a 75 ohm resistor in the (+) lead to cap.; what would that do?

Carl




Thank you for the help,
Carl

[peteski]

As Point353 mentioned  in the reply #1 to this thread:
Also, if you're going to use a capacitor with a value on the order of 100μF, I'd also install a second resistor - with a value of about 50-100Ω - between the blue wire decoder output and the capacitor to limit the current when the decoder output is first enabled.


Basically when the capacitor is discharged and voltage is applied to it in that state, it will accept a very high current (it acts as a short circuit). As it charges up the current drops to zero when its voltage is the same as the voltage supplying power to it.  In  a circuit like this that high current last very short time (milliseconds). The resistor is added to limit that zero-charge current so it doesn't damage the bridge rectifier.  The diodes would likely survive that peak charging current, but it is safer to add that resistor.

When the track power is interrupted, the capacitor starts powering the decoder through that resistor, but since the decoder itself and LED do not consume much current, the voltage dropped across that resistor will only be few volts.  A diode could also be added into that circuit to pass current and result in only 0.7V drop while the cap is supplying power, but I don't think it is necessary. Actually a 47 ohm, 0.25W resistor would work well, and result in even smaller voltage drop when the cap is supplying power to the decoder, while still limiting the cap's charging current.

[carlso]

Thank you Sir ! I appreciate your knowledge and help.

Carl

[carlso]

As follow up:

I installed 2-100uF, 20v caps. I wired as Peteski showed above and the smallest resistor I had was 110 ohm 1/4 W so I used it. The blue line to the LED's has a 5.1K smd resistor. Tested last night and it works well.

thanks to all for input,
Carl

[peteski]

As follow up:

I installed 2-100uF, 20v caps. I wired as Peteski showed above and the smallest resistor I had was 110 ohm 1/4 W so I used it. The blue line to the LED's has a 5.1K smd resistor. Tested last night and it works well.

thanks to all for input,
Carl

Excellent Carl!  110 ohm is a non-standard (not often used) resistor value (typically they are 100 ohm and next value is 120 ohm), but there is nothing wrong with using a 110 ohm resistor.  Also, FYI, you could use 2 110 ohm resistors connected in parallel to create a 55 ohm resistor, but again, if 110 works, then leave things alone. If you use a 5.1k resistor in series with LEDs, then the LED current is likely less than 2mA (very low).

How about the ability to program and read CVs on the programming track?  That still works?

[Steveruger45]

Hi Carl
Was reading this thread with interest.  If you need a small coil in series with your keep alive to prevent possible issues with programming and/or reading CV’s on programming track like Pete mentions may happen then I can save you some thread searching.   The chokes aka Inductors as per photo should work.  I used these on sound decoders and they work fine. [ Guests cannot view attachments ]

[carlso]

Steve - thanks for the info, I shall file for future use.

Sir Pete - Yes the 110 ohm I purchased approx. 15 years ago from Radio Shack and never used any of them until now. You are right the markers are dim but dim enough to see and not overpower the caboose. Back "in the old days" the markers were not very bright. I have a real issue with our diesels that have such bright head lights, especially the older models. The new big mamoos have very bright head and ditch lights, so no issues.

Thanks for your help,
Carl

I forgot to mention that I actually had programmed this decoder prior to install so do not know if the cap may/may not cause an issue. I'm sure I will have an opportunity to find out.