Coach Lighting Stay Alive

adamski

Registered
14 Jul 2016
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kent
Good Morning

Sorry if this thread has been covered I have found a few bits of info but was wondering if my idea would work. So Iam looking at putting some stay alive's in some coaches I have and was thinking of use a bridge rectifier straight of the tracks directly into a dc buck board. Setting the output of the buck board to 4.8v and soldering a 5V 1F super cap direct to the board and then running 2 wires up to a short strip of 5v led tape. Very simple only 3 components does anyone see any problems with that or a better way ideally like to use super caps as will store a lot more power as flickering lights drive me mad.

Is there a board out there that does not need the bridge rectifier 1st ?

Many thanks

Adam
 

Gtarling

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26 Aug 2019
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Grantham
Seems to me that what you describe should work OK. You need the bridge rectifier on the input to the buck converter in order to ensure that the latter is always fed the correct polarity.

Gordon
 

Brixham

No buffers were hurt at this sign
27 Aug 2010
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It may be worth adding a small value series resistor to limit the ‘inrush’ current. Otherwise a rake of these coaches could cause issue with your track power overloading when applying power. Something like 30 ohms, ok I know that’s not an easy value to find, but use 3 100 ohm resistors in parallel.
Malcolm
 

adamski

Registered
14 Jul 2016
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kent
It may be worth adding a small value series resistor to limit the ‘inrush’ current. Otherwise a rake of these coaches could cause issue with your track power overloading when applying power. Something like 30 ohms, ok I know that’s not an easy value to find, but use 3 100 ohm resistors in parallel.
Malcolm
Where would you attach these ?
 

Brixham

No buffers were hurt at this sign
27 Aug 2010
1,241
71
Pop the resistor either before the bridge rectifier, or before the dc dc convector. Assuming a max track voltage of21v, and say 35 ohms, for a brief instant 21/35 amps will flow, some 600mA. Multiple coaches could overload a digital system, the only way round this is to increase the resistor value to maybe 100 ohms.

Malcolm
 

Dan

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28 Jan 2010
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Eastern MA
What type of lights and how many? leds can be controlled with much smaller caps.
 

Greg Elmassian

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8 Mar 2014
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I'd put the resistor on one leg of the super cap.

If you are trying to reduce the turn on current "spike", that is caused by the cap charging.

Putting a resistor on the input of a DC to DC regulator will present a varying voltage to the regulator circuit as the cap current changes.

The regulator SHOULD be able to handle that without issue, but design wise you want your added component to affect the cap directly. Since this is a low current situation, you don't need the traditional diode in parallel with the resistor to provide full voltage on discharge.

Greg
 

Matthias

Registered
13 Apr 2019
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Switzerland
www.beathis.ch
soldering a 5V 1F super cap
Hi Adam,

such a super cap has a too high "initial ESR" (inside resistance) and it will not work. You have to take a super cap with a very low ESR (approx. 0.2 Ohm or less). I use the HV0810-2R7105-R from EATON for example. Such super caps have a max voltage of 2.5V or 2.8V and you have to make a serial connection of the super caps.

The current limitation is a very important point, because otherwise your booster will switch off your system - it looks like short circuit (very high current). I have approx. 20 wagon with such a solution and i had to lower the loading current down to 25mA per wagon.

I use a LM317 as a current limiter. There is a resistor to define the current to load the super cap with the LM317. I made my own solution with 8x1F oder 8x3F:

Energiespeicher (page in german - sorry)

Regards

Matthias
 

Greg Elmassian

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OK, I'm really confused by your post Matthias.

First can you explain why, in a very low current drain circuit, a low ESR cap is needed? Clearly coach lighting is low current and also the current demand is steady, not "spiking"

Next can you explain why you use a voltage regulator as a current limiter? What is wrong with a simple resistor? (Or a current limiting IC?)

Also I think you missed the elegance in his idea, that by putting the super cap on the 5v supply, he can do it with a single cap, whereas you use 8 capacitors in series to achieve a 20 volt supply.

I think his solution is simple and by deciding to use 5 volts and an inexpensive regulator, with the addition of the current limiting resistor, he has a compact, simple, inexpensive solution.

Greg
 

Matthias

Registered
13 Apr 2019
15
1
Switzerland
www.beathis.ch
Hi Greg,

the usage of the LM317 as a current limiter is official - have a look in the data sheet of this IC. You need only a resistor and a diode - very smart. The current limitation is working with high caps (e.g. 10F) too and there is no special effect, when you switch on the power. The solution with a simple resistor to limit the current is not optimal with super caps - my opinion.

ESR (Equivalent Series Resistance): This internal resistor is in serie with the cap. The speed of the voltage drop is lower, if this resistor is smaller, because the current is less. See more details about ESR in the Internet.

Regards

Matthias
 

Greg Elmassian

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I know it can be used as a current regulator, I was in the industry before the 317 came on the market. I mentioned the cost and unnecessary complexity. Clearly a LM317 costs more than not using one at all, just a resistor and diode as I mentioned.

That was my question, why you need to make a controlled current limiter vs. just a simple inrush resistor.

I also know what ESR is, I have a degree (actually several)... and again that is not what I asked, I asked why you think you need a low ESR supercap.


Both times you avoided the question I asked, and I know you are a smart guy, so I am curious why are you avoiding answering the questions?

I'm not taking you to task, but I'd like some justification and reasoning for your statements. I'm asking for reasons.

This is a forum, where it's ok to question reasons, not just take someone's word as "gospel" without reasons and justifications.
I'm honestly trying to see what your reasoning is, and why you have these 2 opinions.

Greg
 
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adamski

Registered
14 Jul 2016
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45
kent
Thank you all for your replies I am going to build my basic version today using some components I have in my workshop as I said I wanted to keep it cheap and small it only to keep the lights on as they go over the points at low speed or over a piece of dirty track. I will upload a picture once I have built it.
 
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