Electornics help needed! Lights in coaches with capacitors!

[Dylanlewis2000]

Hello all,

After a bit of help with electronics. I recently acquired a set of starter set coaches with a view to put lighting in them. I have decided to go for track powered lighting as I have a habit of leaving batteries in devices and over the winter they corrode, so with this in mind i have purchased a pair of track pickup wheels.

Using strip LEDs i have successfully installed lighting in both of these coaches however when they go around the track they flicker. To remedy this i was going to install a capacitor, and as i have had a bit of spare time this weekend, i thought that i would give it a crack.
Looking online i can't see any definitive answers to i took a trip down to Maplin to purchase some capacitors. The 'manager' of the store didn't have a clue what would be the best for my solution to i decided to take a range of different capacitors to try with 1000uf 25v being the biggest.

I have wired the 1000uf up and although it does not solve the flicker problem it does seem to hold charge, however this only works with analogue. When i plug the MTS in and try the same coach, it seems to have a bit of a paddy and the safety shutoff kicks in.
So I'm after a bit of advice. What capacitors do i need (size and type) and how should they be wired in?

 

[rjstott]

MTS is basically an Alternating Current system whereas capacitors (in storage mode) are DC devices. I assume you have just wired the capacitor across the AC from the pickup wheels. This won't help and is why your MTS cuts out. You need a diode rectifier to convert the AC to DC and feed this to the capacitor and the lighting. I suggest you do a search of the web. There are lots of circuits illustrated by various people that will demonstrate what you need to do. One alternate that I use is a cheap function decoder which allows you to control the lighting from your master station. Such a decoder includes all the circuitry to produce a DC voltage to feed you capacitor and lights.

Richard

 

[Dylanlewis2000]

Thanks for the reply.

Would something like this work? http://www.maplin.co.uk/6a-silicon-rectifiers-46413 < Link To http://www.maplin.co.uk/6.silicon-rectifiers-46413

 

[rjstott]

Yes but a bit pricey. The standard for this would probably be a 1N4002 at about 8p each. You might want to include a resistor between the diode bridge (four diodes) and the capacitor to limit the current. 1Kohm would limit 20 volts to 20milliamps. Depends on what your LEDs need. Anyway as I said before, best to model the circuit on something tried and tested!

Richard

 

[Dylanlewis2000]

How would i model the circuit?

 

[DaveB2]

Veroboard or one of the solderless prototyping boards might be good choices.

 

[rjstott]

http://www.robertstrains.net/lighti...ers.optusnet.com.au/nswmn/flicker_free_JB.htm < Link To http://www.members.optusn...mn/flicker_free_JB.htm uses a Zener diode to fix the LED supply voltage, again in my opinion unnecessary. The regulator in both these circuits is more suited to DC/AC analogue systems where the track voltage will vary with desired locomotive speed whereas for DCC and MTS the AC signal is an essentially fixed value.

Richard

 

[DaveB2]

Is the point of the Zener in the second circuit to ensure the voltage on the capacitor never exceeds it's rating? I seem to recall these large capacitors for memory retention can be intolerant of over voltage (by even a small margin) and the sort of energy density you're talking about can result in attention grabbing results if provoked

 

[rjstott]

It would do that yes but as I mentioned I think it is to provide a constant voltage to the LEDs in analogue circuits where brightness would increase with increased track voltage. MTS is supposed to be constant 24 VAC so whilst your 25V capacitor is marginal it should be OK because you'll lose a volt (approx) across the diode bridge at least and if you add a series resistor then there is additional volt drop, depending on the ratio of that resistor to the LED series resistors. I know this sounds complicated so do look at solutions that I suggested.

Richard

 

[DaveB2]

your 25V capacitor is marginal it should be OK

Except it's a 5.5v cap

 

[rjstott]

showprofile?memid=936 < Link To Dylanlewis2000 in the original post mentioned his 25V 1000microfarad capacitor. Silly me transposing people, websites and capacitors. Yes the capacitor must be rated above the voltage it is expected to work at AND most electrolytics are also polarity sensitive.

Richard

 

[DaveB2]

Silly me transposing people, websites and capacitors.
Richard

Very easily done, I didn't reference the link I was commenting on either.
How about forgetting the bridge and living with simple half wave and it's inefficiencies? The output voltage will be lower and I'm sure we can live with the ripple.

 

[rjstott]

Yes, you could. The capacitor gets rid of the ripple in both cases and the half wave rectifier is 50 percent as efficient as the bridge. The Uhlenbrock 73900 is 25 euro from Conrad and controls 4 outputs, so that is lighting, rear lamps and perhaps table lamps in a dining car plus something else?. 'Flickerfree' sell three units for 54 dollars which includes DCC switching at a very reasonable price? The site is a bit unclear about the extras but they seem to include LED mounting strips and other goodies?

Anyone used these?

Richard

 

[Dylanlewis2000]

i still have no clue as to what to do. :-(

 

[gregh]

Here's the circuit you need:

The bridge rectifier from Maplins you need is one of the 2 I have circled - a hopeless website so I can't tell their part number.

Here's their web address:
http://www.maplin.co.uk/bridge-rectifiers-19088

As for the capacitor size, I doubt 1000 uF will be enough. You'll have to measure the mA that the LEDs take. The cap size also will depend on how long the wheels have lost contact for and the cap has to supply the load.
But here's a guess:
For every mA of load you will need 250 uF to cover a 1 second loss of power. eg if you want to cover flickers of 1/10 sec and need 40 mA load you will need 1000uF.
And get a voltage rating of over 35V as if you use AC as in DCC the peak voltage that the cap charges to can by pretty high.

 

[tramcar trev]

Thats looks a good solution Greg... If this had been a tram then flickering of the lights would have been essential for realism Another solution would be to have a really big capacitor in the circuit i.e. a NiMh battery charged by the power and supplying the lights.....

 

[Dylanlewis2000]

Ok. Well on Friday with the manager she suggested that 450uf would be enough... needless to say i will be taking them back.

 

[Dylanlewis2000]

I would just like to say a big thank you to all the contributors to this thread.

I have just come back from Maplins and installed the rectifier.

Here is a link to the video
video= http://www.youtube.com/watch?v=KKsPODm7WAY Awesome stuff!

 

[DaveB2]

Looks like you've got it working successfully to me.
For mine I'm planning on something like Trev's suggestion, 3 x AAA rechargeables, and I may "borrow" the idea I saw on here to use the LEDs out of flickering candles from the pound shop to mimic the effect of oil lamps. Sounds odd I guess but I don't have digital and it means they'll run even when parked.

 

[gregh]

I would just like to say a big thank you to all the contributors to this thread.
I have just come back from Maplins and installed the rectifier.
Here is a link to the video
video= http://www.youtube.com/watch?v=KKsPODm7WAY
Awesome stuff!

So what size capacitor did you finally use?