Low voltage AC to DC converter

  Toneman 09:55 02 Feb 2010

I would like to run crossing lights on my railway layout using the 15 volt AC supply to the track, but need to convert it to 9 volts DC. Haven't been able to find a simple circuit, and wonder if any of you hams can point me in the right direction. I presume that all I need is a simple full wave rectifier circuit, don't think stabilised voltage is necesary. Thanks in anticipation.

  PP321 10:08 02 Feb 2010

15 volts after rectification/smoothing will be around around 19-20VDC (no load) so your going to need a simple voltage reg, using a 7808 or 7809 voltage reg, very simple to do.

here is a pic using a 7805 but its the same circuit.

click here

  Toneman 10:37 02 Feb 2010

Thanks for your very quick reply. I had intended to use a voltage dropping resistor to get the voltage I need. What sort of diodes should I use, don't think a very large current is needed.

  PP321 12:09 02 Feb 2010

Well, you get a 0.7v drop across common rectifier diodes (1N4001 type) , lamps (not LEDs) work on ac or dc so you could (in theory) use 6 diodes in series to drop the voltage down to a suitable level and then power the bulbs? if it DOES have to be dc then you'll have to use a bridge rectifier and smoothing cap as well as the diodes, but you will need more voltage dropping diodes as the output from the bridge rectifier after smoothing will be higher.

  Toneman 13:20 02 Feb 2010

Thanks. I wasn't sufficiently explicit. I need the 9 volts DC to operate a (home made) PCB to work amber and then flashing red lights on the level crossing. At present I'm using a 9 volt battery but keep forgetting to switch it off!

  skeletal 14:29 02 Feb 2010

You already use a home made circuit that includes varying and flashing lights.
You use a 9V battery.

I suspect that your circuit does not consume much current so, although I would normally recommend you actually work out the current requirement before designing a power supply, it is very likely it will be less than 1 A.

So PP321’s circuit should be OK.

If it is, then pretty much any 1A rated diodes would be OK. You can either buy 4 individual ones (1N400x where x= 2,3,4,5,6,7) click here

Or a bridge rectifier (where four diodes are in the same package, probably easier for you as the marking for how you wire it up is on the package and easier to follow) such as the W02
click here

Irritatingly, on the Maplin page for the bridges, the specs are not clear. The W02 is not listed but if you click on the picture you can see the round one is marked “W02”. For 1A this small one will be OK.

As to the “resistor idea” I would recommend you don’t do this (nor series diodes).
From your description, the load varies and you don’t say what other electronics is in your circuit. When (if) none of your lights are on, the voltage drop across the resistor will be at its lowest and thus the voltage across your PCB at its highest. At this point is there anything that could be damaged by a high(ish) voltage? When all the lights are on, the voltage will be at its lowest; when they flash, any other lights will flash in sympathy (i.e. one set of lights on, another set slightly dims).

Also, I’m not into miniature railways, but don’t they use LEDs these days? If so, are they just LEDs or are they supplied with a series resistor? This is important because you can’t just wire an LED across a power supply, it will blow up. I assume that you must have sorted this out otherwise even a battery would blow them up.

Do the lights have any stabilisation in them? If so, the effect I describe will be reduced.

Also, of course, although I suggest “the proper way” you may well decide to use your simple resistor method (or series diodes where the effect I describe is less) and put up with the reduced performance. Personally, I would have to do it “properly”...but that’s just me!

If you do build the circuit, it would be worth putting the 7809 on a small heat sink.


  Toneman 15:47 02 Feb 2010

Many thanks for your explicit info, which I think I have digested. On the basis that there is always more than one way to skin a cat, I have searched through the family's discards and found an old mains battery eliminator giving 9 volts which appears to do the job. I shall have to extend the low voltage lead and accept yet one more item in the mains sockets. However, this seems to be the simple answer to my problem. I have saved your info for further consideration if I decide to give it a try. I know about series resistors for LEDs, the crossing lights connect straight on to the PCB (I think), but if I go on to carriage or platform lighting I shall bear this in mind. Again, many thanks.

  PP321 15:49 02 Feb 2010

I would allways do it "properly" as well, all the components are readily available, probably cost less than a fiver, as Skeletal said, pop the 7809 reg on a heatsink. Even for an absolute beginner, this circuit should be a doddle, also put a 1.5AT wickman (or similar) fuse on the AC output of the transformer, just to be safe.

  Toneman 16:11 02 Feb 2010

Thanks, I shall certainly give your suggestion some consideration...I quite like dabbling with circuits and PCBs - well, copper clad boards...

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