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Lighting transformers


Mallard

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I suspected there was some kind of start up problem. What sort of 12v bulb would be needed. Some auto type? Or a 12v 25W halagon?

I'd use a 21W car brake light bulb as it is less sensitive to over voltage than the halogen, you may get away with a smaller bulb say 5W but play & see,

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Thanks for the suggestions. Things are becoming clearer but still one small doubt.

 

If i use an auto bulb will it start the transformer installed directly after the 12v AC outlet or must it be after a bridge rectifier or similar.

 

I assume it would be connected in parrellel.

 

As you can tell from the above: I really am going through a learning curve. So all advise is much appreciated.

 

Incidentally amoung th same inherited confusion there are a number of ac/dc adaptors in good condition, spec. Input 100-250v ac 50-60HZ 0.35a. Output +5v dc 2.0amp.

If anyone would like one they are free to first come first served. I would want postage. Please PM me for full details.

 

BobM

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Thanks for the suggestions. Things are becoming clearer but still one small doubt.

 

If i use an auto bulb will it start the transformer installed directly after the 12v AC outlet or must it be after a bridge rectifier or similar.

 

I assume it would be connected in parallel.

 

As you can tell from the above: I really am going through a learning curve. So all advise is much appreciated.

 

Incidentally among the same inherited confusion there are a number of AC/DC adaptors in good condition, spec. Input 100-250v AC 50-60HZ 0.35a. Output +5v DC 2.0amp.

If anyone would like one they are free to first come first served. I would want postage. Please PM me for full details.

 

BobM

 

The bulb just needs to be on the low voltage side to make the electronics work & stabilise. Is the output DC or AC?

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Hi

A cars 12 volt 21 watt reversing lamp will provide an initial start up a loading - assuming the PSU needs a load??

Its down side is that it will be on all the while the PSU is on and its a very bright light plus it gets very, very hot!

 

If your layout lighting uses filament lamps - Yard, station and building lights etc, there will be sufficient loading anyway. If a pre load is needed?

 

I would try the PSU to see if a loading is needed.

 

If the PSU output is 12 volt ac then use a 10Amp bridge Rectifier to convert it to dc such as a JO4 type or you can use a larger current rating one without problems. You will lose a small amount of voltage -1.2v but thats not such a bad thing where filament lamps are used.

 

The PSU is quite a big one, so it will be essential to fit suitably sized (current rated) fuses on all lighting circuits or other feeds taken from the PSU or the bridge rectifier.

Failure to fuse each feed could should a short circuit occur in a feed result in the wiring for that feed becoming seriously over heated and perhaps even catching fire! So fusing each feed is essential.

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If the PSU output is 12 volt ac then use a 10Amp bridge Rectifier to convert it to dc such as a JO4 type or you can use a larger current rating one without problems. You will lose a small amount of voltage -1.2v but thats not such a bad thing where filament lamps are used..

 

Not quite, Brian. The 12v is a RMS value, (Root Mean Squared), not to get overly complicated, a kind of average. When rectified to DC, the result is the AC voltage multiplied by 1.4, (a close approximation, and good enough for this purpose), and this will give 16.8v.

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Thanks for the info. I don't fancy the hot 12v -23watt bulb continually burning. Could I use a high power resistor instead. I see I can buy a 20w resistor for about £1. If this is a option would it best be in series or in parallel with the supply, and before or after the bridge rectifier. Many thanks in advance for all replies. I realize I really am milking this in my ignorance.

I appreciate the advice about protection. All supplies will be protected by appropriate fuses.

BobM

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Not quite, Brian. The 12v is a RMS value, (Root Mean Squared), not to get overly complicated, a kind of average. When rectified to DC, the result is the AC voltage multiplied by 1.4, (a close approximation, and good enough for this purpose), and this will give 16.8v.

What!!

Input a measured ac (RMS) voltage into a bridge rectifier and the dc output will be LESS than the input ac volts (RMS).

 

All ac measurement are normally quoted as being the RMS value, as its what is normally measured! Peak ac voltage is very rarely ever stated or used except in some calculations etc.

 

For example, take a bog standard bridge rectifier and input (our often found in model railways) 16v ac (RMS) and then measure the no load dc out voltage on the rectifier, it will be less than the supply v, at around 14.8ish volts dc.

 

So input 12 volts ac and the output on the dc side of a bridge rectifier will be less at around 10.8volts dc ish.

 

BTW. I do know what RMS stands for and also what Peak voltage is too! :O

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What!!

Input a measured ac (RMS) voltage into a bridge rectifier and the dc output will be LESS than the input ac volts (RMS).

 

 

Nope, sorry, the rectifier sees the AC peak voltage, which is more than the RMS voltage. It converts this into pulsed DC at that peak voltage.

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Um, the output from a bridge rectifier is only the same as the AC peak voltage if the rectifier feeds immediately into a large capacitor. (It's actually a little less because of the forward conduction voltage drop in the diodes, as noted by Brian.)

In the absence of a capacitor the output will be the average of the pulsed DC, which goes from 0 to AC peak and back. It's of the same order as the RMS voltage.

 

Based on some of the earlier posts, suggesting the need to load the output with a car bulb "to make the electronics work and stabilize", it seems we are not talking about a simple transformer/rectifier combination here. The behaviour of the output depends on the design. Perhaps the safest thing to do would be to load it with the high-wattage resistor and measure it with a meter, which will take care of any averaging required. A 10-ohm, 20 watt resistor would do. Note that light bulbs have variable resistance, which is maximum at full brightness.

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Nope, sorry, the rectifier sees the AC peak voltage, which is more than the RMS voltage. It converts this into pulsed DC at that peak voltage.

 

Tosh! I suggest you try it!

16v ac (RMS) in equals approx 14.8ish volts dc out. :O :o

 

Only when you add a smoothing capacitor will the dc volts increase.

No smoothing was ever mentioned in my posts! Its simply not needed for lighting power. :rolleyes:

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I don't fancy the hot 12v -23watt bulb continually burning. Could I use a high power resistor instead. I see I can buy a 20w resistor for about £1.

The resistor will get equally hot if it is of the same resistance as the bulb, which I suspect is the general idea here, i.e. to provide a minimum load. At least a bulb gives an indication that power is applied, and that the bulb is working. A resistor won't tell you much unless you touch it. :O

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100% correct Geoff. ;) ;)

Sorry, not quite.

 

Using full wave rectification the average Dc output will be 0.9 x Vac unless a capacitor is fitted then it will be 1.41 x Vac.

 

The peak DC value is still 1.414Vac and may be enough to damage anything rated for .9Vac.

 

Andrew Crosland

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Going back to the PS, it sounds as though the type is unknown, there are basically two types, transformer to rectifier and switched PS, where the voltage comes from a small transformer at a high frequency, (as used in computers etc.) It is rectified to DC by a regulator which normally needs a load.

  • A transformer type needs no load, ( but there are exceptions).
  • A switched PS usually needs a load, (sometimes arranged internally) to make the regulator circuit run correctly. Some switched supplies are self regulating, and bullet proof, some need a load all the time on the output.

The type must be identified to be sure, it will need examination, and a quick test to check it. Transformer types are heavier, and are usually easy to identify.

 

As switched types really need a load, you can add the load in parallel to any switched type. Most ex computer supplies are switched types and must be loaded to give the stated output, and not blow. Car bulbs, (12v), are suitable loads for most types.

 

Hope this helps explain the usual advice not to use switched supplies unless loaded, at which point they work fine. As usual if in doubt be very careful, mains kills!

Stephen.

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Thanks for all the replies. The pdf of rectified current / voltage was very helpful and answered what would have been a future question.

 

From my own suspicions and experiments I began to realize I had two switched mode power supplies that needed some sort of starter load The problem was that I did not know such a condition existed. I do now, thanks to your replies. I think the auto bulb idea has to be the way forward, Apart from the heat generated I was not too keen on the bright light that will be generated. However, painting them black will help kill a lot of the light and I guess it should help with the dissipation of heat. Any suggestions on the type of paint would be helpful.

 

BobM

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Just looked inside one of the "Electronic Transformers" as used for 12v halogen bulbs and the output is a separate winding straight off a toroidal transformer which seems to be part of a HF oscillating circuit.

This one was rated 105VA at 12v AC output.

 

There doesn't seem to be any sort of SMPS type I.C. involved just a bridge rectifier across the mains feeding a circuit with one small tranny and two TIP types with a few discrete components.

 

It looks VERY basic!

 

Keith

 

Edit:

Had a closer look. It is not a switched mode supply. It is a transformer coupled oscillator which seems designed to just limit the max output to 12v under varying loads. The minimum load is stated to be 30VA and the device can be connected to a dimmer to reduce the output voltage. I would suspect that with less than 30VA (read approx 30W as it's power factor is stated to be 0.99) that the voltage output could rise.

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That has really bamboozeled me Melmerby. I understand about half what you are telling us. However the bottom line seems to me to be that the outlet from the ps needs to have a preinstalled 30 Watt load. I can live with that.

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That has really bamboozeled me Melmerby. I understand about half what you are telling us. However the bottom line seems to me to be that the outlet from the ps needs to have a preinstalled 30 Watt load. I can live with that.

 

Sorry if I was too technical.

 

From what I can ascertain it is not a switch-mode supply as there seem to be insufficient components for the necessary control circuitry that that sort of supply would have.

 

I would suspect that with less than the minimum load the voltage could rise above what is safe for a Halogen lamp (they do not like over-voltage and die pretty quick!) . It may be OK with other types of lighting. I am guessing it will probably still start without any load but I may be wrong. I will have a play if I can find the time.

 

It seems to be a high frequency oscillator driving an output transformer, which drives the lamps.

This just applies to the one I am able to get into, it is possible that other makes of Halogen lamp controllers could be switch mode.

 

Keith

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