I have a Bachmann Model NO 6607 HO, N Scale hobby transformer. Input 120v 50/60 HZ – output 17 VDC 20 VAC. Max output 7 VA total. I run postwar trains and use ZW 275 Watt transformers but was wondering if I could use these transformers for accessories such as street lights and lighted towers. The street lights use 1445 and 1447 bulbs and since the AC puts out 20 VAC would that be too much power and make the bulbs to hot and burn/melt anything. I also am using 2 022 switches at about 15-16 volts so this would probably be too much power for those. Lastly can I use the Variable DC side for anything? If I can how many of these lights can be put together without “burning down the house.”
I am electrically challenged.. Thanks again for all your help.
Harold
The problem is not too much power but too much voltage. Unfortunately, you don't have enough power for more than about 4 lamps. But there is a way to get around the voltage part of the problem: Divide your lights into two equal groups (presumably 2 and 2). Power each group from the 20-volt AC output through a diode, with the diodes connected in opposite directions. That is, the stripe on one diode goes toward the transformer and the stripe on the other diode goes toward its group of lights. This will reduce the 20 volts to 14 volts, which is just right for those lamps.
You may find suitable diodes inside the power supply, used for the DC output. You could salvage two of them. Or you could get something like these from Radio Shack: http://www.radioshack.com/product/index.jsp?productId=2036268
Bob Nelson
Or you could wire them(the lights) in series, that would cut the voltage without doing anything else. However if one bulb burns out the others will not light.
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Two in series on 20 volts would be substantially dimmer, at 10 volts apiece. That might suit you, but you probably wouldn't want more than two in series.
Thanks Bob and Sir James,
For 2 street lamps I don't think it is worth it. I have an old 1034, I think that is the number of the transformer that will probably work better. Now my job it to go back to your (bob) notes on how many of these bulbs for a 75 Watt transformer. I don't like doing things in series as I have done that and when one goes out they all do and that is not fun. One more thing though. What is the difference between a diode and a bridge rectifier. I am using the bridge rectifier for my incline and if all I want to do is drop the volts would that also work. Thanks again for all the help and suggestions from you guys.
The difference between a diode and a bridge rectifier--good question! "Rectifier" is the name often given to a diode that is used for a power circuit, as opposed to a "signal diode". The 1N4001 (which I suggested above) through 1N4007 are 1-ampere diodes that are about the smallest that would be called rectifiers. Whether or not a diode is called a rectifier, it acts like an electrical check valve, in that it lets current flow only in one direction. This property is useful for, among other things, converting AC to DC.
A bridge rectifier, called "Graetz bridge" in some other languages after its inventor, is a combination of 4 diodes arranged in a ring, that can do a more complete job of converting AC to DC than a single diode can.
But the bridge rectifier(s) that you're using are not converting any AC to DC. They are simply a convenient package of 4 diodes used to reduce the voltage a little. The way this works is that, even when a diode is conducting, that is, letting current through, it doesn't do a perfect job. It lowers the voltage by about .7 volts. If you wire 2 diodes back to back, in what is called an anti-parallel circuit and put that combination in series with your transformer, you get almost the full voltage, since one or the other of the diodes is almost always conducting. I say "almost" because you actually lose about .5 volts due to the .7-volt loss in the diode. (Why it's .5 and not .7 is complicated and has to do with how AC voltage is calculated.) If you put 2 such diode pairs in series, your voltage goes down by about 1 volt. So the bridge rectifier is not doing its normal job of rectifying--it's just losing a little voltage in a place where you had more than you wanted.
On the other hand, the way I suggested using the 1N4001s above really is rectification. A single diode, even a perfect one, in series with the transformer lets only one half of the AC waveform through, which happens to reduce the voltage by about 30 percent, that is, from 20 volts to 14 volts. The reason for splitting the lamps into two groups and using separate diodes turned in opposite directions for each group is so that the transformer is always driving something, even if it is a different set of lamps on each half-cycle. This is for the transformer's benefit, since it would be damaged if it didn't have such a balanced load, drawing current on every half-cycle. This is a very crude kind of rectification, called "half-wave", but lamps don't care at all about that.
As for your problem of how many lamps you can drive, I would assume that your transformer can put out about 5 amperes. At their rated voltage of 14.4 volts, those lamps draw 135 milliamperes. So divide 5 by .135 to get the answer: 37 lamps.
Bob, I am sorry but I don’t understand. Let say I want to light 8 lamps, I would need to split them into 4 groups of 2 and I would need 8 diodes, 2 for each of the 4 groups. Or would you still just do 2 groups of 4 lamps each and 4 diodes, 2 for each group. I think I will be saving this for experimenting after the holidays but figured I would try to find out how to do it now.
Thanks again for all your help.
Two groups of four, with one diode for each group. You never need more than two diodes. The idea is for one diode to connect its group to the transformer for the positive half-cycle and the other diode to connect its group for the negative half-cycle. The transformer can't sense which group it is driving at any time, so it's the same to it as driving one group of lamps all the time. So the current waveform has the same symmetry as if there were only one group directly connected, making life easy for the transformer. But each actual lamp is connected only half the time, which is what reduces its voltage by 30 percent.
Be sure that the diode for one group has the end with the ring (the cathode) connected to the transformer, but the other diode has its other end, without the ring (the anode), connected to the transformer. Otherwise both groups will be connected at the same time, putting a serious strain on the transformer, which is what we're trying to avoid by splitting the lamps into these two groups.
Harold,
If you want to use the Bachmann transformer for lights, just use the variable DC output. Connect each lamp across the transformer DC terminals, and adjust the voltage for proper light output. You can connect over 30 bulbs as Bob states, and still have power left over. Feel the transformer for heat after 15 minutes or so, and if it is too warm, just lower the voltage, or unload a few bulbs.
Larry
Larry, I was talking about the 1034 when I threw out that number. The Bachmann is rated at only 7 volt-amperes, which can supply only 4 lamps, and that's stretching it a little.
Sorry, I misread the 7VA for 7Amps, and that is where I got my number from.
Bob, if Harold used the 1034 transformer for his lights, and used the variable output, would he need the diodes, and of what benefit and use would they be? As I understand your posts, they would lower the voltage by 1volt if used with a fixed output that was a bit high. As I see it, none would be needed using the variable output of the transformer to set the proper voltage. I can visualize them being used at the input of a light/accessory that you needed to run at a slightly lower voltage without affecting the rest of the lights/accessories connected to the same output.
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