Bridge Rectifier

[BestSnowman]

I've been reading the latest Model Railroader and I had an idea after reading the article on LEDs. If you can use a bridge rectifier to convert AC to DC for an LED would it work to do the same for a DC motor?

I've got a couple of project locomotives that don't have decoders yet as I want to finish other work first (and haven't decided on a decoder) and while I don't run them often when I do run them for testing the buzz gets kind of annoying. I'm taking a guess that the buzz is just the AC running through a DC motor. Would it work to have a bridge rectifier to cover the power to DC so we don't get the buzz but still be able to operate it on address 0?

[ABC]

I'm pretty sure that wouldn't work, but I bet Jim can explain why (he knows everything). I think this kind of is it: DCC is not steady AC, its like a sine wave, what would happen is the DCC would cause the loco to only go one direction at max speed.
-I was close, I couldn't quite remember oh well

[rich1998]

it would probably work but go full speed in one direction.. see the link below. the dc coming out of the bridge would be only one polarity and roughly around 12 volts pulsating dc with the ac input to the bridge. jim can give a better explanation. he is good with words.
the top diagram is what your are thinking. notice the plus and minus leads do not change polarity. the bridge only converst ac to dc. does nothing else. when the controller is on there is always ac pulses on the rails.
on jims website he has photos of what the dcc pulses look like on the rails.

http://tinyurl.com/yzohpap

lex

[BestSnowman]

That makes sense, after I posted I had a thought that it would probably be full power. Didn't think about the fact it would be one direction only though.

[Nigel]

the information I posted, was not correct - thank you Jim for identifying the flaws in my logic.

[Jim Banner]

ABC, you embarrass me with your compliments.  As it happens, I just posted on another thread about how DCC can provide constant power to decoders, variable dc power to analogue locomotives, and a DCC signal, all at the same time.  The link is below:

http://www.bachmanntrains.com/home-usa/board/index.php/topic,10509.msg88426.html#msg88426

Jim

[jward]

let me get this straight. you say model railroader says you need a bridge rectifier to convert ac to dc to run a led (light emitting diode)??

a bridge rectifier is 4 diodes. a diode is an electronic device that will pass electric current in one direction but not the other, in effect converting ac to dc by blocking half of the ac cycle. an led is a diode that emits light. what do they need the bridge rectifier for?

remind me not to buy model railroader any more........lol

[RAM]

Yes a LED is a diode, but you need a constant current power supplies for a LED.  There for you want DC current.

[jward]

a bridge rectifier isn't going to give you a constant current power supply. it WILL give you a constant polarity power supply, but the diode itself will do that. the only thing a bridge rectifier would do in that case is make the ac ripple 120 hz instead of 60 hz. that, and effectively lower the voltage by about 1.4 volts. anything else would require filtering and/or amplification which gets alot more complicated.

[Joe Satnik]

Dear All,

If I remember correctly, strong reverse voltages stress LEDs.   

If you feed an LED (un-rectified) AC, it should have a regular diode in series (elephant style) with it to take the reverse voltages.

AC hot~---Resistor-->l---->l----AC common~
                                    LED

Hope this helps.

Sincerely,

Joe Satnik

[Jim Banner]

And the big cigar goes to Joe Satnik for being right on about reverse voltage.  A series diode will block the reverse voltage.  A diode in parallel with the LED but with the opposite polarity (reverse parallel connection) will shunt the reverse voltage around the LED.  But only a single diode is needed in both cases, not four in a bridge.

The only advantage of a bridge rectifier would be that the diode is on continuously while a single diode leaves it dark for alternate half cycles (does that mean it is a DED - dark emitting diode - during those half cycles?)  However, because of persistence in the human eye, being on for half cycles appears to be just as bright as being on continuously.  Thus if you used a second LED in reverse parallel with the first, both diodes would light and each would appear to be as bright as a single diode lit with a bridge rectifier.  Seems you can get something for nothing - twice the apparent light for the same total power.

So what is the maximum reverse voltage allowed on LED's?  Typically, about 7 volts.  It is listed as PIV or Peak Inverse Voltage on diode specification sheets.

Jim