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I'm not sure what you mean by "older", but the Realtrax and Scaletrax ITADs I've seen can be powered by DC or AC.

I have dis-assembled and drawn and posted schematics of the MTH ITADs on OGR for the common good.  If you have some oddball application such as powering the ITAD with DC but need to switch AC - or whatever - I can probably help you out.

A bridge rectifier in and of itself will not reduce the buzzing in a solenoid.  That is, the voltage still rises and falls 120 times per second except the pulses are all in the same direction (positive or negative) rather than alternating positive and negative.  You need to smooth the pulsating DC with a capacitor...or use a DC power source such as a DC-output wall-wart.

@stan2004 posted:

A bridge rectifier in and of itself will not reduce the buzzing in a solenoid.  That is, the voltage still rises and falls 120 times per second except the pulses are all in the same direction (positive or negative) rather than alternating positive and negative.  You need to smooth the pulsating DC with a capacitor...or use a DC power source such as a DC-output wall-wart.

It may reduce it some, depending on the characteristics of the solenoid.  Since the frequency is effectively doubled, sometimes that helps with the buzz.  Also, the fact that the polarity isn't reversing may be a factor as well.

OTOH, I do agree that I've had issues with relays just using a bridge and adding the cap did the trick.

Charles,

I've also been working on an isolated rail detector using a bridge rectifier and I can validate what Stan2004 and GRJ are advising you.  In testing I was using a 'kludged up' bridge rectifier thru an optical isolator and reading an Arduino input pin.  With no smoothing capacitor the readings were all over the place.  I installed a 330uF / 35V capacitor and the readings became solid digital 0's and 1's (or 0's and 1022/1023 when reading analog).  The attached schematic is what I plan to use on my layout.  Hope this helps or at least provides some food for thought.  Best of luck to ya'.

Barry

Isolated Rail Detector

PCB

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@barnun posted:
In testing I was using a 'kludged up' bridge rectifier thru an optical isolator and reading an Arduino input pin.  With no smoothing capacitor the readings were all over the place.  I installed a 330uF / 35V capacitor and the readings became solid digital 0's and 1's (or 0's and 1022/1023 when reading analog).

FWIW, in your situation driving an opto isolator, you could use much smaller caps for filtering.  However, the large caps do give you a nice long time delay after the train leaves the insulated section.

@barnun posted:

...

2. More importantly, it's what I had laying around.

What is the optoisolator you show in your schematic?

Note that if the purpose of the bridge-rectifier is AC-to-DC conversion at signal levels/currents (as opposed to power levels/currents) you might consider replacing the bridge rectifier with an AC-input optocoupler.  For your application these 4-terminal devices (2-terminal AC in, 2-terminal DC out) would then perform both functions, 1) AC-to-DC conversion and 2) electrical isolation. An AC-input optocoupler will be maybe a nickel or so more in cost than a DC-input optocoupler.

But, to your point, if you already have the components lying around...

Untitled

Separately, I think you're dodging a bullet when you tie together the 2 bridge rectifiers as indicated above.  A bridge rectifier does not provide electrical isolation.  It is electrical isolation that "allows" you to tie together the grounds of different electrical circuits.  You dodge the bullet in this case because your input resistor limit the current but if, for example, Track 1 and Track 2 AC power are out-of-phase, then the diode(s) in the bridge rectifier will electrical short the two tracks.

Again, if this was all constructed with stuff you had and it works for you as built then so be it.

OGR being a discussion forum that is...

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@stan2004 posted:

What is the optoisolator you show in your schematic?

Note that if the purpose of the bridge-rectifier is AC-to-DC conversion at signal levels/currents (as opposed to power levels/currents) you might consider replacing the bridge rectifier with an AC-input optocoupler.

I've successfully used the LTV-8x4 series optocouplers with track level AC inputs behind a current limiting resistor: https://optoelectronics.liteon...0series%20201509.pdf

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