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Dumb auto reverse question

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  • Member since
    February 2002
  • From: Reading, PA
  • 28,193 posts
Posted by rrinker on Monday, June 1, 2020 5:19 PM

 I'm still trying to figure out how that circuit can even work, you have two MOSFETs in series, on each leg of the DCC, with the gates controlled by the same source. So both will turn on and off at the same time, and since they are in series, both will pass current only one way.

                                   --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

  • Member since
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  • From: somerset, nj
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Posted by gregc on Monday, June 1, 2020 6:24 PM

i believe these are p-chan mosfets with their drains connected.  source-to-drain, drain-to-source.  when properly biased, only one mosfet is turned on during each phase of DCC and current flows thru the body diode of the other mosfet.

greg - Philadelphia & Reading / Reading

  • Member since
    February 2002
  • From: Reading, PA
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Posted by rrinker on Monday, June 1, 2020 7:25 PM

 The parts list at the bottom says all 4 are the same N-channel part. Perhaps that is an error. If they were opposite types, it would work like you say, though selection of the proper part is kind of important than. My servo controller uses a MOSFET as the reverse polarity protection on the DC input. Less loss than even a schottky diode - with the input polarity correct, the MOSFET is turned on and with a really low Rds, even if the circuit draws a couple of amps (which it doesn't), the loss is less than the forward voltage of any diode.

                                      --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

  • Member since
    July 2009
  • From: somerset, nj
  • 3,092 posts
Posted by gregc on Monday, June 1, 2020 7:50 PM

rrinker
If they were opposite types, it would work like you say,

it's not a DC circuit.   the voltage and current reverse polarity every DCC phase change.

greg - Philadelphia & Reading / Reading

  • Member since
    July 2009
  • From: somerset, nj
  • 3,092 posts
Posted by gregc on Wednesday, June 3, 2020 3:58 PM

NCEs new AR10 auto-reverser illustrating 4 pairs of mosfets used to connect the output terminals to either input terminal and not a whole lot more (see DCC specialties Frog-AR)

 

IRFZ44NS

PSMN3R0

i see an axial leaded 100 (??) (brown black brown gold) resistor set above the board for better cooling presumably used for short detection, but think even 1 Ohm might be too high for that purpose.

the orange things on left look like pulse transformers often used for detection.   maybe they detect a short based on the amplitude of the coil voltage.

greg - Philadelphia & Reading / Reading

  • Member since
    February 2002
  • From: Reading, PA
  • 28,193 posts
Posted by rrinker on Wednesday, June 3, 2020 5:35 PM

 Yeah, it looks like they are using the donut style transformers for short detection. The PSX-AR has a pair of transformers on it as well.

                             --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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