Feeling very dumb over this but needed to get some clarification. I have a control board with what I thought was a shorted diode. I pulled it and it tested good. Testing across the 2 points the diode had been connected appear to be shorted (kinda defeats the purpose of having a diode?). 2 questions came to mind. Is there a short elsewhere or can a diode be connected intentionally like this?

 

Have a schematic for the control board or a exact model id?

 

Have a schematic for the control board or a exact model id?

No schematics or info online available. The board is from a stone polishing machine (220v) from Italy aprox 50 years ago. Rabbit hole here I go.

 

No schematics or info online available. The board is from a stone polishing machine (220v) from Italy aprox 50 years ago. Rabbit hole here I go.

Clear pictures at least please. There are lots of application in circuits where the DC resistance is very low while the AC resistance is high and things like rectifier circuits from transformers, etc.... There are also anti kick-back/snubber diodes to stop back-emf voltages for coils in relays, contractors, sidelines, etc... that operate on DC voltages.

So yes, a diode be connected intentionally like that.

 

Here are pics. Red arrow is location of diode in question . Thanks for that info.

 

Here are pics. Red arrow is location of diode in question . Thanks for that info.

Looks like some sort of control circuit (coppia -> couple) with likely a AC to DC control signal transformer for feedback. Don't see anything that looks bad other than some hacked soldering and board mods.
https://en.wikipedia.org/wiki/Coupling_(electronics)

My 2c guess would be that the short reading is normal if the diodes all read good. Not going to trace it all out for you.

 

Ok, thank you for looking. I just tried doing an ohms check across the diode connections and it's not reading dead short or even close so I believe what you had said - it was designed that way. Before asking/starting this thread - I had replaced 2 resistors, 2 transistors and 4 out of 8 diodes on the bridge rectifier. The board had been worked on by someone else before I got it.

 

Feeling very dumb over this but needed to get some clarification. I have a control board with what I thought was a shorted diode. I pulled it and it tested good. Testing across the 2 points the diode had been connected appear to be shorted (kinda defeats the purpose of having a diode?). 2 questions came to mind. Is there a short elsewhere or can a diode be connected intentionally like this?

What the value of readings on multimeter?,
Can be the doide and can be something connected to the doide is short, and can be normal readings as long as you measuring components in circuit
You say the doide is good, then look for something else around is shorted
Are you measuring on doide mode or ohm mode?
Please send photo of readings and board
Regards

 

Here are pics. Red arrow is location of diode in question . Thanks for that info.

Can be the transformer coil readings
Please isolta the transformer for. The circuit and Re-measure

 

Ok, thank you for looking. I had to replace 2 resistors, 2 transistors and 4 out of 8 diodes on the bridge rectifier. The board had been worked on by someone else before I got it.

Good that you have found the faulty components
That's all?, or anything more faulty?

 

Since there is at least one relay in the area of the diode, it is quite possible that the diode is being used as a snubber across the relay coil to absorb the voltage transient generated when the relay is de-energized. The diode is reverse-biased while the relay is energized but becomes forward-biased when the relay inductance causes a voltage spike when the relay driver turns off. This is an extremely common practice.

 

Since there is at least one relay in the area of the diode, it is quite possible that the diode is being used as a snubber across the relay coil to absorb the voltage transient generated when the relay is de-energized. The diode is reverse-biased while the relay is energized but becomes forward-biased when the relay inductance causes a voltage spike when the relay driver turns off. This is an extremely common practice.

Thank you - that makes sense.