2 transistors conected in serie and a mosfet

[Guffy]

At my workplace we are trying to fix a znc machine and we've come across a circuit that we dont really know what it does. If anyone knows the porpuse of this circuit, i would really like to know it.
Thanks,
Pedro

 

[Harald Kapp]

• Do you have part numbers?
• Is that simply an LED or is that an optocoupler made from LED + transistors?
• Are you sure about the polarity and emitter/collector connections of the bjts? Having the emitter of a PNP at -12 V is unusual.
• The polarity of the N-MOSFET also looks incorrect. usually the source is at low potential, drain at high potential. As shown, you have the source at 65 V and regardless of the state of the bjts, which are presumably meant to control the gate voltage of the MOSFET between -12 V and + 12 V, tge gate source voltage will be negative (Vgate < Vsource) and the MOSFET will be permanently off. Even it being a depletion type.
  Are you sure the MOSFET is a depletion type? High voltage depletion MOSFETs are rare and in my experience are seldom used. Typically you'd expect an enhancement MOSFET here.

Assuming that the LED and the bjts form an optocoupler, the circuit could be mean to drive the MOSFET between on and off. But ist sure doesn't in the configuration shown (I'd almost bet).

 

[Guffy]

• Do you have part numbers?
• Is that simply an LED or is that an optocoupler made from LED + transistors?
• Are you sure about the polarity and emitter/collector connections of the bjts? Having the emitter of a PNP at -12 V is unusual.
• The polarity of the N-MOSFET also looks incorrect. usually the source is at low potential, drain at high potential. As shown, you have the source at 65 V and regardless of the state of the bjts, which are presumably meant to control the gate voltage of the MOSFET between -12 V and + 12 V, tge gate source voltage will be negative (Vgate < Vsource) and the MOSFET will be permanently off. Even it being a depletion type.
  Are you sure the MOSFET is a depletion type? High voltage depletion MOSFETs are rare and in my experience are seldom used. Typically you'd expect an enhancement MOSFET here.

Assuming that the LED and the bjts form an optocoupler, the circuit could be mean to drive the MOSFET between on and off. But ist sure doesn't in the configuration shown (I'd almost bet).

As i didnt know alot about transistors and mosfets i had to google it, so it can be wrong. But take in mind that this is from 1997, so some of this can look diferent compared to modern circuits

 

[AnalogKid]

Please post photos of the assembly; wide shot to show the surrounding components, close-up of the ones in the schematic, bottom side of the pc board, etc.

ak

 

[Kabelsalat]

Schematic looks incomplete to me.

I'd think the BJT should actually be photo transistors. Maybe it's supposed to be different light paths so that the two BJT's are located on different locations? But the base connection between those two doesn't make sense to me.