Heathkit IP-18 Circuit Explanation Help

Thread Starter

guitardenver

Joined Jan 24, 2009
31
Just for fun, a friend sent this to me to help understand this circuit. But I can't seem to fully figure it out. Circuit is below, I added part designators to help discussion.





Here is what I think of the circuit, please correct anything that is wrong.

D1 makes a half wave rectifier and charges C5, to smooth it out to near DC, D2 regulates to 20V and R1 limits the current through D2. This 20V provides voltage to R3 voltage divider which sets the output voltage of the power supply.

R3 controls the current through the base of Q3, which in turn control the current through the darlington pair formed by Q3 and Q4, which controls the voltage of the power supply output by changing the voltage drop across Q4 Vce. But that would also control the current limit of the output.

My guess is that Q5 is the feedback in the system for current control. The voltage across R6 and R7 controls the current through Q5 base, which in turn controls the current through the darlington pair base. But that would also change the output voltage right?

Q1 JFET is in a constant current source configurations. I am not sure what is going on there.

D7 is there for protection if an outside voltage source of opposite polarity and D6 is there for protection against an outside voltage source of same polarity. Not sure to the current path in that scenario.

D4 and D5 create a full wave rectifier and C1 and C2 smooth it out to DC.

Can someone explain in detail the operations of the power supply. I'd love to learn it well.

Questions:
1) What is the purpose of D3?
2) What is the purpose of R5 and why was it placed in between C1 and C2 and not before or after?
3) Q1 JFET is in a current source configuration, what is the purpose of this?
4) Why is C4 there?
5) Why is the output of the power supply connected to the positive node of C4? Wouldn't that affect the current through Q2 base?
6) How does D6 protect the circuit?
7) Is R8 there for a minimum load requirement?
9) What is the purpose "External Programming" and how does it work.


Thanks for the help. I hope this circuit is simple enough to explain in detail but complicated enough to be fun.
 

Dodgydave

Joined Jun 22, 2012
11,285
Q1 is a Constant current source around 2mA this is used for the load for Q2,and supply for Q3.
Q2 only turns on when the voltage on diode D3 gets to 1.4V, this pulls Q2,Q3 off and the voltage output drops, turning off Q2, the regulation is set using the preset Vr3,..

It's a Very crude method...


Current limit is set by Q5 and the voltage drop across R6,7.. This turns off Q3,4 by shorting out the B/E..
 
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