Electronics > Projects, Designs, and Technical Stuff
Linear Power Supply based on HP/Agilent E3610A
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StillTrying:
In case it's of any use.
Kleinstein:
You could do the switching with a N_MOSFET, if you choose to switch the other half of the two voltages. It looks a little like a bridge circuit. The MOSFET goes from the most negative voltage to the negative output, with the diode from the middle voltage as an alternative. The controlling comparator is powered from the main supply and testing the voltage over the output transistor. So should be reasonably easy at low voltages (e.g. < 30 V), so the comparator can handle.

Though slightly larger and less efficient, one could still use two separate power transformers for the main power and one small one for the two auxiliary supplies. For the auxiliary supply one can get a way with just a single winding, as there is no real need for a large negative supply something like a -1 V and +5 V or maybe +8 V can be enough. The -1 V could use just two diodes as shunt stabilization. The down side might be limit in the voltage between the outputs.
timsu:
I kind of got it working.
At around 7V the taps should be switching, the comparator U5 does this.

But I still have some trouble controlling the MOSFET. With some opamps/comparators there is some weird oscillations. The LT1077A worked, but it is not rail-to-rail, so it does not fully turn the MOSFET M1 off.
Kleinstein:
You don't want an rail to rail OP for the MOSFET control, as the output voltage would be to high. A comparator like LM393 with a little hysteresis should do it. The gate voltage could be from the lower 10 V supply.
I would compare the collector side of the power transistor to something like 3 V higher than GND (at emitter resistor).  One  might need to check how the current from the hysteresis flows, but I won't expect trouble there.
timsu:
I can't quite get it to work.
I used the LM393 as comparator.
The inverting input is collector-3V (with a diode drop).
The noninverting input the emitter voltage.
Output of comparator with a pullup to the 10v low tap.

Because the comparator has such a high imput impedance, a parallel resistor is required, to have a diode drop.
If the comparator switches to on, the voltage goes on only 3V higher than in the off state (100R pull up), which turns the fet not complete on.
Lower pullups make the circuit oscillate.

Maybe it is obvious but all the different voltages which different reference to each other confuse me :-//

Edit:
I tried an different circuit with an optocoupler, but even here I could not get over 3V Gate-Source Voltage.

If i disconnect the gate it goes up to 10V

Edit2:
I got it working
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