Electronics > Projects, Designs, and Technical Stuff
Linear Power Supply based on HP/Agilent E3610A
Kleinstein:
With FET OPs like the like LF412 (HP design), TL072 or TLC272 there is no real need to balance the input impedance. This also applies to some other OPs with compensated input bias (like OP27) or those with a relatively low bias and not so high input impedance (e.g. LT1013). So there are a lot of exceptions where you don't need balanced impedance. The 10 K value is about right to prevent excessive currents that might damage some OPs and for the typical 1 MHz GBW OPs it's low enough you don't have to look at parasitic capacitance so much. Also bias currents are often not that critical at this resistance and thus no real need for balanced impedance with most OPs. So it a natural choice if not critical.
The LM393 has a limited drive capacity, so one should not use the 100 Ohms to make it drive some 40 mA, which is more than it can.
kridri:
When I was looking to the schematic I don't see the point of adding resistor R15 (parallel to the Current sink). When you add a resistor parallel to it. It won't be constant anymore and at a higher voltage, the current increases. With more losses! Please can you explain me why you did it?
timsu:
To slowly discharge the output caps when the device is turned off or at very low voltage.
It does not have to be exactly constant current.
kridri:
--- Quote from: timsu on May 28, 2016, 06:05:19 pm ---To slowly discharge the output caps when the device is turned off or at very low voltage.
It does not have to be exactly constant current.
--- End quote ---
Thanks for the reply and the explanation. If forgot that a bench supply must be turned down. Silly me!
Robaroni:
I have an E3610A that the current adjust seems to drift. That is, I can't set the current to zero. I tried to recalibrate the supply but after awhile it drifts above zero and can no longer be set to zero.
I swapped out the current opamp U4 as I didn't see anything that stuck out as being bad but it didn't fix the trouble. It might be something as simple as the wire wound current pot breaking down but I was wondering if any of have seen this problem before I dig it apart.
Thanks for the help,
Rob
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