Electronics > Projects, Designs, and Technical Stuff
Lab Power Supply - The Lost Current
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jaycee:
Seen this circuit before. To be honest it's not a very good one, and those opamps are probably seeing way too much voltage across them!

Take a look at the schematic for the ELV 22532 power supply. A good approach used by many of the big manufacturers is to have a seperate "bias" supply for the opamps, which floats around the output. I've used a tiny little 9-0-9 transformer from an old bedside digital clock along with 7805/7905 regulators to create such a supply before. Things are vastly simpler and performance is much better with this sort of topology
xavier60:
My idea of connecting the integrator capacitors to the other side of the ORing diodes was too unstable. I want the current limit fast acting yet stable. I'm experimenting with an idea that enables the feedback through the integrator capacitor  only after the CC op-amp has taken control.
radoczi94:
Hi Everyone!

Sorry for the late reply, I had a critical exam last friday, studied my brain out. It was succesful, so rewarded myself with a scope,ordered a Rigol 1054Z. It should be here tomorrow.

On the weekend I was playing around with the PSU. I removed the CC opamp from the socket, and put on a 12V 10W chinese LED as a load. With this config, I measured 0,5A on the shunts and 1 amps on the output. So the CV opamp was oscillating (or something nasty like that). Then I replaced it (NE5534) with a 741, turned on, the result was 1 (or both) shorted 2N3055 and a smoking chinese LED. Doh. I just lost my temper, whacked out the industrial heatgun, desoldered everything from the pcb and threw it in the scrapbin. :horse:


--- Quote from: jaycee on February 12, 2018, 01:35:36 am ---Seen this circuit before. To be honest it's not a very good one, and those opamps are probably seeing way too much voltage across them!

Take a look at the schematic for the ELV 22532 power supply. A good approach used by many of the big manufacturers is to have a seperate "bias" supply for the opamps, which floats around the output. I've used a tiny little 9-0-9 transformer from an old bedside digital clock along with 7805/7905 regulators to create such a supply before. Things are vastly simpler and performance is much better with this sort of topology


--- End quote ---

I decided to build that supply on breadboard and play around with it,try to change small things and measure everything. What I want to do is replace the TIP14x transistors with 1 driver and 3 or 4 end transistors, use a more adequate reference than the 7805, and replace the opamp with a more skookum one. I will put together a transformer tap switch circuit too.


--- Quote from: C on February 11, 2018, 04:22:44 am ---Many ways to stop or limit Oscillation.

Rate of change positive could be different from negative.

Multi-stage feedback.

Areas of circuit where frequency response is different.

Positive feedback to get large change with negative feedback for small change.

For an analog or you could have two. A min OR to control output & a MAX OR to prevent swing to rail.

Many ways other then just slow.

--- End quote ---
I will make some research on things like these, thanks. Will need to do some experiments to see what are the exact effects of these methods.
C:
So you should now have a scope.

Think of the old time test equipment.
  Back then it was very poor, but the old timers built some fantastic stuff using poor. Very stable circuits that had power supplies in the ball park with a lot of noise.
A voltmeter or current meter was treated as a poor slow speed analog display that was not very accurate. A scope was treated as a higher speed video display, again not very accurate.

With every thing poor first high res voltmeters where voltage comparators. They built these by comparing resistors to build a voltage divider. They compared the voltage reference to a better voltage reference.
The result was a bunch of ratios. This resistor is 2x that resistor, this resistor matches that resistor..  The 2x was hard until you had two matching resistors and then very easy. As the comparator got better the degree of match got better and the resistor divider got better, but it still was all ratios.
Assuming that the D'Arsonval galvanomete was repeatable, and by using a Wheatstone bridge you could compare(voltage, current, resistance).

If you use your brain some you can work with uncalibrated test equipment using it more as a comparator. Good numbers are then just a way of passing information.

Old timers also used positive and negative feedback. The trick for this is balance. At static stage you want positive feedback countered by negative feedback.

Think of a switcher power supply, it is just on or off. What makes it work is Time( the rate of change). Good use of analog can act like switcher for large changes and analog for small changes.

If you think of an op amp output as a pot, then if you have a pot input you get a scaled pot output. But here you are forgetting that a op amp output is really two pot outputs. You only get a pot output if both output pots change correctly and you are still forgetting the output load.

Going a step further the op amp feed back is like a . seesaw( teeter-totter) and output is more like a set of springs.
In physical world a seesaw can bend and bounce. Load changes changes spring deflection.   

A min parts circuit has to work harder or be not as good as a many parts circuit. Many parts also has problems of it's own.The total result is what you want to be as good as possible.

A lot of old time transistor circuits would often use differential circuits. This signal goes positive while a second signal goes negative. This can be cheap while opening up options for control.

Really think about your control circuit.
When you have a big output current change, you have a lot of current change on 2N3055 base. Do you want the op amp handle all this change or would it be better if the circuit around the 2N3055 handled some of this change directly.
Do you want source change all handled by op amp or some handled by circuit around 2N3055.
Your output stage is based on current, think current.



chris_leyson:
Stumbled across an interesting blog from Peter Oakes about building a bench supply https://www.element14.com/community/groups/test-and-measurement/blog/2014/09/15/the-modular-bench-power-system-the-essential-diy-build-for-every-ee-student-and-old-timer-alike
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