The transistors in the current mirror, and differential amplifier will benefit from having thermal contact, to keep the temperature difference as small as possible, thereby minimising the DC output offset. I have taken the best picture I could, of this coupling for one channel, in my amplifier.
I have found this tool indespensible, when finding unforseen errors in complex makefiles.
I have built and tested, the revised version of, both the positive, and the tracking negative regulator. The new version is here:
I have not included Q12, since it was part of the original current limiting circuit. The pass transistors Q5 and Q7, are mounted off-board, on a suitable heat sink. R18 adjusts the maximum output voltage, and R17 adjusts the voltage of the negative tracking regulator.
I will do some more testing of the power supply under different load conditions, and post the results.
I started a thread on diyAudio, here. Some of the feedback, I got, made me revise the circuit, to accommodate an off-board current limiting circuit. Along the way, I adjusted a couple of things, and added current mirrors to the differential amplifiers, since I could cram them in there, and they should better the performance. I have designed and etched a PCB, and tested the positive half of the circuit.
When testing is done, I will post the revised schematic, and PCB. Stay tuned.
This is the final schematic, everything is now tested, and working. Had a little trouble, until I realized I had put a PNP transistor in place of Q6.
Refering to the figure in LAB Power Supply taking shape here is the run down on the last parts.
- is the current limiting circuit on the positive rail. When the voltage across the current sense resistor R1 rises above 0.65V the transistor, Q3, will begin to conduct, stealing base current from the pass transistor. This will make the output voltage fall, until the voltage loss in R1 is back at 0.65V. The value of resistor R1 sets the current limit, using Ohms law it is easy to calculate resistor value for a 1A current limit, (R=U/I), R1=0.65V/1=0.65Ω. The closest value is 0.56Ω, which gives a current of 0.65V/0.56Ω=1.16A.
- is the negative current limiting circuit. Q10 and R14, is the mirror circuits of Q3 and R1. Since the negative rail tracks the positive one, the negative voltage will drop, when the positive current limit kicks in. The positive rail, is not tracking the negative, and the positive voltage, will not drop, when the negative current limiter kicks in. Arguably, there will be no over-current problem, on the positive rail, if the positive current limit is not activated. For completeness, Q6 will turn down the voltage of the positive rail, when the difference between the positive and negative rail becomes large enough to make the voltage at the junction R9, R13 more than 0.65V
Well it seems the power supply is working, I have made a PCB design,
that I will perfect, and publish when done.
I have tested the negative tracking regulator today, it differs from the positive regulator, in that it it measures the voltage at the positive output, and adjusts the negative rail accordingly.
Referring to the block diagram in the last post, I will describe the new blocks.
- is a differential amplifier just like 2. The first input is taken from ground, the second from the voltage divider H, that samples the midway point between the positive and negative rail. The midway point is essentially 0V, as the positive and negative real should have equal opposite voltages. The difference amplifier will keep this midway point close to zero, by controlling, through the pass transistor 7, the voltage of the negative rail.
- is a darlington pass transistor like 3.
- is the voltage divider creating the midway voltage for 6.
Everything is working, and the next step is to test the current limiters, which is hopefully going to happen at the beginning of next week.
A have had an old untrustworthy 317/337 based power supply as my test unit, since it was build in 1987. Over the years I have tried to improve the poor thing, at first, without really knowing enough, to improve anything. It has come to the point, where I blame this unit for a lot of things, that might as well be faults, in the circuits I am testing.
I have been working from these specifications:
- Easily adaptable to higher voltages (and currents)
- Input voltage about +/- 22V
- Variable output voltage between about +/- 2- 20V
- Output current limit at about 1A
- Adjustable with a single multi-turn pot
- Predictable performance
- Readily available parts (what I had lying around)
- The new PCB must fit in the place of the original, with only a modest level of violence.
I have been through some op-amp and/or 317/337 based designs, but they all failed in different ways. In the end I am prototyping the following circuit.
The whole thing works in the perfect world of spice, and the coloured parts has been tested as the prototype seen at the top of this post. Since I have designed this from scratch, I will describe the circuit, but first lets take a look at a functional block diagram of a standard linear voltage regulator, from National Semiconductor Application Note 1148:
The positive side of my circuit is mostly a discrete implementation of
the above. VREF equals A in the schematic below, the error amp is
section B. The pass transistor is the darlington Q1, Q2, in block C,
these are connected a little different than the block diagram above,
mostly to save parts, and make life easier for myself in the current
limiting department. Voltage divider R1, R2 equals the voltage divider
R4, R7 in block D below.
- is a twist on a standard zener reference. R2, D1, R5, and C4 is the standard circuit, only it is not connected to ground. R2 limits the current through D1, and could easily be increased, I would suggest 3.9kΩ for a current of about 5mA through D1. R5 and C4 is there to filter out zener noise. D2 and R11, forms a negative counterpart, to D1 and R2, and I would suggest 3.9kΩ for R11. The junction between D2, and R11 is at -5.6V due to D2. Q9 is there to buffer the -5.6V from D2 and R11, due the 0,65V drop in Q9, the emitter voltage is about -5.6V+0.65V=-4.95V. C5 and R10 is a filter just like C4 and R5. D1 is referenced to the -4.95V, instead of the usual ground connection, this means that the junction between R2 and D1 should be at a stable -4.95V+5.6V=0.65V, which serves as a reference voltage,
- is a simple differential amplifier in place of an op-amp. The transistor version was chosen, since most op-amps have a maximum supply rating of 36V. With the BJT version, scaling to a higher voltage, is a simple matter of choosing the right transistors for Q4 and Q5. In my prototype I have used BC337 types. One of the inputs of the differential amplifier is connected to the reference voltage from R2, D1, through the filter R5 and C4. The other input is taken from the voltage divider R4, R7, which is a fraction of the output voltage. The output is taken between the collector of Q5, and R3.
- is the pass transistor, the difference signal from block B, which is the difference between the reference voltage, and an adjustable fraction, of the output voltage, is used to control the voltage drop of Q1. Q2 makes this a darlington transistor with a current gain large enough, that the difference amplifier can drive it at maximum current load, on the power supply output.
- is a simple voltage divider, the difference amplifier will adjust the output voltage, via Q2, Q1, to make the voltage at the connection between R4 and R7 equal to the reference voltage from A. R4 is the pot on the front panel, and R7 is a trimmer for initial fine tuning.
This is as far as I have gotten until now. I will describe the rest of the circuit as it is prototyped. For now this is a positive supply adjustable from 0.70V to 6,6V running off a couple of 9V batteries.
Generated on 2018-03-06 19:14:41.121146