Old Gossen power supply. Help me understand how it works. - Page 1
Forum section: electronics / projects

author: ktulu
author_rank: Regular Contributor
timestamp: April 20, 2020, 12:21:18 pm
content: Hi. I've just recently bought and repaired an old Gossen Konstanter psu.Here is the simplified schematic:It uses a rather high voltage auxiliary winding for the control circuit, which I don't understand why.It is around 90V, and uses 2 large resistors, and 2 series 8.2V zeners to get the stabilized 16V.The first 6W power resistor get so extremely hot, it dissipates around 5.5W. They used the same trick for the fan. It has a large 12W series resistor that dissipates more than 14W! The temperature reaches 300 degrees C. (I took the motor apart, cleaned, lubed and it runs smoothly)The full schematic:I did some cleanup for better visibility:I have a few questions:What is the role of R103 in the auxiliary power supply?Why they needed R105, R106? (2x5W power resistors) These also gets extremely hot.I have a feeling on how the voltage control stuff works, but don't understand exactly.I know that it compares the output voltage with the set voltage (given by the fine and coarse potentiometers and the 7V reference voltage) somehow. But can't wrap my head around the fact, that the aux psu sits on top of the main psu's output. What reference point should I take in the circuit?

author: exe
author_rank: Unknown
timestamp: April 20, 2020, 01:52:51 pm
content: Quote from: ktulu on April 20, 2020, 12:21:18 pmMy best guess is R102 and R103 are voltage reference (along with D101 and D102). I think there are two resistors because it's easier to dissipate more power this way.As of R105 and R106, I didn't find them on the schematicFound them. I think they are to discharge the smoothing cap when not powered. May be also help with power on/off glitches, Idk.

author: ktulu
author_rank: Regular Contributor
timestamp: April 20, 2020, 02:48:23 pm
content: You should right click the pictures and open in new tab to get full size.R101 is the larger 6W 680Ohm power resistor, R102 is 120Ohm/2W. They are the current limiting resistors for the zeners. But R103 is only 10 Ohms. It lowers the ground point of T301?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 20, 2020, 06:16:23 pm
content: The transformer voltage is really unusually high. It could be a remnant from an older tube based version. Otherwise there is no real need for a high voltage there. The larger R102+R103 the better is the regulation of the zeners. So it helps a little, at the price of more power but less parts. From today's perspective this looks odd, but back than parts were expensive.The circuit principle is still quite similar in many modern lab supplies. I know it is quite confusing.The logical reference point is the positive output = negative side of the auxiliary supply.The transistors T301+T302 compare the voltage from the feedback with this level. So the regulator is kind of a negative side regulator, even though the transistors are more to the positive side. The power stage kind of a current amplifier. The voltage is only controlled by the feedback.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 20, 2020, 08:11:15 pm
content: I particularly like the way that the resistor symbols are different to indicate the power rating.Belastbarkeit der WiderständeI have noticed this trend in other, older German schematics.Is this practice still being followed?

author: ktulu
author_rank: Regular Contributor
timestamp: April 21, 2020, 10:29:47 am
content: Ok, thank you. That makes sense.I'm thinking of getting rid of the control part, and replace with a modern opamp based circuit that has current adjustment capability too.Still hesitating a bit, because it is in mint condition inside and outside.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 21, 2020, 03:12:18 pm
content: If the unit is in mint condition, and it is now operating, personally I would keep it as original as possible.There is value in restored old electronics. Even if it is only nostalgia.But that is only my personal opinion.You could replace all the internal guts, and keep only the original external case.