Hi,
I have picked up recently a vintage Farnell LT30-01 dual bench power supply from a well known auction site.
It has two fully independent channels capable of delivering 30V at 1A max.
I tested it today and I think one channel is not behaving correctly (maybe usable but I do not think it is ok).
I want to use current limiting facility.
I set both channels to CV (the switch underneath).
I set up first the desired voltage to say 5V. Then I set up the current limit. I connect 10Ohm to the output. and set the meter to show the current.
I expect to get 0.5A which I do. Then I adjust the current limit downwards until the measured current starts to drop. This is my limit setting.
I verify the operation by replacing 10Ohm load with a smaller one. The meter shows still 0.5A but the voltage drops accordingly to Ohm's law.
This is what I call proper operation.
The other channel: I set up the voltage, connect my 10Ohm resitor and proceed with the current limit setting. With this channel I can set the current limit to any value between 0-1A and the PS seems to increase the voltage if I exceed 0.5A which I do not like very much.
Of course I can still set up the current limit to 0.5A (and verify that the voltage is still 5V) and I can test the limiter by trying a lower value resistor - which seems to work ie t keeps the desired current but the voltage drops.
Still, somehow I do not like the idea that if I accidentally turn the current limit pot my voltage will increase too.
Is the unit faulty?
It is not my policy to take the advantage of the seller/force him to pay return postage etc using fleabay buyer protection scheme. BUT I also do not wish to pay for a unit advertised as working and then spend time fixing it...
I'd rather be soldering amplifiers in this time.
I have picked up recently a vintage Farnell LT30-01 dual bench power supply from a well known auction site.
It has two fully independent channels capable of delivering 30V at 1A max.
I tested it today and I think one channel is not behaving correctly (maybe usable but I do not think it is ok).
I want to use current limiting facility.
I set both channels to CV (the switch underneath).
I set up first the desired voltage to say 5V. Then I set up the current limit. I connect 10Ohm to the output. and set the meter to show the current.
I expect to get 0.5A which I do. Then I adjust the current limit downwards until the measured current starts to drop. This is my limit setting.
I verify the operation by replacing 10Ohm load with a smaller one. The meter shows still 0.5A but the voltage drops accordingly to Ohm's law.
This is what I call proper operation.
The other channel: I set up the voltage, connect my 10Ohm resitor and proceed with the current limit setting. With this channel I can set the current limit to any value between 0-1A and the PS seems to increase the voltage if I exceed 0.5A which I do not like very much.
Of course I can still set up the current limit to 0.5A (and verify that the voltage is still 5V) and I can test the limiter by trying a lower value resistor - which seems to work ie t keeps the desired current but the voltage drops.
Still, somehow I do not like the idea that if I accidentally turn the current limit pot my voltage will increase too.
Is the unit faulty?
It is not my policy to take the advantage of the seller/force him to pay return postage etc using fleabay buyer protection scheme. BUT I also do not wish to pay for a unit advertised as working and then spend time fixing it...
I'd rather be soldering amplifiers in this time.
I agree, the first channel seems to behave as I would expect.
However, I can't quite follow what's happening in the second channel. Are you saying:
1. You set output at 5V.
2. Apply 10 Ohm load, then reduce Current Limit control until supply enters CL operation.
3. If you next increase CL knob, supply voltage rises above the initial setting 5V setting?
If I've interpreted correctly, this is faulty behavior. If I'm misinterpreting, please set me straight.
Perhaps Seller didn't notice this odd behavior, but IMO you're not abusing him. I suggest conferring with him and coming to mutual understanding about return versus compensation for self-repair effort--- if you want to consider that option.
Good luck!
Yes, you interpreted it correctly the voltage does rise in step 3:
Thanks for the confirmation. I have also convinced myself that one channel does not behave correctly.
I have contacted the seller. I would prefer to send it to him to repair, but the unit is pretty heavy so it is not cheap to send back and forth.
So I proposed I will take a heavy discount and deal with the fault. Not my preferred solution, I have never done this but they say you can repair these units with a spanner. Maybe I can learn something.
It looks like constant current operation: you chose desired current in and it adjusts voltage accordingly.
Thanks for the confirmation. I have also convinced myself that one channel does not behave correctly.
I have contacted the seller. I would prefer to send it to him to repair, but the unit is pretty heavy so it is not cheap to send back and forth.
So I proposed I will take a heavy discount and deal with the fault. Not my preferred solution, I have never done this but they say you can repair these units with a spanner. Maybe I can learn something.
I enjoy troubleshooting from a schematic, especially when someone else has to do the heavy lifting! 😀
If you can post a service manual, I'll try to help.
P.S. Your might make sure a service manual is available before fully committing...
If you can post a service manual, I'll try to help.
P.S. Your might make sure a service manual is available before fully committing...
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Mine is LT30-01. There are two schematics inside the manual. The one I have is closer to LT-30-2.
My understanding so far is:
you set up the desired voltage with pots P101/102, this sets the voltage at non inverting input of IC1 (which is just a buffer?)
The first half of IC2 controls the voltage, the second half is for the current control. They both drive the base of VT3 which in turns controls the output transistors through VT101.
R12 is the sensing resistor? Or R204?
you set up the desired voltage with pots P101/102, this sets the voltage at non inverting input of IC1 (which is just a buffer?)
The first half of IC2 controls the voltage, the second half is for the current control. They both drive the base of VT3 which in turns controls the output transistors through VT101.
R12 is the sensing resistor? Or R204?
You're going to have it fixed before I even come up to speed!
R204 is part of sensing current flow. R12 is part of voltage regulation loop. I'm afraid I'm still early in comprehension stage.
R204 is part of sensing current flow. R12 is part of voltage regulation loop. I'm afraid I'm still early in comprehension stage.
I believe you've got it exactly right. IC1 is a bit more than just a buffer. I'm still pondering it.
What about switch S203, which I understand is on the PCB, underside of case? (Note especially bottom half of page 6 in manual.) If I understand correctly, it should be closed so that voltage control can assert itself. Is it in the same position as the working supply? If it appears to be closed, it's probably a good idea to confirm that it's actually conducting.
If this not the problem, I'll think of next a experiment.
Yes, I was thinking about the cv/ci switch at the bottom. It is certainly in the same position as the working part and it is the cv position. But I should indeed check whether it works.
Hi
The switch seems to be fine (I measured the resistance between I Out and -O/P both in CV and CI positions and comparing it to the working channel) All seems to be ok.
The funny thing is that ONCE you set up the desired voltage AND current limit then everything works fine ie the unit will take care not to go above the preset current nor voltage. I verified it by loading the output with a lower value resistor. But when you increase the current limit then it will ignore the preset voltage. It looks like current limit has priority over voltage. Thinking of it I wonder what happens if I load the output with a higher value resistor - I haven't tested it but I kind of know the answer and that is worrying.
In the correctly working channel the preset voltage takes precedence ie the unit will try to deliver the required current (between 0 and current limit) without increasing the voltage if the current limit is too high ie higher than V/R_load). The highest current delivered by the working channel is minimum of V/R_load and the currentLimit. In the faulty channel the highest current is whatever is set by current limit. If necessary the voltage will be increased to V_required = R_load * CurrentLimit (or rather maximum of that and 30V which is the max voltage)
The switch seems to be fine (I measured the resistance between I Out and -O/P both in CV and CI positions and comparing it to the working channel) All seems to be ok.
The funny thing is that ONCE you set up the desired voltage AND current limit then everything works fine ie the unit will take care not to go above the preset current nor voltage. I verified it by loading the output with a lower value resistor. But when you increase the current limit then it will ignore the preset voltage. It looks like current limit has priority over voltage. Thinking of it I wonder what happens if I load the output with a higher value resistor - I haven't tested it but I kind of know the answer and that is worrying.
In the correctly working channel the preset voltage takes precedence ie the unit will try to deliver the required current (between 0 and current limit) without increasing the voltage if the current limit is too high ie higher than V/R_load). The highest current delivered by the working channel is minimum of V/R_load and the currentLimit. In the faulty channel the highest current is whatever is set by current limit. If necessary the voltage will be increased to V_required = R_load * CurrentLimit (or rather maximum of that and 30V which is the max voltage)
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I had concluded the switch was probably working or you wouldn't have been able to achieve step 1, post #2. I was wishing for a zero-effort solution. 😉 Ii's clear that we have to dig deeper.
Mentally, I think of a graphical load line plot: Initially, you set supply voltage to 5V, no load; then you attach a 10 Ohm load and get 5V, 500mA (Point A); next you reduce CL control and, following the resistor load line, arrive at maybe 2V, 200mA (Point B); finally, you again increase increase CL control but instead of returning to Point A, the supply climbs the resistor load line and arrives at say 8V, 800mA (Point C).
Now I'll try to predict what should be happening in the circuit at Points A and B and guess what will be found at abnormal Point C. On the schematic, let's take the encircled 0V symbol (anode end of Z4) as voltmeter reference.
At Point A:
the supply should be in voltage regulation. IC2-pin 1 will be about 2.2V. IC2-pin 7 should be near near the opamp +15V rail.
At Point B:
the supply should be in current regulation. IC2-pin 7 will be about 2.2V. IC2-pin 1 should be near near the plus rail.
At Point C:
I presume the supply must still be in current regulation, buy why? Probably, IC2-pin 7 will be about 2.2V. Probably, IC2-pin 1 will remain near the plus rail, but why didn't it drop to a lower voltage and resume voltage control?
To search for the defect, I suggest getting the supply into state C; the voltage control section of IC2 pin 1 will present an erroneously high voltage. I suspect probing the pin 1 section of IC2 and IC1 will present voltages that don't quite make sense. Be on the alert for opamp supply rails that drop relative to the nominal +15V/-5V rails.
On the good channel, repeat the same exercise to see dynamics of a properly operating supply.
Let me know what you discover and about results that don't make sense. (And if you need clarification.)
I'm sorry for my slow response.
Steve
Mentally, I think of a graphical load line plot: Initially, you set supply voltage to 5V, no load; then you attach a 10 Ohm load and get 5V, 500mA (Point A); next you reduce CL control and, following the resistor load line, arrive at maybe 2V, 200mA (Point B); finally, you again increase increase CL control but instead of returning to Point A, the supply climbs the resistor load line and arrives at say 8V, 800mA (Point C).
Now I'll try to predict what should be happening in the circuit at Points A and B and guess what will be found at abnormal Point C. On the schematic, let's take the encircled 0V symbol (anode end of Z4) as voltmeter reference.
At Point A:
the supply should be in voltage regulation. IC2-pin 1 will be about 2.2V. IC2-pin 7 should be near near the opamp +15V rail.
At Point B:
the supply should be in current regulation. IC2-pin 7 will be about 2.2V. IC2-pin 1 should be near near the plus rail.
At Point C:
I presume the supply must still be in current regulation, buy why? Probably, IC2-pin 7 will be about 2.2V. Probably, IC2-pin 1 will remain near the plus rail, but why didn't it drop to a lower voltage and resume voltage control?
To search for the defect, I suggest getting the supply into state C; the voltage control section of IC2 pin 1 will present an erroneously high voltage. I suspect probing the pin 1 section of IC2 and IC1 will present voltages that don't quite make sense. Be on the alert for opamp supply rails that drop relative to the nominal +15V/-5V rails.
On the good channel, repeat the same exercise to see dynamics of a properly operating supply.
Let me know what you discover and about results that don't make sense. (And if you need clarification.)
I'm sorry for my slow response.
Steve
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Hi Steve,
thanks for your elaborated answer. I appreciate what you are doing here. Don't won't worry about being slow. I am the slow party here, I can do the tests only late in the evening...
Greg
thanks for your elaborated answer. I appreciate what you are doing here. Don't won't worry about being slow. I am the slow party here, I can do the tests only late in the evening...
Greg
I measured today the voltages around IC2:
working channel bad channel
Scenario A
pin 1 1.5V 14V
pin 7 14V 1.5V
Scenario B
pin 1 14V 14V
pin 7 1.2V 1.2V
Scenario C
pin 1 1.45V 14V
pin 7 14V 1.8V
However in the bad channel in Scenario C I see 0.3V at pin 3 and 0V at pin 2.
I started wondering why that could be. I turned my attention to R12 specified as 33k in the schematics. My supply supplies 1A unlike the one in the schematics which can do 2A. So I was not surprised to measure 14k in the working channel. However in the bad channel I measured only 1.7k. I measured also R15 in both channels, these are 1k as specified.
So my current suspect is R12. Shortened C3 seems unlikely? It would need to behave more like 1k resitor?
Tomorrow I will not be able to do anything (Christmas dinner at work... I know early...)
Will check today if I have spare chunkier 14k resistor in my drawers. Otherwise need to order.
I also poked around IC1.
In the working channel I have 0.37V on both inputs and 0.75V on the output.
I the bad channel I have 0.5V at inputs and 1.1V at the output, so I think this is reasonable.
Let me know if I am wrong in my analysis.
working channel bad channel
Scenario A
pin 1 1.5V 14V
pin 7 14V 1.5V
Scenario B
pin 1 14V 14V
pin 7 1.2V 1.2V
Scenario C
pin 1 1.45V 14V
pin 7 14V 1.8V
However in the bad channel in Scenario C I see 0.3V at pin 3 and 0V at pin 2.
I started wondering why that could be. I turned my attention to R12 specified as 33k in the schematics. My supply supplies 1A unlike the one in the schematics which can do 2A. So I was not surprised to measure 14k in the working channel. However in the bad channel I measured only 1.7k. I measured also R15 in both channels, these are 1k as specified.
So my current suspect is R12. Shortened C3 seems unlikely? It would need to behave more like 1k resitor?
Tomorrow I will not be able to do anything (Christmas dinner at work... I know early...)
Will check today if I have spare chunkier 14k resistor in my drawers. Otherwise need to order.
I also poked around IC1.
In the working channel I have 0.37V on both inputs and 0.75V on the output.
I the bad channel I have 0.5V at inputs and 1.1V at the output, so I think this is reasonable.
Let me know if I am wrong in my analysis.
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You did not see because I have heavy fingers! I posted before I typed in the voltages. I immediatelly edited but you must have seen the original version.
There is one more point I do not understand.
The R12 measured values are as I reported 14k and 1.7K.
But I still see the color codes on my retina: orange, orange, orange, red: 33K just like the schematics says.
There is one more point I do not understand.
The R12 measured values are as I reported 14k and 1.7K.
But I still see the color codes on my retina: orange, orange, orange, red: 33K just like the schematics says.
I probably had to disconnect C1 switch (ie CV/CI switch) in case anything else is fried and is in parallel to R12...
I think you may have found it. Your suspicions seem well founded. I toy with the possibility that C3 that might be resistive and confusing the resistance measurement. You might measure in-circuit resistance seen at C3 terminals to see if all seems consistent.
Fingers crossed!
Fingers crossed!
Good channel could show 14K rather than 33K because of shunt resistance paths. But there has to be a reason for widely different in-circuit resistance seen across R12 between channels.
Still toying with possible C3 leakage--- with unit powered, there's likely some voltage present across R12. But there should be no voltage across R15; any DC across R15 indicates leakage through C3.
Still toying with possible C3 leakage--- with unit powered, there's likely some voltage present across R12. But there should be no voltage across R15; any DC across R15 indicates leakage through C3.