Author Topic: B&K Precision 1651 PSU adjustment  (Read 434 times)

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Offline beowulfenatorTopic starter

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B&K Precision 1651 PSU adjustment
« on: May 12, 2021, 09:29:54 pm »
I have bought this used PSU from ebay and did the calibration as it is described in the documentation. It went through fine, but I can't help but wonder why the range of their adjustment potentiometers is so big? For example, on the adjustable channels the zero needs to be set within 20 millivolts, but the overall range of the adjustment pot is several volts. Because of this, even a tiny nudge of the pot causes the value to change by tens of millivolts.

The pots are in series with a constant resistance, for example a 4.7K pot in series with a 1.5K resistor. Why is it designed this way (and not a bigger constant resistor and smaller adjustable resistor)? Why make the pot so big and so adjustable, when it seems to only make it harder to adjust?
« Last Edit: May 13, 2021, 09:29:50 am by beowulfenator »
 

Offline Alex_Baker

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Re: B&K Precision 1651 PSU adjustment
« Reply #1 on: May 14, 2021, 06:21:36 pm »
Surprised nobody has responded to this thread yet,

I am not very experienced in electronics but I have a thought, the potentiometer (SVR9?) looks like it is a part of a relatively complex resistor divider network connected to that op-amp, by changing the two resistor values like you mentioned you would be changing the whole system, its not as simple as the tow resistors adding together. in other words, when you turn that pot you are not changing resistance you are changing voltage (as far as the op-amp is concerned), and by changing their values you affect the range of voltage that the pot can output to the op-amp.

That's my guess, They probably could have made it easier to adjust but ultimately they most likely wanted something simple and cost effective. 
 

Online Kleinstein

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Re: B&K Precision 1651 PSU adjustment
« Reply #2 on: May 14, 2021, 09:20:18 pm »
The circuit looks rather old and a bit odd. When adjusting the full scale range this would also effect the zero setting. So the adjustment range for the zero needs to be quite large to allow for the different full scale voltage settungs. The resistors in the old time also had quite some tolerance, especially the resistance of the pots that can enter in the circuit.

So the circuit is just not the best, but probably just good enough. It was a complicated task to change a board - so don't fix it if you don't have too.
They may have started with a 20 trurns trimmer and only later changes to a simple one for cost cutting.
 


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