Farnell Bench Power Supply Replacing Pots

[RussellS]

Hi, I've been looking for a UK based electronics forum to ask this question and I came across this one so I thought I'd ask here.

I have just aquired a Farnell LT30-2 Dual Channel bench power supply (0-30v 2A) with current limiting facility. I think it has seeen a lot of use but everything appears to be working fine on it. The only problem is that the 6 potentiometers for adjusting the voltage (course & fine) and the current limit are a bit on the loose side and have absolutely no mechanical resistance when turning them. I would like to replace all the pots but I'm unsure what to replace them with. The original pots are Spectrol Reliance 5k wire wound ones but I cannot find much information on them such as the wattage etc. The markings on the pots themselves is MW-9-0-102 & MW-9-0-253. However, the parts listing in the Farnell service manual lists all 6 pots the same as Spectrol MW9/0/502.

What I would like to know is if I need to replace the pots with wire wound ones or would 5k linear carbon track ones be suitable. Also what wattage would I require.

I have provided a link below to the circuit diagram.

https://dl.dropboxusercontent.com/u/324 ... es_psu.pdf


If anyone has any advice or suggestions I would be very gratefull.

Russell

[Pauldf]

Looking at the schematic P101 and P102 are on a 6.2v rail in series with a 2k pot (P2). As the pot set point is unknown I'll ignore this and take it as it's minimum value i.e. 0.
Similar with P103, is in series with P3 and also on the 6.2v rail.
P101, P102 and P103 are all 5K, hence 6.2v/5000R= 1.24mA
I^2 x R = 1.24mA x 1.24mA x 5000R = 7.688mW (or 0.007688 watts) so anything above this value will do.
I'd stick with wire wound if it is going to get much work, most of the 5k wire wounds are 1w or above which is plenty.

[RussellS]

Thanks very much for looking at this for me and for your advice.

I'll see about getting some wirewound pots to replace them with.