Changing Pots to multiturn in a 30V/3A PSU

[ym58]

Good point.  I just checked it and the indicated current is within 20mA.

well, this tends to proove that replacing the 6.8K pot STRAIGHT with a 10K alters the current capacity ...
by chance, let us know if you re-open the PSU case and try to parallel the "CURRENT" multiturn pot with a 22K resistor coz I'm pretty concerned about it too (I am still awaiting the multiturns that I ordered last week from HK).

[ym58]

I have finally received my two 8.00USD multi-turn Bournes pots from HK (actually, they were sent from S'pore).

Did the swap in the PSU (only the voltage control as for now ...).
Had to unsolder and change physical  location of a thick coiled resistance (used by the ammeter, I guess) that was hindering the new pot from fitting in (the Bournes pot is thicker and longer than the old one) but it finally worked.
Had also to enlarge slightly the hole in the front panel since the Bournes pot has also got a wider thread !

I did not solder a 22K resistor to lower the pot value since it seems to be working fine ... I double checked the output voltage with a multimeter to be sure.

The only ASTOUNDING point is that I now have a 48V PSU instead of a 30V one ... at least on channel #2 (it's a dual PSU).

I am not quite sure that was not the case BEFORE I changed the pot since it's a second-hand PSU purchased a month ago, maybe the previous owner had already changed something (transformer ?) in the PSU ... but that was quite of a (good) surprise !
However, don't take it for granted if you also hack this PSU on your side ... !!!

[JacquesBBB]

The only ASTOUNDING point is that I now have a 48V PSU instead of a 30V one ... at least on channel #2 (it's a dual PSU).

This is probably because of the change from  6.8 k to 10 k for the total resistor of the pot.
30/6.8*10 =44

It is like if you were changing the RP2  220 ohms  tension Max spanning pot  into a 3 k pot.

I would not recommend to leave it like that. All the components of this PSU are barely enough for the existing  max value of the voltage. Increasing it to 48 V will be an additional source of breakdown, in particular for the capacitors.

If you need 48V, it is better to pair the two outputs.

[Kleinstein]

The 48 volts are likely only available without load and enough voltage in the grid. Under load and with 10 % less voltage from the grid the available volatage will be lower by something like 20-30%. So the higher voltage are not really usefull, but up to about 40 V might work.

Wether a hhigher output voltage is more stress to the components depends on the circuit. In a simple linear regulator its essentially only the output caps seeing the higher voltage. So this usually is not problem.
A higher volatage can be a problem if a preregulator is used. In this case one should not increase the voltage by much.

[JacquesBBB]

The output capacitors are rated 50 V, but like all components, not of the best quality.

[ym58]

This is probably because of the change from  6.8 k to 10 k for the total resistor of the pot.
30/6.8*10 =44

I understand your perspective, but it does not explain why I should get 48V instead of, say, 44V !
The resistor/potentiometer tolerances are way below this 44V versus 48V discrepancy !

Moreover, I am not quite sure that changing the 6.8K (yellow in the following diagram) with a 10K pot would necessarily lead to a higher output voltage !

Honestly, I feel very lazy to reopen it (you've done it so you know what I mean : this PSU is just so messy inside !) to ascertain that either the pot value should be reduced to a proper one ***OR*** (and that is an option) the previous owner has already changed the (red, see below) transformer to a higher range one (>50V secondary).

What d'you think ?

[JacquesBBB]

The pot is not the right one. I think that the voltage pot is the RP5 pot, which is in series with RP2.

[Kleinstein]

The set voltage is derectly proportional to the pot (RP5). The is also a trimmer PR2 to adjust the maximum voltage. This may have been set to a little more than 30 V before.

Especially with a higher voltage transformer the maximium current should not be set higher than original. Even a lower setting may be needed.

[ym58]

The set voltage is derectly proportional to the pot (RP5). The is also a trimmer PR2 to adjust the maximum voltage. This may have been set to a little more than 30 V before.
Especially with a higher voltage transformer the maximium current should not be set higher than original. Even a lower setting may be needed.

The pot is not the right one. I think that the voltage pot is the RP5 pot, which is in series with RP2.

Got you both, but still ... remember what FrankT once said earlier in this topic :

I have a slightly different model, a Sinometer HY3005D-3, 30 volts, 5 amps.
It had the same 6.8k pots, so I replaced them with 10k multi turn.  Voltage still goes up to 30v, but the current is now limited to 4.4A.

I am not sure his PSU design's quite the same as mine, but his point deserves some clarification ... which I will definitely do as soon as I open the box again.

... and this shouldn't be very long since I am expecting my 4-digit ammeter/voltmeter any time from now     !

[Bsupport]

So I had this LAB PSU and also found the voltage tuning too sensitive. I didn’t want to use the 10turn because the endless turning when setting voltages so I thought of a way to improve that by adding a ‘Fine’ pot meter with a low value in series with the current pot as many othe PSUs have. I checked several diagrams from the net but they all seemed a little different from mine, I guess I have a newer version, the PSU I have has a 10K pot for the voltage setting.
The solution would be to add a 1K pot in series with the 10K pot. But I didn’t want the extra holes, there wouldn’t be any space anyway, so I went for a stacked pot with 10K and 1K:  no chance of course! As a test I soldered a 1K resistor in series with the pot to see the effect: you can’t have the full scale anymore since the total will be 11K. So be it, I would cut off at the end, giving me 29V.
So I ended up with the following: Bought two 10K stacked pot’s and two 1k pot’s from the ALPS type from our Chinese supplier.
When the pots came in I checked the values just for the interest.
Although the stacked pots were marked 10K both didn’t go any further than 9.1K and the 1K pots also didn’t really get any further than a 9.8K.
This turned out to be perfect since in the combination I still had my 10K resistor there!

I drilled out the rivets and removed the lower decks.
Did the same on the 1K pots.
Then assembled the 1K deck on the stacked pot together with the wiper.
Added the cover from the stacked pot and put the rivets back in again.
This really worked! I can now fine-tune my voltage within 10% of the set value.

Added some pictures of the stacked pot assembly and the fitting in the PSU.
I guess you can do the same with the Amps knobs as well.
Hope this helps other to improve this otherwise fine PSU!