10 turn potentiometer question.

[davelectronic]

A while ago i picked up an TLC3780 step up step down converter, with the intention of putting a cheap variable bench power supply together. I've removed the pcb 10 turn voltage potentiometer, and the 10 turn current potentiometer. That went fine after wicking away the solder for clean holes for wires to panel mount 10 turn potentiometers.

After measuring both potentiometers, the voltage potentiometer is 500k and the current potentiometer is 200k. Firstly i would like to keep the precision of 10 turn potentiometers, but panel mount components.
But I'm finding here in the UK there are none available at those values. The project is the one that Great Scott has done on YouTube. He's potentiometers are from Europe and the US going by his list of parts availability. So two questions really, firstly could i use a lower value for these 10 turn potentiometers, or is that idea not feasible ? And if that's not possible, is there supplier of 10 turn potentiometers here in the UK where i am ? I've tried ESR electronics one of my go to components sellers. Also RS here in the UK, and ebay os a non starter, most 10 turn potentiometers on there are not much above 10K.
Thanks for reading, any help appreciated.

[andy3055]

100k: https://www.amazon.com/s?k=100k+multi+turn+potentiometer&ref=nb_sb_noss

500k: https://www.amazon.com/Multi-turn-Trimmable-Trimmer-potentiometer-handle/dp/B01G2Z9IAE/ref=sr_1_2?keywords=500k+multi+turn+potentiometer&qid=1572410421&sr=8-2

[MosherIV]

https://cpc.farnell.com/search?st=10%20turn%20potentiometer

I have used CPC in UK many times, without problems.

I  am surprised you cannot get multi turn potentiometers from RS. Normally, you are overwelmed with choice.
Just checked, and yes - 219 parts to choose from
https://uk.rs-online.com/web/c/passive-components/variable-resistors/potentiometers/?applied-dimensions=4294873067

Edit:
Can you use lower values?
Depends on the circuit. If the pot is just being used to generate a voltage - yes.
If the pot resistance is used in circuit, then no, not without modifying the design.

Yes, I see what you mean about the 500K

[davelectronic]

Thank you for the replys, first links are US i think, so no idea of delivering and import charges. Other following links, none of these go over 100k, so the 200k and 500k are not stocked. Just another thought, if the closest value i can find is say 50k to 100k, can i increase the potentiometers value with another 1% fixed value resistor in series with the potentiometer  ? Just a thought. Although i don't know how the potentiometers value and travel and value would play out.
Thank you again for your help.

[SteveSi]

I had the same issue when trying to remove and extend 200K and 500K pots on a power module.
These points may be of use to you:
1. Some (cheap?) wirewound multi-turn pots have 'no contact' spots - this obviously affects the voltage\current on your output and can be disasterous!
2. Add another 10K/20K pot for a fine voltage control - the voltage adjust pot is 500K and the centre wiper is connected the pot end terminal so it is easy to add another small pot in series - just be careful not to exceed the total resistance of 500K. Only 2 wires are neeed to connect these two (coarse+fine) pots.
3. 200K and 500K are high impedance. In my power module, the 200K CC pot is actually in series with a 1.6M resistor on the PCB - so it is very high impedance. Extending the connections to both pots using long unshielded wires is a bad idea! I found that just by putting my hand near the unshielded wires to the front panel pots caused the PSU module to pick up noise and increased it's total current consumption to 1A! Touching any of the pot wires insulation caused even more issues. I used shielded cable (old USB cable) and connected the braid to 0V at the PSU end, and to the pot enclosure at the other end (check for shorts). This cured most of the problems. Just be careful before putting it into a (metal grounded) case not to touch any of the pot wires.
4. For the current CC pot, use a normal pot but fit a large knob for better control.
I can't give you exact values because my unit stopped working after playing with it a while. I ordered a replacement but when it came, it only went up to 28V instead of 30V (the 20-turn pots were actually 460K and not 500k!). I was so disgusted that I returned the module. I came to the conclusion that this 'BOOST\BUCK – ‘DC5-32V LTC3780 Automatic Lifting Pressure Power Step Up/Down Constant Voltage’ [Out:  80W 1-30V] was not suitable for an adjustable bench power supply as it was just too unstable.
I am investigating other boost\buck modules and combinations, but so far have not found anything which actually works without going unstable or generating voltage spikes on power up\down, etc.    See also my blog - http://rmprepusb.blogspot.com/2019/11/is-d3806-boostbuck-dc-dc-power-supply.html
I am coming to the conclusion that a DPH3205 module https://www.banggood.com/custlink/GmGKsh7ggM might be worth the extra expense after all (if you want to use with an ATX PSU).

[t1d]

I am coming to the conclusion that a DPH3205 module https://www.banggood.com/custlink/GmGKsh7ggM might be worth the extra expense after all.

As this is in the beginner section, I will point out that you may want to investigate the difference between Linear and Switch-Mode power supplies and when it is appropriate to use each. My general short-take on it is this... You can use a linear supply for all applications, including those where switch-mode might be allowable, but, you must use a linear supply, when a switch-mode is not allowable. For a beginner's first bench power supply, I think it is a better investment to obtain a Linear power supply. This all pertaining to DC supplies, of course. Just saying...

As for substituting a lower resistance pot and an additional resistor for a single pot, you will loose part of the operational adjustment, and therefore the range, of the circuit. I think the deficiency would be rather proportional... Let's say you use a 400 ohm pot and a 100 ohm resistor, for the 500 ohm pot. Seems to me that you would loose about 20% of the adjustment range, at the lower end, or the upper end, of the adjustment scale, depending on how the circuit is designed. PS, I did not investigate your circuit.

[SteveSi]

no - as I explained, the voltage 500K 20-turn pot is wired as a simple variable resistor (one end is shorted to the wiper)
so it is fine to wire in another smaller pot wired in the same way.

pin 1 is connected to pin 2 (wiper)
wires go to pins 1&2 and another to pin 3

i.e. simple series resistor.

For the CC pot however, it is not wired in this way and so it is a lot more difficult to add a fine pot.

P.S. re the DPH3205 I meant that it would be best if you have to use an ATX PSU as a power source. I agree that a linear PSU is the best option for a reliable bench supply.

[andy3055]

If you have to use a series resistor, split the value in 2 and use on either side of the pot so that your adjustment range is in the middle rather than to 1 side.

[tunk]

I don't really know much about this, but you might be able to use
lower resistance potentiometers if you replace some resistors on
the board. This may be some help:
https://wiki.beyondlogic.org/index.php?title=WD2002SJ_LTC3780_Synchronous_Buck_Boost_Converter_Ebay_Automatic_lifting_pressure
If I understand the schematic correctly, you could replace R9 with
a 2.8k resistor and use a 100k potentiometer. Then again, this may
blow up your board.