Good Sam Club Open Roads Forum: Mini trim pot to big potentiometer
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 > Mini trim pot to big potentiometer

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wnjj

Cornelius, Oregon

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Posted: 01/22/20 09:02am Link  |  Quote  |  Print  |  Notify Moderator

Tom_M wrote:

[image]
Connect all three pins. Circuit board is double sided so traces can be on both sides.

So you didn’t remove the small pot? You can’t just put two equal sized pots in parallel since the resistance will be reduced significantly, especially if you leave the on-board one turned anywhere but to 8k ohms. Best case you get 4kohms max.

You can get close if you use a much larger value for the added one. For example if you parallel a 100kohm pot, twist the small one to 8k and the large one to 100k you will end up with 1/(1/8 + 1/100) = 7.4k max down to 0 when you turn the big pot all the way down (leaving the small one alone).

However it would probably be simpler to just remove the small one.

Also since there are traces on top, did you confirm there isn’t one to the 3rd pin there?

DrewE

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Posted: 01/22/20 09:56am Link  |  Quote  |  Print  |  Notify Moderator

Did you measure the old pot while still in the circuit? There's a very good chance it's not an accurate measurement if that was done; the surrounding circuitry will affect the resistance reading. It does depend on how the circuit is arranged.

I would be mildly surprised if it's indeed an 8k pot; that's not a particularly common value. I'd expect a larger value most of the time. However, it's certainly not beyond the realm of possibility.

(It is also possible, though not at all likely for a simple thing like this, that there are more than just the top and bottom layers of a circuit board, and some traces on the inside where they are hard or nearly impossible to see. This is almost certainly a two-sided board, though, and not a four or more layer board; no need for the extra cost of inner layers for a heater controller gizmo.)





Mobilesport

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Posted: 01/22/20 10:50am Link  |  Quote  |  Print  |  Notify Moderator

wnjj wrote:

Tom_M wrote:

[image]
Connect all three pins. Circuit board is double sided so traces can be on both sides.

So you didn’t remove the small pot? You can’t just put two equal sized pots in parallel since the resistance will be reduced significantly, especially if you leave the on-board one turned anywhere but to 8k ohms. Best case you get 4kohms max.

You can get close if you use a much larger value for the added one. For example if you parallel a 100kohm pot, twist the small one to 8k and the large one to 100k you will end up with 1/(1/8 + 1/100) = 7.4k max down to 0 when you turn the big pot all the way down (leaving the small one alone).

However it would probably be simpler to just remove the small one.

Also since there are traces on top, did you confirm there isn’t one to the 3rd pin there?


The small one was removed


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landyacht318

Near a large body of salty water

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Posted: 01/22/20 11:32am Link  |  Quote  |  Print  |  Notify Moderator

8k ohm is a strange value.

I've never been able to measure resistance of a pot with it still soldered in place.

I've only run one 3 wire pot in the dozens of voltage buckers that i have run remote pots on, and it turns out 2 wires would have been enough as two legs always seem to have continuity between them when I test before removal.

Is your solder perhaps bridging the contacts? I've done that before for zero voltage control effect nut was able to remove the bridge to return full function, but none were on dual sided circuit boards.

Mobilesport

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Posted: 01/22/20 03:14pm Link  |  Quote  |  Print  |  Notify Moderator

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* This post was edited 01/22/20 06:33pm by Mobilesport *

wa8yxm

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Posted: 01/22/20 03:30pm Link  |  Quote  |  Print  |  Notify Moderator

Mobilesport wrote:

@landyacht318

Bought it from EBay, the seller told me that it is a 8kohm even though it reads 10ohm on the side of the pot.
I bought it and confirmed it with my ohmeter 7.9kohm
Here's were I bought it from.


There you have it marked 10 ohms.

Put a VOM (Multi-Meeter) in resistance on it and MEASURE It. Do not take the seller's word for it.


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Mobilesport

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Posted: 01/22/20 06:37pm Link  |  Quote  |  Print  |  Notify Moderator

Sorry ,not 10 ohms ,,, I meant 10k ohms

* This post was edited 01/22/20 07:17pm by Mobilesport *

Mobilesport

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Posted: 01/25/20 02:16pm Link  |  Quote  |  Print  |  Notify Moderator

Got it to work using TomM picture , I had to wire center pin of the mini pot to the center pin of the new bigger pot ,
Then the pin in Toms picture that is labeled left I had to wire to the right pin of the big pot.
The pin in Tom M picture that is labeled right i had to wire to the left pin of the big pot.

Dave5143

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Posted: 01/25/20 02:40pm Link  |  Quote  |  Print  |  Notify Moderator

Glad you got it to work. I was going to mention that there are basically two different types of pots. Linear taper and audio taper. Linear is probably what you want for this application. Audio goes up in a logarithmic manner as you adjust it.


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Mobilesport

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Posted: 01/25/20 10:18pm Link  |  Quote  |  Print  |  Notify Moderator

Pictures of it finished
Its just a piece of 1 inch PVC conduit pipe with a metal washer glued to the end with The Amazing goop, the potentiometer is mounted in the metal washer.
The PVC pipe was painted navy blue and then glued to the original case that held the little circuit board.
Going to try to find a knob with numbers.

I highly recommend this heater
[image]

[image]

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