# Replace potentiometer with 2 resistors.

#### Bod

Joined Sep 18, 2016
301
hi,
Assuming that the designer has used a 40k from pin #4 to 0V. I would attach a 220k from Vout to pin #4, then check Vout.

E
With a 220K ohm resistor between pin 4 and Vout, I measur about 120mV on the output.

#### Bod

Joined Sep 18, 2016
301
1.25 is 1/0.8

In the example given R2 is 40.2k
1.25 x R2 = 1.25 x 40.2k = 50.25k

Can you find a 40.2kΩ resistor on your board? I don't see one. I see 8.2kΩ.
There is no 40K, just an 8.2K which you found. So does that mean I should use 10.25K in the calculations? (1.25*8.2K = 10.25K)

#### ericgibbs

Joined Jan 29, 2010
10,184
hi,
Image..

#### Ylli

Joined Nov 13, 2015
965
Remove the pot and place a 43K resistor between the indicated points
.
Should get you 5.0 volts out.

#### Bod

Joined Sep 18, 2016
301
Remove the pot and place a 43K resistor between the indicated points
.View attachment 207857
Should get you 5.0 volts out.
Ok, so, I was doing exactly what you said and I had soldered a 43K resistor between those two pins. As I was clipping the crocodile clips on, I had to move the resistors out of the way otherwise it would short. Me being me, I applied too much force and ripped off both pad. Luckily it was my test board so i was just looking at other place I could connect it to:

Each coloured squiggle matches up to the other coloured squiggle. As you can see, where the vias lead should mean I should be able to put the 43K resistor across the bottom right pin and the wiper pin of the pot.
The reason I am showing you this is because I have actually already tried that and unfortunately it didn't seem to work.
I may try it out on my second board because it could be that I have damaged a component on here.

#### Ylli

Joined Nov 13, 2015
965
Where you show 43k in blue is fine too, as long as the pad and connections to the 82K and the IC pin 4 are intact. . It looks like a fairly simple circuit, and the 43K should give you 5 volts. If not, as you say, there may be something else wrong on the board.

When you power it up, what voltage are you seeing at pin 4?

#### Bod

Joined Sep 18, 2016
301
Where you show 43k in blue is fine too, as long as the pad and connections to the 82K and the IC pin 4 are intact. . It looks like a fairly simple circuit, and the 43K should give you 5 volts. If not, as you say, there may be something else wrong on the board.

When you power it up, what voltage are you seeing at pin 4?
2 things:
Firstly, not only did I strip the pad and the trace, I also managed to remove the metal on the end of the SMD resistor, so that's good...
I've switched to my second board and the better news is that it is working. The 43K resistor gives me 9V on the output so I guess I need to crank up the resistance a bit but at least I know it was a damaged board not some dodgy wiring.

#### Bod

Joined Sep 18, 2016
301
It's all working now so thank you!

I hadn't tried a 43K yet because I was under the impression it was the wrong value (because the other board was broken). I redid the calculations an got 43K again so I soldered them on and it gave me 5V out.

Thanks again!

#### ericgibbs

Joined Jan 29, 2010
10,184
hi bod,
Good to hear.
If the 8k2 had been a 40k, you would have used a 211k for 5V

As it is a 8.2k, that ratio is 8k2/40k = 0.205 so, 211k * 0.205 =~ 43k

E