Boost Regulator 4-25V Fixed Output

I have purchased a small quantity of item #799 Boost Regulator 4-25V and want to replace the trim pot with fixed resistor(s) to have a constant 18V output. Can you provide the process to do this?
I understand you offer this service for high-volume applications but I need it for the ones I currently have.

For future orders, how is “high-volume” defined?

Thank you,


The regulator is trying to get the feedback voltage to be approximately 1.24V. To fix the voltage, you would remove R3 and replace R1 and R5 with a resistors that give the feedback voltage with the VOUT you desire. I attached images of the relevant part of the schematic and the component locations below.

  • Ryan


High volume depends on what you want the price to be and how quickly you want them. We make these 40 to a panel, so the multiple will be something like that (allowing for some failed units), and a little under 120 units would be a good starting point for us if we already stock the resistors you want on there.

- Jan

Thank you Ryan and jan.
My solution was to remove R3 and replace R5 with a 27K ohm resistor.
Simplified math to find the value for R5 with a desired output voltage of 18V:
Determine voltage ratio: 18/1.24 (feed back voltage) = ~14.52
R1 is 360K ; (1/(14.52 - 1)) * 360,000 = 26,627
Nearest common value 27,000;
Find the current with the feed back voltage and common resistor ; 1.24 / 27000 = 0.0000459
Find the output voltage to achieve the the feed back voltage:
0.0000459 * (360,000 + 27,000) = 17.8v, This works good for my applicaiton and the parts I have on hand.

Thank you again,

I am glad you got it working! Thank you for sharing your calculations.

- Ryan

Could someone recommend a good pair of resistor values for a fixed 12v output level? Also, what tolerance % would you suggest?

Thanks in advance,

Hi, Mark.

If you take off R3, you are left with a simple voltage divider with R1 and R5. The tolerance to use depends on your requirements. Here are the pairs of common 1% resistor values that are within 0.1% of the correct divider ratio to achieve a fixed output of 12V and have a combined resistance greater than 10K Ohms and a individual value no more than 360K Ohms:

r1: 10500 r5: 1210
r1: 12400 r5: 1430
r1: 13700 r5: 1580
r1: 15800 r5: 1820
r1: 17800 r5: 2050
r1: 19600 r5: 2260
r1: 24300 r5: 2800
r1: 24900 r5: 2870
r1: 25500 r5: 2940
r1: 26100 r5: 3010
r1: 27400 r5: 3160
r1: 33200 r5: 3829
r1: 34000 r5: 3920
r1: 41200 r5: 4750
r1: 105000 r5: 12100
r1: 124000 r5: 14300
r1: 137000 r5: 15800
r1: 158000 r5: 18200
r1: 178000 r5: 20500
r1: 196000 r5: 22600
r1: 243000 r5: 28000
r1: 249000 r5: 28700
r1: 255000 r5: 29400
r1: 261000 r5: 30100
r1: 274000 r5: 31600
r1: 332000 r5: 38300
r1: 340000 r5: 39200

Depending on your exact requirements, values close to these might work okay.

- Ryan

I used the r1: 41200 r5: 4750 pair and the resulting output was 11.8v, well within the 10% tolerance of the device being powered. Thanks for your help!