# 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,
Kalkmore

Hello.

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

Hi.

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,
Kalkmore

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?

Mark

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!

Mark