# Small math problem, I have no clue on how to do the same thing with new numbers.

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#### David_ Hello.

I was going through the steps found at
http://www.simonbramble.co.uk/techa...er_supply/digital_control_of_power_supply.htm
to calculate the values for three resistors used in a scheam to control the output of a regulator.

Anyhow, there is R1, R2 & R3.
It ends up like this:

4.2/R1 + 2.5/R3 = 0.8/R2

Which fills in as

4.2/3.03R3 + 2.5/R3 = 0.8/10kΩ

And from that somehow I should ba able to work out a value for:
R3 = 48.576kΩ
R1 = 147.187kΩ

But I don't understand how 48.576k was derived so I can't take anything from this but I am guessing that its simple and the author felt it to be self explanatory.
I am embarrassingly bad at math and just playing around with the numbers gave nothing please advice.

Regards

#### esp1 Hi.
4.2/3.03R3 + 2.5/R3 = 0.8/10

(4.2/3.03)/R3+2.5/R3 =0.8/10

1.386 + 2.5 = (0.8/10) *R3

(1.386 +2.5)/0.08 =R3

3.886/0.08 =48.575K

• David_

### David_

points: 2

#### David_ Thank you, when some one write something divided by 10k I would always think that that means that I should divide by 10000.
But now after having seen how you solved that think that perhaps 10k in such a situation always tells me that its 10 k's and not a short for 10,000?

#### ark5230 When all units are Kilo Ohms the equation becomes dimension less and can be solved easily, finally units can appropriately be associated.
What you say, writing 10000 for 10K is also perfect.

• David_

points: 2