MT3608 with wrong resistor value


I bought a few boost converters on ebay, they are based on MT3608, up to 30V, ~ 500 mA max which is fine for a lot of applications. Of course they are cheap.

But they dont work, Vout~=Vin whatever the potentiometer says.
Let's see why

They do look like this :


The variable resistor is "102" i.e. 1kOhm, the fixed resistor is 222= 2.2k
The circuit works like this

 Vout
   1k variable ==> FB
   2.2k
Gnd

Vout is adjusted so that FB is 0.6V

Let's write down the general equation for that circuit

FB=0.6=Vout*(2.2k+alphaR)/(2.2l+R), with :

* R the value of the variable resistor (1K) here
* Alpha the coefficient of the variable resistor between 0 and 1

The module should be able to go up to 30V, with alpha=0

0.6=30*(2.2)/(2.2+R) => R=107 kOhm

Ooops, the variable resistor is 1k (102) instead of 100k (104),  it cannot work with the value

Two options : Replace the variable resistor with the proper value i.e. 100kOhm
Or hardwire the value we want

Let"s assume you want 7.2V as output
Vo=0.6*(2.2+R)/2.2
so with Vo=7.2V
0.6=7.2*2.2/(2.2+R) => R ~ 24 k gives Vout=7.14, good enough for a 7805

If we aim a bit lower at 6.5v
with R=22k, Vo=6.6

The circuit from the top is like this :



So short circuit left and center, put a 22k resistor between center and right and we are done




Verdict : 6.45v, close , should be  good enough :)


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