Introduction: Voltage Boost Module Enhancement
This is a simple enhancement that applies to some of the low cost voltage boost modules.
These are useful if you want to step up battery voltages to a higher voltage like 5V or 12V.
I often use the adjustable type as illustrated as these allow the output voltage to be set by adjusting the potentiometer.
These work well, can handle a decent output current, and are pretty stable.
However, I have found that with the ones I use the adjustment was very non-linear with all the action occurring at one end of the potentiometer adjustment.
Note that this applies to several batches of the modules as illustrated that I have bought. Other versions may already be like the modification I show here and will behave OK.
Step 1: Theory and Modification
Boost converters work by using a switch device (MOSFET) to drive an inductive pump circuit to produce a higher voltage. The output voltage is fedback via resistive divider and compared with a reference voltage typically 0.6V. The difference is used to adjust the switch on/off to control the output.
The stable output voltage is then just the 0.6V multiplied by the resistive ratio of the feedback circuit.
For the modules I use the feedback part is as per the 'Original' part of the diagram. The slider of the 10K potentiometer feeds the feedback point (FB) and a 200R resistor completes the circuit to ground.
This arrangement is a little weird as it means the bottom half of the resistive divider is also varying and it leads to a very non-linear control of the ratio.
The output voltage is then
Vo = 0.6 + (0.6 * X * 10000) / ((1-X) * 10000 + 200)
where X is the fractional setting of the potentiometer ( 0 -> 1)
In my view a more sensible arrangement is shown in the modified version of the diagram where the lower part of the divider is fixed at 200 Ohm and the potentiometer just provides the top part.
The output voltage becomes
Vo = 0.6 + (0.6 * X * 10000) / 200
The graph shows the voltage output you get for the 2 arrangements and as can be seen the original has all the action concentrated in the last 15% of the potentiometer whereas the modifed one is linear across the whole range making it much easier to make an accurate setting.
The modification itself is trivial and consists of just shorting out 2 pins of the potentiometer as shown in the image.