Experment with MP2307 KIS3R33 as buck-boost

There are loads of buck converter modules on eBay sold as KIS3R33 (as one vendor says "not new, take apart from machine") so I bought a dozen to play with since they look like a good way to replace linear regulators (actually going to use one in Roman's small capacitance meter, woo hoo!)

They contain an MP2307 buck converter with support components, and output a nominal 3.3v. Loads of people are selling kits to convert them to a variable psu.

I read about buck-boost converters recently, and wanted to see if I could modify one of these modules to produce a negative voltage.

Original output was 2.29v I think.

The modification was very easy. First pop open the case. Cut a couple of vias using a drill bit to disconnect the input capacitor (10uF ceramic) from the ground plane. Connect what used to be the module's output to the capacitor I just disconnected. This then becomes ground. The input stays the same, the original ground now becomes the negative output.

Switch on and hide under the bench...

Output only read -0.22v, so I connected an extra capacitor across it, trying not to be confused about the polarity. Output went down to -3.31v. Connected a 200Ω resistor, voltage went to -3.3v.

Well, I need to play with it some more, but this looks like a great cheap way to get a negative output from a single supply.

Next experiment is strapping a couple of these in parallel to increase the available current. I'm guessing they need to share a single feedback network to do this, possibly also a series resistor with each inductor. Also want to see if I can make negative and positive adjustable tracking outputs.

Interestingly, the conversion to negative output also results in the ability to produce -10v from a +5v supply. Don't see how that works.
Have been attempting to make a tracking adjustment of +ve and -ve modules - very poor results so far.

throbscottle said:

Interestingly, the conversion to negative output also results in the ability to produce -10v from a +5v supply. Don't see how that works.
Have been attempting to make a tracking adjustment of +ve and -ve modules - very poor results so far.

Click to expand...

Negative voltage doesn't really exist. However, it is called negative because it is referenced [usually] to ground.

If you have a +-5v power source, it is really just a 10v power supply with a halfway point.

By the same token, positive voltage doesn't exist either. I spent long enough thinking about existence in general and started to think that I don't exist!

So if negative voltages don't exist.
AND
Positive voltages don't exist.
Now you are going to say that zero voltages don't exist either.
I am going to hold out for irrational number voltages existing.

Can they be referenced to 0v? Do you have to measure them at right angles? Oh no, sorry that's imaginary number voltages!