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Comment by peter_d_sherman | Hacker News Reader

>"You might wonder how a charge pump can turn a positive voltage into a negative voltage. The trick is a "flying" capacitor, as shown below. On the left, the capacitor is charged to 5 volts.

Now, disconnect the capacitor and connect the positive side to ground. The capacitor still has its 5-volt charge, so now the low side must be at -5 volts.

By rapidly switching the capacitor between the two states, the charge pump produces a negative voltage."

That is a strange and weird effect of capacitors!

I never knew this, before this post!

I'll have to experiment with this weird but interesting effect in the future!

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Well, it's absolutely not weird. Capacitors are happy to change current instantaneously but not voltage. It's in their spec: i=CdV/dt. If you charge a capacitor to a particular voltage, and then disconnect the capacitor entirely from its circuit, it still holds that voltage. Swap "+" and "-" and you have a negative output. Couple many capacitors together in an array of switches and you can make nearly any voltage you please, but integer multiples are easiest.

Most all modern rs232 ports use a similar approach to generate the negative voltages needed as well. The ubiquitous max232 and its copies/clones all use charge pumps to convert a +5v rail to +/-12v rails for the line drivers.

Take a look at the max233 cross-sections by @TubeTimeUS. It looks like a normal DIP integrated circuit, but it has four discrete capacitors in the IC package for the charge pumps.

https://twunroll.com/article/1286136812658806785#

Very cool -- I always wondered how they integrated those caps into the package

You can also make positive doublers: long ago I made an EPROM programmer (needs 21 V from 12V) using a logic driven capacitor voltage doubler- basically this circuit:

https://circuitdigest.com/electronic-circuits/voltage-double...

Is this also how switching transformers work?

Not really, except at a high level as capacitors and inductors both store energy. Capacitors store charge as a steady voltage while inductors store magnetic field as a steady current, which results in a different circuit topology.

If you want to learn how switching regulators work, start by studying the basic "boost converter".

The way I see it, this isn't really about capacitors. You could do the same thing, not practically but conceptually, with a rechargeable battery.

In many ways, a capacitor is just a shitty battery

I think conceptually any circuit element with reactance/memory would do, just a matter of cost and spec

I don't think it's useful to think of capacitors as bad batteries. The only point I was trying to make is that a component doesn't have an inherent "ground", so you can reference one pin to any voltage in the circuit.

If you use extremely low leakage switches and capacitors, this technique can be used to add and subtract voltages at very high precision: https://www.analog.com/media/en/technical-documentation/data...

A capacitor adder, or rather, accumulator! And analog too, not digital!

I love it!

Just don't load this accumulator with $FFFFFFFFFFFFFFFF -- as that will generate a voltage so high it will fry the whole system! <g>

No, I'm just kidding! (That was just an attempt at humor! <g>)

On a serious note -- I think the idea is both fasinating and meritorious!

Capacitative adding and subtracting is how many ADC (analog to digital converter) and DAC (digital to analog converter) chips work. There are many architectures, but capacitor arrays that are binary weighted are common, by switching them you can create or modify a voltage for a DAC, or use this to zero in/match this to the ADC input value. A common architecture that does this is called SAR ADC [0], which basically uses a DAC as part of the ADC, and it is often a capacitive DAC.

[0] https://en.m.wikipedia.org/wiki/Successive-approximation_ADC

What I didn't get in the article is how this produce constant negative voltage. From what I understand with my low amateur electronic skills, this would produce a scare signal with -5V/0V, at whatever quick pace.

That would average at -2.5V if we put a low pass filter.

This would work for me with 2 charge pumps, but I only see one large capacitor

The substrate has some capacitance to hold the voltage steady. The diode blocks the 0V so the substrate only "sees" the -5V.(It acts as a peak detector, not a low pass filter.)

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