3 ms·

>"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 ca

>"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!

jhallenworld 6y ago
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-doubler-using-4049-ic https://circuitdigest.com/electronic-circuits/voltage-double...
- dehrmann 6y ago
  Is this also how switching transformers work?
  - mindslight 6y ago
    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".
osamagirl69 6y ago
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.
- kens 6y ago
  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# https://twunroll.com/article/1286136812658806785#
  - osamagirl69 6y ago
    Very cool -- I always wondered how they integrated those caps into the package
cellularmitosis 6y ago
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-sheets/1043fa.pdf https://www.analog.com/media/en/technical-documentation/data...
- peter_d_sherman 6y ago
  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!
  - tails4e 6y ago
    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 https://en.m.wikipedia.org/wiki/Successive-approximation_ADC
jimmyswimmy 6y ago
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.
gugagore 6y ago
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.
- qppo 6y ago
  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
  - gugagore 6y ago
    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.
tardyp 6y ago
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
- kens 6y ago
  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.)

jhallenworld 6y ago

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-doubler-using-4049-ic https://circuitdigest.com/electronic-circuits/voltage-double...

dehrmann 6y ago
Is this also how switching transformers work?
- mindslight 6y ago
  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".

osamagirl69 6y ago

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.

kens 6y ago
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# https://twunroll.com/article/1286136812658806785#
- osamagirl69 6y ago
  Very cool -- I always wondered how they integrated those caps into the package

cellularmitosis 6y ago

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-sheets/1043fa.pdf https://www.analog.com/media/en/technical-documentation/data...

peter_d_sherman 6y ago
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!
- tails4e 6y ago
  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 https://en.m.wikipedia.org/wiki/Successive-approximation_ADC

jimmyswimmy 6y ago

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.

gugagore 6y ago

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.

qppo 6y ago
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
- gugagore 6y ago
  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.

tardyp 6y ago

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

kens 6y ago
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.)