simple 555 polarity inverter voltage droops from very low

[PomPom]

This is the circuit:
https://www.aaroncake.net/circuits/vinvertr.asp

With no load it does fine.

But by the time it gets to 50mA it's down to -3V.

Ideally I'd only exchange components here. I used all the parts shown except I used schottky diodes ( IN5819 ). I had a IN4007 in there before and got basically the same result.

 

[jiripolivka]

For a 555, 50 mA load is too high when you feed a voltage doubler. You will need a transistor output buffer for such load. You can use a two-transistor stage, PNP/NPN, like used in audio amplifiers.

 

[PomPom]

For a 555, 50 mA load is too high when you feed a voltage doubler. You will need a transistor output buffer for such load. You can use a two-transistor stage, PNP/NPN, like used in audio amplifiers.

Thank you for your help. I thought I was well within the 555s current limit and that somehow switching speed/cap values/diodes were the issue. I ended up just sitting another 555 on top of it (for 2x the power output) and it regulates to 6.5 now and i'm pleased with that.

However, I am curious as to what you meant. I kind of understand it, but I don't see how a pnp or npn will turn a voltage negative... it seems like the 555 output will still take a large deal of current draw. Could you draw a schematic on where the transistors would be added?


Thanks again

 

[crutschow]

What is your supply voltage?

 

[jiripolivka]

Thank you for your help. I thought I was well within the 555s current limit and that somehow switching speed/cap values/diodes were the issue. I ended up just sitting another 555 on top of it (for 2x the power output) and it regulates to 6.5 now and i'm pleased with that.

However, I am curious as to what you meant. I kind of understand it, but I don't see how a pnp or npn will turn a voltage negative... it seems like the 555 output will still take a large deal of current draw. Could you draw a schematic on where the transistors would be added?


Thanks again

The NPN/PNP "totem pole" is a standard in most audio power amplifiers. It generates more power by switching between DC rails, not the negative voltage. The negative output is again generated by two diodes like in your schematic.
If two 555s can make a good output under load, then congratulations!
Instead of a 555, there are ICs like 7660 and other modern ones where a saturated coil is used to generate a higher output at any polarity.

 

[crutschow]

Below is the circuit modified with a complementary transistor output driver for more current. I reduced the value of C1 to increase the frequency and reduce the ripple for the given capacitor sizes (also modified some from the posted schematic).

The output is about -8.0V with a 9V source and a 80mA load. The load current can go higher than that, if desired, with some reduction in voltage.

 

[jiripolivka]

Thank you, this is exactly what I recommended! The PNP/NPN buffer does the perfect job!