Simple boost converter with 555

Ummmm, 12-14V on the input converted to 3-5V on its output is not a boost, it is a buck. And if you think 3-5V is an "increase" of 12-14V, then you have other issues that must be addressed prior to working with electronics.

He may mean that the 3-5V is the amount of increase from the 12-14V. Hopefully the OP will respond and let us know if this is the case.

He also didn't tell us how much current he needs to make.

I think we should put a zener diode on pin 5 of the LM555 so that we can make it only cycle if the output voltage is below the requirement.

Well, my english is far from being perfect, but I wrote: ' ... that increases its 12 - 14 V input WITH 3 -5 V on its output...' So I need 12 - 14 V to boost up to 15 - 19 V.

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I have to supply one or two OPAs. So it takes approximately 100mA. Maximum.

Seems to me a simple circuit you could roll together yourself is a charge pump doubler. However, this is not what your spec requires. I think it would be far simpler just to buy a chip that does the job rather than roll your own.

Ah, you meant to say, "that increases its 12 - 14 V input ***BY*** 3

-5 V on its output...".

For a second there, thought you wanted to increase 3-5VDC to 12-14V. (As an aside, the 555 requires at least 4.5VDC, preferably 5V, to operate properly.)

M

Check out

This circuit uses a 555 to take the incoming voltage and add it to itself; input 12, get out 24 . . . then add a 15-18 volt three terminal regulator or use a zener in series to subtract the excess voltage from the output.

A little more effort and there may be a way to use the XS voltage - zener and transistor to switch off the 555 so it would be turning on and off a pulse train. Turn on an NPN transistor to ground and use the reset pin of the 555 to pull low when the output exceeded whatever voltage you want .

4 transistor circuit for 800 ma - change one diode to change output voltage

I have to supply one or two OPAs. So it takes approximately 100mA. Maximum.

100mA is quite a load for a charge pump. Usually a charge can give 10mA or so without to much problem. Better go with a switching step-up regulator, that is, a small coil is needed for an efficiency conversion.

Stranger

That's what I need. But I can't see the maximum output current of this circuit. How can I calculate it?

12-14V seems plenty to run a few op amps. What output swing do you need? And 100mA could run a steel furnace (almost) let alone a few opamps.

I already told you how to do this in my other post. But why roll your own? Would it kill you to buy a 7662? Forget the zener. Follow the doubler with a LDO.

Regarding current, recall that current is a flow of charge versus time.Charge is voltage times capacitance. If you think of the charged cap as a packet of charge that is delivered to the load in a finite time period, you can estimate the current. It is a matter of sizing the capacitors relative to the frequency of the oscillator.

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I would like to build an adjustable power supply. I have a 12V/3000mA transformer. I want to adjust the output 0 - 12 V. I would like to use a simple transistor regulation ( H1061 ). The regulation will be solved by OPAs. As I will need the transistor use as a Darlington, I need to boost up the 12V power for the OPAs to let them drive the transistor to 12V. ( Sorry, my english is not too good. ;-) I hope you can understand me. )

Of course it wouldn't. But I don't have any 7662 on my shelf. Unlike

555. I am a hobbist. So it is a great opportunity to try 555. ( I never used 555 before. )

OK, I will make my calculations.

The maximum output current without external pass transistors will be whatever the 555 can deliver. My 1970's copy of the Linear Databook gives it at 200 milliamps source or sink, with a 600 milliwatt maximum power dissipation.

That is about at the limit for a LM555 based design.

Run a voltage doubler on the output of the transformers to make roughly twice the voltage. It just needs a couple of diodes like

1N400X and a capacitor to do.

Oh, and you have all the fets handy to make the cap fly? And the level shifters to drive the fets. You can probably find some circuits that use diodes where possible to save some circuitry. Granted at less efficiency.

0.1 uF cap across the C-B of Q4 in that circuit, maybe.