555 timer voltage doubler.

[Wayne]

I have been seeing a few circuit that incorporate a 555 IC to generate
pulses at a higher voltage than the input.

Is this reliable? All I am wanting to do, as an experiment, is to run a
chain of LEDs.

It is all hypothetical at this stage as I am only at the planning phase.

 

[Joe G \(Home\)]

I have been seeing a few circuit that incorporate a 555 IC to generate

pulses at a higher voltage than the input.

Is this reliable? All I am wanting to do, as an experiment, is to run a
chain of LEDs.

Why reinven the wheel!

http://www.reconnsworld.com/power_voltdoubler.html

 

[Wayne]

Why reinven the wheel!

http://www.reconnsworld.com/power_voltdoubler.html

That is the site that prompted me to ask.

 

[petrus bitbyter]

That is the site that prompted me to ask.

The circuit is reliable but the voltage will drop fast when you load it (as
stated in the text). I doubt it to be usefull for a chain of LEDS. You'd
better use the power source as is and put the LEDs (or shorter chain of
LEDs) in parallel.

petrus bitbyter

 

[Wayne]

The circuit is reliable but the voltage will drop fast when you load it (as
stated in the text). I doubt it to be usefull for a chain of LEDS. You'd
better use the power source as is and put the LEDs (or shorter chain of
LEDs) in parallel.

petrus bitbyter

I plugged the circuit into LTSpice. The result for a 12v supply had the
output voltage at 20v, and with a single LED down to 18v.

I know that SPICE has a margin for error, but for the cost involved, it
is easier for me to get a 24+v power supply.

 

[ian field]

I plugged the circuit into LTSpice. The result for a 12v supply had the
output voltage at 20v, and with a single LED down to 18v.

I know that SPICE has a margin for error, but for the cost involved, it is
easier for me to get a 24+v power supply.

If you want increased voltage at a decent amount of power, use the 555 in a
self boosting SMPS configuration.

Feed the 555's Vcc via an inductor & forward biased diode (cathode to Vcc
pin - inductor to supply I/P) then drive a MOSFET with the 555's O/P and
connect the drain to the junction of the diode/inductor. You will need
voltage sensing and control of the M/S ratio as even with the minimum 4.5
Vcc, the boosted voltage will easily exceed the 555's Vcc max. A very simple
way of regulation is burst mode, sense the boosted voltage and switch the
reset pin when the voltage exceeds a set value - use a large electrolytic on
Vcc.

 

[ian field]

1N4148s are a lot better than 1N400x, which are SLOW. Im too lazy to
sim/calc the switching frequency, but with a 10nF cap and a 15k (ish)
timing resistor it'll be around 10kHz or so.

make sure the caps are good low ESR caps, eg Rubycon ZLH series. If you
got it from DSE, its probably crap.

and for a bit more grunt, place a complementary emitter follower between
the 555 and the first cap. NPN + PNP, bases tied together connects to 555
with 100R resistor. Emitters tied together connects to cap. NPN collector
to +12V, PNP collector to 0V. BC327 + BC337 is a good start, bigger
transistors = more grunt (technical term)

Cheers
Terry

Complementary emitter followers are great for beefing up the drive current,
but don't forget that each B/E junction costs you 0.7V off the drive
amplitude - certainly significant where the Vcc is 5V and the peak O/P may
be a bit less impressive even with 12V Vcc.

The bipolar 555 can source/sink 200mA which is more than plenty for driving
a diode capacitor charge pump.

 

[ian field]

and a PITA at 3.3V Vcc


depends entirely on the load. OP wants to drive LEDs.....

but C = 220uF, dV = 12V so dT = C*dV/I = 220uF*12V/0.2A = 13ms

OK the 200mA isnt a fixed figure, but what the aforementioned calc shows
is that the current limited output effectively gives a lower bound on the
output impedance & hence regulation.

NB: 12V/0.2A = 60R, so Tau = 13ms.

This suggests that there is a meaningful upper bound on the amount of
capacitance you can drive - making it larger increases Tau, and once Tau
gets up near the switching period, the load regulation gets worse. Im not
going to analyse it, but its pretty obvious Zout will have a minimum for
some value of C, getting larger with smaller C, and asymptotically
approaching Zmax = (impedance of 555 + ESR of cap) for larger C.

high ESR caps and pitifully slow diodes add to the total output impedance.

Toss in an emitter follower (FZT651/FZT751 and now you get 5A), the
effective slew rate goes up (dt = 0.5ms into 220uF), the effective Z555
drops (to about 2R4), Zout drops.....

Cheers
Terry

If the OP wants significant power output then my earlier suggestion might be
useful, make a boost converter by driving a MOSFET to switch the current
through an inductor, the data sheet suggests the 555 guaranteed down to 4.5V
but most chips will go down to about 3V - this is a bit low for fully
efficient gate drive so it may be convenient to supply the 555 from the
boosted rail, the flyback voltage can easily boost the voltage to more than
the recommended Vcc max so I'd suggest using a zener and transistor to shunt
the reset pin as a coarse burst mode regulation - the CV pin can be used to
vary the duty cycle for more precise regulation.

 

[ian field]

thats an understatement. for voltages that low, you need to use a FET
spec'd for operation at Vgs = 2.7V. "ordinary" FETs have Vth = 3-4V or so,
and are seriously unimpressive when Vgs isnt much larger than Vth. I
recently had to replace a 2N7002 with an NDS355AN for this very reason
(Vcc = +3.3V).

In my experience with using a 555 in a self boosting configuration, even a
low gate drive will invoke enough drain current to get things going, from
that point onwards the Vcc rises rapidly. The circuit I built used a MOSFET
liberated from the SMPSU of a scrap monitor - I can't remember the type but
I think it needed about 6 - 8V for full conduction (the circuit was powered
by a 4.8V NiCd pack), the first prototype didn't have any regulation and the
Vcc shot up to about 30V, the spec sheet for the Hitachi 555 I used said Vcc
max 18V so its a miracle it survived. The boosted Vcc was brought under
control by tying the reset pin to Vcc via a 4k7 resistor and shunting it
with a control transistor who's base was fed from the boosted Vcc via a
zener and current limiting resistor. This crude regulation was adequate for
my application so I left it at that, but if the OP is driving LEDs there
might be a noticeable fluctuation in the brightness, so a little extra
circuitry driving the CV pin might be needed to provide continuous smooth MS
ratio control. Since LED brightness is directly related to current, I'd
suggest using an OP-AMP to compare the voltage across a current sensing
resistor with a reference voltage and directly control the CV pin.