[Milsurplus] 12 to 24 volt conversion, the issue of cost

[Jim Whartenby]

Not true, the PWM duty cycle in an NE555 is essentially unlimited.  See:

http://electronicdesign.com/Articles/Print.cfm?ArticleID=11028

for an example of a simple circuit that will allow for a duty cycle from 1% to 99%.  Although a Schottky diode is shown, a germanium signal diode should be as effective.  A Google search will find other circuits for a NE555 PWM.

To close the loop and make the power supply self regulating, all that is needed is to feed back a sample of the 24 volt power supply output to pin 5 of the NE555.  When phased correctly, as the 24 volt supply goes high, the PWM narrows and vice-versa when the 24 volt supply voltage falls.

So cost wise we are talking about a 555 and an op-amp or two, a voltage reference, some resistors and caps plus an e-mode MOSFET or two depending on whether or not you use a transformer or an inductor to make the 24 volts from 12 volts.

So less magnetics, I would guess about $10 or so for parts.
Jim


--- On Fri, 10/9/09, Tom Dawson <wb3akd at earthlink.net> wrote:

> From: Tom Dawson <wb3akd at earthlink.net>
> Subject: Re: [Milsurplus] 12 to 24 volt conversion, the issue of cost
> To: "Jim Whartenby" <antqradio at sbcglobal.net>, milsurplus at mailman.qth.net
> Date: Friday, October 9, 2009, 7:43 AM
> Jim,
> 
> Only drawback to the 555 is the duty cycle limitation in
> astable mode which 
> won;t exceed 50 %.  Given probably losses in the
> switch transistor and 
> inductor, a 12 to 24 boost converter would need more that
> 50% DC.
> 
> Still it is a great place to start and I've used it with a
> Ford ignition 
> coil and TV voltage tripler to power surplus image
> converter tubes.
> 
> 73
> 
> Tom
> WB3AKD
> 
> 
> ----- Original Message ----- 
> From: "Jim Whartenby" <antqradio at sbcglobal.net>
> To: <milsurplus at mailman.qth.net>
> Sent: Friday, October 09, 2009 1:31 AM
> Subject: Re: [Milsurplus] 12 to 24 volt conversion, the
> issue of cost
> 
> 
> You PWM a NE555 using pin 5 which connects to the top
> junction of the three 
> 5k ohm resistors used to set the limits of the voltage
> comparators.
> 
> The 555 should make a usable PWM for the step up
> converter.
> Jim
> 
> --- On Thu, 10/8/09, Tom Dawson <wb3akd at earthlink.net>
> wrote:
> 
> > On your DC-DC converter project, I might recommend
> that you
> > investigate boost converters as the inductors are a
> single
> > winding. I've made
> > experimental converters with just a 555 driving the
> switch transistor.
> > Regulation is achieved via adjusting the duty cycle
> > (something other than a
> > 555 is needed if you want to regulate this way, but
> the 555
> > is a good place
> > to start). National semiconductor has application
> > which greatly notes on
> > their controller chips and boost supplies in
> > general.
> >
> 
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