# Voltage switch problem, most optimum solution ?

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Hi chaps,

240v ac with a 'or' from two sources, turned out ok...

Here's a different problem I've been toying around with
and I dont like the feel of what I've done so far, so looking
to do a little brainstorming, here it is...

Have a single source of power that goes from 9v steady DC to 500v pulsed
(not AC), in that in the first few seconds its a fairly steady 9v +20%,
then starts pulsing at 500v with a duty cycle of 5% but returns to
30v for 20% of the time and 12v for the remaining 75% of the period.
Period is approx 30ms overall but can reduce to 10ms, complication is
the 500v pulse is same period, so the duty cycle for that relatively
goes up as overal period goes down.

With this single source of power I need to draw off about 0.5W and
drive a 'module' at 9 to 11v with some cmos logic along with a few
leds etc.

My problemo is keep the cost down <guh>, no relays unfortunately but
drive this module without having it blow up or be unreliable
because its only power source line goes from 9v steady to
500v pulsed.

The circuit I have so far is a high voltage transistor configured
as a constant current drive into a resistor and 12v zener to spread
the dissipation and filter with a 100uF electro 25VW.
This seems to be ok but uses a few components and there isnt much
gain at the low volts of the constant current source to charge up
the electro quickly enough before the 500v pulses start coming...

So I'm opening this to the public forum to see if any lateral ideas
come up <shrug> cause I have exhausted mine (so far, afaik)  :(

--
Regards
Mike
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Re: Voltage switch problem, most optimum solution ?

What do they do inside Braun shavers?  My old one can take anything from
12VDC to 240VAC through the same plug which is a pretty wide range.

How well can you rely on the duty cycle of the 500V being small?  If you
knew very surely that it would be a small duty cycle then you could try
putting a L-C filter at the input of your circuit with a high value
inductor.  If you follow that with a switched-mode converter then the whole
circuit could be quite efficient.  Might not be cheap though.

Just out of interest, where do you come up with such weird requirements?
Some background info would help e.g. supply impedance etc.

Chris

Re: Voltage switch problem, most optimum solution ?

says...

Interesting, I didnt know that, do they have a small relay to make
it cheap, cause any semi to handle that isnt reliable if its cheap,
unless its low down on the failure/repair curve etc...

Now thats a good idea, the last thing I thought of using there was an
inductor but it does make a lot of sense. The 500v pulse is only for roughly
10 microseconds so that makes it a pretty viable idea, thanks :)

The last one is providing some visual aid to the hearing impaired where
a single row of switches runs an outside light and a couple of fans, they
dont hear the fans and they can't always notice the outside light, having
an inside flashing led on the switch cluster derived from an 'OR' was ideal
as a reminder that something on the cluster was left on...

This one is for a vehicle ignition system, be able to supply some power
to some logic but make it plug compatible so installer doesnt have to run
an extra cable for 12v power, get it via the coil return etc

The 500v pulse is back emf from the coil, so I would guess fairly low
in that the primary impedance is low, I dont want to snub the pulse
if at all possible as it would then reduce the striking voltage to
overcome the spark gap on the sec. The LC seems pretty much the perfect
solution, the output of the LC would be seriously damped when the pulse
arrives as the rise time is fairly short on the 500v pulse and yet have
low impedance to get enough power inbetween pulses, prob wouldnt need an
SMPS as a resistor, clamping zener and cap would be plenty for the amount
of power I would like to supply which is only about 50 or so mA.

Nice one, I should have thought of that <guh>, where would I be without
this newsgroup or your input  :)

Hey CHris, you didnt go to WAIT in WA some many moons ago did you ?

--
Regards
Mike
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Re: Voltage switch problem, most optimum solution ?

No there is nothing mechanical in there except the shaving part.  It charges
a single NiMH cell, but the SMPS can also just about run the motor by
itself when the battery is flat.  I was impressed by the voltage range.

The thing I worry about is that you will need a high value of inductance
combined with the ability to pass a reasonable amount of current without
saturating the core, and also fairly low capacitance would help, I suspect,
since otherwise it will affect the ignition pulses.  If you could get the
current consumption of your load down then that would make everything a lot
easier.  I would consider using the primary of a small mains transformer as
an inductor, but the current would probably saturate it.

There might be an alternative solution with a high voltage MOSFET or TV line
output transistor somehow, but if it were me, I would be trying very hard
to run an extra wire to the normal battery voltage.

No never been to WA. Different Chris I suppose.

Chris

Re: Voltage switch problem, most optimum solution ?

says...

Yeah me too, fascinating, have speculated how to do that, can run a cmos
pwm timer off a few microamps, then just need a hivolt mosfet as regulator,
but heck it would have a really wide range pwm, pfm or ppm drive...

Prob not that much an issue re saturation, afterall the 500v is fairly short
and even if the core does saturate there will be a series resistor, just need
to go through a few iterations. ie Some of the current not snubbed by the
coil will be handled by a series resistor and some dumped into a zener, the
trick will be to get the tree balanced in terms of dissipation whilst allowing
enough current when the pulses arent present, will get to this during week,
should be an interesting exercise :)

I was going to use an MJE13007 as series current regulator, but the LC
instead of those active devices would be heaps more reliable and more
interesting, thanks

no problem, he had identical name and was heavily into electronics, was
a member of Salvation army and knew an old friend Phil - be about 25years
ago now <sigh>

--
Regards
Mike
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Re: Voltage switch problem, most optimum solution ?

1. Are you sure the 500V is a low-impedance source? - EG Put 50 Ohm across
it, and see if it stays at 500V (or smoke for R <1W Diss.)

2. My first thought was a > 50V zener, or other similar fast switch turning
OFF a series switch to a regulator.
There are lots of alternatives.. like having the ">50V ? Detector"
resistor or a 100uF capacitor, (that includes a parallel resistor).
This is essentially an active, non-linear  filter to attempt prenveting
spike reaching low-voltage part of supply.

3. As previously suggested, a passive L-C at front would do at lot of good