# Voltage switch problem, most optimum solution ?

• posted

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 , 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 cause I have exhausted mine (so far, afaik) :(

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Regards
Mike```
• posted

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

• posted

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 , 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 ?

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Regards
Mike```
• posted

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

• posted

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

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Regards
Mike```
• posted
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 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" additionally loading the 500V spike to ground, through, EG a low value 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.

1. As previously suggested, a passive L-C at front would do at lot of good to start with.

Dave Merrett

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