compact HT generator idea
 

Today i took a little trip to the local 99p store to buy some of those useless door alarms. They are magnetically activated beepers that go on your door and suppoedly squeal when the door is opened and the magnet moves away. Although most of the time they just squeal all the time even when the magnet is by them.

But...i just seem them as a cheap source of piezo drivers. Where else can you get 3 piezo drivers for 99p?

Today however i bothered to look at the circuitry instead of just rippign it out. It turns out that its a miniture HV generator to power the piezo driver.

Since piezo drivers are high impeadence i guess it makes more sence to run them at high voltage.

So i connected a scope to the piezo driver. And...what do you know...it was hitting 70v!

It seems that the little IC in the alarm produces a 4KHz pulse which is FM modulated to about 2Hz or so to give the distinctive alarm sound. This is then stepped up to about 70V (loaded by my scope and piezo driver) by an auto transformer.

The entire device is really small.

I was thinking that if the frequancy modulation is removed (or new circuitry is built) the auto transformer may be able to produce about 50-60v with enough current to run a small valve device like a single pentode regen.

With the currents involved i'm even sure that a 555 could drive the auto transformer directly, or if not than a bc108 or 2n2222 will ahve no probelm at all.

The device now runs at 4KHz but i think i will play about with a 555 and see what frequancy it works best at.

An interasting idea here. I'm pretty sure the auto transformer has enough current capability to run a DF/DL96 or simular as a regen. Maybe even a ''real'' pentode like a EF91/95. However, if a DL96 is used than it may even produce enough signal to use a xtal earpiece.

Again the entire device can be made REALLY small and run off a 9v battery, using hybrid construction (solid state HT psu, solid state LT psu using LM317, and hollow state detector).

I guess the question is why not just use a transistor regen. I'm afraid i cannot answer that question :).

Re: compact HT generator idea
 

Hi Adi,
Add a diode and a smoothing cap, then try putting a load on it. If it could be run off 1.5V and managed about 10 mA at 60-70V it could find a home in some small valve portables. I reckon it would need some good screening as well though.
Mike.

Re: compact HT generator idea
 

Hi Gents, that sounds like a reasonable idea, keeping the frequency low should avoid the usual harmonics getting into the signal stages and causing feed back, but for any power to be developed it will need largish components.

A simple watts calculation will show that 70V at 10mA is 0.7 W.
Without any inefficiencies this equates to 0.5 A at 1.5V.

The other approach is to go up to 2MHz plus and the parts can be very small; If the set is typical MW/LW it will not pick anything up. Half decent screening should then stop much radiation to SW sets

Re: compact HT generator idea
 

That sounds like a good idea.

I was thinking of running it on 9V. The current consumption of a typical DL radios heaters is pretty small, and i think even the small TO-92 version of the LM317 wouldnt struggle. A pp9 battery should power it for a reasonable time too.

The alarm thing originally run on 4.5V from 3 1.5v button cells, from which it developed up to 70v.

Is it possible to rig up some kind of simple transistor oscilaltor that would use the primery of the auto transformer as its tuned circuit? That would keep everythgin simple and not use alot of pats.

Re: compact HT generator idea
 

Hi Adi, there are several simple transistor oscillator circuits with feed back to the base via a seperate winding. This may well drift a lot with battery voltage and temp. It probably wouldn't matter in your circuit.
A 555 would also work as would several of the C Mos chips when biased near their mid points.

Ed

Re: compact HT generator idea
 

Hi, Some years ago I experimented using a 555 to drive a transistor/ferrite core combination and after rectification and smoothing, I attached a resistor chain with a pot at the 0V end. The wiper of the pot I took to the reset pin of the 555 and was able to make it modify its pulse width to give some form of stabilisation. I did not progress any further due to lack of spare time but as far as I am aware it was (at least then) a novel way of obtaining a steady DC.
I have not seen it since in any circuit, maybe the control range is too shallow.
A scope showed the pulse width extend when a load was applied, I may have another go at this sometime to see if it could be used to give a stable 90V for battery radios etc.
Les

Re: compact HT generator idea
 

I have tried various experiments with these over the years and I found that as Ed says, to get any useable current at high voltages, the current drain at say 1.5V or even 9V can be quite high and require quite high capacity batteries, due to the I=W*V rule. Quite good fun to experiment with though. You'd be surprised at the emfs a little transformer can generate. I think there are dedicated step-up switching chips to try as well, but if I remember correctly, circuit layout can be quite critical to ensure max efficiency, particularly with the higher frequencies, and low primary voltages. Don't let this put you off though.

Biggles

Re: compact HT generator idea
 

I assume that should read I=W÷V.

Re: compact HT generator idea
 

You're right Dave, it should be. I should pay more attention. I think I originally meant to write W=I*V.

Biggles.

Re: compact HT generator idea
 

Hi Gents, try a Google for "boost convertors". There are plenty of chips around for SMPS. These usuallly require a torroid as the boost inductor. There are a few that charge caps in series and then connect them in parallel. This type is usually only good for about a 6* boost.

Ed