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Old 17th Sep 2015, 5:09 pm   #101
BottleMan
Tetrode
 
Join Date: Mar 2015
Location: Ashford, Kent, UK.
Posts: 93
Default Re: Solid State Vibrator.

Quote:
Originally Posted by davegsm82 View Post
May I also suggest something? the SG3525 and many other clones (KA3525) etc are fully controllable half bridge PWM drivers, usually used in Car audio amplifiers. Not fixed at 50% duty so if your system isn't resonant then you can make substantial savings on primary current.
I am planning to use a spare 3525A to do this job. If you read the original Mallory book on vibrators you will realise that there needs to be some dead-time for it to run efficiently and reliably. During the dead-time the capacitor(s) across the transformer windings are used to (resonate) reverse the voltage so that it is ideally close to zero when the opposite contact makes. Some of you might have come across the term "zero-voltage switching" associated with switched-mode power supplies, used to raise the efficiency. The capacitor will, of course, limit the rate of rise of the voltage across the contact by diverting the current. So the capacitor has two functions. Mallory stipulate the need for dead time as a significant contributor to reliability.
Finally, they mention "soft start". It is possible for the transformer magnetic circuit to have significant remanence and if you start the converter up with a 50/50 duty cycle straight away it's possible for it to saturate and draw BIG currents. Mallory vibrators were designed to start from a low duty cycle to avoid this.
So, I'll use a nice cheap 3525 with inbuilt soft-start and duty-cycle limiting to drive a pair of MOSFETs. Simple.

Graham
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Old 22nd Sep 2015, 8:39 pm   #102
BottleMan
Tetrode
 
Join Date: Mar 2015
Location: Ashford, Kent, UK.
Posts: 93
Default Re: Solid State Vibrator.

More -

I've made it and it fits inside the original can on a piece of Veroboard. It's designed for a positive earth system so uses P-channel MOSFETs. Good h.t. in the radio (220V), fairly quite and runs very cool. Only problem now is to get the front end working properly.

Graham
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