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Chris55000 · 13th Jul 2021, 9:41 pm

Hi!

I've certainly seen a two transformer arrangement in RSGB using two identical 12–0–12V 3A mains transformers – the first (power step–up) transformer is switched by 2 × 2N3055 power transistors, then the primary of the second feedback transformer connects to the ends of the stepup transformer secondary via a 1k5 11W resistor in each lead, with the 12–0–12V winding of the f/b transformer connects to the two bases, the centre–tap going to 12V positive via a 1k 5W resistor and 220 ohm to chassis for d.c. bias. The 2N3055 emitters were common and went to 12V negative via a current feedback resistor of about 0.47 ohm 5W

The article I found it in says the 1k5 11W resistors may have to be adjusted in value to get sustained oscillation without a very large primary current – my drawing software is playing up so I'll have to post it tomorrow!

Chris Williams

13th Jul 2021, 9:41 pm	#12
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,874	Re: HT for a car radio Hi! I've certainly seen a two transformer arrangement in RSGB using two identical 12–0–12V 3A mains transformers – the first (power step–up) transformer is switched by 2 × 2N3055 power transistors, then the primary of the second feedback transformer connects to the ends of the stepup transformer secondary via a 1k5 11W resistor in each lead, with the 12–0–12V winding of the f/b transformer connects to the two bases, the centre–tap going to 12V positive via a 1k 5W resistor and 220 ohm to chassis for d.c. bias. The 2N3055 emitters were common and went to 12V negative via a current feedback resistor of about 0.47 ohm 5W The article I found it in says the 1k5 11W resistors may have to be adjusted in value to get sustained oscillation without a very large primary current – my drawing software is playing up so I'll have to post it tomorrow! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!