Regulated 1.4v supply. - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

Biggles · 23rd Aug 2018, 11:36 pm

I found that driving a 6-0-6v mains transformer in reverse at 50Hz with a simple push-pull oscillator works fine and doesn't produce interference. I even got away with using BFY50's to drive the transformer. Probably not very efficient but it avoids all the hash. I have been through the same problems as others mention while experimenting with all sorts of step-up circuits, and finally went back to 50Hz. My circuit runs from two 6v SLA's. The HT runs from one, and the LT supply is from a separate isolated 6v SLA via a simple dropper resistor and diode limiter. So you need two batteries, but I found that running the HT and LT from one battery seemed to cross couple hum from the inverter, even though there was electrical isolation through the inverter transformer.
Alan.

Argus25 · 24th Aug 2018, 9:36 am

Regarding post #38 by Kalee20,

Those 9mm walls would have really helped.

I found it was not too difficult to filter the interference off the power in & out connections, but it was direct electromagnetic radiation from the supply that was difficult. Also I decided I wanted to house my circuit in a plastic container to keep the weight down.

The method I used to determine if the final design was ok was to use a transistor radio that tuned down to 150KHz, and placed the working circuit board directly on the radio's case to be sure there was no RF energy that could be detected. A ferrite rod based transistor radio makes a great RF sniffer tool.

It was also interesting that the base drive currents to the switching transistors, if increased above a certain threshold, resulted in the sudden generation of interference, even though for practical purposes the the collector-emitter voltages remained less than 400mV in both cases. Tuning the primary winding to control the switching transistion time also helped.

This suggests in the absensence of the required precautions I used for a switching design, or the massive thick case you used, for a bare PCB, probably a pure sine wave supply maybe at 50KHz to 100kHz would be the way to go and not worry too much about efficiency issues. This could be done with a fairly compact transformer and a push pull output stage (biased out of cross over distortion) and an OP amp sine wave generator & driver circuit.

Biggles · 24th Aug 2018, 5:00 pm

I wonder whether you could use one of those bridge amplifier audio chips to drive the transformer in push pull, and with a careful bit of feedback introduced turn it into a 100KHz oscillator all at the same time. The thing would probably refuse to oscillate when you actually want it to. They normally only do that when you don't want it to happen!
Alan.

Boater Sam · 24th Aug 2018, 5:34 pm

That's how this one works,
http://www.la6nca.net/homebrew/kurerpower/index.htm
using a stereo amp chip.

I've not given up yet, but short on time this week and next.
If I can sort some of the ripple on the output and screen the pcb properly it could be OK.
Sam.

23rd Aug 2018, 11:36 pm	#41
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Regulated 1.4v supply. I found that driving a 6-0-6v mains transformer in reverse at 50Hz with a simple push-pull oscillator works fine and doesn't produce interference. I even got away with using BFY50's to drive the transformer. Probably not very efficient but it avoids all the hash. I have been through the same problems as others mention while experimenting with all sorts of step-up circuits, and finally went back to 50Hz. My circuit runs from two 6v SLA's. The HT runs from one, and the LT supply is from a separate isolated 6v SLA via a simple dropper resistor and diode limiter. So you need two batteries, but I found that running the HT and LT from one battery seemed to cross couple hum from the inverter, even though there was electrical isolation through the inverter transformer. Alan.

24th Aug 2018, 9:36 am	#42
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Regulated 1.4v supply. Regarding post #38 by Kalee20, Those 9mm walls would have really helped. I found it was not too difficult to filter the interference off the power in & out connections, but it was direct electromagnetic radiation from the supply that was difficult. Also I decided I wanted to house my circuit in a plastic container to keep the weight down. The method I used to determine if the final design was ok was to use a transistor radio that tuned down to 150KHz, and placed the working circuit board directly on the radio's case to be sure there was no RF energy that could be detected. A ferrite rod based transistor radio makes a great RF sniffer tool. It was also interesting that the base drive currents to the switching transistors, if increased above a certain threshold, resulted in the sudden generation of interference, even though for practical purposes the the collector-emitter voltages remained less than 400mV in both cases. Tuning the primary winding to control the switching transistion time also helped. This suggests in the absensence of the required precautions I used for a switching design, or the massive thick case you used, for a bare PCB, probably a pure sine wave supply maybe at 50KHz to 100kHz would be the way to go and not worry too much about efficiency issues. This could be done with a fairly compact transformer and a push pull output stage (biased out of cross over distortion) and an OP amp sine wave generator & driver circuit.

24th Aug 2018, 5:00 pm	#43
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Regulated 1.4v supply. I wonder whether you could use one of those bridge amplifier audio chips to drive the transformer in push pull, and with a careful bit of feedback introduced turn it into a 100KHz oscillator all at the same time. The thing would probably refuse to oscillate when you actually want it to. They normally only do that when you don't want it to happen! Alan.

24th Aug 2018, 5:34 pm	#44
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Regulated 1.4v supply. That's how this one works, http://www.la6nca.net/homebrew/kurerpower/index.htm using a stereo amp chip. I've not given up yet, but short on time this week and next. If I can sort some of the ripple on the output and screen the pcb properly it could be OK. Sam.