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Old 10th Jul 2023, 8:10 am   #1
Keith956
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Default 90 VDC radio HT supply.

I've been working on a 90V supply for battery powered radios for a little while, inspired by work at https://www.dos4ever.com/battery/battery.html. The original idea was to use a PIC to generate non-overlapping signals for a half bridge converter, running at the low frequency of 100Hz so as not to generate interference on MW and LW.

The original had the nice feature that it sensed when the radio was turned on or off, and only ran the inverter when on, saving battery life and eliminating the need for a second on/off switch.

It's powered by 6 1.2v lithium cells providing 7.2v and a 3.6v centre tap for the PIC. Current draw when the radio is off is a mere 7uA as the PIC goes into a deep sleep mode. When the radio is turned on, the current pulse that occurs by the 7.2v charging the radio's smoothing cap is enough to trigger an interrupt to awaken the PIC, and start running two non-overlapping PWM signals that drive the mosfet based inverter. Similarly when the radio is turned off the PIC goes back to sleep; a pulldown transistor discharges the output so the smoothing caps are not left charged to 90v.

In addition, the output voltage is sensed and compared to an internal reference, with the PWM duty cycle being adjusted to regulate the output over battery voltages of 7.2v down to around 4.5v.

If anyone's interested I could package up the schematic/gerbers and C code for the PIC.
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Old 10th Jul 2023, 9:49 am   #2
Wellington
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Thumbs up Re: 90 VDC radio HT supply.

I'm far from an expert in these matters, but that seems a most interesting and elegant approach. Well done
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Old 10th Jul 2023, 10:20 am   #3
Lloyd 1985
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Default Re: 90 VDC radio HT supply.

I like that, I have several battery valve radios that I could build these for! I have built some rather crude inverters before that seem to work ok, and I trigger them from a reed relay on the LT line. With mine you can still hear a bit of rumbling noise when tuned to a quiet bit of MW.

Regards
Lloyd
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Old 10th Jul 2023, 6:15 pm   #4
Chris55000
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Default Re: 90 VDC radio HT supply.

Yes please Keith!

My friend has a few valve battery portables and we could use the plans!

Chris Williams
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Old 10th Jul 2023, 9:23 pm   #5
Keith956
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Default Re: 90 VDC radio HT supply.

Chris, I'll try and package up the design files in the next few days.

Keith
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Old 11th Jul 2023, 7:13 am   #6
Keith956
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Default Re: 90 VDC radio HT supply.

The design data is available for download from :

https://www.peardrop.co.uk/downloadf...adioSupply.zip

If you want a PCB, I can get a batch made up at JLCPCB, they will work out as £1 each plus P&P.

- Keith
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Old 11th Jul 2023, 11:29 am   #7
kalee20
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Default Re: 90 VDC radio HT supply.

The discharge transistor is a nice touch, and avoids sparking at the radio's off/on switch if switched off and then back on a moment later.

The PWM - I'm not saying it won't work, I'm sure it does - but with peak-rectifying secondary, I'll bet that efficiency is rather low at high input voltages where duty cycle will be wound back, and the circuit is relying on transformer resistances and smaller charging intervals to reduce output voltage. Have you done any measurements? I'd be interested to see them!
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Old 11th Jul 2023, 11:43 am   #8
Keith956
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Default Re: 90 VDC radio HT supply.

Yes the efficiency is nowhere near a good switchmode supply, about 50-60% at best. Much of that is losses in the transformer it seems, which is just a common or garden 240v to 9+9v, 5VA one.

The duty cycle is around 25% for a 7.2v supply, increasing up to the maximum 50% as the supply voltage drops at around 4.5v.
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Old 11th Jul 2023, 8:09 pm   #9
Ed_Dinning
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Default Re: 90 VDC radio HT supply.

To Tick a few efficiency boxes and aid recycling, there was a design published in one of the magazines a few years ago that used the multi winding mains transformers that were common in VCR's. They also recycled these transformers.

Somewhat heavier and not so elegant as your design

Ed
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