Toroid PSU voltage?

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sinus trouble
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Toroid PSU voltage?

Post by sinus trouble » Mon Jul 23, 2018 4:21 pm

Greetings fellow Necks! :)
I have this problem that's been racking my brain for quite some time!
Just for experimental purposes, A 100w (@ 28v) amplifier is the test load! (50 to 60w would be plenty for this test)

The first test was using a 15vac 120va toroid, rectified and followed by a 22000uf 63v cap!
@ no load, I was getting a nice 24v
As I crank up the drive to the amplifier, Current steadily increased and voltage dropped to around 20v
All good and no sign of hums and buzz lol

The second test which I am scared to carry out would involve a 25vac 225va toroid, rectified and followed by a 22000uf 63v cap! However @ no load, This produces around 36v!
Although I know this voltage will decrease with load, Do I risk knackering the amplifier? lol!!
I am as stupid as I look! :|

Albert H
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Re: Toroid PSU voltage?

Post by Albert H » Mon Jul 23, 2018 8:22 pm

It depends on the breakdown voltage of the device in the PA. I used to use 7805 regulator ICs for the FET bias supply, derived from the 28V rail. If the supply voltage went much above 30V, the regulator would die smokily, usually failing badly so as to put the >30V straight on to the gate of the FET, destroying it instantly. I had this problem on a site in Eastern Europe, where the mains Voltage (nominally 220V AC) could go as high as 255V during periods of low load!.

The real solution is to use a regulated supply. This necessitates big power transistors in the supply lines to your PA, but the upside is that you can use the regulator transistor to turn the supply to the PA off if the SWR goes nasty or if the link is lost (or you deliberately want to remotely shut down the rig!). My FET PAs used to have supply regulators using either 2N3055 or 2N3773 transistors. I rated the 3055 at about 5 Amps maximum each device. The 3773 is much more expensive, but can control about 9 Amps without stress. These days I use big low frequency FETs for the regulator, as they're pretty efficient and don't get as hot as the old fashioned transistors.

Another approach I've used to power higher Voltage FETs is to use a step-up switched-mode module. There are some cheap Chinese ones around that really work well, give clean DC and quite a lot of power with little heat.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Toroid PSU voltage?

Post by sinus trouble » Mon Jul 23, 2018 9:38 pm

Cheers Albert! :)
I totally overlooked the possible effects of mains voltage fluctuations until you mentioned it!
Just for clarification, The amplifier is one of those notorious Chinese ones with an MRF186 lol
MRF186.PNG
I think it makes more sense now? Technically if the device is out of conduction, it should withstand overvoltage as long its not ridiculously high? In theory that's great! But not in a practical sense lol!

I dunno if you recall? A while back I made a basic regulator which got so hot that it burnt me lol!!
Anyways I did manage to source a nice heatsink for it!
After my injuries, I never did do anymore testing lol!!
I shall dig it out and post some more pics soon! :)
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I am as stupid as I look! :|

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Re: Toroid PSU voltage?

Post by sinus trouble » Mon Jul 23, 2018 9:57 pm

Albert H wrote:
Mon Jul 23, 2018 8:22 pm
Another approach I've used to power higher Voltage FETs is to use a step-up switched-mode module. There are some cheap Chinese ones around that really work well, give clean DC and quite a lot of power with little heat.
Switched-mode technology these days is quite impressive! I just cant help going back to the times of inefficient heavy London pirate rigs!

I wish I could get hold of one! I wouldn't even use it! I would probably just stare at it or put it in a display cabinet!! :lol:
I am as stupid as I look! :|

Albert H
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Re: Toroid PSU voltage?

Post by Albert H » Tue Jul 24, 2018 12:04 am

The heaviest rig I remember was just 120 Watts out (a lot for the early 80s), and used two BLY90s - seriously over-run - in the final! It was a 12V PA, so it pulled about 20 Amps, and wasted about half of the energy supplied as heat. There were two huge blowers - mains powered - and an enormous toroidal transformer. There were also four smoothing capacitors that were about the size of paint pots! The exciter was in a small diecast box, with another diecast box strapped to it containing the PLL circuit. The whole thing was in an ex-Army 6U 19" rack box, and took two of us to lift it!

The opposite of that was a rig thrown together to get a station on the air in a hurry from the "Norland" block at Shepherd's Bush. This was a frugal, barebones job, without even a PA box or heatsink! The PA was screwed with self-tapping screws to the underside of the cold water tank at the top of the block, and the water-tank provided the cooling. The Power supply was just screwed to a piece of wood, and consisted of a car battery charger transformer, a bridge rectifier (that was also screwed to the water tank) and a huge smoothing capacitor. The PSU gave about 15V - I'd tested the PA at home on 13.8V, so it needed a quick re-match up on the block. I remember lying in the filth under the water tank, tweaking the output stage for maximum power.... It did about 60 Watts, and in those days - with the quieter band and lower noise floor, it could be heard in much of London. The aerial was a super-J mounted to the communal TV aerial pole, and the link receive aerial was a Band IV Yagi pointed down at Sinclair Road.....

It lasted for two weekend broadcasts, then Brian Gotts sussed its location and went up there to remove it. He (or one of his minions) unscrewed the PA from under the water tank, and flooded the top three floors of the block!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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