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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th May 2019, 8:52 am   #21
M0FYA Andy
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Default Re: Power MOSFETs

David, it's an ongoing problem trying to find a good way of generating 1600Hz 80v power in a reliable way, and without costing an arm and a leg! I have some suitable aircraft alternators which I could drive with a mains motor (they require to run at 6000rpm), but it does need some fairly heavy engineering and will be noisy. The other problem is avoiding developing a power source becoming the major project in its own right, when the intention all along is to work on the vintage radar equipment!

Andy
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Old 12th May 2019, 9:23 am   #22
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Default Re: Power MOSFETs

I have a colossal PA amp rated for 3kW, with switching for bridging, it's just a little heavy, about 30kg
I can get another if needed
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Old 12th May 2019, 9:43 am   #23
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Julian,

Is that (or they) going spare?

Andy
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Old 12th May 2019, 9:53 am   #24
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Default Re: Power MOSFETs

Yes, sorry I sounded ambiguous. PM me to discuss
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Old 12th May 2019, 10:59 am   #25
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Default Re: Power MOSFETs

When transistors with a metal tab are screwed to a flat surface, especially if they have a sil-pad insulator, that deforms, the metal tab can warp in the zone around the screw head, so you only get good thermal contact there with a small surface area. This problem has become very bad with the new generation of 78xx (to-220) cased regulators from the far East where the metal tab has been thinned right down (presumably to save on copper).

There are two ways around it, one is not to fix the transistor by its mounting hole, and use a clamp instead over the whole body. The second is to abandon the soft grey sil-pad insulators and go back to old fashioned mica insulators and thermal grease which is very unfriendly for mass production, but fine for home projects. Of course the mating surface must be flat and uniform in all cases.

There are more tricks; the screws & nuts should always have spring washers and never under any circumstances use a nylon (plastic) screw and nut, though it might seem tempting from the electrical perspective, they stretch with heat and thermal contact is lost over time. They made this classic mistake in the power supply of the Processor Technology terminal computer.
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Old 12th May 2019, 11:26 am   #26
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Julian, I've sent a PM.
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Old 13th May 2019, 8:17 am   #27
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Quote:
Originally Posted by M0FYA Andy View Post
David, it's an ongoing problem trying to find a good way of generating 1600Hz 80v power in a reliable way, and without costing an arm and a leg! I have some suitable aircraft alternators which I could drive with a mains motor (they require to run at 6000rpm), but it does need some fairly heavy engineering and will be noisy. The other problem is avoiding developing a power source becoming the major project in its own right, when the intention all along is to work on the vintage radar equipment!
One possibility and it worked for me generating 400Hz power (three phase) for airborne equipment was to use a 750W rated VFD, with a home crafted filter on its output, which could easily be transformed up and down and it gives a good sine wave when filtered. Some VFD's may go to 1600Hz, I'm not sure about that. Here is the project which may give some ideas how to solve the problem. The VFD has all sorts of programmable features and these are helpful (not just for powering motors) but for other tasks as well:

http://worldphaco.com/uploads/3phase.pdf
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Old 13th May 2019, 9:18 am   #28
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I have looked at VFDs as a possibility, but drawn a blank at 1600Hz.
The highest frequency I could find was 400Hz (not surprising, as that is eight times 50Hz, and safely running a motor at eight times rated speed sounds optimistic to me), also they are software-controlled and use custom chips, so not amenable to modification?

Andy
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Old 13th May 2019, 9:47 am   #29
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Default Re: Power MOSFETs

Why not use a Class D design? That would have no problem producing power at 1600Hz with next to no power dissipation. For example https://www.diyclassd.com/product/ucd180hg/4 which does 180W into 4 ohms with very low distortion https://www.diyclassd.com/img/upload...cD180HG_R4.pdf

92% efficiency.

For £50 (equivalent). They have versions all the way to 700W for around £150.

Craig
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Old 13th May 2019, 10:38 am   #30
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Quote:
Why not use a Class D design? That would have no problem producing power at 1600Hz with next to no power dissipation. For example https://www.diyclassd.com/product/ucd180hg/4 which does 180W into 4 ohms with very low distortion
If they produce 180W into 4 ohms it suggests the equivalent rms voltage is only roughly about 26V. It could still work if the output voltage was stepped up with a suitable transformer.
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Old 13th May 2019, 10:47 am   #31
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Default Re: Power MOSFETs

Just noticed that David made the same point about Class D in post 20.

OK - what voltage/power do you need?

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Old 13th May 2019, 11:55 am   #32
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I pointed out in my article using the VFD, if an amplifier is used for the task (be it any class of amplifier A,B,D) it will be very important to not run it open ended, meaning from some uncontrolled amplitude sine wave signal source. It ideally would have feedback around it that samples the output voltage and gain levels it to some particular output voltage. Or it could be a recipe for a very nasty over-voltage event. A good way to do this is with a multiplier IC like an AD633 where a DC level into one port can easily control an AC level passing through it.
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Old 13th May 2019, 12:00 pm   #33
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Default Re: Power MOSFETs

I'm not sure I understand that. Any amplifier, and we are looking at an audio amplifier, is defined in terms of the maximum output power into a defined load in the range (broadly) 2 to 16 ohms.

So, what is it that you need in those rather straightforward terms? Then there are a bunch of us who can advise.

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Old 13th May 2019, 12:23 pm   #34
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Default Re: Power MOSFETs

Quote:
Originally Posted by M0FYA Andy View Post
Our application is a bit unusual, rather than audio use we want to drive it with a 1600Hz sinewave to produce power to operate vintage radar equipment.
What is loading the sinewave - from an electrical circuit and power transfer perspective?
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Old 13th May 2019, 12:33 pm   #35
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Default Re: Power MOSFETs

Avionics and Radar equipment he said, so it'll all be going into transformers and rectifiers, maybe a few fans and possibly synchros, possibly a motor for scanning a dish.

They probably need a sinewave, and the ability to handle erratic loads.

Avionics is my day-job. I'm just glad all my stuff is DC powered

David
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Old 13th May 2019, 2:27 pm   #36
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Default Re: Power MOSFETs

Quote:
Originally Posted by Argus25 View Post
(snip) if an amplifier is used for the task (be it any class of amplifier A,B,D) it will be very important to not run it open ended, meaning from some uncontrolled amplitude sine wave signal source. It ideally would have feedback around it that samples the output voltage and gain levels it to some particular output voltage. Or it could be a recipe for a very nasty over-voltage event...
I don't think a power engineer working at the CEGB would agree, they do not take feedback from customer loads, even the biggest loads, to control the network voltage

A supply network, like a power amplifier, uses internal feedback to maintain a constant OP voltage, which equates to minimum source impedance. The PA amp I have offered uses NFB to keep the OP voltage constant and determined only by the IP signal and thus to avoid "a nasty over-voltage event"
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Old 13th May 2019, 2:51 pm   #37
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Default Re: Power MOSFETs

What sort of power factor does the load present to the amplifier?

If it's a complex mixture of AC motors, fans, rectifiers etc., is there a possibility this is upsetting the audio amp in question?

Pete
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Old 13th May 2019, 3:51 pm   #38
M0FYA Andy
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OK, to go back to the beginning! The requirement is for a power source to power, on the bench, WW2-era aircraft radar equipment such as the H2S ground-mapping radar fitted to the heavy bombers. These systems operate on 80 volts single phase AC, at nominally 1600Hz frequency (and 24/28 volts DC), unlike later aircraft systems which work on 115 volts @ 400Hz, often three-phase.
On the aircraft this power would come from an engine-driven generator (alternator), so the frequency actually varied from, say, 1000Hz to 2400Hz. The output was maintained at 80v RMS by a carbon-pile regulator controlling the generator. Although nominally sinusoidal, I suspect it was a bit rough!
The load in the radar equipment is primarily 'mains' transformers providing the usual EHT, HT and heater supplies to the valve electronics, and often dedicated heater transformers.

One option to provide this supply is to use an 'audio' amplifier, whose input is a 1600Hz continuous sinewave, in much the same way as one would be used to drive a vibration table for environmental testing of equipment, hence this thread.

I hope this clarifies things, although I'm aware there are a lot of questions asked above!

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Old 13th May 2019, 8:27 pm   #39
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Default Re: Power MOSFETs

Hi Andy, a nice old vibration table valve amplifier? There are also modern versions that will go up to a KW.
80V could possibly be in their output voltage range.

Ed
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Old 13th May 2019, 9:12 pm   #40
M0FYA Andy
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Ed, there was an impressive pair of 1000 watt valve vibration table amplifiers for sale recently, each in a 19" rack about six feet tall! Unfortunately missing what I suspect were rather exotic milk-bottle-sized output valves.
However there is no way I could accommodate such beasts, unfortunately (or maybe fortunately)!

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