The high value of capacitance is required for dynamic regulation: you have
jumps in the current from about 2A quiescent to about 10 or 12 Amp at full
output. Remember, the output impedance is quite close to 50 ohms at 320 V.
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Vic K2VCO
Sent: Wednesday, June 05, 2013 7:58 PM
To: amps@contesting.com
Subject: Re: [Amps] Advice needed for SS amp
Well, if nothing else, this project proves that you have guts (I used a
different word, but remembered that this list is moderated)!
Why such a large capacitor for the power supply? Surely 680 uf, one tenth of
what you plan to use, would be more than enough, even given the low
impedance of the load. The extra capacitance just makes the PS more
dangerous in the event of a short or arc, and will take longer to discharge
when you turn it off.
You might need to take some extra precautions to protect your diodes from
transients if the PS is connected directly to the line.
On 6/5/2013 9:07 AM, Bill Turner wrote:
> I'm thinking of building a solid state 1500 watt amp using some of
> MicroSemi's high voltage MOSFETs. This is a new kind of project for me
> and I would appreciate any comments you might have which would help me
> avoid any blunders.
>
> The MicroSemi ARF1500 looks attractive because of its high power
> capabilities and relatively low price. Digi-Key carries these for $205
each.
> A single one is rated at 750 watts output so I would use two of them.
> I'm thinking of water cooling.
>
> MicroSemi data sheet is at
> http://www.microsemi.com/existing-parts/parts/81770 and there are
> other versions with similar ratings.
>
> Notice the unusually high power supply voltage. I'm thinking of
> running a full wave bridge rectifier directly off the 240 VAC line,
> thus eliminating the power transformer and giving about 340 VDC
> no-load. The amp would have to be isolated from ground of course, with
> input and output coupling via toroid transformers and PTT switching done
by a well-insulated relay.
>
> A set of four 10 amp 600 PIV diodes is available on eBay for less than
> $30 and a 6800 uF 450 VDC capacitor is available for about $50 including
> shipping. There's most of your power supply. Pretty cheap, huh? :-)
>
> Of course with that size cap a step-start circuit would be mandatory
> lest you blow your main house breaker when you power up.
>
> My main use would be RTTY and CW so linearity is not an issue, but do
> you have any thoughts on what the IMD might be for SSB? I can't find
> any data on MicroSemi's website for that. Maybe it's no good for SSB.
>
> Since the output impedance is about 50 ohms, I'm wondering if a 1:1
> broadband antenna balun might serve for the output toroid? Just a thought.
> Might not be a good idea.
>
> Like I said, I'm a complete newbie at solid state amps so all comments
> are welcome. This almost seems to be too easy to be true. Go ahead, burst
my
> bubble. :-)
>
> 73, Bill W6WRT
> _______________________________________________
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> Amps@contesting.com
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--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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