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Author Topic: BLF278 for 2m amp  (Read 2724 times)
KB1GTX
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Posts: 459




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« on: November 30, 2012, 03:34:20 AM »

The question is how well regulated dose the power supply need to be to run a blf278 type amp on ssb 144 mhz.  I have a pair of transformers that when DC filtered are 26 volts each and will draw down to about 22 volts each at 20 amps. these are to be wired sires for 50 volts.

They're from regency 22 amp rack mount supplies that the regulators got really smoked by lightning.
« Last Edit: November 30, 2012, 04:21:43 AM by KB1GTX » Logged
N8WWM
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Posts: 26




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« Reply #1 on: November 30, 2012, 06:09:44 PM »

How much RF power output are BLF 278's rated for? To be safe I say one must have at least twice that rating at 13.8VDC. Of course 2 transistors means 2 times that wattage.

I am just making up a figure here, but say a transistor in AB service cranks out 80 watts of RF. With AB being about 45% efficient, figure you need 80 watts times about a little over 2.
Call it 2.25 to be safe. That means your transformer should be rated at about 180 watts at least and be of sufficient voltage to run the amplifier. Even at 2.25 times you may want it to be heftier a bit. You have a relay, bells, whistles, lights, etc. to run.
I think your transformer would be good, since then numbers add up to 440 watts.

As for regulating it...an unregulated supply with any real draw at all will make an SSB signal change frequency, literally go FM, with audio applied.

There are some very simple but very good regulator circuits using the old 78xx line of 3 terminal regulators. Give me a shout at my email as posted on QRZ and I'll be happy to help.

73 Doug

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KM3F
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Posts: 504




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« Reply #2 on: November 30, 2012, 06:59:43 PM »

From your info; load voltage at 22 volts times 2 =44 times 20 amps = 880 watts.
If the amplifier is 40% efficient that leaves 352 watts for RF out if the amp would do that much with the drive available..
It won't hurt the amplifier except the voltage variations will not be good for IMD etc.
Most radios normally do not run on 50 volts so you will be ok with the radio on it's own supply.
At 26 volts times 2 = 52 volts, you don't have a lot of room for regulation unless you are running the amplifier at less than 44 volts.
In order to have regulation, the supply voltage from the transformer and filters needs to be high enough to drop some across the regulator such that the load current never causes a voltage drop greater than the drop across the regulator circuit with some to spare.
Regulation down to about 40 volts will work but power from the amp will also be down.
Don't try to make it up by driving harder than the devices will handle.
Good luck.

Hello Doug, never heard back how your 6m amp worked out.
Hope everything stayed togather for you.
Happy Holidays.

Ken.
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KB1GMX
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Posts: 772




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« Reply #3 on: November 30, 2012, 07:44:00 PM »

>>How much RF power output are BLF 278's rated for? To be safe I say one must have at least twice that rating at 13.8VDC. Of course 2 transistors means 2 times that wattage. <<

The BLF278 is a dual LDMOS power mosfet rated 300W at 50V..


My call is an unregulated supply will  float to over 60V and drop lower than you may think
depending on the amount of filtering.  It may be putting the life of the device at risk as well.

IF the power supply only varied 5% it would be fine but that supply without regulation is likely
to see 10-15% voltage swings and possible hum at highest currents.  It would degrade the quality
of your signal.  Look at the transistor curves for differnt voltages and then assume as you pull
more power the voltage goes down.. as does the IMD.


Being as I've built the likely amp your doing for 2M using that device my feeling is that blowing
a 120$ transistor while trying to cheap out on the supply is poor economy.  Also with a good
48-50V supply the BLF278 will easily do more than 350W output and about 50% (45-55%) so
you need at least 700-800W supply that doesn't droop under load.


Allison
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KB1GTX
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Posts: 459




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« Reply #4 on: December 01, 2012, 03:10:47 AM »

The power supply is 52 volts no load and drops to 47 at about 8 amps, and i could help that with a few zener shunts.  It's just i've never played with high power fets and just wanted to know how unstable they would be with a little voltage flux.
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KB1GMX
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Posts: 772




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« Reply #5 on: December 01, 2012, 08:24:16 AM »

IF you can keep it in that range your ok.  You need to test to 12-14A as that device
when done correctly will want that (350 W output).

Also look at the amount of ripple.  That needs to be low or you will hear about
hum on your audio.

The popular supply is the 50V ones from HP blade servers.

Allison
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N8WWM
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Posts: 26




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« Reply #6 on: December 02, 2012, 11:32:42 AM »



KM3F Said: "Hello Doug, never heard back how your 6m amp worked out.
Hope everything stayed togather for you.
Happy Holidays."

Hi Ken. That thing is doing just peachy! I sure do appreciate your help with it. You had a few good ideas to deal with the board space issues that I would have just sat there going "duhhhhhh" over. Smiley

And a very happy holidays to you and your family as well.

Whipping up a 10 meter Moxon for the contest next weekend, kind of a present to myself.

73 Doug
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