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Ebay Chinesium. You get what you pay for. At best.

nomadradio

Analog Retentive
Apr 3, 2005
6,935
11,055
698
Louisville, KY
www.nomadradio.com
The old adage "You gets whats ya pays for" never seems to be wrong.

This project is barely begun. It should result in a drop-in replacement for the RCI2995DX "firestarter" amplifier board that uses four IRF520 switchmode MOSFETs as a 100-Watt linear amplifier.

The plan is to use a single 50-Volt RF-rated MOSFET made by NXP called the "MRF101".

But the eval kit for that part still has not arrived.

What did arrive today are a couple of candidate voltage step-up inverters I'll need to get 50 Volts DC in a 13.8-Volt base radio.

Here's the pic for a 600-Watt step-up inverter from the Chinabay listing.

6b3qA5.jpg


And here's what they sent.

gQvmhc.jpg


If I wanted to be a jerk, I could probably get it free by invoking Fleabay's "As Pictured" rule. But I was pretty sure I would need to hack this thing anyway, so I really don't care.

The sharp-eyed reader may spot a small assembly crock in the one they sent me.
Like this:

WLJXUY.jpg


Looks like this from the side.

GG5a9k.jpg


Sure enough, the Sil-pad stays where it was when the pcb is unbolted from the heat sink.

wHaO3w.jpg


A good thing I didn't plan to just hook it up and try with a dummy load right away.

The part that was bolted down is a full-wave rectifier, containing two diodes.

5lJPUz.jpg


The one that wasn't bolted down is the switchmode MOSFET.

1MwjfZ.jpg


Seems like a bad idea. Fortunately I have the hardware to finish building this unit before I power it up.

I did not get a good pic of the markings on the two tiny surface-mount integrated circuits. But for those playing along at home, the controller chip is type UC3843 current-mode PWM controller made by ST Micro.

Found it at alldatasheet.com.

The other chip is a generic LM358 dual opamp.

First task, if it survives a dummy-load test, is to devise a shutdown input. I don't want this thing running while the radio is receiving. A switchmode supply on an open unshielded board like this tends to radiate a lot of unwanted RF. If it only runs while the mike is keyed, this won't matter a lot. But this chip doesn't sport that feature. Gotta be a way to implement a shutdown input, though. Might last longer if the linear's power is only applied after the relay has time to change from receive to transmit.

The listing describes this toy as a 600-Watt step-up inverter. With any luck it's big enough to run a single 100-Watt RF power transistor and be comfortable.

Oh, and before you ask, we're doing the final test on a batch of three dozen Peel-'N-Stick variable modules. They should be packed up and ready to ship this week. Won't sell them without first doing a functional test.

73
 
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You will not see 600 watts of power from this unit running it from 12 Vdc. The conversion efficiency is not there. I know because I bought two of those to run my 750 watt Television Transmitter module I bought for six meters.
It was designed to be more of a regulator than power supply.
 
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That's kinda how I had it figured. The wattage they quote is for the highest-possible input voltage and the lowest-possible output voltage. The bigger the spread, the lower the efficiency, and less useful output.

Just figured a board that says "600 Watt" might cut the mustard to supply a single 100-Watt RF final.

Then again, I bought a couple more described as "1200 Watt". they are larger, but if this one won't do it, those certainly should.

73
 
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If you change the inductor you could improve it some. Make it more of a step up transformer and you will get more volts but it will still be anemic in the amps.
I have not tried it but it may be possible to parallel them.
 
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I finally had a look at the fine print. The upper limit for this kind of converter is the input-side current. For this one, that's 10 Amps. If you feed 60 Volts into it, there's your 600 Watts.

Feed 14 Volts into it and you can pull 140 Watts from it. Kinda marginal for a final stage that should show a 100-Watt (or so) carrier from a whistle on sideband.

Got a couple more of these, rated at "1200 Watts". Same fine print, but the input-current limit for this one is 20 Amps. At 14 Volts, that's 280 Watts More than enough for this project.

Saw that the credit-card charge has gone through for the MRF101AN eval kit. With any luck it will show up before long.

FIlm at 11.

73
 
That is the amp. Short of chasing grains of sand around on a postage stamp with a soldering iron.

That is the single ended kit from NXP. You get two MRF101AN , the board, and the aluminum block. $50.

Nomad and I talk once in a while.
We both thought the brag sheet for the MRF101 deserved a good look even though it needs a 50v supply. Then it became sorting through Chinabay for a boost converter that could supply 8-10 amps @ 50v without catching fire. This wouldn't be necessary for a base. Just get a 50v brick.

My #1 kit is getting put through its paces at the family lab. Kit #2 is here for drive level testing and mobile mock up.

The current issues are getting the boost converter to turn on and off without significant delay. Take my word for it , you don't want it running while receiving. The MRF101 isn't completely linear . Temperature compensating bias needs a few tweaks.

Download the info from NXP and follow along.
 
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My half-defrosted idea to use a 50-Volt RF transistor in a 13.8-Volt radio calls for a boost converter. Might be a better way to come up that 50 Volts DC, but this seemed the simplest.

Just one drawback. That boost converter will generate RF interference to the radio's receiver.

Unless it's shielded and strongly filtered.

Or simply shut down in receive. Still working on the best way to do this. This seems like the simpler solution, compared to shielding and filtering.

Like I said. Half-defrosted, not nearly half-baked.

Yet.

73
 

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