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I have an FT-747GX radio that I am using as a RF generator for a plasma cleaning machine.

In the operating manual on page 3 it states:

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I have a power supply connected to this radio which outputs 12V at 30 amps. Between the power supply and the radio I have a 20 amp fuse, but this fuse keeps dying on me.

What I dont understand is: is the radio pulling more amps than what it says it outputs? If it so, should I increase the fuse to 30 amps?

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  • \$\begingroup\$ Has there been a reason why you've been giving it less voltage than what it needs? (legitimate question, not trying to be rude) \$\endgroup\$ Commented Jan 16, 2019 at 15:45
  • \$\begingroup\$ It was like this when I got here should I have a higher voltage power supply? \$\endgroup\$ Commented Jan 16, 2019 at 15:46
  • \$\begingroup\$ There could be a lot of factors that we don't know about that could cause your radio to draw more current than it should. I'd say that you should probably look at the circuit (i.e. the schematic if you have that available) before deciding to apply more voltage. When you say the fuse "keeps dying", is the fuse already blown or are you saying that you replace the fuse but every time you turn it on, it keeps dying? \$\endgroup\$ Commented Jan 16, 2019 at 15:54
  • \$\begingroup\$ @KingDuken The fuse runs hot after about 10 minutes of usage and then blows. would using a 24V 20 Amp power supply be ok? \$\endgroup\$ Commented Jan 16, 2019 at 15:59

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You're using a 12V supply on a system that needs 13.5V. It says 10% tolerance, but that is still 12.15V minimum input or 14.85 maximum input. Depending on your DC source you may be able to tweak that up a little bit.

Anyway, the given current consumption is for 13.5V. So 13.5V at 19A is roughly 256W. 256W at 12V is 21.3A. This means that for a 100W output you're drawing 21.3 A instead of 19A. So yes, you need a larger fuse or a higher voltage supply. Do not use the 24V supply though!

To answer your other question, yes. Power in is always greater than power out because of efficiency. Couldn't find a datasheet for the product you listed so I can't give an exact number.

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  • \$\begingroup\$ Would using a 24v 20amp power supply be sufficient? It would it be too much? \$\endgroup\$ Commented Jan 16, 2019 at 16:19
  • \$\begingroup\$ Without knowing the details of the radio you're using I can't tell you. I'd have to say no though. The 12V should be fine, 150 mV isn't that much... Just get like a 25A fuse instead. \$\endgroup\$ Commented Jan 16, 2019 at 16:20
  • \$\begingroup\$ To the OP: What size wire are you using between power supply and radio? Is the wire you are using now getting hot during operation? Fuses are there to protect wiring, so you need to make sure the wire diameter is adequate for the new fuse. If the wire you have now is not getting hot, then you probably don't need to worry about it. \$\endgroup\$ Commented Jan 16, 2019 at 17:03
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A 20A fuse on a rail that normally runs 19A is way too close, I would be thinking 30A fuse for this.

However using a radio as an exciter for plasma cleaning is very much the wrong tool for the job, they are seldom rated for continuous service, doubly so at full song, and the typical linear amplifier employed is only about 50% efficient, go class C in one of the ISM bands and you will typically get 80% drain efficiency and MUCH cooler running.

I would bet your radio is only thermally designed for maybe 30% duty cycle, and for SSB service at that.

Also, how does the load impedance change during operation, plasma dynamics normally causes the impedance to wonder all over the smith chart, and I would bet that is not good for the rig.

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