Adapting a Computer PSU

[Crackle]

I remembered I had drawn a graph of power vs supply voltage for the linear amp some time ago .

   

Mike

[ppppenguin]

From 12V to 13.8V (the usual voltage for CB type PSUs) that's a 1dB difference in power.

In radio communication you can get improved results by increasing TX power, improving the antennas and better RX s/n ratio. Increasing the TX power is sometimes called "gold plated decibels". In high power TX systems each decibel costs a lot in terms of the electric bill.

[Crackle]

Jeffrey
I get the feeling you do not think it is a worth while exercise, but happen to think it is.

After all saving old stuff from the scrap heap is what many of us on here believe in, and there are thousands of these PSU's being scrapped every day.
How nice it would be to save some old PSU's and help prevent some more PSUs from entering the market.

Mike

[ppppenguin]

I think you may have misunderstood my reasoning. I'm suggesting that you use that PSU at 12V and it will work OK. With just 1dB loss of output.

[Crackle]

The way you put it, it does not sound much but it would be 10 watts less, and I am not happy to compromise.
Of course it can be used, that is blindingly obvious, but you appear to be saying the project is not worth while, which annoys me as I have now made the case to house the PSU, bought the meter and terminals and aluminium sheet for the front panel and I would like if at all possible to see it through with help from Mark.
If Mark cant help me I am sure he will soon let me know, and the idea of saving another scrap PSU can be scrapped.

Mike

[ppppenguin]

The project is entirely worthwhile. I'm saying it's worthwhile even at 12V. One day you may be able to get 14V from the PSU but in the meantime it will work at 12V with little loss of performance. 10W sounds like a lot but the point I'm making is that in this context it's very little.

That's the objective stuff, a 1dB difference, but I'm not sure how you can subjectively test the difference between 12V and 14V since it's the guy at the receiving end who will hear any difference. Perhaps get a 2V lead-acid cell and arrange a switch to bring it in and out of circuit in series with the 12V PSU. Then ask the guy at the receiving end if he can hear any difference.

[Crackle]

One other thing that is not being considered here is that I had also wanted to be able to have a high current supply that would go down in voltage to say 9 volts or even maybe 6 volts and below.
Mike

[ppppenguin]

Is that for a different application? If so, what capability do you need? There's a big difference between a 3A supply and a 10A supply. It's always easier to help if we know what you want to do with the supply.

A PC PSU is unlikely to be able to cover that sort of range as a variable supply.

[Crackle]

I thought I had buggered the power supply by trying to see if I could slow the fan down with a resistor in the supply.
The PSU would not turn on after a while of trying different combinations of the 4 wires to the fan.
It was resolved when I shorted the power supply output, it must have reset the controllers.
It seems to be working OK again now, with the fan still blowing at full belt, and very noisy.

Mike

[Crackle]

A little update on the PSU.
I have been using it for some time now and it operates very well, no sign of any noise on the output, well not any that interferes with what it is currently running.
I finally found that a 47 ohm resistor in series with the black wire to the fan reduced the mechanical noise from a hurricane to a purr, much nicer now.

The good news is with a little tweaking of the antenna, low pass filter, new connecting cables and a tweak of the transceiver, I have now got the little linear amp up to over 80 watts output. It is rated at 70max so I have also taped a small 12v fan to it to cool it down when it gets a lot of use. I  am very pleased with the power output of the linear, and it makes the need to increase the voltage to 14 volts less urgent, as I dont think the RF power transistor would take it anyway at this power level. The power is confirmed on my MFJ antenna matcher, (switched to through) and also on the meter I fitted to the new PSU. Which shows there are very little in the way of losses in the linear amp.

I have fitted some (very poor quality) 4mm binding posts and a couple of expensive good quality German ones for high current loads.

It would still be nice to be able to vary the voltage but not so essential now. I have left a space on the left of the front panel for a pot should I need to fit one in the future, pending Mark being able to suss it out.
   

The red LED is a reminder that the supply is connected to the mains (standby supply) and the green one shows its on but it is a little superfluous as the backlight on the meter is enough.

Mike