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Old 11th August 2007, 03:33 PM   #1
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Location: CT
Default max rail voltage?

Hello,
I just finished an LM3875 Gainclone kit . Before taking it off the variac, I looked up the data sheet to check max supply voltages(+/-40V). My rails are running at -/+39.5V.Each amp will drive a Fostex FE103, rated at 8 ohms. Is this cutting it too close to the maximum, or is there enough wiggle room for +/-39-40V to safely be used?
TIA
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Old 11th August 2007, 04:01 PM   #2
AndrewT is online now AndrewT  Scotland
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Hi,
what was the primary input voltage when you measured that rectified of +-39.5Vdc?
What is the tolerance of your mains supply?
How many amps are connected to the PSU?
What if only one were running, because you disconnected the others or the fuses blew?
How high could an operating amp run when mains is at it's highest?

Now tell us the data and we can advise.
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Old 11th August 2007, 05:24 PM   #3
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OK, I'll try and answer your questions-
When I measured that 39.5Vdc, the PSU wasn't connected to the amp. With all connected (amp operating) 120Vac in = +/-38Vdc. In order to get to 39.5Vdc, I had to go to approx 125Vac.
I don't know the mains tolerance, but it normally never gets much above 120V.
There will be 2- LM3875s connected to the PSU.
I don't understand your next question
I've never known the mains voltage to exceed 121V, but considering transients etc, who knows.
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Old 11th August 2007, 08:29 PM   #4
AndrewT is online now AndrewT  Scotland
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Hi,
if your nominal mains voltage is 120Vac and you get 39.5Vdc from 125Vac, then if mains runs at +4.2% you are at your 39.5Vdc value. I would have no problem with this as the design voltage.
But, +4.2% is a tight tolerance.

In the UK we have a silly system to make it look as though we and the Continent are on the same voltage and they did this by widening the tolerances and not by changing the supply voltage. With these new wider tolerances our nominal 230Vac is actually between the limits of 216Vac to 254Vac (it's the former 240+6%). I design for 254Vac to ensure nothing explodes or burns out if I happen to be operating during times of low demand when voltages tend to be at their highest.

If your maximum from your supplier is 121Vac, then you have already identified the maximum operating voltage of the PSU.
This becomes the value you use to determine dissipated power during quiescent and power delivery conditions.
You use this to determine the heatsink size. I suggest a lot more than National quote in their table. Try for 1.5times. If you were working to nominal mains voltage I would double the heatsink compared to the Minimum from the National table.
You decide on your operating conditions and design from there.
If you can give a more definitive answer we can get closer than all this conjecture.
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Old 11th August 2007, 09:44 PM   #5
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Thank you very much for your help. Guess I'll give it a go. I used some heat sinks I had on hand. They're not huge and I have no specs on them, but I thought seeing as the amps will be driving FE103's rolled off at 12db/oct below 85Hz, the demands shouldn't be great. By the way, if the rail voltages are a bit high, could you not put a 2-4ohm resistor in series with the driver? The max voltage seems to be impedance dependant.
thanks again
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