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Old 13th February 2006, 05:53 AM   #1
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Default 48v phantom / resistor is way hot !

Way...dude.

So I built this power supply:
http://members.aol.com/ecc81/ps48.html

...and R3 is getting too hot. Finger test: 1 to 2 second ....ouch.
I've bumped it up to a 5 watter....still hot.

Some possibilities :

*I replaced one 220uf with a 330uf because it's all I had.

*Transformer is a flatpack 40v 300ma.....but its output tested as 48v unloaded.

* Had to reduce that R3 from 2.7k to 820 ohms to get a 48v regulated reading (unloaded). (otherwise I got 60v with the 2.7k)

* Am using a 317 regulator, not the TL783.

* I added a 0.1u poly in paralel with the 10u electro at the output.

Nothing else is warming up, not even the trimmer following R3.
------------
How hot is too hot?

Should I remove a 220u (or that 33u) cap?

Should I be testing this under load? What could be used as a load?

(btw...LESSON from a newb:::heh heh, don't use bipolar caps in there...almost had another meltdown )

=RR=
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Old 13th February 2006, 06:06 AM   #2
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I tried placing a 10w 270 ohm plus a 3w 511 ohm in series (=781 ohms)....and the 10w gets a little warm, but the 511 3w still gets too hot.

=RR=
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Old 13th February 2006, 06:49 AM   #3
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Even if you applied the full 48V across R3 (2.7k), you would only get 0.85W dissipation. A 1W part here shoud be OK. It'll have a surface tenperature getting on for 100 degress C though.
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Old 13th February 2006, 06:52 AM   #4
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Ah! I've just seen you've reduced the value of R3. No wonder it's getting hot!!

If you're using a 317, then the chip does whatever it can to maintain the voltage between output and sense at 1.25V. Check the spec of the 317 against the part originally specified and see what the equivalent parameter is. You may need to change the value of the resistor between the o/p and the sense.
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Old 13th February 2006, 06:58 AM   #5
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I've just checked the TL783. That uses a 1.25V reference just like the 317. R3 should be a 2.7k just like the schematic shows. Are you sure that you have really got an 82 Ohm resistor for R1?
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Old 13th February 2006, 07:21 AM   #6
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I'll double check when I get home....thx

=RR=

(unrelated edit : my first post said "33u" should say "330u")
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Old 13th February 2006, 09:29 PM   #7
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Default LM317 Vout max = 37 Volt

Hi all,

the LM317 as only Vout max = 37 Volt and Vin - Vout <= 40 Volt !!

See:
LM317.pdf
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Old 13th February 2006, 09:41 PM   #8
Bazukaz is offline Bazukaz  Lithuania
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Measure the voltage drop across the resistor and then you can calculate its dissipated power.
Maybe the resistor is designed to run hot.
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Old 13th February 2006, 10:02 PM   #9
Tim__x is offline Tim__x  Canada
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Quote:
Hi all,

the LM317 as only Vout max = 37 Volt and Vin - Vout <= 40 Volt !!

See:
LM317.pdf
The datasheet is misleading, the LM317 is a floating regulator, it works by holding a constant voltage across an R, and thus a constant current through a second set R. Voltage drop is all that matters. So long as the input voltage ramps up slowly enough that the output capacitor can ramp up with it all is well.
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Old 13th February 2006, 10:40 PM   #10
anatech is offline anatech  Canada
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RR,
Is it possible you have D5 in backwards?

You can bleed off some excess voltage with a resistor between C1 and C2 anodes. If you choose the value right, the regulator will just drop out at full output current, or just beyond. A short will cause a greater drop and possibly save the 317 regulator. It will also get rid of diode hash some more.

-Chris
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