48v phantom / resistor is way hot !

redrabbit · 13th February 2006, 05:53 AM

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=

redrabbit · 13th February 2006, 06:06 AM

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=

Ouroboros · 13th February 2006, 06:49 AM

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.

Ouroboros · 13th February 2006, 06:52 AM

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.

Ouroboros · 13th February 2006, 06:58 AM

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?

redrabbit · 13th February 2006, 07:21 AM

I'll double check when I get home....thx

=RR=

(unrelated edit : my first post said "33u" should say "330u")

dx.master · 13th February 2006, 09:29 PM

Hi all,

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

See:
LM317.pdf

Bazukaz · 13th February 2006, 09:41 PM

Measure the voltage drop across the resistor and then you can calculate its dissipated power.
Maybe the resistor is designed to run hot.

Tim__x · 13th February 2006, 10:02 PM

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.

anatech · 13th February 2006, 10:40 PM

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

13th February 2006, 05:53 AM	#1
redrabbit diyAudio Member Join Date: Oct 2004	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=

13th February 2006, 09:29 PM	#7
dx.master diyAudio Member Join Date: Jul 2004 Location: Hessen, Germany	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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13th February 2006, 06:06 AM	#2
redrabbit diyAudio Member Join Date: Oct 2004	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=

13th February 2006, 06:49 AM	#3
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	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.

13th February 2006, 06:52 AM	#4
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	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.

13th February 2006, 06:58 AM	#5
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	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?

13th February 2006, 07:21 AM	#6
redrabbit diyAudio Member Join Date: Oct 2004	I'll double check when I get home....thx =RR= (unrelated edit : my first post said "33u" should say "330u")

13th February 2006, 09:41 PM	#8
Bazukaz diyAudio Member Join Date: Nov 2005 Location: Vilnius	Measure the voltage drop across the resistor and then you can calculate its dissipated power. Maybe the resistor is designed to run hot.

13th February 2006, 10:40 PM	#10
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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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