Location of voltage-dropping power resistor

hollowman · 4th December 2008, 09:11 PM

Because I have a slightly "larger" transformer (voltage wise) than that need for an application, I'm going to use a 1-2 Watt resistor (aka "power resistor") to drop about 7VDC. It will be located in/near the PSU rectifier, but before a typical 3-pin regulator.

The value of the power resistor will be ~160 ohm.

Is it "best" to place this power resistor after the smoothing caps (aka main rectifier caps), or between diode bridge and the smoothing caps?

A possible reason why "between" may be better is discussed in this topically unrelated thread on the PFM forum. (Note the position of R1 in the diagram).

Thx for any feedback you can provide!

infinia · 4th December 2008, 09:25 PM

Hi
No doubt about it... the best performance is between rectifier and smoothing caps. Forms lowpass fuction with largest C. Also greatly reduces peak charging and minimizes cap esr requirements.

richie00boy · 4th December 2008, 09:25 PM

It has to go after the caps as that's the only way you can guarantee the voltage fed to the reg won't creep up to the full unwanted amount over times of light loading.

hollowman · 4th December 2008, 10:41 PM

Quote:

Originally posted by infinia
No doubt about it... the best performance is between rectifier and smoothing caps.

Quote:

Originally posted by richie00boy
It has to go after the caps ...

Can't have it both ways

... or can I?
FYI: I'm probab. only going to use ONE smooting cap (~2000 uF).
I could place a small R, say 1 ohm, between diodes and C. And, then, the 160 ohm power R after C (before reg.).

sawreyrw · 4th December 2008, 11:07 PM

I agree with richie00boy. You need a small bypass cap on the output side of the resistor, across the regulator input.

lineup · 4th December 2008, 11:31 PM

One Vbe multiplier transistor
would give a more constant voltage drop.
Actually it will work like a zener.

10 x Vbe is ~ 6.5 volt

I guess the current drawn is like 45 mA. (7V/160ohm)
One TO-126 transistor could do the job.

Speedskater · 5th December 2008, 12:41 AM

If the system is lightly loaded then the 3 terminal regulator should be able to handle the higher voltage and low current. I'll go with "infinia" on this one and put the resistor between the rectifier and the big caps.

john65b · 5th December 2008, 02:15 AM

Why not a pair of CL60 thermistors between the rectifiers and caps (one on pos rail and one on neg rail) ? It will drop 5v - 6v each rail and it serves as a soft start too??

paulb · 5th December 2008, 02:23 AM

Use a CRC. One cap after the diode, then the R, then another cap. A pi filter.

john65b · 5th December 2008, 03:03 AM

I thought a pi filter was a CLC??

Whoops, I thought this was a question on reducing voltage rails to an amp...just saw the regulator reference on the first posting...

4th December 2008, 09:11 PM	#1
hollowman diyAudio Member Join Date: Jan 2007	Location of voltage-dropping power resistor Because I have a slightly "larger" transformer (voltage wise) than that need for an application, I'm going to use a 1-2 Watt resistor (aka "power resistor") to drop about 7VDC. It will be located in/near the PSU rectifier, but before a typical 3-pin regulator. The value of the power resistor will be ~160 ohm. Is it "best" to place this power resistor after the smoothing caps (aka main rectifier caps), or between diode bridge and the smoothing caps? A possible reason why "between" may be better is discussed in this topically unrelated thread on the PFM forum. (Note the position of R1 in the diagram). Thx for any feedback you can provide!

4th December 2008, 09:25 PM	#2
infinia diyAudio Member Join Date: May 2005 Location: Californication	Hi No doubt about it... the best performance is between rectifier and smoothing caps. Forms lowpass fuction with largest C. Also greatly reduces peak charging and minimizes cap esr requirements. __________________ like four million tons of hydrogen exploding on the sun like the whisper of the termites building castles in the dust

4th December 2008, 09:25 PM	#3
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	It has to go after the caps as that's the only way you can guarantee the voltage fed to the reg won't creep up to the full unwanted amount over times of light loading. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

4th December 2008, 11:31 PM	#6
lineup diyAudio Member Join Date: Dec 2005 Location: the north	One Vbe multiplier transistor would give a more constant voltage drop. Actually it will work like a zener. 10 x Vbe is ~ 6.5 volt I guess the current drawn is like 45 mA. (7V/160ohm) One TO-126 transistor could do the job. __________________ lineup

5th December 2008, 12:41 AM	#7
Speedskater diyAudio Member Join Date: Sep 2002 Location: Lakewood, Ohio	If the system is lightly loaded then the 3 terminal regulator should be able to handle the higher voltage and low current. I'll go with "infinia" on this one and put the resistor between the rectifier and the big caps. __________________ Kevin


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4th December 2008, 11:07 PM	#5
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	I agree with richie00boy. You need a small bypass cap on the output side of the resistor, across the regulator input.

5th December 2008, 02:15 AM	#8
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	Why not a pair of CL60 thermistors between the rectifiers and caps (one on pos rail and one on neg rail) ? It will drop 5v - 6v each rail and it serves as a soft start too?? __________________ Pillage Before You Burn.

5th December 2008, 02:23 AM	#9
paulb diyAudio Member Join Date: Jun 2001 Location: Calgary	Use a CRC. One cap after the diode, then the R, then another cap. A pi filter.

5th December 2008, 03:03 AM	#10
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	I thought a pi filter was a CLC?? Whoops, I thought this was a question on reducing voltage rails to an amp...just saw the regulator reference on the first posting... __________________ Pillage Before You Burn.

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