R-value in my simple PSU - diyAudio
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Old 2nd August 2005, 09:31 PM   #1
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Default R-value in my simple PSU

Hello

I have made a PCB for a simple 317/337 PSU
http://www.ettnet.se/~tobias/diy/opabuf/schema_psu.jpg

On the PCB there's only a RC and not RCRC, one of this boards is gonna drive a one-channel OPA627/BUF634 combo @ +/-17V

My question is: Is 15ohms a bit to high in the "RC"? should it be like 5ohms or something instead?

Trannies would prpbably be 2x18VAC or 2x15VAC if the voltagedrop isn't to high.

Cheers
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Old 3rd August 2005, 08:25 AM   #2
Loial is offline Loial  Sweden
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Hi
The size of the resistors set the LP-filter value for the RC combination, to knock out 50Hz, the lower values of R - the higher your low pass frequency cut-out gets.
At 5 ohms, your -3 dB is at approx. 26 Hz, and at 15 ohms, it is 9 Hz. Quite a difference.
Then of course, it is interesting to know how much current your the OPA627/BUF634 will draw, to minimize the losses in these resistors.
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Old 3rd August 2005, 10:06 AM   #3
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Hello Loial

Well the combo can put out about 250mA i guess but as it's gonna be used as a preamp for a chipamp the currentdraw would not be big.

Hm, so a 2.5 ohms could work out fine then? but then it would be aiming for the 50Hz ripple? Thought I read that a higher value in an CRC would make a better overall PSRR (or what it's called)

Thanks
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Old 3rd August 2005, 04:45 PM   #4
Loial is offline Loial  Sweden
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I would say you should use a resistor as large as possible, to cut of the 50 Hz from the voltage.
Though a larger resistor also means larger voltage drop...

If it is used as a preamp there should'n be a problem using the 15 ohm resistors.
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Old 6th August 2005, 08:18 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
since you are deleting the second RG, then I suggest that you add the two inline resistors together to give 30R (or use 33R).
The time constant is only 18mS (15R) and 40(33R) so there is some leaway to increase R further. Maybe try 47R.
At 10mA draw it loses half a volt. One opamp could draw 20 to 50mA into low loads at peak and volts lost before reg would be 2.4Volts at peak current (15Vpk into 400R).
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Old 6th August 2005, 10:21 AM   #6
cerrem is offline cerrem  United States
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What you have is two consecutive first order RC filters..
THe result is a DOUBLE POLE filter..with a -40dB rolloff from the POLE.... In your case the pole is at 8.8Hz....
You are trying to filter 100Hz not 50Hz...since you have a full-wave bridge.....
You already know that you will be -40dB down at 88Hz..so figure it at 100Hz....
Don't sweat over this....
The input cap is figured based on the minimum DC input voltage to the linear regulator...since you have a PEAK of the ripple is refered to MAX voltage..the cap size you choose will change the Vmin of the ripple.... Also too big of a cap will shorten the conduction time/angle on the diodes and increases current peaks... The EMI and Power factor need to be assessed...

Chris
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Old 6th August 2005, 11:49 AM   #7
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Default Re: R-value in my simple PSU

Quote:
Originally posted by tobias_svensk
Hello

I have made a PCB for a simple 317/337 PSU
http://www.ettnet.se/~tobias/diy/opabuf/schema_psu.jpg

On the PCB there's only a RC and not RCRC, one of this boards is gonna drive a one-channel OPA627/BUF634 combo @ +/-17V

My question is: Is 15ohms a bit to high in the "RC"? should it be like 5ohms or something instead?

Trannies would prpbably be 2x18VAC or 2x15VAC if the voltagedrop isn't to high.

Cheers
The resistor value isn't particular sensitive. The highest value is determined how much voltage you can afford to loose. 1-10 ohms is pretty normal to achieve some filtering. The best way to figure out the optimal value is to test.
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Old 6th August 2005, 11:53 AM   #8
Eva is offline Eva  Spain
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These regulators and op-amps already show pretty high PSRR figures at low frequencies so I think there is not much left to improve. Humm due to rail ripple will be probably below noise floor anyway.

Poles may be more useful when placed around 150..300Hz, so that line harmonics and EMI get attenuated without sacrifying too much efficiency. Such poles are obtained with R values in the range 2r2..4r7 for 1200uF capacitors.

Splitting resistance between +Vcc/-Vcc and ground may be useful for EMI isolation.
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Old 6th August 2005, 01:22 PM   #9
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you might find that it is less expensive, (it will also take up less real estate) -- to use a tracking pre-regulator -- National will show you how in the LM317 product folder -- a tracking pre-regulator was used in the later versions of the Super-Regulator.
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Old 7th August 2005, 02:38 PM   #10
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Thanks for all your answers! very useful.

Perhaps I just go for a 5 ohms, it's not like it's a super psu anyway

Have another schematic I was wondering about, would this be a better psu?

Thanks
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