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Old 26th May 2013, 09:15 PM   #11
Art M is offline Art M  United States
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So I was wandering around the Texas Instruments LM317 data sheet per the regulator in discussion. I need a bit of guidance as to how the shown AC regulator circuit functions with AC applied to both Vi terminals ? The circuit might have some function as a raw Pre regulator before a Bridge ?
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Old 27th May 2013, 02:48 AM   #12
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It would probably work as an AC "preregulator" to some extent, but it just as probably isn't a very effective or efficient method. The bridge and filter caps don't need this regulation.
The AC clamp circuit functions quite normally - the 120Ω + 480Ω resistors set the regulated voltage at 6V. The output waveform is more of a chopped triangle wave due to the input sine wave reaching the regulator operating point. Then the voltage ramps to the regulated 6V, remains there for a portion of the cycle, and ramps back down.
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Old 27th May 2013, 04:54 AM   #13
Art M is offline Art M  United States
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The regulators will only pass in the Positive direction, how can the Negative return path be completed by the opposite regulator ?
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Old 27th May 2013, 05:24 AM   #14
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Quote:
The regulators will only pass in the Positive direction
Not true; that's why many of the LM317 circuits include the reversed-biased diode from input to output - to provide an alternate current path for input shorts. The AC clamp is both low-voltage (6Vp-p) and low-power (2W). That is probably a circuit requirement. You'll note that at low voltages the datasheet states that the diodes (in/out & adj bypass) are not necessary.
In my analysis I was WAGging it a lot, but it should be a reasonably close description.
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Old 27th May 2013, 05:36 AM   #15
AJT is offline AJT  Philippines
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Quote:
Originally Posted by Art M View Post
The regulators will only pass in the Positive direction, how can the Negative return path be completed by the opposite regulator ?

correct for normal operation, in case of shutdowns or input shorts where a possibility of current flowing in the reverse direction and damage the series pass trannie, a diode or a zener can protect the series pass trannie from damage...
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Old 27th May 2013, 06:16 AM   #16
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Quote:
Originally Posted by sofaspud View Post
It would probably work as an AC "preregulator" to some extent, but it just as probably isn't a very effective or efficient method. The bridge and filter caps don't need this regulation.
The AC clamp circuit functions quite normally - the 120Ω + 480Ω resistors set the regulated voltage at 6V. The output waveform is more of a chopped triangle wave due to the input sine wave reaching the regulator operating point. Then the voltage ramps to the regulated 6V, remains there for a portion of the cycle, and ramps back down.
Indeed, regulating AC doesnt sound like a genius method to me right before being rectified? it might decrease the smoothing capacitors required but i guess, you will still need a regulator circuit after smoothing :\
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Old 27th May 2013, 06:31 AM   #17
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Originally Posted by sofaspud View Post
Typo?
Looks like the LED receives ~15mA; R38 could probably be increased in value.

Maybe not "most of the power," but more than necessary.
For a handful of op amps, either the 26VCT or 30VCT transformers should be more than big enough.
I'd still come as close as i can to regulation voltage assuming the variation in power supply i guess most of the people use 24-30vct rather than one that is near 15v.
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Old 27th May 2013, 09:14 AM   #18
AndrewT is offline AndrewT  Scotland
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a 24Vac centre tapped transformer is the same as a 12+12Vac transformer except that the two secondary winding are joined at the centre tap.

A 12Vac transformer is too low for a 15Vdc regulated supply.
You need very approximately a 14Vac to 18Vac transformer, 15Vac having been proved by many builders to be adequate.
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Old 27th May 2013, 02:48 PM   #19
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i have 13x2 @ 2A one lying in my drawer,i havent measured how much it actually shows unladen. after constructing the circuit, i will let you know if that was sufficient.
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Old 27th June 2013, 06:19 PM   #20
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a 24Vac centre tapped transformer is the same as a 12+12Vac transformer except that the two secondary winding are joined at the centre tap.

A 12Vac transformer is too low for a 15Vdc regulated supply.
You need very approximately a 14Vac to 18Vac transformer, 15Vac having been proved by many builders to be adequate.
Okay guys i built this supply using a 13-0-13 @ 2 amp transformer the rectified and filter voltage is 17.2v on each rail but unfortunately THE REGULATED VOLTAGE IS 14.52 volts. is it okay to drive ne5532 with this voltage or it needs sharp 15v for it?
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