Split Rail Power Supply

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DC_Kid

Joined Feb 25, 2008
796
Is this a valid way to get split rail? Looks like a short to me. I see how to do it with CT xfrmer, but can it be done w/o CT?

If T1 is say ~12v feeding the rectifiers. Is it still just 12v Vcc- to Vcc+, (minus the diodes)?

 
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ian field

Joined Oct 27, 2012
6,539
Is this a valid way to get split rail? Looks like a short to me. I see how to do it with CT xfrmer, but can it be done w/o CT?

If T1 is say ~12v feeding the rectifiers. Is it still just 12v Vcc- to Vcc+, (minus the diodes)?

Its certainly much easier with a CT secondary - I think you can do it by capacitively coupling a pair of BR blocks, but you need big caps for appreciable current and you usually end up with a voltage doubling rectifier. Capacitive coupling gives you something resembling a current limited supply, so its not all bad. The dead simple way is + & - half wave rectifiers. You need bigger reservoir caps to smooth it - but not that much bigger for a few op-amps.
 

crutschow

Joined Mar 14, 2008
25,274
Nope.
That shorts the transformer as you noted.

One way to get two voltages is to ground one of the transformer outputs and use two half-wave rectifiers on the other output as below.
upload_2017-8-11_12-23-30.png
 
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AnalogKid

Joined Aug 1, 2013
8,542
Whatever the size of filter capacitors you were going to use in the full-wave supply, triple them for a half-wave supply (round numbers).

ak
 

#12

Joined Nov 30, 2010
18,210
triple them for a half-wave supply
The math says, "double them".
For peak to peak ripple voltage:
√2 C Er F = I

Half the frequency requires twice the capacitance (for the same amount of current).

I got that formula out of a National Semiconductor Audio Handbook. Then I tried it with a 1% meter and an oscilloscope. It works, +/- 1%:)
 

AnalogKid

Joined Aug 1, 2013
8,542
i can use the crude + - supply w/o caps in a push-pull ckt.
No, you can't.

That's twice you've said/shown a AC/DC power supply without filter capacitors. That is fine for charging NICAD or SLA betteries, but for almost all electronic circuits you need to at least filter the pulsating DC to remove the majority of the ripple, or add a regulator circuit it to remove all of it. Whatever the opamp in post #1 is intended for, it will not work with unfiltered, rectified AC.

ak
 

ian field

Joined Oct 27, 2012
6,539
No, you can't.

That's twice you've said/shown a AC/DC power supply without filter capacitors. That is fine for charging NICAD or SLA betteries, but for almost all electronic circuits you need to at least filter the pulsating DC to remove the majority of the ripple, or add a regulator circuit it to remove all of it. Whatever the opamp in post #1 is intended for, it will not work with unfiltered, rectified AC.

ak
Apparently - lead acid likes smooth DC charge better, its claimed to be the reason for 3-PH alternators.
 

ian field

Joined Oct 27, 2012
6,539
A 3∅ alternator is also more efficient, and requires less copper and magnetic material than a single-phase for the same power output.
The words; "apparently" & "claimed" are probably worth a closer look.

And the context of my comment was light years away from the merits of various alternator design.
 

AnalogKid

Joined Aug 1, 2013
8,542
Isn't the whole idea behind 3-phase is that there is no "hole" in the energy out of a generator or the torque out of a motor or the power into a motor?

Also, I know a "real" battery charger is much more than just a rectifier, but many nicad and LA battery chargers are just that: transformer, diode, battery.

ak
 

crutschow

Joined Mar 14, 2008
25,274
If you sum the power from the 3 phases it is a constant (DC) even though the voltage and current are fluctuating.
That's why the torque is also constant.
 

ian field

Joined Oct 27, 2012
6,539
Isn't the whole idea behind 3-phase is that there is no "hole" in the energy out of a generator

ak
That's more or less how it was explained to me - It wasn't even my idea, yet SOME people think I'm "contrary" for relating what I've been told.

I guess there's some people who just never tire of sniping...................
 
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