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Old 17th November 2013, 06:02 PM   #1
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Default using regulator to limit AC secondary voltage - A possible or reasonable approach?

In my enthusiasm for a "great deal" I have acquired several 500VA transformers that have a center tapped secondary that is rated at 62-0-62VAC with 115VAC applied to the primary. The mains voltage where I live in the USA typically runs at 120-122VAC, and you never know if it will rise another percent or two. This makes the secondary voltage "too hot" for most DIY amplifier applications that I am considering, so I'd like to figure out a way to decrease the secondary voltage without having to do any surgery on the transformer itself.

The transformer has dual 115VAC primaries and, while I could use the transformer with the primaries wired in series to get half the secondary voltage, I would also reduce the rated VA by half. I'd rather not have to go that route.

So, I happened to be reading the spec sheet for an LM317 the other day, and at the end there was this circuit:

Click the image to open in full size.

I never thought of using a linear device for this kind of application. The 317 would be woefully inadequate for limiting my transformer secondary voltage because the device current limit is on the order of 1A.

Then I recalled recently seeing a high current (7.5A) adjustable regulator from Linear Devices, the LT1084:
LT1084 - 7.5A, 5A, 3A Low Dropout Positive Adjustable Regulators - Linear Technology

These are not very expensive, and seem like they could potentially fill my needs.

I am not sure about dissipation. As I understand it, a linear regulator has high dissipation when the difference between Vin and Vout is large and current must flow. When integrated into the linear PS in between the transformer secondary and cap bank, current will mostly flow when the AC waveform is near its peak, meaning that when current is high Vin-Vout is low or zero. Would this work?


Another possibility is something along the lines of the "pre-regulator" transistor circuit that Rod Elliot describes in his project #102, here:
Pre-Regulator

The circuit in his project is meant for powering line level gear from the power amp mains, and uses a zener and some passive components to generate a lower voltage "rail" that can be fed to a linear regulator or used as is. Could I make use of this topology to drop the AC secondary voltage, or the DC voltage after the cap bank, down a few volts???

I'm appealing for advice and assistance since I am not well versed in these sort of circuits, and exactly how happy a transistor or regulator would be under these conditions. I can certainly provide some heatsinking for the devices on the main amp heatsink if need be.

Are there other ways to reduce the voltage in the PS (at the transformer secondary or the filtered DC after the cap bank) a few volts and/or provide an upper limit if none of these ideas are feasible?

-Charlie
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Old 17th November 2013, 06:13 PM   #2
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you can use a transformer on the secondary wired out of phase so the voltages subtract, yes you can use regulators on the output, if you only need to drop a volt or 2, a diode or 2 in series. or you can design with worst case tolerences in mind.
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Old 17th November 2013, 06:41 PM   #3
sreten is online now sreten  United Kingdom
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Hi,

I can see no advantage to that arrangement compared to
building full regulators on the output of the capacitor bank.

In both cases significant voltage drop at high currents =
power dissipation = standard regualtors + pass transistors.

A more unusual approach would be to use single rail amplifiers
off each winding, in BTL mode, eliminating the output capacitors.

rgds, sreten.
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Old 17th November 2013, 08:00 PM   #4
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Quote:
Originally Posted by sreten View Post
A more unusual approach would be to use single rail amplifiers
off each winding, in BTL mode, eliminating the output capacitors.
There are not two separate secondaries, only a single center tapped one. I think that this precludes the dual-mono-in-BTL arrangement, no?
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Old 17th November 2013, 08:16 PM   #5
jcx is offline jcx  United States
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a line bucking transformer on the input need only be rated for ~ % of the big xfmr's VA rating that you want to turn the V down by - good for 10-20% if you already have the right junk box xmfrs

but if you want a factor of 2 you should just series wire the pri and accept the VA loss, console yourself with the thought of the much lower operating B of the core
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Old 17th November 2013, 08:19 PM   #6
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Any practical scheme on the input will be non-sinusoidal. Not sure if there's any downside to that. You might study how Carver did his TRIAC input circuitry on his very efficient amps. I'd just consider this an opportunity to put good high current regulators on each output and have a regulated supply power amp, something not typically done. The voltage drop won't be that large so power dissipation should be manageable. Hey, you got a deal on the transformers. Maybe you'll stumble into some Variacs to run them!
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Last edited by Conrad Hoffman; 17th November 2013 at 08:21 PM.
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Old 17th November 2013, 08:24 PM   #7
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Quote:
Originally Posted by jcx View Post
a line bucking transformer on the input need only be rated for ~ % of the big xfmr's VA rating that you want to turn the V down by - good for 10-20% if you already have the right junk box xmfrs

but if you want a factor of 2 you should just series wire the pri and accept the VA loss, console yourself with the thought of the much lower operating B of the core
I only need a couple of percent decrease in voltage.
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Old 17th November 2013, 08:31 PM   #8
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Quote:
Originally Posted by Conrad Hoffman View Post
Any practical scheme on the input will be non-sinusoidal. Not sure if there's any downside to that. You might study how Carver did his TRIAC input circuitry on his very efficient amps. I'd just consider this an opportunity to put good high current regulators on each output and have a regulated supply power amp, something not typically done. The voltage drop won't be that large so power dissipation should be manageable. Hey, you got a deal on the transformers. Maybe you'll stumble into some Variacs to run them!
So if I understand you correctly, I should just apply the regulation after the caps. I'd only want to regulate the voltage down a bit from its peak and am less interested in the "regulation" aspect. It's more of a over-voltage protection or voltage limiter.

I'm not so clear on what happens under this scenario:
  • Let's say that the cap bank sits at 90Vdc under no load conditions.
  • Now say I set the regulator to hold its output at 85V.
  • Under light load the cap bank voltage may not sag more than 1V. No problem.
  • Under heavy load the current draw through the regulator will likely be enough to drop the cap bank voltage several volts below 85V. The regulator will be running "wide open", e.g. the pass transistor inside of it is just letting all the current through.
Is that OK for the regulator? I don't see why not, but I'm not sure either.
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Old 17th November 2013, 08:33 PM   #9
kevinkr is offline kevinkr  United States
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A bucking transformer on the primary would seem the best way to go, a 6.3V transformer would give you about a 5% reduction and would only need to be rated at 5A or so..
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Old 17th November 2013, 10:05 PM   #10
sreten is online now sreten  United Kingdom
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Hi,

That isn't the way regulators work. "Only" a couple of percent voltage
drop seems pretty meaningless. But FWIW a capacitance multiplier
drops a few volts off each DC line without regulation , and will not
"drop out" like a regulator would if set too near input voltage.

Capacitance Multiplier Power Supply Filter

Talks about increasing 3V to 6V - 10V drop.

rgds, sreten.
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