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:
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
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:
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
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.
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.
There are not two separate secondaries, only a single center tapped one. I think that this precludes the dual-mono-in-BTL arrangement, no?
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
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
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!
Last edited:
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.
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.
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.
I am talking about using a linear regulator that is almost ALWAYS operating in the drop-out mode, that is to say NOT regulating. But it can and will regulate when the voltage is just a bit too high for instance when the amp is under no load or the mains voltage is a bit high. Remember, I mentioned that the regulation is not important to me (I will get this from the cap bank) but I want to make sure that the DC voltage does not exceed a certain value. From the standpoint of simplicity, using a high-current regulator seems to be a simple way to achieve my goal. I am just not certain that operating a regulator continuously in drop-out mode is OK or not. This is what I would really like to know...
Thanks for the link to Rod's capacitance multiplier project. That might just do the trick.
Last edited:
It won't hurt the regulator at all. You seem to want to operate right on the hairy edge of the supply voltage. I'd tend to regulate down from that, sacrifice a bit of power, and know that the circuit was always getting a clean supply. If you're so close to the edge, why not just beef up the amp circuits a bit so you have some safety margin under no-load hi-line conditions?
Perhaps I should just forget about the regulator and just make a CRCRC type power supply and perhaps add in a couple of high current diodes to drop some voltage while filtering at the same time? This approach would probably be fine, and would have the benefit of adding LP filtering to the PS.
The RC combo provides filtering, while the diodes drop a few volts even in the case of no load on the amp. Good?
The RC combo provides filtering, while the diodes drop a few volts even in the case of no load on the amp. Good?
Last edited:
@CharlieLaub,
i suggest you check your load at line voltages of 120-122, what will the off load rails be?
and can your application withstand those conditions?
maximum worst case high line, it will still drop with loading on your secondaries...
you may be having a case of nervousness....
we can't really tell unless we see the complete picture....
i suggest you check your load at line voltages of 120-122, what will the off load rails be?
and can your application withstand those conditions?
maximum worst case high line, it will still drop with loading on your secondaries...
you may be having a case of nervousness....
we can't really tell unless we see the complete picture....
Looks like a version of Rod Elliot's Capacitance Multiplier supply would work well, providing some voltage drop and reducing ripple signficantly:
Capacitance Multiplier Power Supply Filter
Schematic shown below:
I assume I will need to change the transistors because my voltages are close to or above the devices in Rod's schematic can handle.
Any suggestions for replacements for TIP3055/2955 and BD139/140 assuming that the DC voltages will be around +/-90V?
Capacitance Multiplier Power Supply Filter
Schematic shown below:
An externally hosted image should be here but it was not working when we last tested it.
I assume I will need to change the transistors because my voltages are close to or above the devices in Rod's schematic can handle.
Any suggestions for replacements for TIP3055/2955 and BD139/140 assuming that the DC voltages will be around +/-90V?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.