Voltage control for an unregulated psu

[Markgm]

My first electronics project is an unregulated psu, for up to 30 v for an amp, but I figure I'll use up to 25v, 10 amps, for a pair of 5-ohm speakers (125 wpc).

I want to be able to put a knob on the psu to set the maximum voltage from 0 to 30. I think it would be nice to be able to hook it up to a little 12-volt amp as well.

If we're talking about a transformer, rectifier bridge, decoupling caps, how or where would you add a pot (or something) to control the voltage? How capable would it be of stopping the voltage from going over? The next question will be affect on power quality.

TIA -
Mark

[luka]

what kinda supply are we talking about? switch mode or linear?

For linear, the only good way to change is to use variac
next best thing is to make DC regulaton on secontary, in other words variable DC power supply, but this will need to dissapate diffrence in voltage from DC you get right away after trafo, and the output DC... much like linear amplifier

[Mooly]

Originally Posted by Markgm

My first electronics project is an unregulated psu, for up to 30 v for an amp, but I figure I'll use up to 25v, 10 amps, for a pair of 5-ohm speakers (125 wpc).
Mark

I know this has come from your other thread...

First off, 30 vdc (that is a single rail with respect to ground... not a split -/+30v supply) will allow a maximum AC RMS voltage from an amplifier of around,

30 less losses, say 27 volts.

27 volts/2 is 13.5 volts. 27 is the peak to peak value the amp could put out, 13.5 the peak.

The RMS figure is the peak/1.414 which is 9.5 volts RMS.

That would give 9.5*9.5/ 5 or 18 watts RMS as a maximum power output from a normal single ended amplifier.

Originally Posted by Markgm

I want to be able to put a knob on the psu to set the maximum voltage from 0 to 30. I think it would be nice to be able to hook it up to a little 12-volt amp as well.

If we're talking about a transformer, rectifier bridge, decoupling caps, how or where would you add a pot (or something) to control the voltage? How capable would it be of stopping the voltage from going over? The next question will be affect on power quality.

TIA -
Mark

Keeping it very very simple... the transistor (use a high gain darlington) is connected to the wiper of the pot (via a resistor, say 2k2). This allows the voltage on the base to swing from zero to whatever the input is. The zener diode prevents the base voltage going above the fixed level of the zener.

It's very basic, there is no "error correction" and there is no current limiting.

[DF96]

You can't really have a unregulated PSU to supply a small power amplifier, yet also have a knob to change the output voltage. Well, you could just use a big rheostat to drop the excess, but this would be a very poor power supply.

So you need to decide whether you want a simple unregulated supply (designed to produce around 12V for your amp), or a basic regulated supply (which could be as simple as the suggestion made by Mooly in post #3).

While you think about this, can I strongly urge you to learn about mains safety. Your questions suggest you are very new to electronics, so we don't want to lose you or for you to burn down your house. I don't want to put you off, but a mains PSU is not the best first project. Better to start with battery-powered things, or use a bought PSU at first. You can then make your mistakes, and learn from them, in reasonable safety.

[SY]

The more efficient (though neither compact nor inexpensive) solution is the use of a variac (variable autotransformer) preceding the unregulated supply.

I would enthusiastically second DF96's comments regarding safety.

[Markgm]

Originally Posted by Mooly

I know this has come from your other thread...

First off, 30 vdc (that is a single rail with respect to ground... not a split -/+30v supply) will allow a maximum AC RMS voltage from an amplifier of around,

30 less losses, say 27 volts.

27 volts/2 is 13.5 volts. 27 is the peak to peak value the amp could put out, 13.5 the peak.

The RMS figure is the peak/1.414 which is 9.5 volts RMS.

That would give 9.5*9.5/ 5 or 18 watts RMS as a maximum power output from a normal single ended amplifier.

I am planning on +30v and -30v rails. I didn't know you could do it without a minus. I thought each channel would have up to the same voltage, but each getting half the current?

Originally Posted by Mooly

Keeping it very very simple... the transistor (use a high gain darlington) is connected to the wiper of the pot (via a resistor, say 2k2). This allows the voltage on the base to swing from zero to whatever the input is. The zener diode prevents the base voltage going above the fixed level of the zener.

It's very basic, there is no "error correction" and there is no current limiting.

Thank you for that. I'll say now why I thought an unregulated psu would be a good one, was based upon it being the easiest, simplest, cheapest, but also with good dynamics. The idea was to then try to get noise down as a project, and make an inexpensive psu, retaining the unrestrained dynamics, getting low (enough) noise.

In terms on what I am trying to accomplish, would moving to an SMPS be a better idea? I was guessing linear regulated was over-the-top for me. Or, the idea above, with the transistor, what would that psu be called? I've been reading about unregulated, and I'll naturally move on and read, read, read on SMPSs, too.

Thanks all for the (multiple) advice about trying something running off a psu instead of off of mains power. How about sandboxing myself with fuses on both sides - AC in and rectifier output? If it's any relief to hear, I have made control panels for industrial equipment (electronics abilities were not needed for that).

- Mark

[Markgm]

I'll get the first psu straight. I have an Antek PS-3N30 to goof with. The idea was to reduce the noise of it. It has two pairs of secondaries, saying use one pair for 120v operation, or both for 240v operation. It says 300 watts. It's a 300 VA transformer. It has one bridge (STS KBPC3510W). There are DC +/- outs. It says up to 10 amps.

At the output, there are two big Epsilon filter caps (22,000 uF each), and a resistor, all + to -. I don't see any kind of ground lead for an electrostatic shield, but it's a nice big fat toroid. The basic psu board output has terminals to a fuse board, with fuses, LEDs and resistors, and its own terminal block output.

Antek - PS-3N30

[AndrewT]

Originally Posted by Markgm

........ I have an Antek PS-3N30 to goof with. ........... It has two pairs of secondaries, saying use one pair for 120v operation, or both for 240v operation.

Have you mistaken Primary windings for Secondary windings?

If the transformer is a dual primary "universal" transformer then the two windings are rated for 110 to 120Vac each. They are usually specified as 115/230Vac.
The two Primaries are used in parallel for mains supplies of 110Vac to 120Vac (+-tolerances of local supplier).
The Two Primaries are used in series for mains supplies of 220Vac to 240Vac (+-tolerances of local supplier).

Both the parallel 115Vac and the series 230Vac can be wired wrongly by inverting the phase (connections) of one winding.
This could result in serious damage to you or your transformer.

STOP and get experienced help before you injure yourself.

[DF96]

When you say "made control panels" do you mean metal bashing, or designing and wiring mains or higher voltage equipment? I ask because your questions and comments suggest that you have very little knowledge of electronics in general, and power supplies in particular.

Fuses do not provide a "sandbox". They may, just, prevent you from burning down your house but they will not protect you from electrocuting yourself or losing an eye when a capacitor explodes. The fact that you think they might protect you is worrying in itself. Unless you can convince us that you have a reasonable chance of remaining safe we may be reluctant to help. Sorry!

[Markgm]

Originally Posted by AndrewT

Have you mistaken Primary windings for Secondary windings?

If the transformer is a dual primary "universal" transformer then the two windings are rated for 110 to 120Vac each. They are usually specified as 115/230Vac.
The two Primaries are used in parallel for mains supplies of 110Vac to 120Vac (+-tolerances of local supplier).
The Two Primaries are used in series for mains supplies of 220Vac to 240Vac (+-tolerances of local supplier).

Both the parallel 115Vac and the series 230Vac can be wired wrongly by inverting the phase (connections) of one winding.
This could result in serious damage to you or your transformer.

STOP and get experienced help before you injure yourself.

Guys, I am sorry. I should not have said secondaries. I should have said primaries. Off the house AC, there is 120v live and 0v neutral on regular outlets, and 120v live and 120v neutral on 240v outlets, going back to the breaker panel. I have two pairs of primary leads on the transformer, and one pair is for 120v operation, the two pairs are for 240v (230v).

The whole of the unit is assembled and mounted. It needs an AC fuse and switch (and case).

In response to another question, I have made 24v control panels and controller workstations for machines. I would use a controller and hook up switches, encoders, motor drivers and such, and the software/hardware for the PC either running or controlling the controller.

I have seen a DC motor get smoked when hooked up to 3-phase AC. I have not seen a cap blow.