Understanding a simple regulated power supply

[minim]

Hi,

I'm looking to replicate the reverb circuit from an existing amplifier, as I want to utilize an extra tank that I have. It is a different model tank, however, and some funky audible artifacts are present when using it with my existing amp. Hence, I wish to build a circuit from scratch and tweak it to suit this tank. Please be aware, I love spring reverb and couldn't do without it, so this is a labor of love.

What has prompted this query is that I can only source 12V diodes at the moment, and I wish to obtain a fundamental understanding of what's going on so I can build it reasonably confidently.

My main questions revolve around the basic regulated power supply, and pertain mainly to the relationship between A) the voltage output from the rectifier, B) the relationship between the 47ohm resistors and their zener diodes.

Here is a schematic of the power supply:



I have measured the output of the rectifier at around 15.5vdc.

From what I understand, the zeners are there to 'clamp' the voltage to 10V in order to allow for, e.g. fluctuations in wall voltage.

The questions that are driving me mad are:

1) Is the primary purpose of R62 to drop the volts to around 10Vdc, or drop the current to stop the 10V zeners from overheating, or both?

2) If R62 wasn't there, would the zener hold the volts at 10V anyway?

3) If the wall voltage drops somewhat due to demands on the electricity supply, and R62 drops the voltage below 10V, what does that mean for the zener and its role in all this?

4) Does the volt drop across R62 have to come down to a point where it is reasonably close to 10V? Or 10V exactly ideally?

From these questions you may get an idea that my main confusion is, re: R62 and the zener diode...where does one end and the other begin?

Thank you for any and all responses.

Mitch

[R.G.]

What has prompted this query is that I can only source 12V diodes at the moment, and I wish to obtain a fundamental understanding of what's going on so I can build it reasonably confidently.

If the +/- 10V are for opamps, they ought to do fine on +/-12V too.

From what I understand, the zeners are there to 'clamp' the voltage to 10V in order to allow for, e.g. fluctuations in wall voltage.

Well, kind of. They are there to eat as much current as it takes to drop the 15V at the rectifier/cap output down to 10V. If there was no load at all at the output, they would have to eat whatever current flows through the 47 ohm resistors, or (15V - 10V)/47 = 106ma. If the load eats some of this 106ma, the zener eats less of it, until the load is taking the whole amount.

1) Is the primary purpose of R62 to drop the volts to around 10Vdc, or drop the current to stop the 10V zeners from overheating, or both?

R62 is there to drop the voltage down to 10V. It does this by Ohm's law, V(dropped) = current times resistance. So if the filter cap voltage on C34 is 15V, the zener tries to hold 10V, and the difference, 15-10 = 5V has to be dropped across the resistor. Either the zener, or the load, or both, has to eat 106ma for this to happen.

2) If R62 wasn't there, would the zener hold the volts at 10V anyway?

Yes - for a few milliseconds. The current would go through the roof, as the only resistance limiting the current would be the resistance of the transformer windings and the diode resistances, so the zener would overheat and die FAST. It would transform itself into a Darkness Emitting Diode (or DED )

3) If the wall voltage drops somewhat due to demands on the electricity supply, and R62 drops the voltage below 10V, what does that mean for the zener and its role in all this?

There is some load current going out of this mess on the +10V line. That current must come through R62. So R62 drops the voltage by the total of the load current plus the zener current times its resistance, 47 ohms.

If the line voltage sags, there is less raw voltage at C34 - let's pick 12.5V out of thin air as an example - so there is only 12.5V at C34, the zener is still trying to eat enough current to hold its 10V, so the current available through R62 is (12.5V-10V)/47 = 53.2ma. That's all that's available through R62 if the zener can still conduct. So the load current plus zener current has to be less than 53.2ma. This is fine for all load currents until it eats the full 53.2ma. At that point, there is no current left for the zener to eat. When the load current increases to 53.3ma, the zener current goes to zero, and the voltage dropped across R62 becomes 47*0.0533 = 2.5051V, and that is dropped from the 12.5V on C34. So the voltage at the zener drops to 12.5-2.5051 = 9.995, and this is the point where the zener is no longer regulating.

4) Does the volt drop across R62 have to come down to a point where it is reasonably close to 10V? Or 10V exactly ideally?

The voltage drop across R62 is always Ohm's Law: V = I *R. What the zener does is it has zero current through it for all voltages less than its zener voltage (which is in reality variable, a little, but we'll assume it's 10.00000V). When the voltage across the zener hits 10V, it starts conducting current. It will conduct (in theory) any amount of current to keep its 10V constant.

Think of the zener as a pressure relief valve. When the pressure across it exceeds 10V, it bleeds any excess flow. Until it hits 10V, nothing happens. How much flow it bleeds off depends on how hard it's being pushed and through what "resistance". If the pipe leading to the 10V relief valve is really skinny, not much flow can get through. That's what R62 does.

From these questions you may get an idea that my main confusion is, re: R62 and the zener diode...where does one end and the other begin?

It is complicated. The zener and resistor are part of a pair. The zener will start conducting at 10V, but nothing before that. It will then (in theory) conduct any current needed to keep 10V. R62 is what keeps the "irresistable force" of the rectified power from the transformer from burning out the zener, and also what the zener plays off against to limit the total current into the zener and the load. R62 drops the voltage as needed to limit the current, the zener sucks down the current that R62 lets through to keep 10V.

[minim]

Wow. Thanks R.G. for your response, this outlines everything perfectly. I'm going to forge ahead with this sucker.

You have a knack for describing these things in a very clear fashion.

Thanks

[R.G.]

Nah, I just wander around until I can figure out how it's really Ohm's Law.