Using transistor zener regulator to get 24V from 48V

[tombaker]

Hi all,
I recently bought a piece of equipment for my studio that runs of a 48V external regulated power supply that feeds the phantom power inside the box.
The voltage is split between Phantom and needing to go to the preamp cards, so there was a little transistor zener regulator board inside and when measured it gave 22.6V.
The board was quite old and had signs of decay so I removed it and traced the vero it was on.

The circuit when traced looked like this (I believe, meaning that's what I could trace):


For some reason when I simulate it, I'm just regulating the 48V, even though I have a 12V Zener in there. "No S&$t" I hear you saying, but I can't get my head around it.

I've been reading about these regulators and still can't get my head around what I'm doing wrong.

JLM even have a little board like this - http://www.jlmaudio.com/forum/viewtopic.php?f=2&t=83

Can anyone tell me what I'm missing?
Any help would be appreciated.
Thanks

Tom

[R.G.]

Put a load resistor on the output in the simulation.

[tombaker]

Thanks RG.

[PRR]

> when measured it gave 22.6V.

TIP122 is a Darlington. (When loaded, as R.G. sez) the emitter sits ~~1.2V lower than the Base.

If the Zener is 24.0V, then we expect 22.8V out.

The Zener tolerance is likely 5%, so we expect 24V to 21.7V.

It is fairly unlikely that a "24V" circuit is flipped-out by 21V to 24V.

The circuit you traced is very probably right, except you do not show a load.

Not knowing what your load really is, I would throw 1K across it just to give it something to do.

That Zener number appears to be a 9V job, FWIW, but that just gives you ~~8V out until you find a 24V-25V Zener.

I would expect an "un-loaded" TIP122 and 24.0V Zener to show a hair under 24V with just voltmeter loading. For it to show higher in a Sim suggests your sim is set-up wrong or the sim has some strange notion of what a TIP122 does with *no* load.

[tombaker]

Hey,
Thanks PRR. The part number on the Zener is a 9V job, but in this sim I've changed the data.
You're right there was nothing wrong with the circuit that was originally there, I just could read what the Zener voltage was on the board so I thought I'd sim it and see what I needed.

It's not the best sim, it doesn't function the best. I should start using something else but I don't use it often enough at the moment.
Thanks for your feedback and the extra knowledge too.

Putting a load resistor into the sim worked a treat.
Cheers

[PRR]

> It's not the best sim

There is a saying about a carpenter who blames his tools.

And in fact most sim back-ends are SPICE which "often" gives reasonable responses if you ask the right question. And don't stray too far from the "normal" operating zone of the transistors.

In this case, failing to provide a load was a user-error. Something to know for next time.

On the breadboard with real parts and a real meter, I would expect some tiny "load current" to flow, transistor to work a little bit, output no higher than Zener voltage.

But the sim's virtual volt-meters may be _NO_ loading at all. And the sim's model of a TIP may think "no current, leakage pulls-up emitter toward collector", and show collector voltage.

Much sim-work is about knowing the idiot-computer's blind-spots, and knowing how to cross-check the crap it gives you.

> Putting a load resistor into the sim worked a treat.

Yes, and the machine is an idiot. My antique sim actually complains about *certain* unloaded conditions, almost never the ones which matter.

I just tossed a simple version of your sim in my sim. For no load and 24.000...V at Base it shows 24.000002 at emitter. Mine does not give "48V", however I didn't have a TIP122 so I used a 2N6059 power Darlington, which lacks the internal resistors that a TIP122 has.

It also says the emitter current is 284fA -- femto-Amps?? Counting on my toes, I know anything involving femto-Amps is totally bogus in a power supply (in dang near anything outside an eletrometer).

Another clue is to look at the Collector and Base currents, and the direction of this bogus current. All three are flowing "in", which is physically impossible.

In fact it *can't* be flowing ANY Amps into a perfect capacitor.

So this 200fA has to be numerical round-off errors.

Hanging a 1K load gives the expected 22.8V output. Reasonable alternate loads give reasonable numbers.

A 10Meg (real-world voltmeter) load gives 23.98V, which may be nearly correct for such a light load, though I would not bet my rocket-ship on it. Many regulators "need" a load to pull them into operation.

1 Ohm load gives 19.7V, reasonable, but the resulting 19.6 Amps times the 28V lost across the transistor is 560 Watts, which a TIP122 won't stand, and the simulator usually won't tell you something is cooking.

The most important part of a simulator is the human using it.

[tombaker]

Definitely.
That's why I like these forums because if you get stuck you can asks questions and once being helped to find the answer you can back track and join the dots in your own knowledge.
Thanks for your real world analysis very handy to get some extra results.