Zener regulator current.

[timb]

Here's one I've used a few times in the past (and recently on the Scope Pong project):



It's Zener based, only needs three transistors and provides basic current limiting. It could easily be modified to provide a higher output current by changing a few resistor values to provide more base drive and up the current limit.

[MK14]

Thanks TIMB!

I wanted to see your low transistor count, regulator circuit, once you mentioned it in another thread (ping pong anyone ?).

I've been curious about it, ever since.

[davelectronic]

I'm  not sure what circuit in another thread your referring to, the only other two circuits I've built over the last few months where LM317 with single pnp pass transistor. And the bigger one, that's a 7812K with 4 x MJ4502 trasistors. Thanks for posting the scope view of your circuit, and sharing it. I did see recently somewhere (forgot where) zener diodes can be a little noisy as a regulator, andca capacitor for decoupling was recommended.

[davelectronic]

Thanks TIMB!

I wanted to see your low transistor count, regulator circuit, once you mentioned it in another thread (ping pong anyone ?).

I've been curious about it, ever since.

Oh ok Timbs low transistor count circuit, sorry my mistake doh lol.

[MK14]

Thanks TIMB!

I wanted to see your low transistor count, regulator circuit, once you mentioned it in another thread (ping pong anyone ?).

I've been curious about it, ever since.

Oh ok Timbs low transistor count circuit, sorry my mistake doh lol.

Don't worry, it is an easy mistake to make.

I'm VERY impressed with his circuit. Very neat!

From a quick look, despite the fact it is still "zener" based, it actually uses the combined gain of a darlington transistor AND another, plus an interesting configuration, so that it will actually close the loop (i.e. really regulate the voltage), to a fair extent (some non-linearities, obviously). It even seems to have a built in current limit.
Because of its clever implementation, it is using a zener of about 6.25V, which gives really good voltage accuracy (especially as regards temperature stability, as that voltage, usually gives the smallest change per deg C, because of the semi-conductor physics of the device, because of zener/avalanche mode stuff and voltage).
I presume the resistance added to the smoothing capacitor, across the zener, is to increase its ESR (virtually), to improve the stability of the power supply.
Because it limits the heating effects in the zener and transistor which drives the darlington, as they have relatively little current in them, and the pot/trimmer allows most of that error to be "tuned/calibrated" out. The voltage accuracy and stability over time, should be good. Especially for a zener based, circuit.
At the moment, the circuit seems to be designed for around 500mA's or so, output current. Since it seems to be limited at around the 650mA mark.
(Obviously) it could use higher spec parts (especially the main output darlington), to achieve higher output currents. It may even be possible to configure it to use more than one output darlington (to share the heat dissipation), with some thought to the circuit, such as suitably valued emitter resistors, of very approximately 200 milliohms.

[timb]

Thanks TIMB!

I wanted to see your low transistor count, regulator circuit, once you mentioned it in another thread (ping pong anyone ?).

I've been curious about it, ever since.

Oh ok Timbs low transistor count circuit, sorry my mistake doh lol.

Don't worry, it is an easy mistake to make.

I'm VERY impressed with his circuit. Very neat!

I presume the resistance added to the smoothing capacitor, across the zener, is to increase its ESR (virtually), to improve the stability of the power supply.

Thanks!

You got everything pretty much spot on, except for the resistors added to the large (4.7mF and 100uF) caps. I forgot to mention this when I posted the schematic: Any resistor (without a labeled value) attached to a capacitor represent the ESR of said capacitor.

This particular simulation software (iCircuit) doesn't allow you to set the ESR of capacitors, so I always add a series resistor to the larger value caps, otherwise you can end up with pretty wild oscillations during simulation.

But otherwise, yeah, it works pretty much as you've described. This particular implementation is good up to about 500mA (with a heatsink on the TIP110/115) and will start limiting current between 600 and 700mA. This is enough to keep quick shorts from destroying the pass transistor (and potentially whatever you're shorting). If you add an appropriately sized polyfuse to the input it would allow the supply to deal with continuous short circuits without overheating as well.

It could easily upped to several amps by replacing the feedback/Zener transistor with a high gain darlington and using a larger darlington for the main pass element. (And lowering the 1 Ohm current limit resistor to to whatever drops approximately 600mV at the desired limiting current.

[davelectronic]

Well I got round to trying the first posted schematic with a single transistor. I started with a 50VA 15 Volts AC transformer, 17 Volts reading no load. Rectified and filtered 25V 2200uf capacitor, no load 23 Volts, bit high. Using 14 Volt 5 watt zener diode, and trying TIP3055, and TIP142. Both worked well, I used a 100 ohm 3 watt resistor, and tried up to 140 ohms as the Base zener resistor. My off load voltage was a bit high at 14.8 Volts, dropping to 12.8 Volts under load of 1.6 Amps, 20 watt halogen lamps. The resistor dropped 7.9 Volts with no output load, and 3.2 Volts with the 1.6 Amp load. Right at the zener reference the voltage under this load was 13.8 Volts, and not far off that from the emitter of the transistor. I should have wrote some of this down, I measured around the transistor, it was fine. The TIP142 held a bit higher voltage under the 1.6 Amp load than the TIP3055. I know that input needs dropping some what to be more efficient. I was using a breadboard and 4 Amp max load hook up wire. I'd like to close the 2 Volt drop gap from no load to loaded, but I think it might not be possible due to transistor drop, and drop through output wire. Although if I built it as a power supply I'd keep leads as short as possible. Regardless of the 100 ohm 3 watt resistor, or 140 ohm resistor the voltage drop was the same at around 1.8 to 2.0 Volts, although the unloaded voltage was better at 14.4 Volts with no load and 140 ohm resistor. Thought it was worth sharing these findings. 

[MK14]

If I understand your reply correctly, 2 volts is way too high an output voltage drop. Going from no load to significant load.
I suspect it is caused, because there is too little base current to properly drive the power transistor.
But there will be some voltage drop in that type of circuit, because it lacks any real regulation, and the transistor as well as other circuit elements, will have varying voltages across them, as well.

2 volts variation is not very good at all, as regards voltage regulation.

Your experiments have shown why that type of circuit is not usually used at higher currents. Because it has way too poor voltage regulation. At higher currents, the voltage drop across the power transistor (especially a 2N3055), will get bigger and bigger. With no real increase in base current (but maybe some), to try and compensate (closed loop).

Also, breadboards are not good for that sort of circuit, especially the higher currents. They tend to have too poor connections/contacts. You might find some of the 2 volt drop, is because of the breadboards excessive resistances, etc. Opinions seem to vary, but people seem to say that somewhere between 100 mA's and 1 amp, is the limit for breadboards. With 500 mA and less being the most common recommended limits.
Cheap and poor quality breadboards make it worse still.
Maybe you can wire the high current paths, directly with wires, to the components or something. To avoid placing high currents, through breadboards.

[davelectronic]

If I understand your reply correctly, 2 volts is way too high an output voltage drop. Going from no load to significant load.
I suspect it is caused, because there is too little base current to properly drive the power transistor.
But there will be some voltage drop in that type of circuit, because it lacks any real regulation, and the transistor as well as other circuit elements, will have varying voltages across them, as well.

2 volts variation is not very good at all, as regards voltage regulation.

Your experiments have shown why that type of circuit is not usually used at higher currents. Because it has way too poor voltage regulation. At higher currents, the voltage drop across the power transistor (especially a 2N3055), will get bigger and bigger. With no real increase in base current (but maybe some), to try and compensate (closed loop).

Also, breadboards are not good for that sort of circuit, especially the higher currents. They tend to have too poor connections/contacts. You might find some of the 2 volt drop, is because of the breadboards excessive resistances, etc. Opinions seem to vary, but people seem to say that somewhere between 100 mA's and 1 amp, is the limit for breadboards. With 500 mA and less being the most common recommended limits.
Cheap and poor quality breadboards make it worse still.
Maybe you can wire the high current paths, directly with wires, to the components or something. To avoid placing high currents, through breadboards.

Thanks for your reply, sure I understand what your saying, a bit of it might have been the transformer and my wire run was a bit long, over 6" closer to 12" I except this circuit has its limitations, if I can build a 1.8 to 2.0 Amp psu from it I'd feel I've given it a go. 80mA was the current through the 100 ohm resistor. I should have written some more details down as I did it. The no load through the zener diode and resistor ran warmer than when the transistor was under load, then the zener and the resistor ran very cool. Nothing got dangerously hot. I think if I lowered the resistor value, given the same voltage conditions input, the output would have been to high. Say 50 ohms I think the output voltage off load would have been to high. Not 100% sure. I did try the 680 ohm resistor 3 watts as in the first schematic, I only got 10.80 Volts with the same load.

[MK14]

That makes sense. The zener will get hottest (in that circuit), when there is no load. Because it will have to sink most of the current, as the transistor, is not using very much (emitter follower). But at high output current loads, the 2N3055's base, will take almost all the current from the resistor. Keeping the zener really cool.

[davelectronic]

Thanks a gain  MK14, I will put a small project built from this circuit in the near future. Hopefully I can close that voltage differential with a tighter smaller circuit board build with minimal trace lengths and short output wires. Cheers again to everyone for the help. 

[MK14]

Thanks a gain  MK14, I will put a small project built from this circuit in the near future. Hopefully I can close that voltage differential with a tighter smaller circuit board build with minimal trace lengths and short output wires. Cheers again to everyone for the help. 

I've just tried simulating your original circuit, with a 50 resistor (R1) for the zener. It seems to be able to regulate to around a 200 millivolt change, from a 1K load, down to a 7 load. Real life circuits will obviously give more accurate results.
With a 680 for R1, the regulation is hopeless (as expected), dropping to about 6V, from 12V. The 100 R1, is almost as good as the 50 R1.
N.B. The simulated Hfe (gain) of the 2N3055, may be significantly different to the one you are using.

200 millivolts, is not that bad. But a proper closed-loop regulator, would usually be massively better than this.

The simulator (LTspice), is effectively assuming perfect wires/connectors. So some of your voltage drops are NOT included. E.g. Breadboard.

I guess with the real life circuit, you can use a multimeter, to see where exactly any voltage drops, are. E.g. across the 2N3055's Collector/Emitter, etc. Which you have already been doing, as mentioned in your posts.

[davelectronic]

It's very interesting, would a 50 ohm 3 watt resistor get a bit hot ? Given the 7.9 Volts the 100 ohm resistor that was dropped across it. I suppose it would just be double the current, not sure that's right as the voltage across a 50 ohm resistor might be higher.

[MK14]

It's very interesting, would a 50 ohm 3 watt resistor get a bit hot ? Given the 7.9 Volts the 100 ohm resistor that was dropped across it. I suppose it would just be double the current, not sure that's right as the voltage across a 50 ohm resistor might be higher.

With no load, the 50 resistor will have the full unregulated voltage minus the 12V of the Zener across it. Which you are saying is about 8 volts, which sounds reasonable. There will also be a ripple voltage, which you probably are not seeing/observing because you are not using an oscilloscope yet (if I remember correctly from other threads).
So the power formula (using IV for power, then using ohms law by substituting I for V/R) is V squared over Resistance = V^2/R

So the power dissipation in R1, when it is 50 , with about 8 volts across it and ignoring ripple.

= ( 8 x 8 ) / 50 = about 1.3 Watts.

But the unmeasured ripple may make that figure a lot higher, depending on if your measurements including an average of the voltage including ripple, or were the lowest voltage, excluding ripple.
I.e. I.m NOT 100% sure what a multimeter would do if you use the DC range to measure a voltage with lots of ripple on it. (I'd use a scope). Presumably you would get some kind of average or the reading would keep on changing (fluctuating). It may vary between different types of multimeter.
Anyway, when the resistor (R1) would be at its hottest, i.e. NO LOAD. There should be very little ripple, as the voltage will tend to just stay at the peak, and be held up by the relatively unloaded reservoir capacitors.

[davelectronic]

Thanks for the help with the maths, yes your right I don't have a scope at the moment. I will try and invest in a modest one soon. All I can do is try the 50 ohm 3 watt resistor and see how much it drops across it in voltage, and work it out from there. I know a bit more drive to the transistor would be useful. I was lead to belive on some multimeters you can see any ripple component in a DC voltage using a low range AC setting, not sure if that would help on seeing ripple. I've  got a couple of uni- t  meters, one is true rms, the other is not. Also a small analogue meter and another digital non rms meter. I know a scope would be a great asset for this and so much more.

[MK14]

For that particular circuit, not using a scope is fine. It is not even closed loop, and is unlikely to oscillate.
But switched mode power supplies (if you ever go that advanced), would make a much more compelling case, to need a scope. Especially if you are designing them from scratch.
Linear power supplies can sometimes oscillate and suffer from short term effects (like over shooting, just after a sudden load change or initial PSU switch on), so they can also sometimes need a scope.
Scopes are useful tools for learning more about electronics.
But scopes are not essential. I suspect many beginners and casual electronics people DON'T have them.
EEVblog often has free competitions for them (there is one running at the moment), where you can win free scopes (but import duties are probably significant into the UK).

EDIT: But a scope would have allowed you to make sure that the ripple voltage is NOT getting through to the output (and check how much it has been removed, i.e. how clean/smooth the output voltage is), and maybe other possible issues.

[davelectronic]

Yes I will look about for a second hand modest scope, I'm primarily interested in power supply construction, so mhz as opposed to ghz would probably be ok for what I would use it for. Taking into account maximum switching frequency of smps. I wish I could get to grips with the maths a bit better, I can work out quite a bit, but get stuck on more complex multiple equations in some circuits.

[MK14]

On the other hand, don't feel pressured into getting a scope. Sometimes people buy things, because of EEVblog forums, such as expensive Fluke multimeters. Then they end up not using them, for various reasons.
I think if you really needed one, you probably would already have one.

It could end up being never used.

[davelectronic]

It might see some use, more on minimising output noise of different psu circuits. I will keep my eyes open for a bargain. Thanks again for all the help. 

[MK14]

It might see some use, more on minimising output noise of different psu circuits. I will keep my eyes open for a bargain. Thanks again for all the help. 

Ok, good luck on finding a bargain scope. Even the old CRT analogue 20 MHz ones, would probably be good enough, for PSU work. They sometimes pop up for free, or small amounts like £10..£15, often/sometimes more.