design of regulted power supplies

Without a circuit diagram the question is meaningless.

We are good ...but not that good .

Del

ok sorry

and the circuit is displayed as follows:

Q3 appears to be the wrong way around!

The picture isnt that clear but if it's an NPN transistor the emitter should go to the zenner.

If the output drops, Q3 then tends to turn off, less current flows through R1 thus dropping less voltage so the voltage on the base of Q1/2 goes up, thus the emitter (follower) of Q1/2 goes up correcting the low voltage on the output.

As shown.. no current can flow through Q3 as it is reversed.

The zenner vaolage is fairly arbitary, and 5.6v is a good value as these have good temperature stability.

Del

The zenner is the reference.

Q3 amplifies the difference between it's emitter and base. The emitter should go to the reference (top of the zenner) while the base goes to the output (via the potential divider which controls the output voltage and would usually have a pot in there )

Del

Q3 is shown incorrectly (as pointed out by others in this thread). Q3 should be an NPN transistor and the emitter should be pointing down into the Zener diode; the collector should connect to the base of the Darlington pair; and the base should remain where it is shown. In this configuration Q3 operates a common collector amplifier with the gain set by resistor R1. Common collector amplifiers are inverting amplifiers, which is what is needed here in order to counteract any variation in output voltage. The regulator output voltage is controlled by the voltage divider R2 and R3. The Zener acts as a constant voltage reference for the base of Q3. If the Zener were replaced with a resistor, then the voltage across this resistor would change according to the current through it. The operating voltage across Q3 (from collector to emitter) should be at least 1 Volt in order to keep Q3 in the linear region. The output voltage of the regulator circuit is equal to the voltage on the (revised) collector of Q3 plus 2 Vbe drops through the Darlington.

If Q3 is an NPN transistor (and the collector and emitter are reversed), then it acts as a negative feedback amplifier working to correct any deviation in output voltage. If Q3 is a PNP transistor then it functions as a postive feedback amplifier which acts to drive the regulator to one of the voltage rails, either positive or negative.

Ken Stewart

I could image this class series regualtor. if the V1 haS Aengough amplification amount, neednt use darlington.

zener boltage neednt to have a half voltage of Vc3, just keep Vce3 has a , atl lease higher than 3 or 4V enough, which positon at the enlarge state.

i.e. Vc3 - Vce3 = zener voltage where Vce3 =.= 3--4v. as well as Vce1,2, in order that they position at magnification zone.

next you should put down circuit here, in order that others can help you easily.

the third, you should return a reply to others rapidly, like Del, who anxwer your question promptly.

good manners is neccessery.

Q3 should be a PNP so the circuit is correct.

The output voltage is set not just by the zener, but also by the voltage divider on the base of Q3 (I think it's R2 and R3).

Roughly speaking Vz = R3 / (R2 + R3) * Vo therefore Vo = Vz * (R2 + R3) / R3.

It looks like R2 and R3 are about the same value (but it's hard to read your diagram) so this explains why the output voltage is about twice as big as the zener voltage.

I am not including the effects of the voltage drops on the base-emitter junctions (about 0.7V and twice that for the darlington). This will probably change the output voltage a tiny bit.

Maybe you can find a similar circuit on the web?

If it's PNP then the zenner is completely pointless it's the BE junction difference which needs the error voltage across it.

There are countless textbooks (one of which you probably have somewhere) with full and detailed explanations of this basic feedback circuit.

Isn't this our first introduction to feedback when we are learning electronics?

Your advice to search the web is excellent as this is really a 'learning' circuit, in prctice one would use a voltage regulator IC for a far better, cheaper, smaller solution.

the best way to learn about the circuit operation is BUILD IT and get a scope on it, I messed with this stuff 35 odd years ago, and a full understanding formed my introduction to electronics.

Del

Hi all,

Del wins the day, either way, NPN or PNP Q3 is wrong.

Without Q3 and a suitable value of D1 the circuit would probably work fine as a regulated emitter follower.

It looks to me that this will not work much better anyway (with Q3 corrected) as there will still be a variation on V_D1 dependant on the current and ∆R_D1. Would not D1 be better place in the base circuit of Q3 where the current variation would be less?

The answer to Richa Kuma's question (why is zener voltage Vz half of Vc3 of transistor Q3 used as an error amplifier in the circuit?) however is that the error amp is added to improve the regulation. Ignoring the junction voltages D1 sets a reference point of half the desired voltage and R2 &R3 feedback half the actual voltage and Q3 amplifies the error.

Regards

Chas

Chas, are you misleading the top thread?

look at left, cat del must be supprise, where is wrong with my posts?

hehe,

Cat del is right, no wrong. only you wrong. look at your wrong words

Without Q3 and a suitable value of D1 the circuit would probably work fine as a regulated emitter follower

withouit D1, the follower cannt work as good as a regulator!

Q3 would be npn transistor., of cause it can be pnp, but less one use like this way, it has only tiny, very tiny amplification coeficient.( of cause it has somewhat good some where, in fact)

Ignoring the junction voltages D1

you absolutly cannt ignore the voltage variable of D1 at all !!!!, it difines all the presicion of the regulator.

if the d1 is a worst device, the regualtor would be worst one power supply.

hehe, suport catdel

Hi, Chas,

its very kind of you to teach a slang english. Although I can guess this phrase, Im not aware of its exact meanning. Del may laugh at some place.

What I misunderstood is Q3 and D1. I though of your words include d1 after q3. thats why I said, if no Q3 and D1 ...

Now I understand. you are right, if there D1 exists, the whole circuit can work as a regulator without Q3. and can ignore the junction voltage...

welcome to keep on pointing out my apparant mistakes

I vote you as a good answer.

Hi, Chas,

I know this abc function at top of the box. I rarely use it, as it can only correct wrong words, rather than grammar and phrases etc. occassionally I utilize it for those who are very precisian or stick in the mud.

oddly enough, why you cannt speak fluent spainsh but good englsih? are yuou living in a some blocks like chinatown or english speaking district?

I wish I answer the tech issues and those who post the questions can correct my wrong phrases and sentences and synomy. but disappear. none like to do it. they seem not like to pay for the paid.

I think they are ver y self-concern.