How do PSU's work?

Indianatone said:

Britton30 said:

I think here in the US we always called vacuum tubes just "tubes" while the English call them "valves". I remember as a boy waiting to see the tiny red glow to come on in the back of the TV or radio.

Me too, now I listen to the faint whir of my PC's fans.....Stuff was amazing in those days. How we got pictures back from the moon landing and broadcast them around the world, the Coronation of Queen Elizabeth II was the first outside broadcast in the world. Now it is all normal.

Thanks Tony that was real good stuff I started off with vacuum tubes (yes Gary we called them valves LOL!!) and some very basic circuits. It took me a fair while to understand transistors not that I know a great deal but enough to get me by and then the advent of the chips (flip flops etc) and I do remember making a few using those three terminal regulators to provide a stable voltage and also allow a pass transistor for a higher currents but I never dabbled in switched mode.
In fact I came across it quite by accident / fluke when dismantling some old computers at the local tip and saw the current ratings and then did some researching. I was using enormous transformers then for some electro plating PSU's and the max one I could find was a 18v 10 amp secondary, and my circuits were very basic like you said. My first brush with high frequency in a power circuit was with a car transistorised ignition unit and yes I did get some nasty "bites".
Now the problem I was working toward was how do we find a way of discharging those caps before we can actually and safely handle a PSU that has been recently used ie swapping it out for a new or replacement one.
Personally I think it would be the responsible thing to do for the manufacturer to build in some kind of manually discharge circuitry but I guess that would also mean higher costs, but it would minimise at least or fix the problem of the possibility of an individual from getting what is potentially a fatal shock.
The SCR I used in power tool control devices and then the TRIAC which I found easier to use but that could have been a just a fluke knowing me.
I await part 2 as I get the gist of the beast.

I saw "pass transistor" mentioned. Is there a simple description of it and is it different than a normal one that is like a switch?

Britton30 said:

I saw "pass transistor" mentioned. Is there a simple description of it and is it different than a normal one that is like a switch?

Yes Gary it is a high current transistor switch come power transistor I used to use the 3055 in the pass mode all it means ids that the normal voltage regulators (just small chips really can handle up to about 1 amp roughly speaking so if you need a higher current than that you use the regulator to switch on the pass transistor and still maintain the regulated voltage.
The 2N3055 was also used in power stages in audio circuits.
(The potentiometer I forgot can be used to adjust the Vout voltage)

I'm too dense to grasp any of that John. what is pass mode and what is a 3055? Is a pass transistor any run of the mill device?

Britton30 said:

I'm too dense to grasp any of that John. what is pass mode and what is a 3055? Is a pass transistor any run of the mill device?

No you not dense Gary look at the two pics in the previous post and then the pics in this post the voltage comes in from the left and the out volts are controlled by the chip inside the pack in this pic. There are some transistors and resistors in there much like a CPU that take excess volts to ground (the middle pin) ie keep the voltage stable at Vout (on the right)

Now that regulator will handle up to a about 1.5 amps on it's own anything more and it would "fry it"
Therefore you need something like 2N3055 power transistor (that can carry up to 10 amps) to provide the current going to your circuits.
The regulator "adjusts" the transistor to "pass" any required current at only the regulated voltage.
ie say 5 amps at whatever voltage that regulator is rated at.

There are standard voltage regulators commonly 5v and 12v but there are some that can be adjusted to whatever voltage you require but stick with the standard ones for the sake of simplicity.
So whatever volts (within reason) you apply in - only the set regulated voltage will come out
ie 24v in (12v regulator) = 12v out because the other 12v is "got rid of'

Ahh, ok a pass transistor is a volt regulator.

Britton30 said:

Ahh, ok a pass transistor is a volt regulator.

Well not exactly the regulator controls only the volts (pressure) not the amps (current / flow) in the transistors they can carry as much as 10 amps.

That sort of current would destroy the regulator so there are three current restricting resistors between the regulator and the transistor/s to allow volts in but not any substantial current.

Remember resistors control current not volts as such.

Se the retouched pics.
Regulator
If I were to put 28v to volts in (Vin) the 12v regulator chip then keeps the volts out (Vout) at a set voltage = 12v by "leaking" the excess 16v to ground via it's circuitry to ground (the middle pin).

Transistor
It has 28v all the way through to Vout at say 8amps (much too high for the regulator)
So at Vout the regulator again "leaks" the excess 16v to ground via a resistor that stops any high current flowing to ground through the regulator stopping it with R1 or directly to ground stopping it with R2. These two only let volts through not amps.
Thus the current at Vout is maintained at say 8amps but only at 12v so a clever piece of circuitry is able to let the transistors do the work by bypassing the regulator. If it were not for R1 and R2 and R3 the whole thing would burn up.

So using the regulator we achieve the quite precise voltages that we need and the transistors the current.
Next time you peek inside the PSU you will see those large transistors on those heat sinks (the regulators are too but usually much smaller)

I appreciate the effort, but I'll never understand all of this. It got more confusing bringing in the regulator. You say the transistors bypass it, so why have a regulator?

If I plug it in turn it on and it works, it's okay with me.

Britton30 said:

I appreciate the effort, but I'll never understand all of this. It got more confusing bringing in the regulator. You say the transistors bypass it, so why have a regulator?

If I plug it in turn it on and it works, it's okay with me.

That is it G the regulator works fine on its own as in the small pic but it is limited to a fairly small current.

So to get a large current you pass that through the transistors which would not differetiate between the volts in and the volts out the regulator does that job. By using those resistors it stops th high current passing through it (reg) from the transistors.

So in effect you have 28v in at 10 amps and 12v out at 10amps because the reg leaks the excess volts away and dose nothing with the current.

It is a bit like a pressure relief valve (reg) in a high flow (amps) water pipe only it is electricity instead of water. ie 100 gallons a minute at 20 psi at one end a relief valve near the other end that takes that pressure down to 5psi but the 100 gallons still flows.