Transformers v power supplies.

[stockers]

Not being in anyway an electronic person, Can you guys help me understand something.
I understand (but stand to be corrected) that transformers are no longer used (for our trains) and are being replaced by electronic power supplies. I make the assumption that these must be a current switching mechanism that allow full voltage but only for intermittent periods (Pulsed wave?). Am I correct in this?
So how does that effect electronics that don't like Pulsed Wave.

And further - what exactly does 'smoothed supply' mean in relation to all this.

Now I don't need recommendations on which bit of kit I might need, I want to get an amateurs grasp on the technology - if possible.

I once tried to understand 3 phase electricity and I think I am almost there!

 

[sparky230]

this might help

http://en.wikipedia.org/wiki/Switched-mode_power_supply

But a smoothed supply is usally a rectified ac supply, converted to dc via a bridge rectifier of some description, then passed through a smoothing capacitor to smooth out the bumps, it then can be a regulated supply by going through a regulator circuit or device, or if there is no regulation a unregulated supply.

On a switch mode only the ac to ac conversion is done in the switch mode bit, similar to a old fashioned transformer. The ac - dc part is very similar to a conventional power supply

 

[Gavin Sowry]

this might help

http://en.wikipedia.org/wiki/Switched-mode_power_supply

But a smoothed supply is usally a rectified ac supply, converted to dc via a bridge rectifier of some description, then passed through a smoothing capacitor to smooth out the bumps, it then can be a regulated supply by going through a regulator circuit or device, or if there is no regulation a unregulated supply.

On a switch mode only the ac to ac conversion is done in the switch mode bit, similar to a old fashioned transformer. The ac - dc part is very similar to a conventional power supply

Is that explaination available in English, as spoken by the electrically challenged?

 

[sparky230]

For the Electricaly challenged , Instead of having a big lump of iron wound with copper to go from 240v -24, you have a pile of discrete electrical components and a much, much smaller bit of iron with very fine copper to go from 240v -24. you still get plain dc out, but the power supply is much.much smaller and lighter.

Thats all really you need to Know

 

[PhilP]

Or, if you want another attempt to explain how this 'elektrickery' is done:

Ok.. Here goes..

A transformer either steps-up, or in our case steps-down, an alternating current. - The 'wiggly' sine-wave you perhaps saw in your youth on an oscilloscope.
Transformers only work with an alternating (AC) current. The output is dependent on the ratio of turns of wires on the input-side to the output-side of the transformer. The two coils of wires being 'linked' by the magnetic field generated by the primary (input) coupled through the 'soft-iron' (normally flat plates of steel) core which then generated the output in the secondary winding.

That is all a transformer does it either makes an AC voltage higher, or lower at the output compared to the input..

A 'power supply' normally infers more 'cleverness' to make (most commonly) a fixed DC output available.
This can be from a conventional transformer (to reduce the voltage) then a set of diodes (or bridge rectifier, which is four diodes in a 'box') - to make the AC into DC. Then there is either 'just' some big capacitors to smooth the 'lumps' out a bit, or more often, some more electronics to make a fixed voltage available, and possibly limit the current available.
The 'switched mode' bit of a supply tends to make the incoming mains into DC, then chop it into a higher frequency AC. - A higher frequency alternating current can use a physically small transformer to do the same job. Less copper, smaller, cheaper.. There are other ways of doing the 'switched mode' bit, but after that the rectification (converting from AC to DC), smoothing (getting rid of the 'lumps'), and regulation / current limiting (setting the output voltage, and protecting the supply, or load) are much the same.

Does this stream of consciousness help at all??

 

[stockers]

Getting there - thanks

 

[Gavin Sowry]

Getting there - thanks

Ditto.

 

[GAP]

A power supply is best described in its simplest form as 240V ac goes in and 24V DC (or what ever its rated output is) comes out.

In between "stuff happens" how it happens is of no consequence unless you really want to play with big bitey invisible things that hurt if they come in contact with humans (trust me I have done battle with these things and they usually win).

Smoothing is done by capacitors which are really only batteries in disguise, basically a battery is just a real big capacitor.

As sparky 230 previously described.

In a transformer power supply the ac is converted into smaller ac then applied to a circuit which converts it to lumpy DC this then is filtered (smoothed) and there is a circuit (a regulator) that keeps the DC at the same level by looking at the input and adjusting stuff (a governor in mechanical terms).

In a switch mode power supply the ac is converted into DC by a switching circuit and its output is then filtered (smoothed) and there is a circuit (a regulator) that keeps the DC at the same level.

If by "pulsed wave" you may be referring to Pulse Width Modulation (PWM) then the full output of the power supply is available at the output, but a circuit switches the output off and on at a rate determined by a controller, the motor will spin at a speed determined by the amount of on time ie amount of on time, increase the on time and the motor spins faster.

Simplest analogy is switch on and off a light but change the speed which you operate the switch the faster it is switched the more it will seem to be on.

This is the simplest I can make it any questions just ask I'm here for a while yet, well till I go back to teaching boneheaded little apprentices.

 

[stockers]

Thanks - nearly getting it.
One thing Gap. In your description of the 'switching mode', I don't see where the voltage reduction takes place or is it all done in the Regulator?

 

[The Devonian]

For the Electricaly challenged , Instead of having a big lump of iron wound with copper to go from 240v -24, you have a pile of discrete electrical components and a much, much smaller bit of iron with very fine copper to go from 240v -24. you still get plain dc out, but the power supply is much.much smaller and lighter.

Thats all really you need to Know

For the electically/electronically challenged I guess this is the best answer!

 

[stockers]

For the electically/electronically challenged I guess this is the best answer!

Yes it is. But I am also curious and want to know a little more (even if I don't understand it!).

 

[sparky230]

Thanks - nearly getting it.
One thing Gap. In your description of the 'switching mode', I don't see where the voltage reduction takes place or is it all done in the Regulator?

No there's still a transformer in there to do the voltage reduction, but because you've chopped up the voltage and running it at a very high frequency, it only needs to be very very small

 

[stockers]

Thanks - I think I am grasping it now.

Cheers all.

 

[Madman]

Back in the day of toy trains, transformers were used to power trains that ran on AC, while power packs was the term given to transformers that powered DC operated trains. For the most part, back then, Lionel and American Flyer on this side of the pond, ran on AC. All HO scale trains ran on DC. "O" scale train layouts used either/or one of the above. Some "O" scale layouts had a third rail mounted beside the track and the locomotive picked up AC, usually, from that. There were and still are, I believe, "O" scale layouts that use DC current, without a third outside rail.

Another name for a transformer that takes AC current and changes it to DC is a rectifier.

This is my simple explanation along with some more or less accurate information about the history of powering toy trains.

 

[cbeckett]

Far too complicated for me - I do understand coal combustion....

 

[GAP]

Thanks - nearly getting it.
One thing Gap. In your description of the 'switching mode', I don't see where the voltage reduction takes place or is it all done in the Regulator?

In the switching mode the ac voltage is reduced as sparky says without the need for a big lump of iron I could go into switching speed but that would scare the life out of you.

Stick to the basics of ac in stuff happens then DC comes out.

The regulator is there to make sure that the DC voltage remains the same regardless of a changing load i.e. if 1 Amp of current is flowing or 5 Amps is flowing the
voltage remains the same up until the current flow exceeds the rating of the power supply.

Biggest difference is instead of being Arnold Schwarzenger any person can lift it

Probably the simplest analogy for how DC electricity works is think of your garden hose and you are setting a sprinkler on the law, the pressure is the voltage and the current is the amount of water/min that comes out of the tap it is the regulator and the monitor is you while watching what comes out of the sprinkler.

 

[sparky230]

Stockers if switch modes getting complex for you, try understanding Thermonic Valves, I have to brush up on them to sit my Full Amateur Radio exam

 

[stockers]

Not at all Sparky - its the details that I would like to understand (even if some of it goes way over my head) That's why I started the thread in the first place. Reading a Google description is all well and good but you cant stop them and ask a question when your confused. The assorted replies here have added up to a very comprehensive (to my limited mind) explanation
So - switch modes :-

 

[Gizzy]

I am struggling to try to put into words how an electronic power supply works, and I am an Electronic Engineer who has worked on a Technical Helpdesk in a previous career!

There are some very good replies here which I can't really better....

 

[dutchelm]

Stockers if switch modes getting complex for you, try understanding Thermonic Valves, I have to brush up on them to sit my Full Amateur Radio exam

How things change. When I was at college we were shown a transistor in a glass case. "Don't worry lads, they won't catch on, you stick to valves".
Now I don't understand any of it.