IET logo
Decrease font size
Increase font size
Topic Title: PSU or transfomer?
Topic Summary: I have a gap in my knowledge
Created On: 01 October 2017 01:16 PM
Status: Read Only
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 01 October 2017 01:16 PM
User is offline View Users Profile Print this message


Posts: 3842
Joined: 20 July 2006


I did a drawing a couple of weeks' back where the 230 V was transformed to 24V at the upstream of the control panel. the sketch was done by someone else and had a transformer on it.

Sam and Dave came down to look at the panel undergoing replacement and immediately said they'd change that to a PSU (power supply unit). They have now done the work and I am blown away by their knowledge. Fantastic job. Various other changes are noted. I see them as having more up to date knowledge because they work in one of the more modern parts of our environment.

Which makes me want some of their knowledge.

When a transformer and when a PSU? What is going on inside a PSU that is different from a transformer when the requirement is a basic step down?

 01 October 2017 01:53 PM
User is offline View Users Profile Print this message


Posts: 2493
Joined: 07 August 2007

These days the terms transformer and PSU are often used interchangeably, especialy when dealing with consumer grade equipment.

Strictly speaking and IMO, a transformer takes an AC supply and transforms this to a different AC voltage. The output is AC and is usually a fixed percentage of the input voltage.
Some types are continually adjustable and often known as "variacs" though that is a trade name in fact.
Some transformers have tapings for different output voltages, either selectable in use, or for choosing the most suitable one at time of installation. A bell ringing transformer might for example have tapings for 5 volts, 7 volts and 12 volts output.
Other transformers have tapings for different input voltages, for example a control panel transformer might have tapings for 208 volts, 220 volts, 240 volts and 277 volts.

A PSU is usually a transformer and a rectifier and often contains other components to smooth and regulate the output.
The DC output voltage might be fixed during manufacture, or adjustable during installation, or may be user adjustable depending on the requirements.

But I repeat that in common usage the terms power supply and transformer tend to be used interchangeably these days.
 01 October 2017 01:54 PM
User is offline View Users Profile Print this message


Posts: 4549
Joined: 21 November 2008

A PSU is a device to convert the AC power supply into DC for the components it is supplying. So a 24 v PSU for a control panel for example might be 230V AC input with a 24v DC output. So a transformer and rectifier pack basically. It might also have additional bits and pieces like a cooling fan, fuses,etc. Well that's my understanding anyway.
 01 October 2017 02:20 PM
User is offline View Users Profile Print this message


Posts: 100
Joined: 14 February 2007

Agree with the previous replies; "Transformer" implies just the wound component itself, whereas PSU implies an assembly, I wouldn't say it has to imply something with a DC output, but most are. Now while a PSU could be linear (i.e. it has a transformer, a recifier, smoothing and voltage regulation tagged onto it). There is a move these days to switching power supplies that use electronics to do the voltage conversion (I'm not entirely upto speed on the ins and outs, but basically they recify mains to DC, convert to a high frequency with a duty cycle which corresponds to having an RMS of the wanted voltage, then smooth and regulate that - I am sure Mapj1 will correct me here...). Most still contain a transformer but thats for isolation of the output as most cases want it to be electrically separate, but because its driven at khz, rather than 50hz, it can be a lot smaller, and the whole thing is a lot lighter and these days, cheaper to make than a linear PSU.

Sometimes a switching PSU is wrongly called a transformer.... often in relation to things like the power supply units for 12V spot lights... though at least sometimes they call it an "electronic transformer"
 01 October 2017 02:24 PM
User is offline View Users Profile Print this message


Posts: 4549
Joined: 21 November 2008

Yes that right Phoenix, switch mode power supplies - the things that a pest when you have RCD's installed!
 01 October 2017 04:09 PM
User is offline View Users Profile Print this message


Posts: 204
Joined: 22 July 2016

Yes totally agree transformers are generally AD to AC volltage changingwhereas a PSU is for converting mains to some DC level OR alternatively supplying a high frequency output like 400 cycles for avionics or several Kilocycles for HF flourishing lamps. And also keep innings you wouldn't call an 11Kv to 433 volt transformer a PSU would you?
 01 October 2017 04:46 PM
User is offline View Users Profile Print this message


Posts: 97
Joined: 18 June 2011

I would say a transformer is always AC to AC and just coils of wire, often on an iron core.
A PSU has AC or DC input and AC or DC output.
 01 October 2017 06:47 PM
User is offline View Users Profile Print this message


Posts: 204
Joined: 22 July 2016

And of course a PSU can be transformerless like some. SMPSUs or if you go back in history the half wave rectifier type used in old TVs and radios
 01 October 2017 06:50 PM
User is online View Users Profile Print this message


Posts: 734
Joined: 14 December 2004

If you are looking for a good Electronics book, I can recommend The Art of Electronics, Horowitz & Hill. Currently in its 3rd edition.


Clive S Carver GCGI IEng MIET MITP
 01 October 2017 09:33 PM
User is offline View Users Profile Print this message


Posts: 1129
Joined: 01 February 2009

PSUs offer more options than just a tranny. Often having facility for battery supply, relays for this and that, fan cooling, easy connections, fused outputs, AC or DC, neon indicators, and nice white enameled finish too.
Incidentally never put a sticker on a PSU nor a label, not any marking at all. Where possible do not fix to a wall where a ceiling tile will suffice, and for good measure always leave the front door open. Oh and never remove a disused one. By following these practises you will help confuse the he'll out of fellow sparkies
 01 October 2017 10:28 PM
User is offline View Users Profile Print this message


Posts: 9814
Joined: 22 July 2004

- I am sure Mapj1 will correct me here..

no, that is all true, but as I sometimes design the wretched things, I can't resist expanding, so feel free to scroll past this on to the next more interesting post.

Not so long ago it was simple. Transformers on AC were lumps of laminated iron that sat still and buzzed at 50Hz, and on DC were slightly heavier lumps that rotated as a motor-generator.
A transformer took as input, one voltage, and transformed to it to an output at another. The definition was extended to isolation devices, where the input and output were at the same voltage, so strictly speaking no voltage transformation in some cases.
Power supplies were one of the above, maybe with many windings, and a rectifier valve or two, or for low voltages, copper oxide, or for higher powers mercury vapour.
a power supply then typically took in mains, but gave out DC, maybe or maybe not regulated or current limited.
Then along came semiconductors and modern chemistry , and it has all slowly got far more confusing.
It is now cheaper to rectify the mains to DC as soon as it gets into the box, then chop it up at supersonic speed with a pair of high voltage power transistors, and apply the chopped up pseudo AC to a much smaller transformer than would be needed for 50Hz.
Now, if the output is an AC, but at 25khz or something, by one argument it is AC in, AC out, and the voltage has changed, so some makers will call this arrangement an 'electronic transformer'.
Such things are often found driving downlights and similar ELV fittings.
Then if the high frequency is AC is rectified back to DC, it becomes a switch mode power supply, and depending what else is needed it may be controlled to provide constant voltage, or constant current, or clever voltage foldback at some critical current.

I'm not even going to mention piezoelectic transformers, RF matching transformers, or small toy robots, as that simply adds further confusion

Actually, I'd suggest the whole definition is now become so cloudy that it is best to specify exactly what you want - " 24V DC 5A unit"
"12VAC lighting supply", and as at a push a transformer in a box could also be called a 'supply', calling it a "PSU" will safely cover all of the bases, from the simplest to the most complex.

regards Mike
 02 October 2017 03:58 PM
User is offline View Users Profile Print this message


Posts: 591
Joined: 06 November 2010

In addition to Mike's comprehensive answer above. For DC sources vibrators were also used especially for car radios and similar battery supplied equipment. The vibrator was like a buzzer with contacts that reversed the polarity of the output probably getting on for 50 hertz. The resultant square wave could then be transformed for valve HT supplies. As they incorporated contacts and springs they were prone to failure and I had to replace the one in my car radio a few times.

To me a transformer needs copper, none of this dark art transistor stuff.

If you are interested in finding out more please be careful from where you search for DC/AC vibrators. :-)

New here?

See Also:

FuseTalk Standard Edition v3.2 - © 1999-2018 FuseTalk Inc. All rights reserved.