12v power supply design

Going to make a 12v linear supply, always wondered whats the best transformer to use, 12v or a slightly higher voltage one like 15v wonder if anybody knows of a good power supply design info would really to understand fully the transformer side.

I pretty much know how to design one but im not entirly sure how to select the correct transformer,

What's your input power source? AC, DC?

Design it around what you have in the junk box. Or is you junk like mine, too many transformers. If you can design a PSU, then you already know the voltage from the rectifier/filter capacitor will be at peak voltage until current is drawn. What are your current requirements? Any voltage higher than the regulated voltage will have to be dissipated as heat somewhere.


Transformers are usually rated VA, rather than watts, for some odd reason. In fact the current is
the important rating, since heat dissipation goes up as the square of the current, and exceeding the
current puts the core closer to the point of saturation (very bad).

A 10VA 10V secondary is designed for upto 1A rms into a resistive load - ie peaks of 1.4A

From what I recall the important point is to get a large conduction angle(*), so that the peak current
through the transformer is as low as possible - that usually means adding some resistance or inductance
before the main smoothing capacitor.

There must be some online apps for designing such supply.

Quite why you would want a linear power supply in the 21st century is beyond me though, its ancient
tech like steam power and hollerith punched cards these days!!

(*) the phase angle for which the rectifier is conducting - when the winding voltage is larger than the
capacitor voltage. Avoiding a capacitor value that is over-large helps here, but you have to ensure
it doesn’t fall below the linear regulator’s minimum input voltage.

240v ac
ive never really understood when you have 2 winding on the output,
say 2 12v if you parrallel these do you get double the current?
what current do you get when you get 2 12v windings in series you get 24v, same current?

i do have a switch mode supply, but im trying a linor supply to try to reduce noise really

am i correct in saying a 15v 5va winding will produce 330ma peek, 254 rms

so what is the max dc load i can get out of it 254ma?

ac ratings are always rms unless stated otherwise.

You get two secondaries so you only have to make one type of transformer, that will
work with bridge rectifier or a centre-tapped full-wave rectifer. You also get the ability
to wire in series or parallel for the bridge-rectifier case.

ok bit confused about the rms value, is that the max amount of current you can draw from it?

rms is used for ac because you can calculate the average power with it. 240Vrms at 2Arms is 480W. If
you multiplied the peak voltage and current (aka amplitudes) you'd get 960W, which happens to be the
peak power, not the average power.

For sinusoidal waveforms the peak = sqrt(2) x rms.

For more complex waveforms you have to calculate the sqrt (mean (square (x))) over the waveform
to get the rms.

For windings the heat dissipation is related to the rms current (its proportional to the square of the
rms current). So mostly the VA rating of a transformer just tells you the max continuous rms current
you can use without overheating.

For very spikey waveforms the peak value can be many times the rms, and for transformers this
matters because the core can become saturated on the peaks, at which point drastic things happen
(extremely high currents in the primary as the inductance drops to a low value). It is mandatory to
have a fuse on the primary of a mains transformer for this reason alone.

Transformers are usually rated VA, rather than watts, for some odd reason

You have to know the PF of the load.


Power factor, has to do with phase shift in reactive loads, current and voltage or not in phase so P = I * V doesn't work any more.

You have to know the PF of the load.

Only if you are talking HI current and Power and SMPS.
It looks like the OP is looking at a supply under 10A and linear.
PF is pfffffffff! ! ! irrelevant in this case.
Tom... :slight_smile:

Transformers are rated using VA because Volts determines the insulation / isolation requirements and Amps determines the conductor size. Watts is more or less meaningless when designing a transformer.

When using a transformer, knowing only the Watts rating of the load does not provide enough information. For instance, say we have a 10VA rated transformer with 10V, 1A conductor size for secondary winding. Connect a 10VA load and everything is fine. Connect a 10W load at 0.5PF, then the load is 20VA (2 amps @ 10V) and the transformer would overheat and probably be damaged.

ive never really understood when you have 2 winding on the output,
say 2 12v if you parrallel these do you get double the current?
what current do you get when you get 2 12v windings in series you get 24v, same current?

All you need to know is that

VA Primary = VA Secondary

Say the above transformer is rated for 24VA. Therefore, VA secondary is 24. For two 12V secondary windings;

When connected in parallel, the voltage is still 12V. The current available is 24VA/12V = 2 amps.
When connected in series, the voltage becomes 24V. The current available is 24VA/24V = 1 amp.

If you double the voltage, you have half the current available. If you double the current, you have half the voltage available.

Premade psu's are so cheap these days I'd just buy one unless if I wanted to do it as a project in it's own right...



Hi allen i did look couldnt really find much do you know of somewhere?
Found lots of switchmode ones.
A pre build one would be great but its got to be mountable on a flat surface, with propper fixings

Thanks for the help

Why must you have a linear supply ? very sensitive circuitry?



yes, my circuit is monitoring a frequency inverter

That doesn't sound sensitive to me . What power levels is it working at?


hi allan, im measuring 2 phases of 2 3 way motors rinning at about 10 amps, with a current sensor i have tryed this before with a processor connected to it but it just didnt work due to the noise, from the phases
im just trying my best to limit noise using a linear supply as im sure noise is traveling through gnd into the processor