Using a PCB transformer

Hi all. I plan to use a PCB transformer in my next project (240v to 9v). I'm wondering if any additional components will be required - I know I will need a bridge rectifier to convert the AC output and I'm also putting a fuse on the o/p, but is there anything else I should add, such as capacitors?

This is the transformer I plan to use

Possibly useful guide here:

Tell us what quality of DC supply you want to end up with - good enough for a bulb or good enough for a sensitive circuit? Do you care about ripple? What voltage?

You know that transformers usually have output voltage marked that corresponds to RMS output voltage into a resistor at full load? The output voltage can be quite a bit more at low load.

Full wave bridge rectifier (4 diodes), 100uF eletrolytic cap rated for 16V or more.
7805 type regulator, and another 10uF eletrolytic cap rated for 16V or more.

This would be a good regulator for the voltage your selected transformer can supply

Sorry guys I should have said where the power will be going - it will be running a Nano only and the on-board regulator will do the rest of the work (LCD screen and couple of LED's).

Will I need anything other than a rectifier for that purpose or can the Nano V-reg deal with the rest of the task?


I'd still go with the bigger caps, will help the nano regulator do its job better.

Your "9 volt" secondary may produce more than 12 volts to the nano power input which will place quite a thermal load on its regulator. Suggest you use a 9 volt regulator as part of your PSU design. That'll result in much less thermal load on the nano regulator.

Hi all. I plan to use a PCB transformer in my next project (240v to 9v). ... I'm also putting a fuse on the o/p

Rule of thumb: Fuse on the output is optional. Fuse on the input is not.

@ SirNickity - thanks for the input, I will be fusing the input to the transformer also - I guess ohms lay would apply to select a sensible value? It will draw max 200mA, but I have calculated that my circuit won't draw more than 100mA. How would I calculate the input current @240v?

@ jackrae - thanks for the advice. Surely a primary regulator would be subjected to the same stress as the Nano regulator, or are you suggesting to use a larger one to dissipate heat easier than the nano? Would I be better off using a single 5v regulator and to power the entire circuit and the nano?

@ CrossRoads - thanks, if I add a regulator should I just follow the recommended capacitor values from the relevant datasheet?

Size is a real issue as I'm mounting the project in a standard wall socket double back box, but I can hopefully squeeze a couple of extra bits in there :slight_smile:

Unless its a fail-safe transformer - short the secondary and it'll survive by the description in the datasheet "inhrently short-circuit proof transformer" (but I agree put a fuse in, its a good habit)

Input current to output current ratio is the inverse of voltage ratio.

Caps - could try that, and beef them up if the output ripple looks too messy.

In my opinion a fuse on the primary is a waste of time and a failing to understand why the fuse is there. The principle of a fuse is to protect and the only device on the primary side that requires protection is the supply lead from the mains to the transformer. Since you are UK based you are obviously using a 3-pin fused plug. The fuse in the plug protects the supply line and in your case should be rated for 3 amps.

A fuse on the secondary protects the transformer secondary windings from a "downstream" fault and that is where the only fuse that is required should be located.

That's true, the supply will be fused at the plug anyway.

So is the general consensus to use a primary regulator before power gets fed into the Nano then?

That's true, the supply will be fused at the plug anyway.

At my house that would be a 20amp breaker. I have took an amp clamp and saw a 100amp spike on a 20amp breaker before it tripped because of a short circuit!

I would rather blow a 3amp glass fuse on a project than trust my household breaker to trip before a fire occurs.


You missed the point I made - the user is UK based where ALL appliance leads (other than botched illegal ones) have a fuse built into the plug. They can be a pain in the butt when it comes to size (they are large) but at least it gives a relatively fool-proof system (apart fro having too high a rated fuse for the cable in use).

If your mains sockets can pump 20 to 100 amps or so into an appliance lead, what protects your normal small domestic appliances that have mains leads suitable for carrying only a couple of amps.

In the US, our appliances are supposed follow UL Standards. In short, they are supposed to be double insulated and/or have proper grounding methods such as a ground pin on the plug. My microwave has an over temp. circuit to stop it from getting too hot. Basically, they would stop the sale of an electrocution chair to the general public XD.

I do think its good we rely on fuses so much, just a pity so many people don't think about the value they stick in the plugs!

Back to my project, I have just thought... why don't I use a 6v transformer

same footprint, almost same current output and surely will leave less work to do for the Nano's regulator?

Can you get the equivalent of this?
Lot less fuss than messing with a transformer.
Output is also short circuit protected.

If you can use a transformer closer to your target voltage, all the better. A volt or two extra is nice to allow the regulator to regulate when the line voltage isn't quite perfect. However, many transformers are rated at high load (for their target capacity) so light loads will be higher still. Furthermore, RMS AC rectified to DC will result in a slightly higher DC voltage.

Aiming for a 6v AC transformer will give you some cushion if you use a 5v regulator. If you go higher, then using a 9v regulator will shed some of the excess as heat, and then your Nano's onboard regulator will shed the excess from 9v to 5v as heat. This is better for each of the regulators than going straight from (say) 12v to 5v, as the waste is dissipated via two regulators instead of one.

Re: fuses. I would always, always put a fuse on any mains-connected PCB. Just in case. There is all manner of excrement that can occur. Sure, your UK power cord may have a 3A or 5A fuse in it. But if you have (for e.g.) a transformer rated for 250mA, how's it going to feel about passing 3A until the fuse blows? Circuit breakers and even power cord fuses are rated high enough to not trip as a nuisance. A product's own fuse is rated for the expected load of the product, and will tend to be a much lower value. I don't know what the laws are, or even what common practice dictates, but if it were me, I'd pony up that $1 or so for a 250mA fuse. It's cheap peace of mind. Your house, your life, so it's entirely up to you, and I won't arm-wrestle anyone that wants to do otherwise. It's just my not-so-humble opinion. :.

Thanks again all.

@CrossRoads - I could probably get similar but that would not fit in a socket flush mount back box, although I could mount it remotely but that would mean more cables running to the device because I still need to run mains to my project (its a thermostat). Thanks for the idea though its one I haven't yet considered.

@SirNickity - Thanks that's some really useful info there. I think I might pick up a 6v transformer and test it out using only the nano regulator before I finalise a PCB. Also good point about the fuses!