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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 http://uk.farnell.com/myrra/44122/transformer-2va-9v/dp/1689054?Ntt=1689054
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Possibly useful guide here: http://www.ti.com/lit/an/snva006b/snva006b.pdf

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.
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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.

http://www.kpsec.freeuk.com/powersup.htm

This would be a good regulator for the voltage your selected transformer can supply
http://www.digikey.com/product-detail/en/LM2931T-5.0G/LM2931T-5.0GOS-ND/918492
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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?

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





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

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.
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  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 smiley-lol.
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