OllyK

Can anybody point me in the direction of a parts list and circuit diagram to take a 240V AC supply down to 3V (Equivalent of 2 x AAA battery). Yes I know I can buy a plug in transformer that will do this, but I'd like to power of the existing device rather than having to run a separate cable.

Shark Man

No diagrams. But I can give hint on a very simple method:

One transformer, probably a 240 to 12volt is the easiest to get hold of (or 240v to 6v if you can find one).

4 diodes for the rectifier (standard full wave layout)

Couple of smoothing capacitors (across the output supply from the rectifier)

And either a Zener diode and some resistors to act as a voltage clamp, or an off the shelf voltage regulator unit to clamp the voltage at 3.3v, latter will be more efficient though.

Easy (if you can remember the maths)

Actual component values and types will depend on the anticpated current that the circuit will have to cope with and how sensitive the device it powers is to volatge ripple.

OllyK

That's my problem, I don't know what I need or the specs of the components. Following a wiring diagram and using a soldering iron I can handle!

I'll have a rummage on Maplins site and see what bits they have.

TopBanana

Why don't you get a premade unit and install it inside the case of the existing device?

Also, what is the existing device? Could you piggyback off its powersupply?

OllyK

One of these

4 of these

Some of these?

One of these or one of these

Input is 240v 13A standard mains. It's to power a laser on a mitre saw, currently powered by 2 x AAA batteries, which always go flat just when you need them, hence wanting to take power direct from the saw.

OllyK

The only premade unit I can find is a variable one moulded in to the plug. I could dismantle it I suppose, but I suspect there is a lot of gubbins in there that I don't need, and I doubt there will be a lot of space in the case of the saw (see above), so it may be easier to custom make it.

andy97

iTrader: (1)

check the laser is using 3v instead of 1.5v. Its far easier to use a premade unit, for cost and probably size, unless you are good with circuit board design.

You could use a resistor( high value resistance), diode, zener diode and smoothing capacitor if the current is low enough and do away with the transformer. This circuit would mean that in the event of a fault the laser would die and for an instant be connected to live mains potential.

It all depends on how sensitive the laser is to ripple and whether you want to risk a failure.


edit AAA batteries roughly are able to supply 1 Ampere/hour. My laser on my saw lasts a couple of hours, constant use, so current drawn is approx 500mA.
Value of resistor would be 230-3v(neglible) R=V/I 230/~0.5= 460 Ohms Power rating of resistor P=V*I 230*0.5=115Watts

saying that though a 100w resistors arent that big really


Andy

Nigel H

Surely this will be no bigger than a DIY job?

Maplin > Variable Voltage 500mA AC/DC Unregulated Mains Adaptor

AndyC_772

iTrader: (2)

Go and treat yourself to a set of these, rather than messing about building a power supply. They're AAA rechargeable batteries that don't go flat on their own if you don't use them - a marvellous invention.

Shark Man

Transformer = OK 12v or 9v. It shouldn't matter. But I'd probably go for the 9v one.

Diodes = Four 1N4001 will be more than enough (good for 1amp), you could use a lower rated one, but I use these all the time so I know they work.

Errm, Capacitors, personally I use electrolytic (don't know why, I always have done...habit? ): Maplin > Electrolytic They need to be above the voltage rating of the transformer and they need to be fitted in the correct direction. But I'll have to bail on giving the correct uF value, sorry.

No, those Zeners Vz ratings are too high

Alternatively, this: Maplin > 800mA Low Dropout Positive Voltage Regulators which is supposed to be a 3.3v regulator. (Needs two 10uF Tantalum capacitors and 9v transformer according to specs on the PDF sheet). It is a bit higher than the 3volt rating of the laser - but most new AAA cells are 1.6 volts out the packet, and the laser "should" have its own voltage regulator anyway.

Would also reccomend a suitable fuse after the transformer (250mA will do)




Presuming the laser isn't above 100mA (would be interesting if it was ) then there should be no issue

This site should give a bit of info on how to put it together. And give you the maths to calculate the correct capacitor and resistor values :

Power Supplies

Shark Man

Sorry, I said those zener diodes are fine, when they aren't: You need ones with a Vz rating as close to the aimed voltage (3volts).

A BZX79C 3V0 should do the job - ensure its connected the right way round and has a suitable protective resistor: otherwise it'll blow : Maplin > 0.5W Zener Diodes


The datasheet on the maplin site is incorrect: this is teh correct one: (probably no use, but handy to compare the package polarity markings with the polarity of the circuit diagram: http://www.nxp.com/acrobat_download/...ts/BZX79_3.pdf )

OllyK

Well it's running on 2 xAAA and runs on a variable transformer at 3V, but then I guess it may blow up anytime soon!

OllyK

I'd like to see you fit one of those in the handle of a saw!!

That's what I am using and I don't really wan to run 2 sockets, I'd prefer to draw from the saw power supply.

OllyK

That's what I was using, but the way the saw was designed it's a separate battery pack. The great plan is to include a micro switch under the main switch in the handle so a half press triggers the laser. I "could" do it with batteries, just be nicer to not have to worry about them.

OllyK

That looks handy! Many thanks!