ok I see.
My Local electronics store dosnt actually have a search function or datasheets for their MOSFETS. Might have to try and go down to find someone to speak to.

I wouldn't waste my time, unless you know there's a "genius" there. Just Google.

 

Well... This store has been there since i was a boy, so many years. Called up, within a minute I was given this part number

2SJ512
http://www.classiccmp.org/rtellason/transdata/2sj512.pdf

They have that and in stock. Could you possibly just run over the spec for me to make sure I haven't missed anything important?

 

That chip will have about 1 ohm of resistance. Page 3, bottom right graph. That will eat up about 4.5 volts for 4.5 amps of flow and cause a bit over 20 watts of heating. This is why one usually chooses a MOSFET that is rated at a lot more amps that you are going to use. They have less "on" resistance.

At 20+ watts, this is much more likely to die of overheating that any chip previously mentioned.

 

I agree. I read somewhere - and have been using - a rule of thumb that the quoted, continuous current rating when choosing a MOSFET should be at least 4X whatever your expected current will be. Great attention to design could probably narrow the gap but my feeling is, so what? It costs very little extra to go large. For your 4.5A, that's an 18A rating or better.

 

ok thanks guys, they dont seem to have anything higher in a P
I think I'll build a test circuit using the IRF540A that ive got and see how that goes, will try to call the Voltage Regulator guys to see if there is anything wrong with low side switching.
Once completed I'll post up a pic

Many thanks again guys