[AndrewJ]

Gday Fahim

What if the PWM output from the PICAXE was rectified, would this not reduce the RFI?

Yep, but it would kill the Mosfet
To use PWM, you want the mosfet to toggle between hard on or hard off. This gives the minimal resistance drop across the mosfet.
To try and use it like a transistor, ( where output voltage is proportional to base input voltage ), it will overheat ( and die ) quickly
Thus, the major RFI comes from the leads, not the controller.

I looked at the Microzed page, and I found the PICAXE-28 has a few more PWM outputs.

It only has two, hence, for yr 4 channels, its probably better to look at a distributed system.

What is the diffrence between the A and X series?

Different underlying PIC
PICAXE is just a std PIC with a proprietary "basic" interpreter loaded ( hence its slower running ). The A vs the X just signifies what type of PIC chip its based on, and hence its extra capabilities.

Is there a good internet resource for PICAXE and PIC newbies.

I wouldnt suggest PIC straight up, but for PICAXE, there is a yahoo group
http://groups.yahoo.com/group/picaxe/
and also a REvEd forum group
http://www.rev-ed.co.uk/picaxe/forum/
lots of good resources and links there.

Am I correct in assuming the PIC/PICAXE is not a uP but a controller with a uP inside, designed for general applications.

Nope. As per above, the PICAXE is just a raw PIC, preprogrammed with a basic interpreter that knows how to do lots of things ( but not all )
Ie, it has a single command to read a dallas onewire digital temp sensor ( but only allows one sensor per input )
It also has a command to output PWM, etc etc
download the software and have a look through all the examples for whats available.
What isnt available can sometimes be bit banged if necessary.

I designed with at Uni one of the motorolla's, had an address bus, data bus, IRQ's etc etc and you provided the program from a ROM chip. Am I correct to assume then the PIC/PICAXE has all of these inside?

Yep. Its basically the same process, but now you have a human readable code generator between you and all the guts of the PIC
As such, you program at a high level, and it takes care of the rest ( within limits ).
As i mentioned earlier, download the software and read the manuals / tutes on what it can and cant do.

Ie for my basic controller, the program is basically
read temp sensor air
read temp sensor ota
if temp air - temp OTA > 2 set pwm to 100
if temp air - temp OTA < 0 set pwm off
if between, set PWM to a proportional value
This provides a ramping application of power that avoids having the system hunt or thump on/off
( my no 2 circuit also works this way )
ie i can programatically ramp the power based on temp difference.
Also, as it has an onboard ADC, you can measure yr battery voltage
and warn/shutoff the sytem before it damages yr battery.
Lots of options available.

Andrew