On Mon, 3 Dec 2007 02:23:35 -0800 (PST),
wrote:

No. for bipolar transistors there's a characteristic named 'beta'
(or Hfe) which specifies the current gain of the transistor:

Ic
Hfe = ----
Ib

For example, if a transistor is specified as having a minimum beta
of 100 with 100mA of collector current, then the maximum base
current required to get that 100mA of collector current will be:

Ic 100mA
Ib = ----- = ------- = 1mA
Hfe 100
Ok, got that.

For MOSFETs, in this type of application, all that's necessary is to
raise the voltage on the gate high enough to lower the resistance of
the drain-to-source channel to the point where its resistance is
negligible with respect to the load.

Why am I talking about Collectors,Bases, Emitters and you talk about
drains and sources. Are these the same? Why the different naming then?

---
Bipolar Junction Transistors (BJT) and Metal Oxide Semiconductor
Field Effect Transistors (MOSFET) are two completely different
animals, but they each have three terminals with the bipolar's base
corresponding to the MOSFET's gate, the bipolar collector
corresponding to the MOSFET drain and the bipolar emitter
corresponding to the MOSFET source.

For the BJT:

http://en.wikipedia.org/wiki/Bipolar...ion_transistor

and the MOSFET:

http://en.wikipedia.org/wiki/MOSFET
---

And then of course, my next question will be if you can suggest an
appropriate mosfet from my local shop:www.radionics.ie

---
Which circuit are you planning on using?

This seems the simplest. (stolen from your message above)
220AC---[FUSE]-- |
|S1A
O T1 FWB
| O O +-----+
+----P||S---|~ +|---+--------+
| R||E | | | |
| +--I||C-+ | | |+ [SOL]
| | | | |[4700µF] |
| | T2 | | | |100V D
| | O O | | | | IRL3215G--+
+-|--P||S-+ | | | S |
| R||E | | | | |
+--I||C---|$ -|---+--------+ |
| +-----+ | |
O | |
|S1B | |
220AC------------ | | |
| |
GND---------------------------------------------+ |
|
PIC--------------------------------------------------+

Thanks a lot for all your help. I really appreciate it.

---
You're welcome. Use an IRL3215.

Also, just to be safe,

220AC---[FUSE]-- |
|S1A
O T1 FWB
| O O +-----+
+----P||S---|~ +|---+-------+-----+
| R||E | | | | |A
| +--I||C-+ | | |+ [SOL][1N4003]
| | | | |[4700µF] | |
| | | | | |100V +-----+
| | T2 | | | | D
| | O O | | | | IRL3215 G--+
+-|--P||S-+ | | | S |
| R||E | | | | |
+--I||C---|~ -|---+-------+ |
| +-----+ | |
O +--[ZENER]--+
|S1B | 1N4734A |
220AC------------ | | |
| [10K]
GND------------------------------------+ |
|
PIC-------------------------------------------------+


--
JF