switching

[stirling]

I need to switch 18V 10mA via TTL (turn on/off a plasma torch via the parallel port).

A relay seems a bit overkill for this - doesn't it? and I read somewhere (I think) that such low current is not best switched by a SSR - true? or have I made that up?

Thoughts on cheapest/bestest/nicest/properest way to do this?

Cheers

Ian

[manmeran]

18V 10mA

cheapest way is transistor.
on-off

Amir

[Tweakie.CNC]

Ian,

The common SSR's need to switch a/c loads as they require the zero crossing point for switching off but having said that I switch my cross hair pointer on / off with a mains connected SSR which powers the wall wart to output the 5Vdc for the pointer. The SSR, which has internal opto isolation, is powered directly from the LPT port.

Tweakie.

[stirling]

OK guys - thanks.

Tweakie - have you looked at logic level mosfets for your cross hair pointer - they sound like the mut's nuts for that.

Ian

[Tweakie.CNC]

Thanks Ian but I needed -5volts (relative to the frame) because the body of the pointer is connected to + so it was easier, for me at the time, to get the required negative voltage from a wall wart.
Yes I agree that these logic level switching MOSFET's would be brilliant for a lot of applications. (more circuit board routing is on the cards) 

Tweakie.

[manmeran]

logic level mosfets

Using them is not suitable for this issue.
you need only 10ma .you can build a simple circuit by transistor.

Amir

[stirling]

I was suggesting them for Tweakie's laser not my trigger Amir. Keep up 

[manmeran]

sorry. my mistake