How to switch an EL inverter with my HDD LED pins?

I have an EL wire (incorrectly known as 'neon string' I believe) that I'd
like to use as a replacement for the HDD LED on my PC. The HDD LED pins are
usually at around 5v each but one of them (that I'll call the switching
line) drops to about 2v when the HDD is active. The EL wire's inverter
requires 5v at about 200mA.

I think I need to wire an NPN transistor up so that the collector is
connected to 5v, the emitter feeds the +ve side of the load (with the
load's -ve connected to 0v) and the base is connected via a resistor to the
HDD LED header's switching line, so to speak.

On paper it looks super (and simple) but I'm getting stuck on the maths:
IIRC an NPN transistor will allow current to flow from collector to emitter
as long as there's a [proportionally smaller] current drawn from the base. I
can work out that the HDD LED was probably drawing around 30mA and so I
shouldn't hurt my motherboard if I draw around the same current through the
base of my transistor (LED usually has 3V across it when on, so a 100 ohm
resistor between the HDD LED pin and the transistor base will still look
roughly like a LED to the m/bd, right?).

I'm drawing a current through the collector and base, so the transistor
should turn on and allow current to flow between the collector and emitter,
powering the EL inverter.

This is where I start to need help ( / reassurance / slap upside head etc ):

- What transistor should I be using? I have a selection pack in front of me,
witha very useful reference sheet, and I can certainly choose one that is
NPN and will handle 200mA, but what other considerations are there? They
only seem to come in black or silver, which doesn't match the rest of my
colour scheme [joke]
- If the EL inverter has anything fancy going on inside it like transformers
and capacitors and suchlike, should I be providing my one-resistor,
one-transistor circuit with ... ummm.... backlash protection?
- Every circuit diagram I ever look at has caps all over it, some of which
are AIUI to absorb RF and stop it interfering, getting amplified etc - will
my circuit need any? Sort of feel it's lacking without. Not sure what
flavour / value to use.


Any suggestions will be much appreciated before I let the magic smoke out of
my motherboard

Thanks for reading,

h

 

A *PNP* transistor would work with the active-low system, but for *minimum*
risk to the motherboard use a cheap opto-isolator with its diode connected
in place of the case LED, then boost the current-handling with a NPN such as
a 2N2222A, which will switch 600mA or more.

Example circuit at http://www.mexbro.co.uk/bit/optobooster.gif , shown for
12V, for 5V you can drop the base resistor to 1k.

You can add a 1N4148 diode in series with the optoisolator's IR led to bring
the combined forward voltage more in line with a 2V green or amber case led,
but it's not essential, the motherboard's current-limiting will cope with
the lower Vf.

The inverter has an oscillator as first stage, so AFAICS you won't need a
diode across the output.

 

<snip>

That's super - much appreciated and many thanks,

h