The green power button LED for my 1997 G3 MiniTower needs replacing. Does anyone know what voltage LED to use?
Thanks
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Use any LED you want. The LED output has a transistor drive circuit that limits the current dynamically to about 20mAh.
So, I dont have to worry about putting an LED with too high a voltage requirement in there?
no! ... for example running 10v LED on a 5v supply will just result in a dim LED
TOM
I want to err on the side of caution, so, does a 5 volt LED leave enough of a safety margin, or should i go higher?
Logic curcuits are almost always 5V and less. The voltage rating is a maximum figure... see "Ohm's Law".
SCSI Hard Drive activity LEDs do that too... I think mostly at slightly less current.
Ok ,guys n gals, i ran out ot Radio Hack today for an LED. I got a 5mm blue LED with a max. voltage of 4.5 volts. It is working fine and it looks freakin awsome
Oh, btw, the LED cost me about $3.00 and change with the sales tax.
As I mentioned above, the current is limited as part of the circuit. This means that the system increases the voltage until the current being drawn matches whatever it's set at. LED's are not really voltage driven devices (beyond a minimum). They are current driven devices. I could drive one off 120 V if I wanted to without any harm.
The voltage rating of the LED is only the minimum voltage required to get it to light up.
Again this is not relevant since the LED driver in that machine already current limits. The voltage rating is a minimum figure, NOT a maximum figure. Finally, the LED driver circuit in that machine is powered off the 12V rail, so it can raise the voltage up to 12V in order to source the requisite 20mAh of current.
Bob is correct. Current is a maximum figure, voltage will go to what ever is needed to reach that current. Voltage is just a way of regulating current, voltage doesnt kill components, the rise in current do to the voltage being changed is what does.
(Dang, this thing doesn't preview a quote within a quote right)
Yep, I screwed that up... looked at too many "LEDs for Dummies" websites.
My local Ratshack is pretty pathetic about keeping up their stock.
They've never had the 4.5V - 20 mA LED, so I picked up a 5V - 30 mA one.
I haven't tried it yet (too many projects), but I'm thinking that even if works on my 7500/G3 I'm not going to be all that happy... the "typical wavelength is 430nm" would probably be too purple. I'll probably order in some other blue LEDs, just to see what they're like.
430 is well into the purple range. Also those require an extraordinarily high drive voltage, and may be dimmer than optimal on a standard LED driver.