Re: Basic Circuit Question

On Fri, 28 Oct 2005 06:26:08 -0700, faceman28208 wrote:

> I am trying to debug a kit that I bought that isn't working. I am
> trying to test various links on the board with a an LED and resistor
> soldered together.
>
> When I hit the put the ends of the LED/resistor at a point on the board
> with a direct link to the power supply, it lights up (so the obvious
> isn't a problem).
>
> If I move my LED/resistor pair to the next in the chain, a diode I get:
>
> (-V9) ------- (- Diode +) ---- (- LED + ) --- (Resistor) --- (+9V)
>
> and the LED no longer lights. Should I expect it to light up,
> suggesting there is a problem with the Diode? Or is there something
> more to diode that I am unaware of (I just thought they were one-way
> current blockers -- suggesting the above should work).
>
> The Diode is a TN4001. Resistor is 267Ohm. Foward voltage on the LED is
> 3.7.

You have one end of the diode marked "+" here. Did you know that the
band on a diode is usually the cathode (and with the 4001, always),
which in schematics is sometimes marked "+", which that end will be
_inside a power supply_. Outside the power supply, the diode will
conduct when the banded end is more negative than the blank end by
a diode drop, about .7V.

Hope This Helps!
Rich


.

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