From: email@example.com (Tom Bruhns)
Subject: Re: so frustrated!
Date: 30 Oct 2002 10:52:23 -0800
NNTP-Posting-Date: 30 Oct 2002 18:52:24 GMT
I imagine you'll get plenty of answers to this. Some will be
obviously not useful. But read all the others...somewhere in here is
likely to be something that turns on a light for you. Here's my stab
In SCRs, triacs, bipolar transistors, and some other devices, there
are multiple junctions. Normally, the forward bias refers to one
_junction_, not a single terminal. So in an SCR, the junction you
want to forward bias to turn it on is the gate-cathode juntion. To
get to the anode from the gate, you actually go through two junctions.
It's possible to make a "reverse polarity" SCR, but the vast majority
of them forward-bias the gate-cathode junction by having the gate more
positive than the cathode. And if it's not burned out, a junction in
silicon typically begins to conduct significantly around half a volt,
and by the time you get to a volt, the current is quite high... so if
you actually measure -3.5V on the gate an -9V on the cathode at the
same time, that's (-3.5V - -9V) = 5.5V of forward bias, and that's
unreasonable for a single silicon junction.
For an SCR, if the anode is negative with respect to the cathode (but
within the voltage rating of the SCR), you get very little current in
the anode lead, even if the gate-cathode junction is forward biased.
But if the anode is positive with respect to the anode (by at least a
volt), when you forward bias the gate-cathode junction, the anode will
start conducting strongly, and it will supply current into that
gate-cathode junction to keep the SCR "turned on" until you drop both
the anode and the gate current to a low enough value.
In the case of the resistors feeding that junction, when the junction
(the gate lead) starts to conduct current, the current will be limited
by the resistor. It's a balancing act: some of the current from the
"upper" resistor will go to the lower resistor, but some also to the
gate, and that will keep the gate-cathode junction voltage from ever
getting very high.
For some people, it helps a lot to get some parts and a power supply
(even just batteries) and a voltmeter (you can get a DVM these days
for as little as $5!), and try things out, measuring various
conditions and thinking about them till you understand what's going
"David Jones" wrote in message news:...
> I can't figure out why some circuits become forward biased sometimes. I
> would draw you a schematic but the last time I tried it didnt' work. if you
> have a circuit with two resistors in series with the gate of an SCR in the
> middle of the resistors, how do you tell weather the SCR is going to be
> forward biased or not? I can't figure it out for the life of me. lets say
> the gate is -3.5 volts, the cathode is -9volts and the anode is 0. is it
> forward biased or not. the gate is positive with respect to the cathode,
> yet negative with respect to the anode. this may seem simple at first
> glance but I assure you I have tried to learn electronics several times and
> each time I get frustrated and give up so what the hell am I doing wrong?!
> some one give me some peace!