NNTP-Posting-Date: Wed, 30 Oct 2002 23:36:16 -0600
From: "David Jones"
Subject: Re: so frustrated!
Date: Thu, 31 Oct 2002 00:32:16 -0500
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thanks, I think that if I think about things in respect to the cathode it
works out fine. I just don't know if it will always work well to think of it
that way. I guess now that I can get buy with understanding it that way I
can get used to it and will eventually understand it well. thanks
"Tom Bruhns" wrote in message
> Hi David,
> 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
> at it:
> 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
> > 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
> > have a circuit with two resistors in series with the gate of an SCR in
> > 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
> > the gate is -3.5 volts, the cathode is -9volts and the anode is 0. is
> > forward biased or not. the gate is positive with respect to the
> > 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
> > each time I get frustrated and give up so what the hell am I doing
> > some one give me some peace!