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From: firstname.lastname@example.org (R.Legg)
Subject: Re: so frustrated!
Date: 30 Oct 2002 07:20:17 -0800
NNTP-Posting-Date: 30 Oct 2002 15:20:17 GMT
"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!
SCRs have differing gate sensitivities, depending upon internally
present resistive elements shunting the gate junction. Consult the
data sheet for the device that you are trying to use. There are
excellent app notes available for these devices. Consult the mfr's
SCRs are only used to control DC and will block current in the reverse
direction, even after being turned on in the normal direction. Triacs
can be controlled in both directions, using both polarities of gate
drive, with differing gate sensitivity to gate current, depending on
the quadrant. Again, consult the data sheet for the device being used.
Both devices are sensitive to re-applied dV/dT, and may self-fire if
the voltage rises at a rate faster than specified. This voltage can be
controlled with capacitive snubbing, if the load impedance and current
are within a known range of values. dV/dT sensitivity and normal gate
firing sensitivity change dramatically with temperature. Design for
worst case conditions.
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