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Re: SCR based coils (was High-Power Car Ignition Coils)

Normal SCR's will switch on only with positive gate current. The max 
pulsed value of gate current permissible should be quite a good deal 
more than cont. gate current rating - check the spec on the device you 
are using though. There is a fairly obscure scr variant known as a GTO - 
Gate Turn Off thyristor - in this a + gate current switches the device 
on - and this device functions essentially identically to a standard SCR 
except that a - gate current will trigger the device OFF. I tested a few 
of these that I found recently and noted that the gate current for turn 
off is higher than the gate current for turn on. TRIACS are AC switches 
and not only will conduct in either direction but can also be triggered 
by either polarity of gate current - as there are 4 possible operating 
modes here they are referred to as 'quadrants' ie main supp + gate+  / 
ms + g- / ms - g+ / ms- g-
operation in 3 of the 4 quadrants is preferred for most triacs, with + 
gate current (2cases) being 2 of the 3 preferred (I forget which the 
other usual one is). You can get some which are '4 quadrant guaranteed 
', but in any case, the + gate modes are generally faster and/or req. 
less current. SCR's however are much faster than triacs of equivalent 
ratings and so are preferable unless you are req'ing ac switching. dv/dt 
exceeding spec'd max may cause false triggering due to current pulse 
charging parasitic gate capacitance, and also very important is di/dt - 
excess rate of current rise as device switches on can lead to hot spots 
in the device resulting in premature failure. Typical stud mount devices 
may req around 100ma for trig , hockey pucks can require substantially 
more than that. I think a higher gate current will lead to faster turn 
on time, but di/dt rating should be observed - External circuit 
inductances will have a large effect on this, with some very large scr 
systems I have seen using di/dt limiting inductors on each device.
Dan


>From tesla-request-at-pupman-dot-com Mon Jan 11 19:42:55 1999
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>Subject: Re: SCR based coils (was High-Power Car Ignition Coils)
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>Original Poster: "Harri Suomalainen" <harri.suomalainen-at-pp.inet.fi> 
>
>>Original Poster: "Bill the arcstarter" <arcstarter-at-hotmail-dot-com> 
>
>>The question is how to size the gate resistor.  Too large a resistor 
>>won't allow a quick turn-on of the part, 
>
>..or won't allow turn-on of the part at all!
>
>>but too small a resistor would 
>>allow a possibly large gate current. 
>
>Large currents are preferred. You should check the data sheets for 
mininum
>current needed to trigger the device. With large SCR's this may be 
around
>100mA or so!
>
>Current required depends also on polarity. Typically negative currents 
needed
>are smaller than positive current. Negative current is therefore 
preferred.
>
>Then there is the current rise time: scr will shut down when current 
falls
>below
>certain level. This means also that during turn-on pulse current must 
rise
>above this value or scr will turn off as soon as the trigger pulse 
ends.
>
>>This problem appears to be 
>>aggravated by the fact that the voltage across the resistor depends on 
>>the firing angle.
>
>One possibilitity would be using something like a cap charged by the 
rest
>of the system as a "floating supply" for trigger current. There would 
be no
>problems then.
>
>>(The circuit doesn't show the 100 ohm 0.1 uf snubber which is attached 
>>across the triac terminals)
>
>Snubbing is a must. Without suitable snubber inductive spike at 
turn-off
>can kill the scr. If it does not kill the scr there can also be very 
high
>dv/dt
>which is coupled to the gate through small parasitic cap. This can 
cause
>spurious turn-ons.
>
>These subjects are covered nicely on some SGS app notes available at
>their web-site. (Sorry, no url)
>
>>Now I know why most industrial designs use a small pulse transformer 
>>between the cathode and the gate - rather than shunting current from 
the 
>>anode into the gate...
>
>
>That simplifies things. Why would you not use one?
>
>
>


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