Increasing the SCR holding current

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newton brawn

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Hi All !

I have one capacitor discharging circuit with a 0.15 uF capacitor that discharge 230V into a 5uH coil at the rate of 120~360 times per second. The discharge is provided by a SCR, TYN1225, that have a holding current of 100A/us. The circuit worksfine, HOWEVER I want increase the capacitor and reduce the inductance. In order to keep the rate at 360times per second I need increase the charging current that is limited by SCR holding current.
Are there any method of increasing the SCR holding current?
A resistor , cap or inductance bettwen gate / cathode could modify the SCR holding current ?
Thanks in advance for any suggestion.
Newton

EDIT: PLEASE READ "a holding curret of 50mA ", not 100A/us.
Sorry for the confusion.
 
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100 A/µs would be a dI/dt (dynamic current rise) rating rather than holding current. It's limited by the time the thyristor needs to spread the conductive zone over the full chip and can't be arbitrarily increased (at best slightly by raising the trigger current).
 
Shunting the base (anode gate to anode, or cathode
gate to cathode, depending on your SCR's construction)
will increase the holding current to Vbe/Rshunt roughly.

This of course depends on the trigger path having a
decently low access resistance - I have a patent on a
high voltage ESD clamp that used this scheme, but the
layout was critical and of the permutations I tried (a
fully dielectric isolated SCR all in a single tub) some
were "quenchable" via the AG alone, some CG alone,
only one with both being effective. All depends on the
close-in base resistance, which is in series with your
external shunt.

You might look into "GTO" (gate turnoff) SCRs, these
must have a low impedance access to work.
 

Hi FvM !
For sure 100A/us is the di/dt dynamic curret rise,
Thanks for your advise and sorry for my confusion. I just put a note in the post as "edit " to clarify my question.
By the way iI am looking for a method of incresi the scr holding current.
Regards
Newton
 

Yes, a method to increase the holding current has been discussed by dick_freebird.

I don't however understand why there should be a holding current problem in the present application. In C discharge with some amount of inductance, the thyristor will be usually switched off on the first current zero crossing of the oscillating discharge, independent of the holding current.
 


Hi FvM !

Here is the circuit:

In the schematic the SIDAC symbol is a SCR and a SIDAC ( sidadc shunting the anode to the gate )

I am afraid that increasing the capacitor Cd and reducing the Rs the SCR-SIDAC could be continuously conducting.

Thanks for any advise

Newton
 
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SIDAC/SCR combination for AC? That would damage the thyristor.

The circuit is far from usual gas ignition circuits by the Cd and L2 parts, and the behavior is hardly predictable without knowing the exact component parameters. You can either refer to known working ignition circuits or have to evaluate the conditions for safe operation empirically.
 

FvM:

To be more precise, there are two SCR in an antiparallel configuration, and 2 SIDACS. This arrangmentis to allow a higher discharge current.

The circuit as is posted is for ignition of gas discharge bulbs, mercury vapour, or sodiun, but is going to be modified to ignite TIG torches.

For calculation of the transients I have used the posted circuit.

The components values are:
Tt =15:300 turns, in a "C"" ferrite core 2sq cm cross section,
Ct = 0.15uF 500V
Cd = 0.33uF 500V
SIDAG ignition volts = 220V
5 uH is air core, 20-25 turns #22AWG, 25mm diameter air core
and the Xl=60 ohm refers to 60Hz.

If you need more info let me know
hanks
 
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I am not familiar with gaz ignition circuits, so my suggestion is just a guessing.
Is it possible adding a branch in parallel with Rs that consists of a capacitor and a resistor in series?
 

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