Thanks guys,

So........

1. leave the 100 ohm resistor 'R5' between 'Q1' and 'X2' ???

2. Add resister between 'Q1' and ground at a value of 1K ????

3. Capacitor 'C1' - What type of capacitor and value would you advise here ????

Something like this, choose R1 and R2 to have 2.5 in the midpoint at the trip voltage.

Ok, sorry im lost here, how do I do this:

"R1 and R2 to have 2.5 in the midpoint at the trip voltage"

I'm no expert, is there a formula I should use for this ???

Many Thanks
Paul

 

So with the inclusion of the other mentioned resister from transistor to ground, will it look something like this:

 

If the source has high enough resistance so the few doesn´t blow, then it limits the current through the gate so it doesn´t destroy the thyristor,

When an opto-triac is used to control a power triac switching mains; the current limiting resistor is usually only about 100 - 180R or so. If you do the math that isn't enough to protect the gate - that is if the full mains voltage were still there.............but it isn't, within a few tens of uS the triac has shunted that down to about a volt.

And some designers shunt the gate with a capacitor as well as a resistor to keep noise from false triggering. But you don't want any more than about 100nF or it delays switch on.

Thanks guys,

So........

1. leave the 100 ohm resistor 'R5' between 'Q1' and 'X2' ???

R5 isn't a do or die issue - some designers like to include it, but the higher its value - the slower the crowbar activates. Pulling a figure out of thin air, I'd go for around 27 - 33R.

2. Add resister between 'Q1' and ground at a value of 1K ????

1k is a typical "one size fits all" value, much more than that may not damp noise - much lower makes the transistor work harder. If noise and spikes cause false triggering; you can put a small capacitor in parallel with the gate resistor, but it will slow the crowbar action - especially if R5 is large. 100nF would be a good starting point, it shouldn't be far from that value - maybe smaller if anything.

3. Capacitor 'C1' - What type of capacitor and value would you advise here ????



Ok, sorry im lost here, how do I do this:

"R1 and R2 to have 2.5 in the midpoint at the trip voltage"

I'm no expert, is there a formula I should use for this ???

The TL431 "programmable zener" is actually a comparator with a built in 2.5V Vref generator. The circuit you described rolls it off with enough nfb to make it operate in a linear mode. At the desired trip voltage; the potential at the voltage divider should just tip over 2.5V - that makes the TL431 clamp down to about 2V and throw the rest of the circuit into action.

The TL431 is nearly as prolific as the 555, most semiconductor manufacturers offer it.

Between all those manufacturers - there's about 4 or 5 versions of the appnote - I'm pretty sure all of them give formulas for the divider.

Many Thanks
Paul

 

Ian,
27 ohm's and 100nf it is then

Wait a minuet

What actually is 'X1' ???

Zener diodes don't have 3 terminals so what is 'X1' ?

 

X1 is a TL431 programmable zener diode.

Note your circuit is drawn wrong its shorting out the zener.

 

Dodgy,

The circuit diagram is exactly what Kubeek posted on page one, all I did was add the resister 'R6' from 'Q1' to ground.

I'm assuming your referring to the link which goes to ground from 'R4' ??

Should this link be removed ?

Paul

 

If the source has high enough resistance so the few doesn´t blow, then it limits the current through the gate so it doesn´t destroy the thyristor, especially with higher fuse rating and higher voltage.
Right, forgot to draw that one. Also a resistor between the gate and ground is a good idea to prevent triggering due to leakage of the pnp.

Transistor leakage probably isn't a problem unless using a sensitive gate thyristor.

The biggest issue is noise.

 

Hello all,
I have miraculously come across some MC3423's for my Crowbar circuits, lets hope they work eh ?

So I'm fine with the voltage divider side of things for setting my trip voltage but I do have one question which im not 100% sure on.....

Based on the following datasheet:

http://www.onsemi.com/pub_link/Collateral/MC3423-D.PDF

Based on one of my DC-DC unit outputting 12V and the intention to trip at 13V

and also based on the following Thyristor I'm using:

http://uk.farnell.com/on-semiconductor/2n6509g/thyristor-25a-800v-to-220/dp/9557202

Ow, and not forgetting the circuit im using:

https://www.google.co.uk/search?q=MC3423+crowbar+circuit&biw=1680&bih=902&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjD0L_t2enMAhWjKcAKHZryD6UQsAQIKQ#imgrc=xbodhPXdUWl9iM:

I'm not 100% sure how to work out the value (and wattage requirement) of the resistor Rg in the circuit which will compliment my Thyristor's, the resistor between the Thyristor 'Gate' to pin 8 of the MC3423.

Can anyone help me ? I'm not after a straight answer here, I like to learn you know. I clocked the graph in the MC3423 datasheet ref the Rg resistor which only seems to refer to the Voltage (12V would be around 4.7 ohms on the graph). Is it that simple or is this resistor value also influenced by the characteristics of my chosen Thyristor ??

Any help, as usual, would be gratefully received
Paul