Hi All,

I have a crowbar circuit which I want to use as an e-fuse (over current protection not over voltage). Basically the volt drop across a sense resistor is used to trigger the gate of the SCR when the current through the resistor reaches a set level, shorting the circuit and concequently blowing the upstream fuse. However I want to know how quickly the SCR triggers, so I have done some calculations but I am not sure if they are correct. Would someone help me validate these calcs? Vcc is about 16 V, Vth is 1.7 V, and dV/dt is 10 V/us from the datasheet so I make the time about (16 - 1.7) / 10 = 1.43 us. Does this seem correct?

 

What is part number ?

https://www.littelfuse.com/~/media/...cteristics_of_thyristors_application_note.pdf


Regards, Dana.

 

What is part number ?
Regards, Dana.

Thanks for the app note link, I will take a look. The part number of the SCR is MCR08B from ON-Semi.

EDIT: This is a DC circuit by the way. Also, I've scanned the app note but it doesn't seem to indicate how to calculate turn on time.

 

The datasheet shows a turn on time for some specific conditions.

A more involved calculation is shown in this GE SCR manual (section 5) -

https://ia800209.us.archive.org/14/items/GESCRManual1964/GE - SCR Manual 1964.pdf


Regards, Dana.

 

Why not use a proper sized fuse to take care of an overcurrent condition?

 

From the ON semiconductor datasheet:



Additionally, this article might be of some help:
https://www.electrical4u.com/switching-of-on-off-characteristics-of-scr-turn-on-turn-off-time/

 

From the ON semiconductor datasheet:

View attachment 172229

Additionally, this article might be of some help:
https://www.electrical4u.com/switching-of-on-off-characteristics-of-scr-turn-on-turn-off-time/

Hi Dick, thanks for your reply. Duh! I missed that! I was looking at dynamic characeristics just underneath it. So can I clarify that when the SCR receives the correct current and voltage above the minimum threshold at it's gate it will turn on in 1.25 us? Is my calculation incorrect? I would like to validate this work hence my post.

Why not use a proper sized fuse to take care of an overcurrent condition?

@crutschow I am evaluating someone else's schematic, I am not permitted to make changes. FYI there is an upstream fuse that opens when the crowbar triggers.

 

FYI there is an upstream fuse that opens when the crowbar triggers.

Yes.
I meant why is that fuse size not selected for the overcurrent value that the protection is for?
Perhaps the circuit designer needed a more precise overcurrent limit than a fuse can provide.

 

-some text removed for clarity-
So can I clarify that when the SCR receives the correct current and voltage above the minimum threshold at it's gate it will turn on in 1.25 us? Is my calculation incorrect? I would like to validate this work hence my post.

That's the way I interpret the specification.

I am not familiar with the formula, but if you plug in the variables according to the datasheet and the answer comes out close 1.25 us then I would say it has a very good chance of being correct.