can someone help me trouble shoot this?

Thread Starter

tuan.le007

Joined Nov 29, 2016
46
I have these work instructions
upload_2017-9-14_16-23-38.png

upload_2017-9-14_16-24-46.png

I have that circuit. When I apply 10V to A1 and A3. My DMM reads 1.3V right away.
I think my circuit is already shorted somewhere but I don't know where to start.
If my fuse is out (F1), I should read 10V.
can someone sugguest some ideas to troubleshoot on why?
Thanks
 

GopherT

Joined Nov 23, 2012
8,012
I have these work instructions
View attachment 135079

View attachment 135080

I have that circuit. When I apply 10V to A1 and A3. My DMM reads 1.3V right away.
I think my circuit is already shorted somewhere but I don't know where to start.
If my fuse is out (F1), I should read 10V.
can someone sugguest some ideas to troubleshoot on why?
Thanks
Your circuit is a type of "crowbar". The circuit is named afte the crowbar which is a large steel claw-like tool for breaking and prying things apart. And, in context, "dropping a crowbar across that power supply will certainly blow that fuse."

So, it is used to instantly blow a fuse if there is too much current drawn from the supply. In cases where the designer is concerned that a fuse near its limit would take too long to blow or the tolerances of a fuse may be too broad.

This circuit instantly knows when to essentially create a "short" with a big SCR and blow the fuse in an instant.

So, yes, your fuse is likely blown if the Crowbar circuit was set up incorrectly.

Also, SCRs are used in crowbars because they can handle a large amp load (to immediatly pop the fuse) and once they are turned on, they don't turn off until the supply voltage is cut (or the fuse pops).
 
Last edited:

AlbertHall

Joined Jun 4, 2014
10,400
Check for a short-circuit across the SCR (A to K).
If it reads less than 10Ω, you should disconnect the SCR and check it again. If it still reads short then you have found the problem. If it is OK then the short is elsewhere.
 

Thread Starter

tuan.le007

Joined Nov 29, 2016
46
"Also, SCRs are used in crowbars because they can handle a large amp load (to immediatly pop the fuse) and once they are turned on, they don't turn off until the supply voltage is cut (or the fuse pops).

Hi,
So yes, the fuse is bad and I replaced it with a new one.
I am trying to find out if the SCRs are bad or not.
it was mentioned that if the SCRs are turned on, how do I turn it off?
I am not cleared on that
 

GopherT

Joined Nov 23, 2012
8,012
"Also, SCRs are used in crowbars because they can handle a large amp load (to immediatly pop the fuse) and once they are turned on, they don't turn off until the supply voltage is cut (or the fuse pops).

Hi,
So yes, the fuse is bad and I replaced it with a new one.
I am trying to find out if the SCRs are bad or not.
it was mentioned that if the SCRs are turned on, how do I turn it off?
I am not cleared on that
To turn it off, drop the gate voltage to ground AND remove stop current flow from anode to cathode.
 

Thread Starter

tuan.le007

Joined Nov 29, 2016
46
upload_2017-9-18_16-13-14.png

I tried replacing Q1 and that didn't fix the problem.
CR1 is my other suspect.
What do I need to do in order to make sure its that?
 

AlbertHall

Joined Jun 4, 2014
10,400
It's not an AC source, it is running on DC. Q1 (SCR) acts as a crowbar to blow the fuse when tripped under a fault condition. From the circuit diagram, CR1 has a breakover voltage of 8.2V and when that is exceeded it becomes low impedance and dumps the charge from C1 into the gate of Q1 tripping the crowbar.
 
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