thyristor leakage

Thread Starter

Zara Engineer

Joined Feb 6, 2015
53
Hello everyone


I have built a circuit to switch a thyristor* on and off .

The circuit is like the circuit below.



when the gate is on all is right and the device is latched when the gate voltage is removed , but the problem that if I remove the connection of anode (+5v)(with Vcontrol = 5V) the circuit continues to conduct with Voltage of cathode = 0.55V.(VR2).

Maybe this voltage hasn't any effect but why that's happen? is there any leakage current from gate to cathode !

*BT151 :
 

bertus

Joined Apr 5, 2008
20,230
Hello,

A thysistor will keep conducting (once triggered) until the powersupply is removed.
Normaly the thyristors are used in AC circuits.
In DC circuits the thyristor can be used as a latch.

Bertus
 

Thread Starter

Zara Engineer

Joined Feb 6, 2015
53
hello,

ok no problem that the thyristor causes a latch to the device , but my problem here that the gate still conducting and the anode is removed, the result that I found a cathode voltage = 0.55 , what's the reason for this leakage whatever it conducts AC or DC or the device is latching or not!

thank you in advance
 

Thread Starter

Zara Engineer

Joined Feb 6, 2015
53
hello ,

My component is an SCR , I don't have any problem with the basics of scr , my problem is clear , what's the reason of leakage with no voltage applied on anode , voltage only applied on gate and source !

thank you Bertus
 

Johann

Joined Nov 27, 2006
190
It looks like a sensitive gate. Desensitize it by inserting a 1 k Ohm or so resistor between gate and cathode.
 

Thread Starter

Zara Engineer

Joined Feb 6, 2015
53
Hello Johann,

How can I know this sensitivity , is it given on datasheet ? is it for all types of scr ?

I didn't read about that in the scr basics !

but if we take your suggestion in my circuit , it will be like this :

circuit3.PNG

if we do the calculation for circuit we have here a voltage divider : Vk=Vg*(100)/(470+1000+100) = 0.31 V.

so ?
 
Last edited:

Johann

Joined Nov 27, 2006
190
Hello Zara,
I did not check out the specs for your scr, but basically you get scrs with "normal" sensitivity and some like c106D which is a so-called sensitive gate device. It almost triggers when the wind blows over the gate, but should be used in conjunction with a resistor between gate and cathode to prevent false triggering. When I read about your problem earlier on, It just crossed my mind, so I commented.
Is the control voltage ever removed, or is it there all the time?
 

Johann

Joined Nov 27, 2006
190
Try disconnecting on the cathote side instead of on the anode side. This will interrupt both the anode current and the gate current simultaneously and the scr should turn off.
I mean, disconnect the ground instead of the anode.
 
Last edited:

Johann

Joined Nov 27, 2006
190
Hello Johann,

How can I know this sensitivity , is it given on datasheet ? is it for all types of scr ?

I didn't read about that in the scr basics !

but if we take your suggestion in my circuit , it will be like this :

View attachment 98189

if we do the calculation for circuit we have here a voltage divider : Vk=Vg*(100)/(470+1000+100) = 0.31 V.

so ?
Remember the gate to cathode is also a low resistance in parallel with the new 1k.
 

Johann

Joined Nov 27, 2006
190
Rather connect your load between positive supply and the anode and take the cathode straight to gnd. This way there will not be any signal current flowing in the load.
 

Johann

Joined Nov 27, 2006
190
Your scr, the BT151 is not a sensitive gate scr. The problem seems to be the position of your load with the constant trigger level that you have.
As I have mentioned before, rather put the load on the Anode side. Idealy, when you interrupt the anode current to turn the scr conduction off, you should simultaneously remove the gate control also until you need to turn on the scr again.
 

Alec_t

Joined Sep 17, 2013
10,728
If the control input is continuous then there will be a current flowing continuously through the gate/cathode junction of the scr.
 

Thread Starter

Zara Engineer

Joined Feb 6, 2015
53
If the control input is continuous then there will be a current flowing continuously through the gate/cathode junction of the scr.

That is not leakage. That is intentional current that you are supplying through the gate terminal.
whatever the anode ?
is it a characteristic for thyristor only or for bipolar transistor , because I didn't read anything about that in the basics of transistor !
 
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