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 :

 

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

 

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

 

Hello,

Have a look at the attached PDF.
It will give you the theory and schematics for thyristors and triacs.

Bertus

 

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

 

It looks like a sensitive gate. Desensitize it by inserting a 1 k Ohm or so resistor between gate and cathode.

 

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 :



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

so ?

 

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?

 

hello again,

sorry but I edited my reply above please reread it .

no the gate voltage is all the time .

 

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.

 

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.

 

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.

 

hello Johann,

Ok thank you very much, but about the sensitivity can you help more to understand this idea .

 

hello Johann,

Ok thank you very much, but about the sensitivity can you help more to understand this idea .

It is all in the design of the actual scr. Please see page 40 of the following for detailed explanation:

http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF

Hope it helps!
Johann

 

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.

 

OK johann thank you .
I will change the position of my load from low side to high side.

 

It is all in the design of the actual scr. Please see page 40 of the following for detailed explanation:

http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF

Hope it helps!
Johann

very helpful book

 

If the control input is continuous then there will be a current flowing continuously through the gate/cathode junction of the scr.

 

the gate voltage is all the time

That is not leakage. That is intentional current that you are supplying through the gate terminal.

 

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 !