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Forum: Analog Circuits Choose hardware component - thyristor - minimum resistance/ voltage drop


von Hans L. (haled)


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Hello everyone,

I am currently working on a project, where I want to use a thyristor as 
a static switch that once turned on stays in its on (conductive) state. 
The problem is, that I can't find any information about the thyristors 
remaining collector emitter resistance once triggered. The datasheets 
don't provide that value. Also no shop provides any sort or filter 
option regarding this value. I would like to choose the component with 
the minimum resistance/ voltage drop in conductive mode.
I asked a question regarding this topic quite some time ago but it could 
not really be solved and now returning to this project I am stuck again.

I hope you can help me.
Thanks a lot :-)

von mark space (Guest)


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Physics did not change meanwhile. And so you will find an output 
saturation voltage specified in every data sheet, but no on-resisistance 
- this is no MOSFET.

von A-Freak (Guest)


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the voltage drop is with any thyristor somewhere around 1V, the sum of 
one BE-equivalent junction and the saturation voltage of the opposite 
CE-equivalent junction

The exact value depends on your current, also the doping levels where 
there is a trade off between switching speed and voltage

As an example i am refering to the BT145 that i recently used, 
https://de.rs-online.com/web/p/thyristoren/4842836/ where datasheet 
figure 10 lists the on-state voltage as a function of current

von Hans L. (haled)


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Thanks a lot for your answers!!
Pointing me to the datasheet and telling me what to look for with 
"on-state voltage" I could now figure out more what I have to search 
for. As you said all thyristors I could find have a voltage drop of 
around 1V so it doesn't seem to make sense for me to use them in my 12V 
and relatively low current application. I was now thinking about using 
alternatively a MOSFET and somehow create a self-holding circuit for it.
My problem in general is that I want to build a one cable solution 
meaning the switching MOSFET and its controlling circuit should sit on 
the cable that powers (+) the load without any direct connection to 
ground despite through the load itself. Can someone think of a way to do 
this? Maybe with a charge pump and using the on state resistance of the 
MOSFET?
Also please excuse my late reply. I invested some time getting familiar 
with LTspice and trying to come up with a solution myself.
I'm looking forward to possible answers :-)

von stefanus (Guest)


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Too much prose, draw a schematic!

von Hans L. (haled)


Attached files:

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Here is a basic schematic of the problem. The idea is that the switch 
should be not conductive at power up so the µC can boot. Then once 
triggered by the µC, the switch should stay conductive until power is 
cut.

von mark space (Guest)


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I would consider a latching (polarized) magnetic relay.

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