[SOLVED] Need help selecting thyristor

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Zak28

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Need to discharges a capacitor and with LTSPICE I measured the pulse current to be 136.44A RMS as seen in the pic but it peaks @ ~215amps thus my confusion for selecting an appropriate thyristor. The cap peaks @ 166v peak (not rms). The cap will be discharged @ same current every 600ms thus I stated current, potential and duration for thyristor.

A pic of cap discharge pulse.


Greatly appreciated.
 
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You can select the SCR looking at I2t rating. The shown discharge waveform however can't be achieved with a SCR. It's actively terminated by opening the switch, not feasible with SCR. Instead the discharge will stop at the current zero crossing after about doubled time.
 
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    Zak28

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Hi,

I wonder... you say you want to use a thyristor ... but why didn´t you use a thristor in your simulation?
Your simulation doesn´t show thyristor behaviour and nor does it show typical voltage drop of a thristor. (at least it seems so)

I can´t see how a thyristor can create the same current waveform as in your simulation.
You know a thyristor is triggered to switch ON, but then it stays ON until the current comes close to zero. But in your case the current immediately stops.

Are you sure your capacitor can supply and withstand a current of about 200A.

Klaus

Added:
(FvM was faster than me...)
 
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    Zak28

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The current waveform with SCR switch is a half-sine, same peak current. You get an I²t of about 15 A²s, means a small 6 or 10 A SCR can already handle it. I would go for 16 or 25A SCR class to get some margin.

- - - Updated - - -

Here's a mini simulation
 

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  • SCR cap discharge.zip
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    Zak28

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Correct me if Im wrong but I believe 16RIA80 will work just fine as stated second page in datasheet. Capacitor current spike is ~7ms long.
 

Someone deleted a previous post which basically stated those swiches are place holders for scrs. Im really glad I can get away with a tiny (in comparison to the stud package) scr.

So what I need to make sure is a thryistors "Non repetitive surge peak" is just over capacitors current peak for the same or less duration?
 
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Hi,

Someone deleted a previous post
Click to expand...
I wonder.
If a moderator deleted a post ... you get a notification in either case. Automatically.
I assume no other person can delete a post.
In the database for this thread there currently are two actions:
* adding a new attachment (picture upload) for post#1
* replacing the link to the picture with the uploaded picure in post#1
Both modifications were made by me. No other action is reported.

*******
To the circuit:
In my eyes your simulation is useless, because it's behaviour is not realistic.
Start a new simulation.

Klaus
 
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    Zak28

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Its currently a proof of concept and isn't useless, I will be adding details to it later.
 

TYN640 a good choice also TO-220, 1060 I^2T and 480 Amps pk 8.3mS .... 40A nominal 600V ( TYN840 = 800V )
 
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    Zak28

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I choose TN4015H its well above circuit demands.
 

The TN4015H is a good (modern ) SCR, if you are switching ~230A pk it is as well to use the largest SCR for a similar ( or lesser in this case ) price in the same package - this is called good engineering practice - as it costs the client nothing ( or less in fact ) and makes for a more rugged design ...
 
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