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Crowbar Circuit not working

Hi,

 

I'm trying to simulate a simple crowbar circuit (see attached), but the SCR isn't behaving as expected.

 

At any V(AK) over 0.7V, the SCR starts conducting, despite the gate being tied to ground!

 

What am I doing wrong?

 

Thanks 🙂

 

crowbar_01.png

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Hi wt_higo,

 

Multisim usually works off of ideal cases. It seems in this circuit there is a voltage split after the 1000 ohm resistor which feeds into the SCR gate input. Since this is a simulation, it takes ideal conditions such as there being no resistance in the wires, so the circuit may behave differently in real life. 

 

Best Regards,

 

Soloman

Application Engineer (CLAD)
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Hi,

 

  thanks for your input, but I don't understand what you mean, sorry.

 

  Multisim deals in ideal quantities, I know, but this doesn't explain what's happening here - the resistance between ground and the SCR gate should be zero ohms, in that case, with the entire voltage being dropped across the upper 1K resistor - why is the SCR triggering under these conditions?

 

  Thanks 🙂

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Hi wt_higo,

 

Apologies I made a typo in my previous message. The voltage after the 1000 ohm resistor travels to both the gate input, and the variable resistor. The gate expects only a voltage to be triggered, and the SCR has a small resistance associated with it, so it will allow the majority of the current to flow through it.

 

Best Regards,

 

Soloman

Application Engineer (CLAD)
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Hi,

 

  I'm still no closer to getting this working - I've modified the circuit to use a zener instead of a pot to set up the gate voltage, but whatever SCR I choose, it just triggers, even if the gate is left floating!

 

crowbar_02.png

 

  Why is the SCR triggering? 

 

  The 2N2576 should be good for blocking 200V before hitting Vbo - why is a 12V supply forcing it to trigger?

 

  Any thoughts would be appreciated...

 

Thanks 🙂

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Spice simulators sometimes do strange things when trying to establish the initial operating point for very non-linear circuits.  I do not have Multisim, but if you can post a text file containing the model of the SCR, I will take a look to see if I can spot anything.

 

Lynn

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hello every body.

 

it is SCR, and according to its model's behavior    it must get AC voltage in order to close itself after it has been opened once, 

 

according to the DATA it should support DC but obviously it's MS model does not .

 

it 's model is NOT designed to work on DC !!!.

 

check the data pages - http://pdf1.alldatasheet.com/datasheet-pdf/view/87005/MOTOROLA/2N2575.html

 

 

 

I have played around a bit - check the images

01 - DC only

02 - AC only

03- AC\DC toggle with S1 - when you start simulation, always start with S1 in DC position

 

best regards

 

Michael

 

 

 

 

 

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Hi,

 

  thanks very much for your help with this - your circuits are certainly a clever solution to the problem.

 

  I'm not entirely clear why the SCR models have been designed to work under a.c. conditions only - surely this limits their usability rather dramatically?

 

  Does anybody know if any of the SCR models that ship with multisim as standard will work in the configuration I'm looking for?

 

  Other than going through each one in turn and inspecting its model, I can't see how to tell...

 

Thanks 🙂

Message 8 of 10
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Hi,

for exemple check this for info

http://www.sentex.ca/~mec1995/tutorial/triacs/triacs.html

 

and about MS models - it looks like to go one by one is only option, unless somebody can advise something different.

 

good luck

 

Michael

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Hi,  I just have my self double checked.  I am sure neither one of these will work on DC!!!!

 

that because neither SCR or Triac could be closed by interrupting or reduction of control current .

 

through current should get below theshhold to close the device. - it means they work properly only with AC .

 

if you want to use it with DC you have to have to reduce the through current some how

 

with AC it happens 2 times each period!!!

 

the diffrence betwen these devices is only the SCR behaives as diode (conducts one way) when it open, and triac conducts both ways.

 

 

good luck

 

Michael

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