A simple dI/dt confusion, Anyone like to try & solve it ?

[Julie.]

400uF 1mH - runtime of 100ms

Zoomed in over the first 150us:

LTspice is easy to use once you have the model


OK, added the current to the last trace - check out the values - what do you think that will do to an SCR?

Yes that is ~18kA at 7.9kHz

 

[Sanjay kumar]

Ok, I will try.

But please confirm that I can do what I am doing.

Since resistance of coil is very low.
Can It create sparks in capacitors?


In order to get 1uH, Can I use 150mm wire ?

So what is dI/dt of above circuit??

do i need to worry about 1890A/uS rise time??

 

[Julie.]

Ok, I will try.

But please confirm that I can do what I am doing.

Since resistance of coil is very low.
Can It create sparks in capacitors?


In order to get 1uH, Can I use 150mm wire ?

So what is dI/dt of above circuit??

do i need to worry about 1890A/uS rise time??

I edited my previous post to include the current - at initial contact around 18kA at 8kHz or so


As for the suitability of electronic components - not my area!

Basic circuit theory - yes OK at that!

 

[Julie.]

spice model using the figures 400uF and 1mH

 

[Julie.]

Anyway, it was this or helping my husband trim the ivy - swift avoidance of Ivy to be had.. so:

This is the model in LTspice - simple Capacitor of 2000uF with the initial conditions of 945V discharging through an inductor of 200uH and series resistor - critical damping would be at 2x SQRT(2000/200) = 2 sqrt (10) = 6.32 ohm

In each case the switched is closed at 10ms

Case 1 Resistance = 6mOhm
View attachment 61196

As you can see the discharge oscillates as it decays down slowly

Case 2 60mOhm
View attachment 61198

This is the same as before, but the decay down is much quicker

Case 3 - in this case we have R = 6.4 Ohm - i.e. critical damping

View attachment 61199

As to be expected - straight forward decay with no oscilation.

If you want - here is the LTspice model, play to your heart's content.

It is a .txt file - you will need to rename it .asc before you load it into LTspice

It has just been pointed out to me - that I can't calculate!!

The critical damping should be 2 Sqrt (L/C) - I did C/R in error

So 2 Sqrt (0.1) = 0.632 ohm, not 6.32 ohm!

so 0/10 - must improve!

Here is the output for 0.64 ohm anyhow

 

[pc1966]

The thing about simulations that you always have to remember is this:

• If it tells you it won't work, it won't work.
• If it tells you it works, it might work.

In the second case it will work so long as the models used are sufficiently close to the actual components it is built with. The problem here is most manufacturers of SCR, etc, expect them to be used for controlling 50/60Hz power systems, and probably not for high energy pulse discharge, so it is unlikely the model of the SCR is accurate when pushed to extreme values of I and dI/dt.

In some cases you can "read" the SPICE model of a part as it is really a sub-circuit and just a text file you can open. Then you can see how they implemented it as a mixture of basic semiconductor models and added R/C/L to model the real part's packaging, etc.

In the example circuit above the storage capacitor is models as a pure capacitance, in reality it will have some series R & L anyway. Also every part has a blow-up point in surge current, so if you are doing this sort of stuff and want to have more than one bite of the cake you need to check the dV/dt rating of the capacitor as well (which is really a peak-current specification as I = C * dV/dt).

 

[Julie.]

The thing about simulations that you always have to remember is this:

• If it tells you it won't work, it won't work.
• If it tells you it works, it might work.

In the second case it will work so long as the models used are sufficiently close to the actual components it is built with. The problem here is most manufacturers of SCR, etc, expect them to be used for controlling 50/60Hz power systems, and probably not for high energy pulse discharge, so it is unlikely the model of the SCR is accurate when pushed to extreme values of I and dI/dt.

In some cases you can "read" the SPICE model of a part as it is really a sub-circuit and just a text file you can open. Then you can see how they implemented it as a mixture of basic semiconductor models and added R/C/L to model the real part's packaging, etc.

In the example circuit above the storage capacitor is models as a pure capacitance, in reality it will have some series R & L anyway. Also every part has a blow-up point in surge current, so if you are doing this sort of stuff and want to have more than one bite of the cake you need to check the dV/dt rating of the capacitor as well (which is really a peak-current specification as I = C * dV/dt).

Agree, although to be fair, the spice models do allow more characteristics to be added to them than I have, series r & x, leakage r & x and so on.

But absolutely agree with the interpretation of the results, just looking at the voltage, does not give an indication of the current etc.

In the model it shows the voltage oscillating as expected, but only when you look at the current do you realise the magnitude, any current even approaching this could damage components.

Calculate the fault level on a site, again it's a guide and doesn't in itself indicate the suitability of any equipment

 

[Sanjay kumar]

The thing about simulations that you always have to remember is this:

• If it tells you it won't work, it won't work.
• If it tells you it works, it might work.

In the second case it will work so long as the models used are sufficiently close to the actual components it is built with. The problem here is most manufacturers of SCR, etc, expect them to be used for controlling 50/60Hz power systems, and probably not for high energy pulse discharge, so it is unlikely the model of the SCR is accurate when pushed to extreme values of I and dI/dt.

In some cases you can "read" the SPICE model of a part as it is really a sub-circuit and just a text file you can open. Then you can see how they implemented it as a mixture of basic semiconductor models and added R/C/L to model the real part's packaging, etc.

In the example circuit above the storage capacitor is models as a pure capacitance, in reality it will have some series R & L anyway. Also every part has a blow-up point in surge current, so if you are doing this sort of stuff and want to have more than one bite of the cake you need to check the dV/dt rating of the capacitor as well (which is really a peak-current specification as I = C * dV/dt).

Thank you for valuable information!

According to datasheet maximum peak current of single pulse is specified with 1000A per component.
I Am using 10 in parallel

 

[pc1966]

According to datasheet maximum peak current of single pulse is specified with 1000A per component.
I Am using 10 in parallel

You will need to take steps to ensure they share the current equally, so a very small R or similar for each to make that happen (that could be 10 equal moderate lengths of medium-size wire from capacitors to load/switching point).

Also if one capacitor fails the other 9 will dump their energy in to the failed one. I would recommend a metal box to contain any fragments or flames...

 

[DPG]

I think he means 10 x SCRs in parallel. I'm a bit out of touch, but don't you need to use some low value 'balancing' resistors?