Working AC Crowbar Circuit Anyone?

[ACharnley]

Apparently the triac can do it, only google has no circuit diagrams. I can only find the one (which many reposted) and it doesn't work (it's DC).

 

[crutschow]

Exactly what do you want the crowbar to do?

 

[KeepItSimpleStupid]

Your schematic looks like a typical AC dimmer. Definitely not a crowbar. AC or DC might be referring to Universal motors which can operate on AC or DC.

 

[ACharnley]

Agreed. So many comments on the web saying a triac can crowbar AC... no schematics to back it up.

crutschow ... short the AC on over-voltage.

 

[crutschow]

short the AC on over-voltage.

So it will blow a breaker or fuse when that happens?

Do you want to trip on the the peak of the waveform or its average value for the overvoltage detect point?
Tripping on the peak would make it susceptible to noise spikes.

 

[ACharnley]

A voltage on the wave over a determined level, noise spikes are fine. That's correct on the fuse.

 

[eTech]

A voltage on the wave over a determined level, noise spikes are fine. That's correct on the fuse.

Hi

Here's my attempt at a demo
The simulation shows a sudden rise in line voltage between 50-70ms. The fuse blows as shown by the sudden drop of current thru the load.

See OnSemi app note AND8012/D.

eT

 

[ACharnley]

I wonder if it's that simple - damn .

I came up with this using two SCR's!

 

[tomizett]

First circuit looks fine to me - esenntially the same as eTech's in #7. You can adjust the value of the 0.1u cap to trade off noise immunity vs. response time.
It is essentially the same as a dimmer, just that there is no load in series with the triac (except for the fuse).

 

[ACharnley]

LT Spice keeps getting stuck. I'm using the same libraries as yourself (which you'd posted on another thread).

 

[alec_t]

LT Spice keeps getting stuck.

Have you got the latest LTS executable update? My LTS was randomly freezing until I updated it yesterday.

 

[ACharnley]

Build: July 3, 2018 (latest)

It's not randomly freezing, it's going very, very slow!

I've attached the asc file (it's a bit different as I've been playing around to try and make it simulate).

I also wonder why LTSpice copies all the models/symbol files into the users home directory and doesn't sync them.

 

[eTech]

Build: July 3, 2018 (latest)

It's not randomly freezing, it's going very, very slow!

I've attached the asc file (it's a bit different as I've been playing around to try and make it simulate).

I also wonder why LTSpice copies all the models/symbol files into the users home directory and doesn't sync them.

Hi

I had to use a modified version of the triac library and symbol.
I’ll post later this afternoon.

eT

 

[eTech]

I wonder if it's that simple - damn .

I came up with this using two SCR's!

heh...a triac is basically two back to back SCR’s.
Check out the app note...the circuit is in there.

eT

 

[gophert]

ON Semi (Motorola) used to make a Crowbar sensing chip,MC3423

https://www.onsemi.com/pub/Collateral/MC3423-D.PDF

 

[ACharnley]

Hi

I had to use a modified version of the triac library and symbol.
I’ll post later this afternoon.

eT

Thought so, any input to the gate caused LTSpice to die.

 

[gophert]

Thought so, any input to the gate caused LTSpice to die.

Try adding a few ohm resistor in place of the fuse in the model because spice models don't like infinite current circuits. The crowbar is supposed to represent throwing a crowbar across the bus bars - infinite current and blown fuse.

 

[Aswinth_Raj]

I also had a very bad time trying to simulate the circuit on LTSpice of even Proteus for that matter. Finally decided to give the hardware directly a try since it had only minimum components. I also used a much more simple circuit that only employs a thyristor and zener. You can find detials of it here
https://circuitdigest.com/electronic-circuits/crowbar-circuit-diagram

Needless to say you have to use a TRIAC in place of Thyristor if your supply is AC. Also do people still use these circuits? I mean there are better options available not considering the cost though

 

[ACharnley]

I gave up with triacs with inductive sources, they latch on and persist through zero crossing.

 

[KeepItSimpleStupid]

You have to deal with both voltage and current zero crossings.