# Crow Bar Circuits and respective Hybrids

#### iceman11

Joined May 4, 2007
39
Good day all, i have the cricuit at this link http://www.quickshare.co.za/files/r7ahkpbn/pic_1.JPG.html, i am trying ot construct a basic crow bar circuit for AC signals. however, the program i am using (multisim 9) is for some reason not firing the gate of the SCR. and hence i am getting the same voltage at the output as the input whether the input voltage is above the zener break down or not.

My frend sugested the circuit in the above link, and i have mirrored the circuit so that it may work for both halves of the AC signal. however, i am getting this at the output. http://www.quickshare.co.za/files/1uppayor/pic_2.JPG.html from the scope (second on right hand side) HSC2 measuring across the output of resistor R4. the grounded centre line in the centre of the ciruit represents the centre tap, of the transfromer which is the circuits REal World input.

If some one could please explain
1>what circuit this is (it obviously isnt a traditional crowbar circuit)
2>why it is giving me messed up results (also shown)
3>how i can get multisim to simulate properly with a SCR instead of this arrangement (as i wud prefer using a deisgn im familiar with)

thanks

Final Year Elec Eng Student, South Africa

#### iceman11

Joined May 4, 2007
39
oh yea, i kinda figured out that the spikes showed on the HSC2 scope, is the time taken for the SCR to recognise the over voltage condition and then fire, however, what i dont understand is why are there 4spikes/period. shouldn't there be only 2spikes/period, and the begining of each half cycle for the SCR delay in firing ? why are there the additional 2 extra spikes at the end of each half cycle

speedy help wud be much appreciated, i put a lot of effort into making my dilemma a bit easy to understand :>

#### beenthere

Joined Apr 20, 2004
15,819

#### iceman11`

Joined May 4, 2007
39
hi been there, yea i know it has no SCR in it.

I was wondering how the circuit i presented in the attached picture works, cos it seems that it almost does (according to the results pf the scope) with the spikes coming at the end of each half cycle being strange (which i also mentioned). im not too sure of how this circuit works, i stumbled upon it by chance if you will.

regarding a crow bar with a SRC, im completey familiar with how this works, and how the SCR is fired from the zener etc etc. however, on multisim it seems that simulating with an SCR gives major problmes, any ideas why? and should i attach a circuit diagram of this SCR crowbar version that refuses to simulate with meaningful results when using the SCR version? ( i also mentioned above that it seems that the gate of the SCR is not triggered because the voltage seen at the load is the same whether the input voltage is lower or higher than the zener trigger, its like the SCR doesnt fire at all)

this is when i ended up stumbling across the circuit above, which seems to be yielding some kind of sensible simulation results.

thanks, Dushen

#### beenthere

Joined Apr 20, 2004
15,819
I have never used Multisim, so I will have to defer to anyone who might know something about the odd behavior.