Problem with comparator in ramp generator circuit using 741 opamps

[Smitasi]

Hi ppl we have rigged a thyristor gating/firing circuit using opamps 741

the opamp stages include- zcd, integrator, comparator

zcd that detects 0 crossings of 50Hz sine signal from the signal generator
generating a square wave

the is the integrator which gives a ramp output
(4.7K,0.47uF)

After dat is the comparator
Ramp to non inv and votge divider ie ref voltage to inv i/p

From the comparator we are supposed to get pulses based on the comparison b/w the ramp input and the reference input
the ref input is a voltage divider ckt

Till the integrator things were working fyn
but after dat the comparator is not

we are having problems with the kpots(potentiometers) that don't wrk so we eliminated them from the intergator as well

changed breadbords,Ics resistors/capacitors still no use

Could sum1 suggest changes/troubleshooting techniques as 50Hz is not even a high speed signal

Thanks

 

[pranam77]

trigger to ramp schematics

Let me know how are you driving the thyrister gate? A schematic if posted here will speak itself. Good luck

 

[FvM]

r-firing ckt

we are having problems with the kpots(potentiometers) that don't wrk so we eliminated them from the intergator as well

changed breadbords,Ics resistors/capacitors still no use

In other words, you didn't manage to design a working circuit? Unfortunately, if you assemble a number of electronic parts, there's a high likelihood of selecting a non-operational circuit. So, to get help in this regard, you should post your circuit.

 

[Smitasi]

simple op amp ramp circuit 741

plz chk da jpg file

i hav tried installing psice/other sim s/ws bt cudnt

the G, K refer to gate and cathode of the scr

The title of the expt is 'SCR GATING AND FIRING CKT WITH SYNCHRONISED AC POWER SUPPLY'

the signal freq is 50 Hz sine signal from signal genrtr

 

[Smitasi]

ramp wave

As i hav vista n my comp many times pspice,multisim etc havnt got instld n my comp
I wud be very grateful if sum 1 helps me wit dis ckt

also 1 mor thng the -ve 1/2 of the integrator o/p wich is traingular wave is gettn clipped but +ve half is perfect

Thanks

 

[FvM]

design 741 ramp generator

The ramp generator can't work this way. It seems to me, that you designed it from the scratch, without thinking thoroughly about the problem or having a known operational reference design.

Just to mentions two points:
- a 100 Hz sawtooth rather than a 50 Hz triangle waveform is required for symmetrical bipolar phase angle control. If you want to achieve something special, you should clarify the intended trigger and AC waveforms by a diagram.
- an integrator without a reset or a special clipping circuit doesn't give a defined output waveform

Furthermore, a trigger transformer usually gets saturated when driven with a 50 Hz squarewave. Also the power disspiation is probably too high. The common solution is to generate either a single short trigger pulse (of e.g. 20 us duration) or a trigger burst. This is particularly required considering the rather high trigger current of power thyristors and triacs.

I suggest to study the operation of state-of-the-art trigger generators, e.g TCA785, as a reference.

 

[Smitasi]

circuit of ramp generator using 741

Hey it's not a self generating ramp generator
to produce trigeering in sync wit da ac supply
da 0 crosings can be used as ref
hence we r using a zcd integrator and comparator so that only at a particular voltage and 4 small time period pulses r generated

 

[FvM]

ramp generator with opam

da 0 crosings can be used as ref

Yes, but your circuit doesn't manage it. I made my suggestion.Good luck!