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Help me to understand these circuits

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nura100

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Hello Forum,

I am working on a micro based project which controls inductive loads like trafos, etc using phase angle control. the following are two circuits which will be used : one is voltage 0 crossing circuit and second is current 0 cross circuit.


the mains input is between 120VAC t0 240VAC, i know that both the circuits employ Resistor dividers to scale down the mains voltage, i just need explanation on role these components especially the 4V7 zener in second circuit

Thanks & Regards

Arun
 
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In the first circuit, R17 and R20 scale down the voltage from the mains. The resistance values here need to be very high to limit the current going into the base of the transistor. When the mains voltage gets down to about 3.7V..... (100k/(100k+470k) x 3.7 = 0.65V the transistor switches off. The 1N4148 stops any reverse voltage from the mains reverse biassing the transistor base.

In the second circuit the zero crossing circuit is being used to fire a triac. However, a triac, when switched on will have a voltage drop across it (looking at the datasheet of the TIC225 it is 2.1V). The circuit designer has added a 4.7V Zener to increase the trigger voltage of the zero crossing circuit from 3.7V to (3.7V + 4.7V) to ensure that the ON voltage does not stop the circuit from firing. This is what I believe is the function of the Zener.
 
Thank you Bill,

3.7V that's magic figure i wasn't getting, 1N4148 is rated to handle 70V @ 0.3 A, but here it is used on 110 -240 mains, wouldn't it blow up ?
can a normal 1N4007 be used which has equal 0.6 Vfd and can handle 1000V @1 Amp

Both the circuits give Square wave ( 0V -5V) signal to the micro's I/O pins one for Voltage 0 and current 0 crossing.

based on these inputs the micro fires the triac.

second circuit is more complex to understand and i need to dig in a bit

i hope Mike MI throws some light on this circuit


Arun
 
HI Arun

The 70V rating of the diode refers to the reverse breakdown voltage. The base of the transistor will conduct when the mains is positive, limiting the reverse voltage of the 1N4148 to 0.65V. Likewise on the negative cycle, the 4148 will conduct protecting the reverse voltage of the transistor base-emitter.

They both help each other out on opposing phases of the mains
 
Thank you Bill, Now, i get it.

one more thing i wanted to ask; can the 3.7V minimum voltage be brought down so that the sensing is done closer to actual voltage zero crossing ? to do so the resistor divider's values have to be reduced which will increase the current flowing to the base of BC547.

Any suggestion to strike a good balance between component safety and accuracy?


Regards,

Arun
 
Yes it can. If you reduce the 470k resistor, this will 'allow more of the mains through'. I think this is why the resistor is labelled 'FB' - it is designed to be tweaked to change the feedback (FB).

Reduce it to 100k and your cross over voltage should be nearer to 1.3V. I would not want to go any lower than 100k though.

240V rms = 240 x 1.41V peak = 340V. 340V/100k = 3.4mA peak current into the base. This should be more than enough current to put into the base of a transistor.
 
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