SCR power control

savvej · Nov 14, 2011

Please help me with the attached pic.
All what I as trying was to understand SCR control.
It consists of a full wave bridge rectifier(Input 230V AC) whose output is fed to a series combination of a 2Mohm resistance(by having 2 ,1M Ohm resistance in series) + 1 LED connected to anode of SCR(BT151) and then cathode of SCR connected to other end of output of rectifier.I decided to trigger the gate(though foolish when working with a frequency of 50Hz but still) to the joint of 1M and 1M expecting that the LED will glow.But after giving 2-3 pulses there was a big flash and the SCR broke open.Why did it happen this way?I have attached datasheet as well as circuit diagram.

Futher,I was about to implement SCR phase angle control using Atmega 32 by following aplication note AVR 182 and AVR422.
What I havent got yet is,how is the SCR is triggered.The datasheet says Igt=5ma and Vgt=0.6V but these are for VD of 12 V.How do these translate when working with 230V.

savvej · Nov 14, 2011

this is the scr control which I will be trying now :

savvej · Nov 14, 2011

zero dectector using optocoupler:

alec_t · Nov 14, 2011

there was a big flash and the SCR broke open.Why did it happen this way?

I guess because either or both '1 megohm' resistors was/were in fact a much lower value. Unless the resistors were correctly rated for 230V AC use they could have arced internally. Or perhaps their markings were incorrect or open to misinterpretation?

Edit: BTW, 1N4001 diodes as shown in the schematic do not have a high enough voltage rating. They should be 1N4007.

savvej · Nov 16, 2011

just wanted to confirm regarding the working of the optocoupler(wrt the circuit attached) when you give sinusoidal input to it. When the input to the optocoupler is a few volts(2V),the phototransistor starts conducting in saturation(fixing output(collector of phtotransistor) to around 4.3 volts and then maintains the same voltage till the AC waveform again drops till around 2 volts when it swicthes off.this cycle keeps on repeating.so there will a squre wave output at the collector of the phtotransistor.

*courtesy for the circuit : electo-tech-online user "Boncuk"

savvej · Nov 16, 2011

Regarding zero crossing detector circuit

In the attached circuit:

just wanted to confirm regarding the working of the optocoupler when you give sinusoidal input to it. When the input to the optocoupler is a few volts(2V),the phototransistor starts conducting in saturation(fixing output(collector of phtotransistor) to around 4.3 volts and then maintains the same voltage till the AC waveform again drops till around 2 volts when it swicthes off.this cycle keeps on repeating.so there will a square wave output at the collector of the phototransistor.

Also why is the schmitt trigger circuit attached at the output?
The zero crossing detector is to be used for power control to heater via SCR/Triac.

* PS the circuit attached is given by electo-tech user "Boncuk".

alec_t · Nov 16, 2011

I don't know how you get the 2V and 4.3V figures. Let's assume Vcc is 10V (?). In order to saturate the phototransistor the collector current will have to be ~ 10V/47k ~ 0.2mA. According to the 4N25 datasheet the current transfer ratio (CTR) at 10mA is ~50%. If we assume (probably wrongly) the CTR is also ~50% at 0.2mA then the photodiode current will have to be ~ 0.4mA. So with 100k in series with the diode the bridge rectifier voltage will need to be ~ 0.4mA x 100k + diode drop (~2V), = 42V.
Because the mains voltage is sinusoidal the opto-transistor won't always be saturated; its collector current and voltage will also have a sinusoidal component. That is unsatisfactory for a zero-crossing detector and is why the Schmitt trigger stage is added....to give an abrupt change in output.

savvej · Nov 16, 2011

Thanks @alec_t for your reply.I tried simulating the circuit in alike PSPICE software, and got following graphs attached.I have changed the input circuit a bit.At the input I am using 230-12V transformer and its output is fed to the bridge rectifier.Also I have changed the input resistance to LED as 250 R ((12rt2 -2)/(Imax=60ma) =249 R).Everthing else is same.The only difference is that the output has sharper edges.So is the schmitt trigger necessary?

Boncuk · Nov 16, 2011

Here you can see clearly why a Schmitt-Trigger is the best way for a zero crossing detector.

The yellow trace is the one at the collector of the optocoupler. The blue one is at the output of the Schmitt-Trigger.

Boncuk

savvej · Nov 16, 2011

The other advantage of schmitt trigger is that the user can control the pulse with adjusting it.For the simulation I carried out,I found that the circuit works for varying pulse widths for capacitance max 120nF. The pulse width in above circuit without schmitt trigger is around 1ms and when I can change the capacitance to 100nf I get reduced pulse width .Also the pulse gets shifted relative to the zero crossing point by changing the capacitance at the output.According to me the pulse width should not be a concern,as if I have configured uC interrupt pin to detect edges,the pulse width factor does'nt come much into consideration(unless the width is close to the clock frequency).So the only use which I think is that by changing the capacitance I can shift the pulse generated and in my case for capacitance=70nF I got perfect zero crosssing detection.Attached are the graphs I got via simulation.
Please correct me if I am wrong.

savvej · Nov 16, 2011

@Boncuk Sir for the values which you have said and the modified circuit which I am simulating,I am not getting the waveform as obtained by you.Attached is a zoomed snapshot of the graph obtained by me.

savvej · Nov 16, 2011

What is users say on this circuit? Anyone tried it before? : **broken link removed**

alec_t · Nov 16, 2011

The zero crossing detector is to be used for power control to heater via SCR/Triac.

If you will be using burst (whole cycle) control, rather than phase angle control, could you use an opto-triac with a built-in zero-crossing detector, such as an MOC3163 ?

savvej · Nov 17, 2011

pic

pic_of_zero_crossing_detection problem.

3v0 · Nov 17, 2011

duplicate

3v0 · Nov 17, 2011

As long as the zero crossing detection is uniform (the same each time) you can adjust for it being early or late in software.

You may loose a bit of range but for heating it will not matter.

Also there is very little power at a few degrees on either side of the zero crossing.

There is no problem.

EDIT: Once you have the hardware setup you can run an experiment to find the lowest number that will turn on the power in the wanted cycle. Hook a lamp up the the AC and the first number that give full power is the lowest number you can use to activate AC. Note that larger number result in less power.

savvej · Nov 17, 2011

kk.Got your point.

SCR power control

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