SCR dropping voltage

[vaibhav]

Can somebody help me? I am trying to build circuit with scr's but whenever i am connecting these scr's to 415v ac then it is dropping the voltages & motor is running with 111V after gate signal is on corresponding to that scr.

 

[Harald Kapp]

An SCR is a unidirectional device. It passes current only in one direction, but is closed to the other direction. So only 1/2 of your AC is passing and your effective voltage is accordingly low.
Use a triac for 2-way conduction.

 

[vaibhav]

Thank you for replying Sir. I am using two scr's of BT150-600R antiparallel in each phase.So it should give 415V ac output,but we are getting 111V ac (phase to phase)& 96V (phase to neutral).

 

[Harald Kapp]

Show us your schematic.

How do do you measure the voltage? With a voltmeter or multimeter? Do you have access to an oscilloscope to visualize the waveform of the voltage? Does it show a full sine wave?

I suspect that your antiparallel arrangement is not operating as you expect. An SCR needs a positive gate-cathode voltage to trigger. So while your control signal may well trigger one of the scrs, the other will not see the required positive gate-cathode voltage because the cathode is not connected to the control signal's lower potential.

 

[vaibhav]

Here I have attached the circuit ,please see that & please give me suggestions.

 

[Harald Kapp]

We are giving 1.2volt to each gate of each scr for triggering at respected time.

1.2V from gate to which point in the circuit?
An scr requires the trigger voltage to be applied between gate and cathode.

 

[vaibhav]

1.2V from gate to which point in the circuit?
An scr requires the trigger voltage to be applied between gate and cathode.

We have 5V source which we are dividing up to 1.2V & connected to gates.How to give Trigger voltage 1.2V between G & K ,bcoz there is possibility of back emf in each half cycle.

 

[duke37]

The normal method to trigger a SCR with a floating cathode is to use a pulse transformer with the secondary connected to cathode and gate.

You must get the polarity right, positive gate to trigger on.

A triac will switch the supply in both directions with a trigger in either direction.

 

[vaibhav]

The normal method to trigger a SCR with a floating cathode is to use a pulse transformer with the secondary connected to cathode and gate.

You must get the polarity right, positive gate to trigger on.

A triac will switch the supply in both directions with a trigger in either direction.

Sir we are giving positive 1.2V to the gates with respect to ground & we have added resistor between Gate & Cathode terminals. Is it wrong?

 

[Harald Kapp]

Sir we are giving positive 1.2V to the gates with respect to ground

Where is ground in your circuit? It is not shown in your pdf. Also you cannot have the sources of both scrs connected to the same ground, as the scrs are antiparallel.
This is the third time I tell you that the trigger voltage has to be applied between gate and source of the scr. Do you know how an scr works?

Since the sources of boths scrs are on opposite sides, duke's tip to use a transformer is a good one. Here it is shown how to do it.

Alternatively you could use just one single scr and put it on the DC-side of a bridge rectifier. Then you have only one gate to control. Put the load and the voltage source on the AC side of the bridge. Thus the load will see AC but the scr wil have to switch ony DC. But there still remains the problem of potential isolation between the driving circuit and the high voltage circuit. Therefore the transformer is still the better solution.

 

[(*steve*)]

You could also use an optocoupler to provide current to the gate (from the anode).

Triacs are often triggered in a similar way.

edit: beware that this won't allow for triggering right from the zero crossover and you need to be careful about maximum gate current.

 

[vaibhav]

You could also use an optocoupler to provide current to the gate (from the anode).

Triacs are often triggered in a similar way.

edit: beware that this won't allow for triggering right from the zero crossover and you need to be careful about maximum gate current.

Where is ground in your circuit? It is not shown in your pdf. Also you cannot have the sources of both scrs connected to the same ground, as the scrs are antiparallel.
This is the third time I tell you that the trigger voltage has to be applied between gate and source of the scr. Do you know how an scr works?

Since the sources of boths scrs are on opposite sides, duke's tip to use a transformer is a good one. Here it is shown how to do it.

Alternatively you could use just one single scr and put it on the DC-side of a bridge rectifier. Then you have only one gate to control. Put the load and the voltage source on the AC side of the bridge. Thus the load will see AC but the scr wil have to switch ony DC. But there still remains the problem of potential isolation between the driving circuit and the high voltage circuit. Therefore the transformer is still the better solution.

Sir we have used ZCD circuit for sensing the R & B phase & giving it to the controller pin.Controller will sense this & generate pulses at respective times for respective gates. We have connected voltage divider network for 5V to 1.2V to controller pin (connected to gates) & other ending point of divider network to gnd which we have created artifitially with the help of resistor network.Now problem is after switching on the supply 415V AC, voltage is passing to the motor but motor is not running only giving sound.Whenever we are switching on the 5V adapter then motor is running at slow speed & it is showing 111V at the motor connections.So please tell me why this SCR is dropping the voltage & also one of the SCR is burning with resistor. We have connected Y phase directly to the motor terminals.

 

[vaibhav]

You could also use an optocoupler to provide current to the gate (from the anode).

Triacs are often triggered in a similar way.

edit: beware that this won't allow for triggering right from the zero crossover and you need to be careful about maximum gate current.

We are triggering scr with the help of microcontroller.

 

[Harald Kapp]

You repeat talking about a connection to gnd, but your circuit diagram (PDF) is incomplete. It doesn't show where gnd is and thus your textual explanations are of little help.

We need to see the complete control circuit. At least the part that contains the trigger voltage to the gates and that ominous gnd you keep talking about without showing us.

Also, I repeat (the umpteenth time), the cathodes (K) of the two anti-parallel scrs are connected to different potential. Since you have to apply the trigger impulse between gate and cathode, obviously this cannot be achieved by connecting the trigger voltage between gate and gnd. Because that would mean that gnd is on both sides of the two scrs and thus would create a short circuit.
So at best gnd can be connected to one side of the scr which in turn makes it clear that the trigger impulse can work only for that scr, where gnd=cathode. On the other scr gnd=anode and therefore this will not be triggered correctly.

 

[(*steve*)]

OK, we need a circuit diagram for the whole setup so that we can see how you're doing this.

Then we are going to ask you to use an oscilloscope to show some voltages. The first one will show the voltages across each of the three windings of the motor. WARNING this can kill you and it can easily blow up your oscilloscope so make sure you know what you're doing.

 

[vaibhav]

You repeat talking about a connection to gnd, but your circuit diagram (PDF) is incomplete. It doesn't show where gnd is and thus your textual explanations are of little help.

We need to see the complete control circuit. At least the part that contains the trigger voltage to the gates and that ominous gnd you keep talking about without showing us.

Also, I repeat (the umpteenth time), the cathodes (K) of the two anti-parallel scrs are connected to different potential. Since you have to apply the trigger impulse between gate and cathode, obviously this cannot be achieved by connecting the trigger voltage between gate and gnd. Because that would mean that gnd is on both sides of the two scrs and thus would create a short circuit.
So at best gnd can be connected to one side of the scr which in turn makes it clear that the trigger impulse can work only for that scr, where gnd=cathode. On the other scr gnd=anode and therefore this will not be triggered correctly.

Thank you sir for taking interest & helping us.Here I have attached the file which contains the controller circuit with gates of SCR's with problems which we are facing.

 

[Harald Kapp]

The attachment is missing.

 

[vaibhav]

The attachment is missing.

Sorry sir i thought it's attached. please find the files attached.

 

[duke37]

Still no ground connection.

The gates will be leaping up and down relative to one another at the full mains voltage. How will the microcontroller cope with this?

 

[Harald Kapp]

What is so difficult about showing us your gnd connection? Or showing us a moderately complete circuit diagram?

- How is G2 connected?
- How are the cathodes of T1 and T2 connnected?
- How are the microcontroller's ground and the high voltage ground connected?


As you show it, the microcontroller is not isolated from the high voltage. There is only a 10 kOhm resistor which is insufficient to ensure safety. This circuit definitely looks dangerous!