About MOC3083 driving thyristor

qq770785791

About MOC3083 driving thyristor [Copy link]

 

Question 1: Can MOC3083 control the on and off of thyristors in a DC environment?

The circuit is shown in the figure, which controls the soft start of the thyristor after the rectifier bridge.

maychang

[Can MOC3083 control the on and off of thyristors in a DC environment]?

Generally speaking, thyristors will lose their on-off control function in a DC environment.

However, in the circuit shown in the figure, the DC is not at the position of the thyristor, so it is possible to control the on and off of the current.

maychang

I saw [Question 1] in the first post, but not [Question 2], [Question 3]...

maychang

To prevent the thyristor from losing control in a DC environment, in this diagram you only need to place the thyristor before the rectifier bridge, at any AC input end of the rectifier bridge.

maychang

【Control the soft start of the thyristor after the rectifier bridge】

Maybe you are wrong. The thyristor is not a soft starter here. The soft starter is RT200. This component is marked with NTC 22D-11, which is a negative temperature coefficient thermistor. The thyristor is connected in parallel with this thermistor. The function (the purpose of the circuit designer) may be that the thyristor is turned off when power is turned on, and the thyristor is turned on after the startup process is completed, short-circuiting the thermistor.

maychang

There is another suspicious point in this circuit: the thyristor in this circuit can only be triggered by the optocoupler when the voltage across the thyristor reaches a certain value. However, due to the presence of the NTC, the voltage across the thyristor is unlikely to reach a level that can be triggered.

qq770785791

maychang posted on 2023-7-9 10:13 [Control the soft start of the thyristor after the rectifier bridge] Maybe you are wrong. The thyristor is not a [soft start] here. The one that plays the [soft start] function is R ...

Yes

qq770785791

maychang posted on 2023-7-9 10:35 There is another suspicious point in this circuit: the thyristor in this circuit can only be triggered by the optocoupler to conduct when the voltage across the thyristor reaches a certain value. But because...

Now I suspect that the thyristor is not turned on reliably because of the voltage difference between the two ends of the thyristor.

maychang

qq770785791 posted on 2023-7-9 11:15 Now I suspect that the thyristor is not reliably turned on because of the voltage difference between the two ends of the thyristor

Yes. The thyristor must be turned on with the left end positive and the right end negative. The voltage across the thyristor must reach the thyristor gate trigger voltage after R200, the voltage drop of the optocoupler, and the voltage divider of R204 (the gate itself will also shunt R204). In this way, the voltage across the thyristor may be several volts (4~5V?) before the thyristor can be triggered. If the voltage across the thermistor does not reach this large, the thyristor cannot be triggered.

maychang

qq770785791 posted on 2023-7-9 11:15 Now I suspect that the thyristor is not reliably turned on because of the voltage difference between the two ends of the thyristor

I have seen a relatively high-power switching power supply that uses a small transformer to trigger the thyristor after soft start. The trigger voltage and current required by the thyristor gate are completely provided by the small transformer and have nothing to do with the voltage across the thyristor. This triggering method will of course not cause the thyristor to fail to trigger due to insufficient voltage across the thyristor.

chunyang

The cut-off condition after the thyristor is turned on is that the current flowing through it is less than a certain threshold, so in the general "DC" context, the thyristor cannot be used to control the opening and closing. Once the thyristor in the original poster's picture is triggered, it will only be cut off when the load current is small enough or the input voltage is removed.

The thyristor in the original poster's picture is not to achieve soft start, but to remove soft start. Soft start is achieved by RT200, and the end of soft start is achieved by thyristor, that is, the normal power supply current mainly passes through the thyristor circuit. It can be seen that as long as the load exists and the input is not powered off, the thyristor does not need to be cut off, and the trigger circuit is turned on but not off.

chunyang

There are technical problems in the design of the OP's circuit. MOC3083 is a thyristor type output with a large on-state voltage drop, which is not suitable for such use. In addition, the value of RT200 is generally not too large, so there will be problems with the triggering of the thyristor. An external trigger power supply should be used for reliability.