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Isolated power supply for Triac Gate

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adnan012

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hi,

I want to implement a ac/ac speed controller for 1/2 HP induction motor. I have issues in driving the gates of THYRISTORS.

There are 8 THYRISTORS as shown in the converter file. Is it a good idea to make 8 isolated 5v dc supplies using 8 small transformer , rectifier bridges and linear regulators such as lm7805 and connect them as shown in the Proteus ckt (transformer and regulator is replaced by a battery for simplicity also only one sided leg is drawn in Proteus ckt). Is there any other way to drive the gates safely ? I don't want to use pulse transformers.

I have used this method in a ac/dc matrix converter with mosfets (bidirectional switch configuration) to drive a 24v volt dc motor without any problem. I want to know that this method is useful for high voltage application?

Regards
 

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Adnan; Why don't you want to use a pulse transformer?

In the past, I've successfully used gate drive transformers. It provides the individual SCR with a hard-trigger pulse.

In my experience, the soft-trigger characteristics of optocouplers sometimes fail to latch the SCR.
 
Thanks for reply.

Is there any software tool and tutorial available for the designing o pulse transformer quickly ? I have toroid from old switching power supplies (input common mode choke). I do not have datasheets for these toroid.
 

Micrometals website has useful design tools. You may want to check there.

In the past, I've cut the ferrite rod antennas in 2 cm lengths. Wound as many turns as possible on two layers. This is the primary. Add a layer of kapton or mylar type as insulator. Now wind again as many turns as possible, single layer. This is your secondary. As long as you keep the starts and finishes identified, you can phase the transformer properly.
 

Use an opto triac.
An opto triac will 'hard fire' the main triac. ( Edit - this applies to Triacs. I've not used it with Thyristors but I suspect the following still applies ).

A hard fire means that the triac gate is ALWAYS being driven when required to be on.
Soft trigger is the transformer type - which relies on the latching characteristic of the triac. This is the least reliable triggering - unless the trigger pulse is constantly repeated during the required 'on' time.

If the main triac becomes non-conducting because of a noise spike or a large inductive load then it will not retrigger if a single pulse is used by a transformer.
An opto triac will always be triggering the main triac - and will re-trigger it if it tried to cut off.
All professional Dimmers use this method and I've designed a fair few.

It's worth making the resistor in the gate drive to be a fusible one - so if there's a problem you shouldn't get catastrophic damage to the opto. ( I.e. exploding packages ).
E.g if the main triac fails open circuit then the opto will try to drive the load on its own - not good.

If you are phase controlling then you need to make sure your zero cross detector is reliable.
If not, then you can use an opto triac that incorporates a zero cross detector - but I don't know how good these are if the mains is very noisy. Dimmer design doesn't use this type of course.
 
Last edited:
rowifi said:
Use an opto triac.
An opto triac will 'hard fire' the main triac. ( Edit - this applies to Triacs. I've not used it with Thyristors but I suspect the following still applies ).

A hard fire means that the triac gate is ALWAYS being driven when required to be on.
Soft trigger is the transformer type - which relies on the latching characteristic of the triac. This is the least reliable triggering - unless the trigger pulse is constantly repeated during the required 'on' time.

If the main triac becomes non-conducting because of a noise spike or a large inductive load then it will not retrigger if a single pulse is used by a transformer.
An opto triac will always be triggering the main triac - and will re-trigger it if it tried to cut off.
All professional Dimmers use this method and I've designed a fair few.

It's worth making the resistor in the gate drive to be a fusible one - so if there's a problem you shouldn't get catastrophic damage to the opto. ( I.e. exploding packages ).
E.g if the main triac fails open circuit then the opto will try to drive the load on its own - not good.

If you are phase controlling then you need to make sure your zero cross detector is reliable.
If not, then you can use an opto triac that incorporates a zero cross detector - but I don't know how good these are if the mains is very noisy. Dimmer design doesn't use this type of course.
Click to expand...


You make several points here that are worth clarifying.

1) If an SCR (like the poster's circuit) is driven with an optoTRIAC, a reverse polarity could be applied to the gate if one is not careful with the firing sequence.

2) The best gate drive strategy depends on the load being driven. On a dimmer, which is a resistive load, I agree an optotriac is definitively the best way to go.
However, in a motor load, where back-EMF occurs, it could give a soft trigger characteristic (the available gate drive current is a function of the difference between the anode and cathode voltage during the non-conducting state).

3) You are correct that in a transformer drive with a single pulse, the TRIAC or SCR may unlatch. Therefore, transformers should be driven continuously with pulses, what is called a picket-fence drive.
 
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