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[SOLVED] Can a triac be used as an SCR?

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Hi all, can a triac be used in place of an SCR for a DC rectifying circuit? if so, should the AC side go MT2 or MT1, how should the gate drive be organised? is there a danger of conduction the wrong way?
 

Hi,

is there a danger of conduction the wrong way?
Click to expand...
Yes. In opposite to an SCR the triac may be conductive in both directions.
Besides this problem you may use a triac instead of an SCR.

Klaus
 

Also.....
whereas the SCR triggers in a single quadrant, and with the Triac you can trigger it in several quadrants.

The voltage drop across the Triac may be higher too.
 

The trigger voltage has to be applied between G and MT1, MT1 is acting as cathode and MT2 as anode when replacing the SCR.

There are basically two ways to trigger the triac with unwanted polarity
- applying trigger voltage while MT2 is negative against MT1
- self triggering due to high dv/dt

On-state voltage of triac should be similar to SCR with same current rating.
 
Dear all, from the gate to A1 (MT1) there is P-type silicon forming a resistive bridge, on the BTA40-600B this is about 50 ohm, for 1st quadrant firing (usually the easiest) the gate is pos wrt MT1, and MT2 is pos wrt MT1.

Surprisingly little gate current is required to trigger in this quadrant (<< 50mA in data sheet - depending on temp) and the MT2 terminal can go far negative (with gate current applied) and not trigger the device.

dv/dt is an issue for the triac compared to the modern cathode gate SCR - snubbers needed, and Tj < 105 degC.

Our measurements show <1V Von for this device - similar to a good SCR,

and the pricing is very favourable - < $1 for medium quantities.
 

Your solution takes advantage from the fact that most standard triacs don't trigger well in the 4th quadrant and need at least a considerably larger gate current. "Overhead" trigger might still occur.
 

Just FYI, we had an issue on the "24VDC" rectifier - with the triacs (one of) triggering ON for reverse connection of 24V battery and switching the mains Tx ON/OFF - particularly OFF, as that is when the largest dv/dt is produced on the LV sec side - cured with 1uF 100V across the gate and MT1 (control gates lines were lifting CM w.r.t. the MT2 terminal) - now a safe system ... 105 degC T limit on the TRIAC heatsink.
 
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