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Topic: BT-138 and inductive loads (Read 917 times) previous topic - next topic


Apr 25, 2013, 07:04 am Last Edit: Apr 25, 2013, 07:07 am by lemming Reason: 1
I am looking at turning a small 240Vac motor off and on using the circuit described in http://arduino.cc/forum/index.php/topic,51671.0.html which uses a BT-138 Triac. (I will only being using two states: full on or 'full' off.)

If I was driving a DC motor (inductive load) using a Mosfet I would need to use a diode to stop on/off spikes from  damaging the semiconductor.

Obviously this is not possible with an AC motor. Do I need to worry about spikes damaging the BT-138?


You need to add a snubber circuit if you dealing with inductive load.  The resister and capacitor in series across the load in the first circuit is nothing but the RC snubber.


Thanks Lavan.

Do I take it form your comment "..is nothing but the RC snubber" that you do not consider this snubber is adequate?


No.. No.. What I meant was the RC  circuit ( .01 mfd and 39R in series) in the link you mentioned is a snubber. So you no need to add anything if you are using the same circuit.


Switching an AC inductive load with a triac largely avoids generating a back emf in the load when it switches off, because a triac will only turn off when the current is near zero. However, a small amount of current (a few mA or tens of mA) will still be passing when the triac turns off, so there sill still be some back emf. Depending on the capacitance of the load, this may be enough to turn the triac on again. That is why a snubber is recommended when switching an inductive load with a triac, unless you have established (e.g. using an oscilloscope) that it is not necessary with a particular load.

Note that the capacitor used in the snubber must have an AC voltage rating sufficient to take the mains voltage. The resistor should also have a sufficient voltage rating, a 1W metal film or wirewound resistor would typically be used.
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Thanks guys for the explanation.

Thats clarifies the issue.

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