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How does one trip a relay given only triac output?

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Gaudeamus

New Member
Hello everyone! I am a new member. I hope someone is willing to help me with the following, seemingly simple, question:

I need to trip a relay (coil can be any voltage you like, AC or DC) whenever there is output from a triac. The triac's output is used to drive a 120VAC exhaust fan motor. I have access ONLY to the AC motor's leads (i.e., the triac's variable AC output).

I cannot use a 120VAC coil relay because on slow motor speeds, the triac's output is not sufficient to trip the relay. Consequently, I need a device or circuit with a wide voltage range in order to handle the very_low-to-120VAC variable output from the triac.

Alternatively, one can try to detect current flow to the motor. Whatever the method, I need to trip a relay whenever there is any output from the triac, i.e., whenever the motor is running at any speed. Once again, I have access only to the motor's two leads. How can I achieve this?
Thank you.
 
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Boncuk

New Member
Hi,

connect a diode in series (e.g. 1N5407 (800V, 3A)) with the load and use the voltage drop to termine if there is current flow. The voltage drop is constant independent of the triac output voltage and current flow across the load.

Boncuk
 

Gaudeamus

New Member
Thank you for your suggestion. However, I prefer not to change the waveform into the motor. The diode in series will yield a half-wave rectified signal. Is there some other equally elegant/easy way?
 

Hero999

Banned
An AC input solid state relay?

Solid state relays often trigger over a very wide 'coil' voltage.

You could make your own using an isolator and a transistor.

Does the relay output need to pass AC or DC?
 

MikeMl

Well-Known Member
Most Helpful Member
Will the motor tolerate a slight reduction in voltage? (~3V) If so, a low-value current-shunt resistor with the LED of an opto-isolator across it (with a current-limiting resistor in-series with the LED) will detect the current . You can select the output side of the opto-isolator to be an AC or DC circuit, relay or transistor switch as required.
 

crutschow

Well-Known Member
Most Helpful Member
Will the motor tolerate a slight reduction in voltage? (~3V) If so, a low-value current-shunt resistor with the LED of an opto-isolator across it (with a current-limiting resistor in-series with the LED) will detect the current . You can select the output side of the opto-isolator to be an AC or DC circuit, relay or transistor switch as required.
The problem is you need to drop 3v at the minimum current level which would likely make the drop much more than that at the full load current.

I suggest just connecting a rectifier and capacitor across the motor windings to provide a DC voltage which will drive a solid-state relay when the motor is energized. You would add a resistor in series with the relay input with resistance chosen to detect the desired minimum motor voltage. If the relay current becomes too high with the maximum motor voltage you can add a zener from the relay input to ground.
 

Gaudeamus

New Member
Thank you all very much for your solutions!

Carl: I hope to make your suggestion work, as it provides me both the isolation and the functionality I need. Thanks again.
 
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