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# Help on triac circuit to control fan from 2 mains signals

#### paultowlson

##### New Member
I am looking for a simple triac circuit to control a 24W fan. The signal will be from 2 other fans. When either of the small fans is activated (and giving a 240V mains signal), the triac circuit needs to turn on the main fan.
Should I use some kind of OR function, or use two triacs in parallel (not my ideal choice)?
I'm more used to low voltage digital circuits, so I'm not sure how to reduce the 240V AC signal down to the 3V or so for the triac gate.

thanks for any help

Solution
To control a 24W fan using a triac circuit, you can use a simple optocoupler-based design. This circuit can be triggered by a 240V AC signal, which is used to turn on the fan. You can use a single triac to control the fan. This write-up can give you a rough idea.

To control a 24W fan using a triac circuit, you can use a simple optocoupler-based design. This circuit can be triggered by a 240V AC signal, which is used to turn on the fan. You can use a single triac to control the fan. This write-up can give you a rough idea.

This might be useful to you :

Regards, Dana.

I am looking for a simple triac circuit to control a 24W fan. The signal will be from 2 other fans. When either of the small fans is activated (and giving a 240V mains signal), the triac circuit needs to turn on the main fan.
Should I use some kind of OR function, or use two triacs in parallel (not my ideal choice)?
I'm more used to low voltage digital circuits, so I'm not sure how to reduce the 240V AC signal down to the 3V or so for the triac gate.

thanks for any help

To control a 24W fan using a triac circuit, you can use a simple optocoupler-based design. This circuit can be triggered by a 240V AC signal, which is used to turn on the fan. You can use a single triac to control the fan. This write-up can give you a rough idea.

Thank you. This seems overly complex as I don't need to control speed, just simple on-off. I've seen gate signals derived from the same live as the load, just dropped through a resistor. In my case, the live signal is from a different load but the same neutral supply.

This might be useful to you :

Regards, Dana.

I suggest a couple of 240 V relays. Connect the each relay coil in parallel with one of the small fans. Connect the relay contacts from both relays in parallel and use those to control the main fan.

That would certainly work, though my preference is for a solid state solution.
thank you

I am not sure what you need, but I sketched up this.

I am not sure what you need, but I sketched up this.
View attachment 141413
thanks. Is the control box representing one of the small fans? The signal from these 2 fans is 240V. I'm unsure of where this fits in. Also, I have 2 fans giving a signal so I need an OR function. Can I achieve that with an opto coupler?

The simplest thing I can think of using off-the-shelf parts is a basic, cheap, solid-state relay that needs a DC control voltage as a power switch, then a couple of 5V USB power units running from the two other fans.

Connect both 5V negative wires to the SSR control negative, then use diodes (1N4000 series, 1N4002 or 1N4007 - whatever is easiest or cheapest) from each 5V positive to the SSR control positive.

The diode cathodes (banded end) should be to the relay.

The 5V units only need to be very low current, but 5V 500mA types may be cheapest.

You don’t need the diodes. A simple 5V PSU won’t mind having 5 V on the output when there is no mains supply.

A google search and a selection:

A google search and a selection:
If R1 connected to the 240V signal instead of to the load, that I think would work. Not sure how to achieve an OR function though. There are 2 240V signals - the main fan should come on if either signal is on. I know I can do it at 5V with an OR gate, but it means isolating mains from DC with opto couplers. So, 2 opto couplers from the 240V signals, the 2 low voltage DC signals then into an OR gate thate controls the triac to switch the load on.

I am looking for a simple triac circuit to control a 24W fan. The signal will be from 2 other fans. When either of the small fans is activated (and giving a 240V mains signal), the triac circuit needs to turn on the main fan.
I would just use a few SSRs with 240 VAC control lines. You don't really have much of a load to switch. I would just use a 240 VAC controlled SSR. If either small fan is on the large fan will run.

Ron

Thank you for all the ideas. The solid state relay was the simplest as I had easy signal lines at 240V to activate the relays. Whichever fan is on activates the inline booster fan.

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