Relay - Capacitor Needed?

[jocanon]

I am building a theater room in my house with a projector that will be ceiling mounted inside a hush box to keep the noise and heat the projector generates out of the theater room. In order to do this without over-heating the projector inside the hush box, I had to build ducting with an in-line duct fan to pull cool air through a filtered vent from the ambient air in the theater room, through the hush box to cool the projector and suck out the hot air, and then ultimately dump it into an adjacent room.

So I have everything done (or at least on paper), except I just need a way to turn the in-line duct fan off and on when the projector is on. I was going to use a thermostat switch inside the hush box until I realized the projector has a 12v trigger that comes on whenever the projector is on. So my plan now is to connect a 12v relay to this trigger and use it to switch the 120 VAC mains power to the in-line duct fan. Is this as simple as using this switch by soldering the coil leads to a cord that I plug into the 12v trigger and then using the contact leads on the relay to switch the mains power? Or should I put a capacitor between the 12v trigger and the relay to smooth it out? If yes, do you know what capacitor I should use? Or is there something else I need to do that I have not thought about? Thanks for any help with this:

https://www.mouser.com/ProductDetai...EpiMZZMtSzCF3XBhmW1upDI8uprbL%2b6ui%2bElcbnE=

Another question I had is the above is rate for 250 VAC. I am in the US so I use 120 VAC, does the 250 VAC rating mean it can go up to 250 VAC so 120 VAC would be fine because it is less than 250? Or do I need to use this one that is rate at 120 VAC? Actually, the below is rates at both 250 VAC and 120 VAC (my guess is both would be fine, the first one just didn't put the 120 VAC rating on it even though it would work):

https://www.mouser.com/ProductDetai...=sGAEpiMZZMtSzCF3XBhmW0jFivbULW3rPEZOlpMvDjM=

 

[alec_t]

Is this as simple as using this switch by soldering the coil leads to a cord that I plug into the 12v trigger and then using the contact leads on the relay to switch the mains power?

It could be, IF the trigger output is 12VDC and is rated to provide enough current for the relay's coil demand. A catching diode across the relay coil would be needed if the 12V is provided by a solid-state switch rather than by internal relay contacts. Do you have a schematic for the projector, or at least the full spec of the trigger section?

Or should I put a capacitor between the 12v trigger and the relay to smooth it out?

No. That won't be of use for a DC relay coil.

does the 250 VAC rating mean it can go up to 250 VAC so 120 VAC would be fine

Yes.

 

[jocanon]

It could be, IF the trigger output is 12VDC and is rated to provide enough current for the relay's coil demand. A catching diode across the relay coil would be needed if the 12V is provided by a solid-state switch rather than by internal relay contacts. Do you have a schematic for the projector, or at least the full spec of the trigger section?

The projector is a BenQ HT 1075 model, but the manual is no help. I took some of my own measurements this morning. When the projector is first powered on, the trigger maintains a constant 12.2v. Then on power down, the projector's internal cooling fans run for about a minute or so after the projector is turned off in order to draw remaining heat off the lamp. During this time the 12v trigger continues to be active but instead of 12.2v during this cool down time it reads about 12.4v. I am not sure why the minor difference, but I tried it a couple times and got consistent results each time. I don't think the minor difference in voltage between normal operation and cool down of the projector will matter though. I just want to give you all the info I have, even if I think it is irrelevant just in case I am wrong and it is in fact relevant.

As far as the amount of current the 12v trigger can provide, I cannot find any information on that. In looking at the datasheet of the first relay I linked to in my initial post, it looks like it only draws 400mw which I am guessing is small enough to not be a problem, but that is just a guess. The trigger is made to control external devices (i.e. raise and lower a screen, turn speakers on, etc...). It is not made to provide much in the way of power, just a minor amount to trigger a device.

 

[MikeMl]

0.4W/12V = 0.2A. If that is coming from a "logic output" inside the projector, that might be more than the projector can drive. Try it, and measure the voltage across the relay coil when the projector is running... If the output voltage stays near 12V, you are likely ok...

Diode across relay coil: cathode(and top end of relay coil) to projector output, anode (and bottom end of the relay coil) to projector ground.

 

[alec_t]

0.4W/12V = 0.2A.

?? I make it 33mA. Even so, that may be more than the trigger is intended to source.

 

[MikeMl]

?? I make it 33mA. Even so, that may be more than the trigger is intended to source.

I must have hit the wrong button on the calc...
0.03333333333333333A, actually

 

[alec_t]

I don't think we need worry about the 17th decimal place value .

 

[jocanon]

Diode across relay coil: cathode(and top end of relay coil) to projector output, anode (and bottom end of the relay coil) to projector ground.

Suggestions on specific diode to use are welcome

 

[AnalogKid]

Although they cost a little more, I think your total short-term effort and long-term grief will be less with a solid state relay. Only a few mA to activate, no emf spikes to contain, all screw driver installation if that's appealing...
Hockey puck or mini hockey puck, around $7.50:
https://www.electronicsurplus.com/relays/solid-state-relay/sort-by/price/sort-direction/asc/show/72
https://www.goldmine-elec-products.com/prodinfo.asp?number=G21136
https://www.midwestsurplus.net/contents/en-us/d55.html
https://www.surplussales.com/Relays/RESSPlug-1.html

I don't see anything about fan power or current in previous posts. What is the steady state fan current? For any switch of any kind, double that as a starting point for the switch rating.

ak

 

[jocanon]

Good point

...
I don't see anything about fan power or current in previous posts. What is the steady state fan current? For any switch of any kind, double that as a starting point for the switch rating.
...
ak

Good question, to answer, I don't think fan power should not be an issue. It is less than 1/5 an amp draw:
https://www.globalindustrial.com/p/hvac/exhaust-fans/in-line-duct-fans/in-line-duct-fan-106-cfm

So what are the benefits of a solid state relay. I am assuming it is just plain more reliable, correct? If yes, then I would probably prefer a solid state relay for just a few more dollars.

 

[AnalogKid]

I like them, especially for an application with lots of activations per day, where a normal relay lasts only a few months. Your use is considerably more benign. When the controller for my sprinkler system pump died back in the 90's, I replaced the switch with a surplus Grayhill mini-hockey puck. 10 A relay switching a 6 A motor, around 200-300 activations per year, still running. I think the low activation energy fits well with your control signal, and it needs no protection diode or current limiter resistor or anything else.

ak

 

[jocanon]

would this be a good one:

**broken link removed**

 

[jocanon]

Or maybe I will go with this one for some head room in the output current - it goes to 10 amps instead of just 2.5 like the above one (even though the fan says it only uses 0.38 amps).

**broken link removed**

edit:
I got this one. Here is a link to it on Mouser so you can see what I got even after the link above no longer works:

https://www.mouser.com/ProductDetai...=sGAEpiMZZMtq49AUx5G37x4sUP8FBQ5O5nWu5w7eSY8=