Hi, I have a unit with a couple of consoles in that is getting a little warm when the doors are shut.
As I have a load of spare 12v 0.4A PC fans, I thought I'd have a play at making a cooling system for it.
I haven't done anything like this for a long time and I'm struggling a bit tbh.

I don't want it active all the time and so it seems to make sense to activate them only when the TV was on. As the TV has a USB port, I was thinking of using this to power and also trigger a 5v relay which when closed, activates the fan.
The fan (or fans, as this should vote with 4 if needed) itself would simply be connected up to a 12v 2A power supply left over from an old Sky box.

Does this sound sensible?
How should I go about linking the USB as a trigger for the relay? Would I just link between the + and the signal pins or should I pop a resistor in there?

This is the relay I was thinking of getting: https://m.banggood.com/2Pcs-5V-1-Ch...odule-For-Arduino-p-1366337.html?rmmds=search

In the long term I'd probably want to trigger this with a temp sensor, but I think thats a bit much for me to get my head around at the moment.

Cheers

 

In the interest of keeping it simple I would use the USB port 5 volts to drive a common opto isolator and use that to turn on a single MOSFET to run the fans. Not saying using a relay is a bad idea, merely how I would do it. For that matter you really don't even need the opto isolator. Just let the 5 volts on the USB port turn on a logic level N channel MOSFET and switch the low side of the fans. Not much to it. Even with each fan drawing 400 to 500 mA all four will come in under 2 amps.

Another advantage is if you ever want to control fan speed based on temperature the same MOSFET can later be adapted to using PWM to control fan speed based on temperature rise.

Ron

 

Thanks for the quick reply.

I think I was miss-using the term 'relay' as an overall term for things that separates two circuits such as mechanical relays and common opto isolators (which is another term for a optocoupler isn't is?)

The MOFSET method looks really nice and simple (this is starting to bring back memories now), but I was looking at keeping the circuits separated with the optocoupler so that there was less chance I could fry the TV if something went wrong.

I could buy a optocoupler and MOFSET separately and connect them up, but if I'm reading it correctly, doesn't the item I linked to before already essentially do this? (I'm posting here again as previously I had accidentally put the mobile link for which doesn't show all the details nicely on PC: https://www.banggood.com/2Pcs-5V-1-...337.html?akmClientCountry=GB&cur_warehouse=CN).

 

Yeah, if you want or feel you need isolation you can use an optocoupler. It's an option. The small relay cards you linked to are also a good option as they are modular making for easy incorporation into your scheme. Using a jumper they also allow for high or low level triggering. They won't allow PWM but if all you want is fan On/Off they will work fine. The coil current is about 72 mA per unit so you can drive at least one from a USB port. Maybe two depending on the USP max port 5 volt current. This is the relay data sheet.

There are any of several ways to get where you want to go.

Ron

 

In the interest of keeping it simple I would use the USB port 5 volts to drive a common opto isolator and use that to turn on a single MOSFET to run the fans. Not saying using a relay is a bad idea, merely how I would do it. For that matter you really don't even need the opto isolator. Just let the 5 volts on the USB port turn on a logic level N channel MOSFET and switch the low side of the fans. Not much to it. Even with each fan drawing 400 to 500 mA all four will come in under 2 amps.

Another advantage is if you ever want to control fan speed based on temperature the same MOSFET can later be adapted to using PWM to control fan speed based on temperature rise.

Ron

You can’t drive a power MOSFET switching 12 VDC directly with 5V. The difference in voltages wont allow Vgth to be satisfied. You need a BJT or N channel logic level MOSFET as a driver. Such as a 2N7000. In this case, the driver’s gate is connected to the 5V signal and the drain is connected to the power FETs gate and 12V via a pull up resistor. I just had to solve this problem.

 

Cheers for the link to the data sheet.

The coil current is about 72 mA per unit so you can drive at least one from a USB port. Maybe two depending on the USP max port 5 volt current.

I believe USB gives 5v at 500 mA (newer version can give more) btw, so it should be more more than adequate.

Still a noob with a lot of this sorry, but is the info you gave me from figure 6 on the data sheet?
So if for example, if in the future I were to get one of these rated for 12v and powered it from a 12v supply it would pull about 30 mA?

 

You can’t drive a power MOSFET switching 12 VDC directly with 5V. The difference in voltages wont allow Vgth to be satisfied. You need a BJT or N channel logic level MOSFET as a driver. Such as a 2N7000. In this case, the driver’s gate is connected to the 5V signal and the drain is connected to the power FETs gate and 12V via a pull up resistor. I just had to solve this problem.

I understand what you are saying using the 2N7000 as a driver. What I don't quite get is why just using an N channel MOSFET for low side switching of the fans won't work? You are telling me that I can't use for example an IRL 540 N Channel MOSFET for low side switching of a 12 volt fan applying 5 volts to the gate?

Ron

 

Cheers for the link to the data sheet.



I believe USB gives 5v at 500 mA (newer version can give more) btw, so it should be more more than adequate.

Still a noob with a lot of this sorry, but is the info you gave me from figure 6 on the data sheet?
So if for example, if in the future I were to get one of these rated for 12v and powered it from a 12v supply it would pull about 30 mA?

Yes, that relay is available in several coil voltages and the 12 volt coil; version has a current draw of about 30 mA.

Ron

 

Yes, that relay is available in several coil voltages and the 12 volt coil; version has a current draw of about 30 mA.

Yes! I'm learning!
Thank you very much.