4N25 Schematic check

[Amps]

Hey guys.

I plan on controlling 2 fans, 1 Intake and 1 exhaust, for cooling in a computer case. I want to have one DPST switch that when in one position will cause an 8v (eventually a temperature controlled circuit instead) to power only the Intake fan and when in the second position, both fans will receive a full 12v.

I decided to use 4N25 optoisolaters simply because it would isolate the 2 fans completely but I've never used something like this before.

I made this schematic in Circuitmaker2000 and it seems to work on there but I really have no idea what the tolerances of this circuit would be or if maybe I'm doing something wrong.

If you guys have any suggestions, please lemme know.

 

[MP1]

Backwards. You would normally connect power to the collector. These are NPN transistors.
Are you aware the fans have two different voltages? Are they different fans?

MP

 

[Amps]

Both fans will be designed to run at 12volts but the intake fan will be scaled down when the switch is in the first position to run close to 8volts.

 

[MP1]

One other thing I thought about that was not mentioned in your post:
Have you checked the data sheet for the 4N25 to see if it is capable of the current required by the fans?

MP

 

[Amps]

Well see, thats why I had to come here to ask. I can't make sense of anything on the datasheet as far as limits goes. I'm just not into the lingo

The fans will probably be drawing no more then 1.5 amps total (the exhaust will be 2 high cfm fans) and I'm really not sure if that will burn up the chips, which means I'd need to find a high current version of the 4N25.

 

[sec]

I really don't understand what the optoisolators are for. Isolating the fans? Isolating them from what, or isolating what from them? If you were using the output of some sensitive IC to drive the fans, then I could see it, but a simple switch does not need isolation from the fans, nor do the fans need isolation from the switch!

Get rid of the optoisolators, and use the switch to switch the intake fan between the 8v source and the 12v source. Don't make things more complicated than they need to be.

BTW, all of the computer case fans I have around here have a current draw of 0.1 A or so. Much less than what you estimated. Check the labels of your fans; the current should be printed on it.

 

[MP1]

It is always a good idea to use an optoisolator when connecting a device capable of EMF to a computer.
But sec brings up a good point. Is the 5V source coming from other switching circuitry or is this just part of the power supply?
At the very least, you should include some power diodes across the terminals of the fans so the EMF does not cause a momentary short on your computer power supply. Then you could just use the switch as is. If the 5V and switch is representative of a microcontroller or some other similar circuitry, then you must have the optoisolator or other type of protection.

MP

 

[Amps]

Well see, I'm not just switching from 12v to 8v. Lemme see if this helps:

Switch Position 1:

-Intake Fan powered by a temperature control circuit. Approximately 6-10 volts varrying between 25-40 degrees celcius will be applied to the intake. The 8v in the schematic was just a place holder so I could test the circuit in my software.

-Exhaust Fan is off, no power.

Switch Position 2:

-Intake AND Exhaust fans are both receiving a full 12 volts.


My original reasoning for using the 4N25's was because I couldn't figure a way to controll both fans with one switch but have one fan controlled by 2 sources and the other off/on without crossing the power lines somewhere. With the isolators, I don't have that issue.

 

[MP1]

Here is a control circuit (switch) that can be used with most any small PNP transistor such as 2N3906. When the resistor to the base is connected to ground (or close) the transistor is in an "ON" state.
The diagram shows the switch used to turn on a relay but it could be used the same for a motor such as used in a small fan.

MP

View attachment 35248

 

[Amps]

Thanks a lot MP. I've considered all the options and I think that for the sake of simplicity, I'm going to go with 2 DT Relays connected to a DPST switch. Here's an image of the schematic I've figured out.

The only thing different from the final schematic is the variable resistor to the LM317 will be a 500ohm thermistor.

One thing I'm not sure about but have heard about is back EMF for the relays. Apparently I should toss a Diode in somewhere?

 

[MP1]

Yes, just like the example in my last post. That is what the diode is there for. This is also a relay driver. By the way, how much current does the relay require to turn on the coil? Do I recall that the switch is not really a switch? That it is another device? You should check to be sure you have enough current from this device to power the relay coils.

MP

 

[Amps]

Ok, here's the revised circuit.

The circuit will be controlled by a regular switch so current limitations shouldn't be an issue.

Thanks for all the help MP ;D

 

[audioguru2]

Amps, your revised circuit shows 12V relay coils powered by only 5V.
Use 5V relays.
When you turn-on the computer, it will be cold and the switch or temp.-controller will apply only 8V to 1 fan. Will it start running on this low voltage? Most fan controllers apply full voltage for a moment to get the fan started then ramp the voltage down to a lower running voltage.
As MP stated in his 3rd reply, use anti back-EMF diodes across the fans.

 

[MP1]

audioguru, good eyes! Yes. The relays will not work if they are 12 VDC coils and only have 5 VDC to activate them

MP

 

[Amps]

Oops, thats a small error on my part, I meant to put 5v relays in, just mis-clicked.

Typical ambient temperature in my case will be about 15-20 degrees celcius. The way I have the resistance set up, that means that the controller will put out about 4.5-5v. These fans spin up at around 3.5v and it won't take long for them to hit the operating temperature of about 25-30 degrees.

You mention putting a diode across the fan but I've never had to do that for a project such as this before. Can you explain? I've got the 2 across each coil on the relays.

 

[MP1]

You will have the same EMF effect when the motors for the fan come on like what you have when the coil of the relay is energized. It is a good plan for all motor type circuits. You would not see a difference in the way your circuit operates. It is for protection, like adding a fuse.

MP

 

[audioguru2]

That's right, MP. Please let me elaborate.
The motor has coils of wire around a core, just like a relay. They are inductors. When you energize an inductor then a magnetic field is built-up around the core. When you remove the power source then the collapsing field induces a high voltage across the coil. That is the principle that some DC-DC step-up converters use.
This high voltage is the reverse of the applied voltage and could damage a driver transistor. In Amp's circuit, the high voltage could spark and cause pitting of the relays' contacts or jump to adjacent circuitry causing damage there.
A reverse-connected diode arrests the induced voltage.

 

[Amps]

Ok, thats something I'll consider but I'll have to do more research. The motors in the circuit are just place holders for computer fans (circuitmaker didn't have a fan). I'm pretty sure that most computer fans have protection against that sort of thing otherwise everytime I switched on one of my many fans, my power supply would hiccup ;D

 

[MP1]

Some computer fans do have the diode built into them. If you can reverse the direction of the motor by switching the wires, then it does not. If you can switch the polarity of the wires and it will not run, then there is a diode built into the fan.

MP