Hi

My question is related to the project I'm working on, discussed in this thread:
http://forum.allaboutcircuits.com/showthread.php?t=85422
However, I've now strayed from that thread's original title, so thought it best to start a new thread.

I have a 5v device powered separately from a 5v cooling fan. I want to use 2 switches:
1 - 2-position rocker to turn both the fan and the separately powered device, ON/OFF, together.
2 - 2-position rocker to toggle the fan between FULL SPEED and HALF SPEED.

Please could someone take a look at the following diagram and tell me if it looks OK?

Do I need to add a diode after R2, at the point marked with a red arrow?

Anything else I've missed, or is this diagram just completely wrong?

Thanks in advance.

 

Switch #1 (left most) could be single pole instead of double pole as shown, since the +5V supply line is the only thing being switch there. So you could get two identical simple switches.

Or you might consider a double pole, 3 position rocker switch (DPQT? I think) to replace the two switches. The "device" pole would have 2 of the positions wired together so it would only be off in one position and full on at the other 2 positions. The fan pole would have the resistor on one "on" position and a direct connection on the other.

 

Switch #1 (left most) could be single pole instead of double pole as shown, since the +5V supply line is the only thing being switch there. So you could get two identical simple switches.

Or you might consider a double pole, 3 position rocker switch (DPQT? I think) to replace the two switches. The "device" pole would have 2 of the positions wired together so it would only be off in one position and full on at the other 2 positions. The fan pole would have the resistor on one "on" position and a direct connection on the other.

Despite the device and fan being on two separate circuits (for reasons I won't bore you with ), I really just want to use a single ON/OFF switch for both, for aesthetic/user-friendly reasons.

Your idea about using a single, double pole, 3p rocker is an interesting one. Hmm, I'll give that some thought. Thanks!

So, no diodes needed anywhere, then?

 

...I really just want to use a single ON/OFF switch for both...

I think you misunderstood me. I agree you need only one switch for both devices. I pointed out that switch could be the same simple type you drew for the fan speed selection, so that both switches could be the same simple type.

And no, no diodes required as long as your "device" is not a power source.

 

I think you misunderstood me. I agree you need only one switch for both devices. I pointed out that switch could be the same simple type you drew for the fan speed selection, so that both switches could be the same simple type.

And no, no diodes required as long as your "device" is not a power source.

Sorry, I don't understand how I can use a single pole switch to power two separate circuits that use two separate power sources?

Good to know no diodes are needed. Thanks.

BTW, the 'device' is actually an Android TV stick/dongle, like this one:
http://www.amazon.co.uk/gp/product/B00AFVO1EA/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1
It comes with firmware that runs the CPU at 1.2Ghz but I've flashed it with modded firmware that allows the CPU to run at the full 1.6Ghz. Since the device comes with no active/passive cooling (not even a heatsink on the SoC!), my project is a to make a new (bigger) case for it. I've already fitted heatsinks.

 

Labeling your power sources something like 5vDC(1) and 5vDC(2) may avoid confusion.

 

Labeling your power sources something like 5vDC(1) and 5vDC(2) may avoid confusion.

Ah, yes - good point.

 

If noise is an issue and assuming your fan has no protection circuitry, a diode is an excellent way to reduce noise from the fan motor (assuming the two 5v input sources are linked). A capacitor across the fan inputs adds even more protection. If the power sources are completely separated, no diode is needed.

 

If noise is an issue and assuming your fan has no protection circuitry, a diode is an excellent way to reduce noise from the fan motor (assuming the two 5v input sources are linked). A capacitor across the fan inputs adds even more protection. If the power sources are completely separated, no diode is needed.

Can you explain what you mean by 'protection' and how a capacitor helps, please? I'm kind of new to all this stuff.

The two power sources are completely separate: the device is powered by a 5v 2.1A adapter; the fan circuit is powered via USB, plugged into a mains powered hub.

 

Would this switch be OK as an ON/OFF switch for both power sources, keeping them isolated?
http://uk.rs-online.com/web/p/rocker-switches/4954074/

 

I guess the diode at R1 is a LED to indicate "Power on". Your circuit is OK - go ahead and do it.

 

I guess the diode at R1 is a LED to indicate "Power on". Your circuit is OK - go ahead and do it.

Yes it is. Thanks.

 

Sorry, I don't understand how I can use a single pole switch to power two separate circuits that use two separate power sources?

Ah, I've finally grasped that you have separate +5V supplies. Yes of course you do not want them to be connected directly. Forget my comments about using a simple singe pole on/off switch.

The switch you linked to will keep them isolated.

Here is an example of a 3-position switch that, I think, could do all your switching chores in one package. Another.

 

Ah, I've finally grasped that you have separate +5V supplies. Yes of course you do not want them to be connected directly. Forget my comments about using a simple singe pole on/off switch.

The switch you linked to will keep them isolated.

Here is an example of a 3-position switch that, I think, could do all your switching chores in one package. Another.

Wow, that's a couple of serious switches!

Unfortunately, I can't use a switch like that because my project case is only small and has limited 'clearance', inside. TBH, I've been struggling to find switches that will fit. Why do switches all seem to have a large depth measurement, compared to their width and height?!

Anyway, I've decided to stick with a 2-switch circuit, mainly to ensure I don't accidentally switch off the power when changing the fan speed.

I've also decided to use a 3-position on-off-on rocker switch for the fan, so I can turn it off when I leave the device idling. It doesn't really get hot then so the fan won't be needed.

After much searching I've managed to find a pair of suitable, matching (looks-wise) switches, which I've just ordered, together with a resistor kit and a multimeter!

Thanks for all your help with this - I'm learning more as I go along.