Very basic question, should this (attached picture) circuit work? If yes, you are wrong. It doesn't. If no, please explain how I can make it work.

Its a simple kinda home-made 12V operated DC air cooler with a fan (using Denso blower motor) consuming ~9A i.e. ~100W power and a small water pump consuming ~1A i.e. ~10 watt power. I want to control its motor and pump on/off function from a single-pole 3-position rotary switch. Position 1: off. Position 2: only fan works. Position 3: fan and pump work. In the given circuit, I used 10A10 diode which gets heated due to large flow of current i.e. ~9A when at position 3. Should I use some other diode (with higher current flow rate) or it can't work at all in this way? Using a 2-pole 3-position rotary switch is an easy option, I know. But just for curiosity, I want to make it work that way.

 

Very basic question, should this (attached picture) circuit work? If yes, you are wrong. It doesn't.

No, you are wrong. It does. According to your own description, the circuit *works* just fine. Your complaint is that the diode gets hot, not that the fan or pump do not run when intended.

You are pumping 9 A of current through a surface-mount diode. Of course it's going to get hot, probably hot enough to melt your fingerprints (that is a real thing in electronics). According to the datasheet, at 9 A it is dissipating over 5 W. That's enough to unsolder itself from a pc board. Change to a 20 A stud-mount Shottkey diode mounted to a small heatsink and see if thing cool down.

Rule of thumb for electronic component reliability - always double the requirements for the minimum component ratings:
12 V circuit - use 25 V capacitors, 25 V diodes, 25 V transistors minimum
9 A circuit - use 20 amp diodes, 20 amp transistors minimum
1 watt resistor - use 2 watt minimum (it still will be to hot to touch)
etc.

ak

 

To avoid having to add a heatsink for the diode heat dissipation, you can use the attached circuit, which uses a switched power P-MOSFET to control the current to the fan.

The P-MOSFET blocks the current from the fan to the pump, but when the pump is energized it turns on the MOSFET to also deliver current to the fan.
Note that the drain-to-source current direction through the P-MOSFET is reversed from normal for a P-MOSFET but that's okay since a MOSFET can conduct equally well in both directions when ON.

The P-MOSFET should have an on-resistance of ≤10mΩ with a voltage rating of ≥40V in a TO-220 or similar case, such as this inexpensive automotive rated device.
That device has a maximum ON resistance of only 3.1mΩ, giving a dissipation @9A of 251 mW, which a TO-220 case can easily dissipate without a heatsink, even at underhood temperatures.
The ON voltage drop is also less than 50mV, much less than even a Schottky diode has.

D1 and D2 should be added to suppress any transients from the motor inductance when they are turned off.

The LTspice simulation shows the load current and MOSFET dissipation for the three switch positions with a similar MOSFET to that referenced.
You can see that the pump is ON for only one position (2) but the fan is ON for two positions, (2 and 3, the two overlapping lines at 9A).
There is no current in either device, of course, for the first (1) switch position.

The circuit does require two resistors and a transistor along with the MOSFET so you are trading circuit complexity for not having to add a heatsink. Your choice.

 

Or simply add a relay....

 

I like both solutions (post #3 & 4).

What kind of 12 volt pump are you using?

I set up a hydroponic system to keep my wife's plants alive through the winter. I used a 12 volt windshield pump motor powered from a 12 volt transformer. The transformer was switched on by one of those cheap settable (not programmable) timers. The kind with the slide switches - you push them down for "Time ON" and up for "Time OFF". They were settable in 30 minute increments. It didn't take long for my system to fail.

It failed because the windshield washer pump motor was designed to deliver high flow rate for a short burst, and not meant to run for 30 minutes at a time. So if you're planning on using a windshield washer pump - you're not going to be happy, they overheat and self destruct (melt).

My latest hydroponic system consists of two reservoirs. One under the plants to capture overflow and store water, the second is for watering the plants (gravity feed through drip irrigation). I used a dishwasher water ejector pump (NOT HOUSED IN THE RESERVOIR) to pump water up to a 4 inch by 12 inch PVC tank. One inlet, one outlet and one overflow. The overflow allowed the water to fill the gravity tank full without forcing water to go somewhere. Excess water simply drained back into the main reservoir while the water stored in the gravity tank would trickle down through the drip irrigation system to the plants. As they took water, any excess overflowed the plate under the plants and dripped back into the reservoir. And I used a programmable time instead of a "Settable" timer. The pump only had to run for 1 minute (shortest interval possible), thus, not running the risk of overheating a pump from prolonged running.

If you have a better pump for your system I'd love to hear about it.

As for your statement saying this system you built doesn't work - it doesn't work because of some fundamental problem it fails because the engineering behind it was faulty. In short, you didn't build it robust enough for the purpose. Hence, my concern for your pump; which may be your next obstacle.

Good luck. Oh, and for ease of construction, I'd opt (personally) for Blocco's solution. Relays are plentiful in junk yards and in automotive stores. Still, if you want greater dependability - Crutschow's solution should be more dependable. Relays fail over time. So will the P-Channel MOSFET, but it will last longer than the relay.

 

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Crutschow's solution should be more dependable. Relays fail over time. So will the P-Channel MOSFET, but it will last longer than the relay.

Yes it may fail, but likely not in the OP's lifetime if the MOSFET is not subjected to any operating conditions that are not well within its rated limits.

 

Also wondering what kind of rotary switch you have. Pumping 9 amps through it - then it can't be like the ones you find in electronics, they are designed to handle about 250 mA.

 

Also wondering what kind of rotary switch you have. Pumping 9 amps through it - then it can't be like the ones you find in electronics, they are designed to handle about 250 mA.

If you used a relay like blocco suggested, then the switch current is only the pump plus fan relay coil current (about 1A).
If you want even less through the switch you could, of course, use two relays.

 

Why don't we flip the motor locations?

 

Maybe there's a way to pretend that we changed motor locations?

 

Why don't we flip the motor locations?

If you first turn on the pump then you can only turn the fan on in ADDITION to the pump. The user may not wish to run the pump. So the fan switches on first. Then if you desire - add the pump. Otherwise there's no need for a rotary switch. Just a switch that turns the fan and pump on and off.

 

It's the medication tony, at least that's what I tell everybody. It's always a good idea to check me.

 

It's the medication

Think I may need some too. I jumped in on a post and responded with something you already said. Guess I should have read all the posts before chiming in.

Oh, and there's a humming bird feeding from the feeder. TINY little cuss.