As I was watering the garden yesterday, I noticed the rapid heating of the water in the hose from the sun. I thought if I built a thermistor based comparator circuit to cycle pool water through a black hose as it rose over 100 degrees, It would extend the swimming season, and be a cheap and neat project. The pump is a RULE 360, and uses 12Volts, 2.5A. I would use a rated power supply for mains isolation, and the 120vAC is on a GFI.
I have some IRF510's and TIP30's. The TIP30 is maxxed at 3A, so I was leaning toward the IRF510, and a 3906 pnp and LM339 comparator to drive it. (in the circuit, the switch is replaced by a LM339, and sinks very little current) Will this work? Multisim likes it, but I've found that simulations are nice for learning, but expierence makes it work. Any suggestions would be welcome. Thanks in advance.

 

It may work but you need connect the PNP transistor properly.
Swap the emitter with collector.
And connect R2 directly to the base.
And change R1 = 10K; R3 = 10K; R4 = 100.
And don't forget to add the positive feedback to you comparator.
And to decrease the power dissipate in the MOSFET use different type of the MOSFET (IRF540, BUZ11).
Or connect them in parallel.

 

Unless this is a VERY small pool...You can not heat water up fast enough to warm up a large body like a pool.

 

It may work but you need connect the PNP transistor properly.
Swap the emitter with collector.
And connect R2 directly to the base.
And change R1 = 10K; R3 = 10K; R4 = 100.
And don't forget to add the positive feedback to you comparator.
And to decrease the power dissipate in the MOSFET use different type of the MOSFET (IRF540, BUZ11).
Or connect them in parallel.

Thanks, that's an improvement! @ 2.5A, thats 29 watts dissipated right?

 

Unless this is a VERY small pool...You can not heat water up fast enough to warm up a large body like a pool.

Now there you go ruinin my 'speriment!!! Actually, I was also playing with the idea of using a soleniod valve and garden hose to automatically top off the pool with free hot water.... The pool is around 18000 gals I think. That's a little over 100 gallions per 1/4" Hardly seems worth it.... I might build it anyway...

 

Solar pool heaters are very efficient because you only need a few degrees change. That eliminates most of the energy lost by re-radiating from the collector and pipes. Unfortunately, a garden hose doesn't represent very many square feet of sun absorbing area. A piece of black plastic laid on the surface of the water would turn the whole pool surface into a collector! If you have a circulating pump running, that will break up the stratification of the hot layer.

May I mention cetyl alcohol? If you don't make a black plastic surface cover, a few drops applied to the surface of the water supresses evaporation when nobody is in the pool. That might give you a few extra days of play time.

 

Our recently stained deck gets so hot, you can't walk barefoot on it, I thought about a 1.5" ABS coil 50', and the bilge pump. That is around 4.5 gallons. That cycles the entire pipe in a little under a minute. I figure it would take 1/2 hour to reheat. I'm already using a solar cover, so anything extra is a bonus.

 

Thanks, that's an improvement! @ 2.5A, thats 29 watts dissipated right?

No, If we look to the datasheet for IRF510
http://www.ee.nmt.edu/~wedeward/EE443L/FA99/IRF510.pdf
Rds(on) = 0.54Ω
Ptot = 2.5A^2 *0.54 = 3.4W
But we know that Rds(on) will increase its value with junction temperature rise. (figure 9)
So for Tj = 100°C Rds(on) ≈ 1.6 *0.54 = 0.87Ω
So the Ptot = 2.5A^2*0.87 = 5.5W
And this is a lot heat so you need a heatsink. But if you change the MOSFET then the heatsink is not needed anymore.

 

We had a solar heater on our pool a few years back. My wife got it so I would use the pool with her, but water being cold is only one of the reasons I don't like pools. Water is also wet.

So she orders this thing and it comes (as I remember) two 25 x 2 feet black plastic mats made of lots of tubes (something like these). Laid then out side by side, the filter pump ran the water into this thing instead of dumping right back into the pool. It also came with some plumbing items to

The instructions recomended turning the bypass of in the morning, and back on at night. As long as the sun was up it would make some hot, but I very rarely remembered to turn it on or off. You could feel warmer water going back into the pool.

 

A friend put lengths of black plastic pipe on a black shingled garage roof to heat water for his pool. Just remember to add a timer to the pump, the system also acts as a radiator at night to cool the water if you don't.

 

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No, If we look to the datasheet for IRF510
http://www.ee.nmt.edu/~wedeward/EE443L/FA99/IRF510.pdf
Rds(on) = 0.54Ω
Ptot = 2.5A^2 *0.54 = 3.4W
But we know that Rds(on) will increase its value with junction temperature rise. (figure 9)
So for Tj = 100°C Rds(on) ≈ 1.6 *0.54 = 0.87Ω
So the Ptot = 2.5A^2*0.87 = 5.5W
And this is a lot heat so you need a heatsink. But if you change the MOSFET then the heatsink is not needed anymore.

This is new material to me, Please correct my misconceptions... Power is calculated by two methods... I^{2}R, and VI. Since Mosfets have internal resistance, I^{2}R is used. Is this standard for most semiconductors? Not because of voltage applied?

 

This is new material to me, Please correct my misconceptions... Power is calculated by two methods... I^{2}R, and VI. Since Mosfets have internal resistance, I^{2}R is used. Is this standard for most semiconductors? Not because of voltage applied?

Yes we can use P = I^2*Rds(on) for Mosfets but only when Mosfet work as a switch.