How to reduce voltage to drive tiny DC water pump?

[jdiamond]

I realize this post isn't quite in line with this forum, as it's about a tiny 3.5 watt DC water pump for use in a fountain, not the typical heavy duty water pumping stuff. So feel free to redirect me to a more appropriate forum.

Here's the problem: I want to run the DC water pump directly off a solar panel, but the pump is only rated from 4-12 volts operation, and the 5 watt solar panel can hit 18 volts. The best solution would be one that "caps" the voltage as 12 volts, but if that doesn't work, I need at least voltage reducer. I realize I could open up the solar panel and try to rewire the cells in parallel to get 6-9 volt output, but this reduces the time it could run the pump, since it would need to hit 8 source volts to get the minimum 4 output volts.

Here's the problem I ran into. It seems like no one sells these anymore. Most of the regulators these days are for battery charging - they actually measure the battery's voltage and won't work without a battery. The only other form I find voltage limiters in is silicon chips, and I'm not up to making my own circuit board.

Does anyone know of a ready made, water proof module that can cap Voltage at a max level, specifically 4-18 Volts in -> 4-12 volts out, running at just 0.3 amps and 5 watts?

Alternatively, would it be possible to simultaneously charge a battery AND run the pump off that battery? It seems like it wouldn't work, because the battery terminals would see the higher voltage of the solar panel.

Any advice you have is so worth it.
Thanks!
- Jeff Diamond

[Wy_White_Wolf]

Cheapest and easiest route will be to get a 6v solar panel.

[inetdog]

Cheapest and easiest route will be to get a 6v solar panel.

Second cheapest and easiest route would be to get a high power zener diode that can handle the power and just put it in parallel with the panel. Since the panel is acting as a current source, it will dump as much current (up to Isc) into the zener at 12 volts. Anything which the pump takes will just result in less current flowing through the Zener.
Of course that is a huge waste of power unless you also put a series resistor between the diode and the panel.
Marginally more practical would be to put a zener in series between the pump and the panel which drops about 6 volts and puts the voltage seen by the pump into the range of 18 - 6 = 12 volts.
The voltage output of the panel will not vary with amount of sun, just with temperature, so you would get full use.

Similarly, if you rewire the panels to a lower voltage configuration, it would actually deliver more current sooner in the day and would run the pump better. So there is your optimal solution.

[Mike90250]

Second cheapest and easiest route would be to get a high power zener diode that can handle the power and just put it in parallel with the panel.....

Yep, what he said.

[Sunking]

Well I say none of the above. Just the 5 watt panel is all you need. Adjust the tilt angle at noon so you get about 12 to 13 volts and call it done.

[inetdog]

Well I say none of the above. Just the 5 watt panel is all you need. Adjust the tilt angle at noon so you get about 12 to 13 volts and call it done.

With the pump running!

[jdiamond]

Tllting the panel...

Thanks so much for your advice, everyone. I'll measure the actual voltage coming out of the panel, and first determine if I can just make it "less inefficient" as suggested, and then I'll go from there.

- Jeff

[Sunking]

Thanks so much for your advice, everyone. I'll measure the actual voltage coming out of the panel, and first determine if I can just make it "less inefficient" as suggested, and then I'll go from there.- Jeff

Any method mentioned so far is just making it less efficient.

[jdiamond]

Efficiency Losses...

You're absolutely right. I guess I should have said the methods which require the least hardware first. The only one I'm guessing that would be efficient might be a circuit that kicks in when the voltage exceeds 12V and acts as a current booster, dropping the voltage but keeping the power the same.

[Sunking]

You're absolutely right. I guess I should have said the methods which require the least hardware first.

I think I met that criteria. Hers is the deal if the motor is rated for 12 volts like a battery, it can handle 15 volts. A 5 watt panel will never generate its rated capacity. In fact I doubt it could ever generate 4 watts. The motor resistance will overload the panel and i doubt even if optimized sun angle is used would ever over supply the motor. I would be looking at a 10 watt panel.

[jdiamond]

Thanks!

Thanks - that's good advice.

I already ordered the 5 watt, but when it arrives, I'll measure its voltage out in different scenarios and post the results. Some people had measured over 17 volts from it. The reason I was cautious about the motor is it has a rating range of 4-12 volts, which makes me think it has less headroom than a purely "12 volt" motor. If the power isn't great, I'll get a second 5 watt and wire them up in parallel.

It occurred to me that a very common complaint from these fountain kits is that after a little while, they just stopped working. I can't help but wonder if the motor's burning out because they just direct wire it. But we'll see.

What started all this has been my experience that whenever you buy an integrated solar kit, the included solar panel is so small it doesn't work. Maybe this is because the solar panel is the most expensive part. For example, in the fountain kit we got, they supplied a 1.5 watt solar panel for a 3.5 watt motor. Of course, it never worked. When I tried to call the company on it, they handed me this long BS story that to paraphrase was, "actually, solar panels don't actually work - it's a myth that they work. If you hooked up a 100 watt panel to it, it still wouldn't work. That's just solar."

- Jeff

[Sunking]

Thanks - that's good advice.

I already ordered the 5 watt, but when it arrives,

Make sure you take the voltage measurement with the motor connected. Otherwise if the panel is not connected to anything will read open circuit voltage (Voc) of about 22 volts with moonlight or a flashlight.

[jdiamond]

Ouch...

Thanks for the tip - it won't be easy to measure it hooked up, because the pump uses one of those DC co-axial plugs like a headphone jack. But I should be able to get at the panel wire leads before they tie into the DC plug.

I'm a little confused though - man I wish I took EE in college... if I measure the voltage difference with the motor attached, why would that look different?

[Sunking]

I'm a little confused though - man I wish I took EE in college... if I measure the voltage difference with the motor attached, why would that look different?

The voltage. Solar panels are current sources, not voltage. The voltage will depend on the amount of current the panel can generate and the resistance the load offers.

[jdiamond]

Thanks. That clears up a lot of misconceptions I've had about solar panels. I had viewed the situation as their voltage varied with time, and the attached load drew a certain amount of current, but I hadn't really thought of solar cells as maintaining a fixed current...