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I am an electronics noob and I'd like to create some air circulation through my friends cellar using a salvaged 12V 0,8A brushless DC computer fan and a suitable PV panel. I have searched Ebay for 12V solar panels but their V-maximum is mostly in the 17-18V range and that worries me.

  1. What PV rated wattage would be ideal, how would I calculate?
  2. Will 17-18Vmax kill my 12V fan?
  3. If yes, then do I need a step-down buck converter or should continue looking for a 12Vmax panel?
  4. Does a step-down converter help? Does it help or hinder in cloudy weather?

Id like to keep it cheap and simple but any suggestions to make it better are very very welome. Thank you in advance!

This is the fan: Delta Electronics AFB1212SH-F00

These are two PV panels that I am considering, a 10W Eco-worthy and a 20W GWL/Sunny mono (not enough reputation for link).

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  • \$\begingroup\$ Please share a link for a representative solar panel to facilitate discussion. If you have additional data about the fan, please include that as well. For example, does the fan have a label showing rated power or current? Even better, do you have the ability to power on the fan and measure the current consumption? If so, please do so. If nothing else, then please post a picture of the fan. \$\endgroup\$
    – mkeith
    Jun 25, 2015 at 19:34
  • \$\begingroup\$ This is the fan: digikey.com/product-detail/en/AFB1212SH-F00/603-1336-ND/2560460 \$\endgroup\$
    – Sven
    Jun 25, 2015 at 20:34
  • \$\begingroup\$ Most small PV panels are either 5W or 10W rated output. Here are two: eco-worthy.com/catalog/… and ev-power.eu/Solar-Panels/… \$\endgroup\$
    – Sven
    Jun 25, 2015 at 20:43
  • \$\begingroup\$ Next time, kindly add the extra information by editing the original question. \$\endgroup\$
    – mkeith
    Jun 25, 2015 at 21:00

4 Answers 4

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The Fan is rated at 0.53A at 12V. Voltage range of 7-13.8V. If you select a nominal 12V solar panel with an output short-circuit current of around 0.5A (and certainly no more than 0.53) you will be OK. Yes, the maximum panel Voltage may be 20V or more, but the panel cannot achieve that voltage when a load is attached.

The solar panel has a V-I curve. In other words, the current and voltage of the panel are always on a well-defined curve (under constant lighting conditions). Likewise, the motor will have a V-I curve. When you connect them together, they will find the single point of intersection of their V-I curves.

As long as the voltage at that single point of intersection does not exceed the maximum motor rated voltage (13.8), then you are OK. Unfortunately, we don't have the V-I curve for the motor or the solar panel.

But, we know that the motor will consume 0.53 Amps at 12V. And, every solar panel publishes its short-circuit current (aka, Isc). So if you use a solar panel which can only supply 0.53A (Isc <= 0.53A), you don't need to worry. Because at any lower current, the voltage will be below 12V.

The panel you linked to has an Isc of 0.69A. So if you use that panel, there is some potential for problems. There is still a chance that it will be OK, but you would have to try it to find out. If you can find a panel with a lower Isc, that will be safer. If you can find a panel with a published V-I curve, you can check the current at 12V to see if it is OK. If you want to use the panel you linked to, you should be prepared to burn up some extra power somehow (if needed), using one or more power diodes in series or resistors in series, or using an LDO regulator. Just enough to keep the voltage under 13.8 in full sun. Personally, I would not use a DC-DC converter in this case just because of the complexity and cost. I am also not totally sure how the DC-DC will behave when the lighting is low. It may due some weird thing where it cycles on and off in an irritating way.

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  • \$\begingroup\$ Oh wow, thanks for sharing. This is really helpful! Ok, I will look for a panel with a Isc<=0.53A and if that fails, I will also get a cheap DC-DC step-down regulator like this LM2596. Since I sincerely hope my friend won't be living in his cellar, the second option might not be that bad what ever the regulator might do :) \$\endgroup\$
    – Sven
    Jun 26, 2015 at 5:42
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Supplying a 12V fan with 17-18V is a bad idea. Using a linear regulator to convert the solar panel voltage to 12V is also a bad idea, since you will just waste energy (18V-12V)*0.8 = 4.8 Watt. The step-down converter you mention will convert the solar panel voltage to 12V with minimal loss. To answer your question 4, the load (converter with fan) won't affect how the solar panel handles cloudy weather. The clouding will however affect how much power the panel can put out. At some cloudy point the step-down regulator will require too much power to keep the 12V regulation, which will lead to the 12V output decreasing = fan stops.

Step-down converter boards are possible to buy from most shops supplying electronic components or kits. Search for instance for a LM2596 board like this: Step-Down

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  • \$\begingroup\$ This is an overly simplistic answer. The fan load curve has to be super-imposed on the solar panel load curve to see where the operating point is. There may be no danger if the panel is matched to the fan. \$\endgroup\$
    – mkeith
    Jun 25, 2015 at 20:55
  • \$\begingroup\$ Great tips, thank you very much! What I still don't understand is that when it is cloudy and the input voltage drops too low, does the step-down regulator kill the output or will it just decrease the voltage? \$\endgroup\$
    – Sven
    Jun 25, 2015 at 20:55
  • \$\begingroup\$ It is indeed a simplistic answer to a simple question, by as he put it "electronics noob". Feel free to post a more appropriate answer. @Sven, this depends on how the regulator board is designed. Some has undervoltage lockout = kills output at low input voltage. Some designs just fail to regulate and puts out whatever is put into the regulator, perhaps minus some voltage drop across the regulator components. \$\endgroup\$
    – Dejvid_no1
    Jun 25, 2015 at 21:09
  • \$\begingroup\$ I believe a step-down with UVLO will cycle on and off in low light. Once it turns off, the panel voltage will rise. I am not saying this is necessarily a problem, but it would irritate me if I were in the room with the fan. \$\endgroup\$
    – mkeith
    Jun 25, 2015 at 21:23
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If you know the impedance of the fan during its operation, you can connect it to the PV panel as long as you add an additional load to the series loop. If you add a load or resistor in series that takes on 5V from the PV panel, the fan will operate without issue. Since you are using free energy from the sun, the power loss introduced by the resistive load is irrelevant. Your fan will work.

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    \$\begingroup\$ Adding a load/resistor would mean that I would also need to buy a bigger panel and that would most certainly be more expensive than a buck converter. It is an idea however, thank you. \$\endgroup\$
    – Sven
    Jun 25, 2015 at 20:49
  • \$\begingroup\$ Well, if the load is sized correctly, it will just dissipate the extra power so that the fan is protected from over-voltage. Due to the nature of the V-I curve, you could also add a parallel load, now that I think about it. \$\endgroup\$
    – mkeith
    Jun 25, 2015 at 21:24
  • \$\begingroup\$ I understand I need to grab a book on the topic but, if you don't mind @mkeith, would you briefly explain how it would dissipate just the "extra" power? I presume it would need to be wired in a specific way, no? \$\endgroup\$
    – Sven
    Jun 26, 2015 at 7:36
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    \$\begingroup\$ If you put one or more diodes or resistors in series with the fan, they will dissipate the power as heat. The amount of power dissipated can be calculated using P=IV. It is hard to predict ahead of time because as previously noted, the solar panel enforces a particular V-I curve. So adding series elements will move you to a different place on that curve. In other words, both voltage and current will change. Easiest thing to do is obtain two or three diodes with a high current rating. If you find the fan voltage is > 13.8, add a diode in series. re-check, and add another one if needed. \$\endgroup\$
    – mkeith
    Jun 28, 2015 at 18:31
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Wouldn't it be simpler to hook up a Charge controller to the PV panel which will accept a large range of PV panel voltages (enusre you get one for your PV panel voltage range), add a 12V battery (around 10 Ah), and hook up the motor to the 12V output from the charge controller. This will allow the fan to be run when the sun is not shining. The battery will be the largest cost, probably around $30, and the charge controller will cost around $15 (see ebay for both). Finally, most charge controllers today also have a 5V USB output so you can charge your phone if desired.

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