Conditional fan cooling for analog solar panel regulator - Page 1
Forum section: electronics / projects

author: enut11
author_rank: Super Contributor
timestamp: July 17, 2022, 09:35:03 pm
content: I built a modified version of this 22v solar panel to 12v battery regulator by adding a PNP power transistor to allow up to 2.5A charge current.When the 12v lead acid battery is low in voltage, 10 or more volts has to be dropped by the regulator. This results in 25 watts or more of heat dissipated by the power transistor until the battery voltage rises to 13.5v and the current reduces to a trickle charge.I am currently using a 12v fan to cool the heatsink but, as the fan is only needed for part of the time, I am looking for ideas on how to switch the fan on only while heavy current is flowing and to switch off when the 12v battery is fully charged. Essentially, a current sense switch.enut11

author: chuckb
author_rank: Frequent Contributor
timestamp: July 17, 2022, 09:59:50 pm
content: A simple, normally open, thermal switch (50C?) mounted to the heatsink can control the fan.One example -uxcell KSD9700 Thermostat, 50C Normally Open Temperature Switch Thermal Switch, N.O 5A Metal Temperature Controller

author: enut11
author_rank: Super Contributor
timestamp: July 17, 2022, 10:08:06 pm
content: Thanks chuckb. I considered that. The downside I believe is the large thermal hysteresis resulting in overheating and overcooling.I was hoping for an idea to monitor and act on the charge current.enut11

author: themadhippy
author_rank: Super Contributor
timestamp: July 17, 2022, 10:41:08 pm
content: monitor the voltage between the meter output and the battery,as the current increases so does the voltage.

author: Seekonk
author_rank: Super Contributor
timestamp: July 17, 2022, 10:58:55 pm
content: I've been thinking of this for a refrigerator motor for a chest fridge at my camp. Fridge only operates in daylight so each morning the runs are extra long. I think in the normal short duty cycles the mfg uses the pump assembly as a large thermal mass. So, a little extra cooling with a fan could help. Only been thinking about this for 6 years. Thinking of a thermistor and a FET bonded to the motor and just use the varying 12V as the reference. The FET getting hot, lowering gate threshold voltage and the thermistor would give it a double whammy.I've used cheap switching regulators for solar applications. It is easy to monitor the panel voltage and fake the reference pin into thinking the output is too high, throttling down the regulator and keeping the panel near power point. Little heat and extra power on poor days.

author: enut11
author_rank: Super Contributor
timestamp: July 18, 2022, 03:50:55 am
content: Quote from: themadhippy on July 17, 2022, 10:41:08 pmthemadhippyI can't see how this would give a useable signal. Would you please elaborate?enut11

author: enut11
author_rank: Super Contributor
timestamp: July 18, 2022, 03:53:01 am
content: SeekonkUsing a thermistor is a good idea, perhaps in conjunction with a BJT as they are easier to set up?enut11

author: SeanB
author_rank: Super Contributor
timestamp: July 18, 2022, 08:54:50 am
content: Use a zener diode, resistor and transistor to detect input voltage, and then use a transistor to control the coil on a small low current relay, so that the fan turn on transient is enough to start it. Fan runs off the battery output, but will only turn on when there is at least 16V across the pass transistor, so there is going to be enough current to run it. Zener probably 16V, 2k2 resistor in series, 10k across base emitter of the transistor, pretty much any small signal transistor like BC108, PN2222 will work. Turns on when there is high voltage, cooling the heatsink and also the battery, which will help with life of the SLA pack as well.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 18, 2022, 01:08:47 pm
content: Just connect the fan across the transistor.

author: themadhippy
author_rank: Super Contributor
timestamp: July 18, 2022, 01:37:02 pm
content: Quoteyou get a voltage that changes proportionally to the current being drawn,maybe amplify it a bit and have it compared to a reference voltage ,if the measured voltage is greater than the reference voltage it switches on,if its not it stays off

author: Zero999
author_rank: Super Contributor
timestamp: July 18, 2022, 01:57:12 pm
content: Have you measured the solar panel voltage, when the current is 2.5A? I would have thought it would have fallen.How much current does the solar panel actually output in full sun?I've done a similar thing before, but I skipped the current limiting circuit and booster transistor.You only need one diode and you should put a zener diode across the LM317 to protect it against high voltage spikes from distant lightning strikes.

author: enut11
author_rank: Super Contributor
timestamp: July 19, 2022, 06:43:40 pm
content: No. I am still setting up the solar panel so all simulations have been done with a power supply. The panel is rated at 200w - see pic.Thanks for the zener tip

author: enut11
author_rank: Super Contributor
timestamp: July 19, 2022, 06:49:04 pm
content: Quote from: NiHaoMike on July 18, 2022, 01:08:47 pmFor the power transistor, the voltage will fall with rising current - nogo.

author: Zero999
author_rank: Super Contributor
timestamp: July 20, 2022, 07:35:26 am
content: Quote from: enut11 on July 19, 2022, 06:49:04 pmI think the idea is to use some of the power which otherwise would have been wasted in the transistor to power the fan. The voltage across the fan falling a bit shouldn't matter.

author: Seekonk
author_rank: Super Contributor
timestamp: July 20, 2022, 02:19:24 pm
content: FET is easy to use and gives lowest part count, thermistor, pot, FET.This is a nice learning project, things get hot. The lesson should be, don't design things that get hot and go for efficiency. You wouldn't see a design like this in the commercial world.

author: enut11
author_rank: Super Contributor
timestamp: July 20, 2022, 06:53:12 pm
content: Quote from: Zero999 on July 20, 2022, 07:35:26 amBut the fan would still be running when the battery is fully charged. I would like the fan to stop when not needed.

author: enut11
author_rank: Super Contributor
timestamp: July 20, 2022, 07:07:42 pm
content: Quote from: Seekonk on July 20, 2022, 02:19:24 pmTrue. I do have a small 'commercial' solar battery controller but it wanted to charge the LA battery to 15v, even when set to 'SLA'.Also, I wanted to monitor the voltage and currentfromthe solar panel and the commercial unit did not offer this.I realise that a linear regulator is inefficient but the energy is essentially free :-)I will try the FET idea soon. At the moment we are up to our ears baby sitting.enut11