Capacitor Question for You Electronic Gurus

So, I've got that Davis Vantage Pro 2 weather station outside here that uses a solar panel to power the wireless system. The panel charges a capacitor during the day, and is SUPPOSED to use that stored energy at night. Once the capacitor is drained, a 3v battery takes over. When you start going through batteries every 4-6 months (should last 3 or more years), us Davis owners know it's time for a new capacitor.

I understand how they work, but I also know nothing electrical is a simple cut-and-dry deal.

The stock, factory installed capacitor is 2.7v & 10uF capacitance. That 2.7v rating in a 3v system could be the reason why this is such a failure-prone component of the Davis systems. A search will find hundreds of posts all over the net on this problem.

Now, going with my "bigger is better" instinct, I want to install a capacitor with a higher voltage rating, and more capacitance. I understand this may increase the physical size of the capacitor itself, but, my main question is ... is this just a simple deal of getting a, say, 6v capacitor and 20uF capacitance? In other words ... is there an electrical downside to going TOO big? Also, should I stick with the "tank" or "can" shaped original design, or can I try different types? (I forget the names of all of them ...) My reasoning here is that I don't want to do this twice.

This unit is outside, operating 24/7, 365.

I would say a lot has to do with how it is wired into the circuit. I wouldn't go any higher if the unit was working fine. You will have to get a 6.3v or higher cap as more common the cap a lot less expensive. Just rip apart some junk electronic thing and steal a cap that is close to original as possible. 10uf/6.3v 22uf/6.3v, if none of those look for 10v, 12v or slightly higher.

GREAT idea - you just reminded me that I have a bunch of electronic junk out in the barn I saved for that purpose. Free is for ME!

Half the reason my barn is so full is because I forget what the hell I was saving the stuff for ...

I've re-capped a couple old tube amps and it was always higher voltage is of no concern. Lower voltage and capacitance changes weren't good. That's working with audio equipment so capacitance changes could do some weird things but I can't tell you what they are now.

I can tell you that caps are generally not much good (at least in trying to maintain spec) after 15 years, sometimes less.

That's good to know. If I only have to re-solder another cap in there once in 15 years, I'm good with that.

Davis probably used the 2.7v one for the same reasons most companies use inferior products - to save money. .06 cents x 100k units adds up I suppose.

I wish I understood what I know. I'm leaning with the bigger, higher, is OK. The one thing I guess I'd ponder on is: Why are some caps AC & some DC? I guess I'd stick with DC in this case.

I'm just talking here 'cause I really don't know....but might the be using a less than 3 volt cap because it won't put out more than 3 volts? If you put in a 9 volt cap, is it possible it will put out too high a voltage & fry things? I don't know...just brain melting.

Freddy, there is really no difference between AC and DC capacitors. They both store energy. In a DC circuit the cap will charge and that's it until the circuit voltage falls below the caps charge, then it will release it's charge.

An AC circuit will see a constant charge/discharge when the current changes directions. Think of it as a buffer, used a lot to filter out AC from audio like radios. An old radio will hum if the caps are bad.

As for the voltage it's kind of like a safeguard. A 3 volt cap will store up to 3 volts without damage. Higher voltages going in will smoke the cap.

There's more to it but that's the gist from what I understand. Anyone feel free to correct if I spoke incorrectly.

From what I've been reading, I guess I could install a 50v capacitor ... or 200v ... because it will only charge up as high as the voltage supplied. So if 3v goes in, 3v is what you get - no more, no less. Only problem would be fitting that beer can in the little case that the board sits in.

I do know what happens when you feed them MORE than their rated voltage .... they POP - like a zit, only much louder.

I found this out 13 years ago playing around in my apartment in Phoenix with a 12v 400W power inverter, and a 1960's battery charger. Turns out, the capacitors didn't like 16v hitting them, and they puked cardboard all over the inside of the case. Lots of smoke too.

See what happens when I get left alone in a room with electronics?

SMITTY wrote:So, I've got that Davis Vantage Pro 2 weather station outside here that uses a solar panel to power the wireless system. The panel charges a capacitor during the day, and is SUPPOSED to use that stored energy at night. Once the capacitor is drained, a 3v battery takes over. When you start going through batteries every 4-6 months (should last 3 or more years), us Davis owners know it's time for a new capacitor.

The stock, factory installed capacitor is 2.7v & 10uF capacitance. That 2.7v rating in a 3v system could be the reason why this is such a failure-prone component of the Davis systems. A search will find hundreds of posts all over the net on this problem.

I don't think you have a correct understanding of how the solar cell, battery charging circuitry likely in intended to work. I know nothing about the Davis products and a quick look at their web site didn't didn't help much. No schematics at all. When the solar panel is producing max output, it usually more than the battery can accept, that's why you need a regulator. A well designed regulator also can increase the voltage so the battery can still be charged at low light levels.

Typically the solar cell charges the rechargeable battery, via some control circuit. It's important not to overcharge or over temperature the battery. Doing so would reduce the battery life, or in the case of Lithium batteries cause a fire. Not a possibility here because the available power is to low.

It's likely the capacitor is used in the regulator circuit. There are two many different designs to guess what's used. The capacitor is almost surely a polarized DC capacitor. Higher than rated voltage is OK and somewhat desirable. More important is the type, electrolytic, film or tantalum and the temperature/environment rating. There is also the new, only about a decade old ultracapacitor. These actually store considerable charge. They are expensive and are low voltage, around a few series battery cell volts .

Get some more info on the circuit and/or the OEM capacitor specs and type. I'll be able to make a recommendation. A photograph of the capacitor will help. Except for the ultracapactor the price will be inexpensive when brought from an electronics components distributor. Shipping will cost more than the capacitor.

OK... ive heard it all now Smitty's got his own solar weather station and he raises and slaughters his own beef too

anyways... not get off subject to much here but are old capacitors worth anything if they can be swapped out to anything like that? I almost grabbed a huge box of these left at the dump by someone (they looked like 50 cal. casings) damb dumps closed till wed now but im going back to try and grab them if you think they have value?

Nah, if they're old they really aren't worth anything. That is unless they came from old Fender tube amps. Then someone will want them for the sound. I personally don't know why, it's a nostalgic thing. Leo Fender wouldn't have put old caps in his stuff.

Thanks Yanche. It'll be a while before I get some pics of the board - I'll have to get up on the roof here and get the thing apart. In the meantime I'll see if I can find pics that somebody might have taken while doing the same job ...

The battery for this unit is a 3v CR123 Lithium non-rechargeable battery. The solar panel charges only the capacitor. The battery kicks in when the cap is "empty".

Found a pic of the capacitor. Looks to be the electrolytic type if I'm not mistaken. Scroll down the page to see it: http://www.weather-watch.com/smf/index.php/topic, ... 90.30.html

If you salvage a cap from somthing you should make sure its discharged before you screw with it much or else you might get the piss shocked out of you. Dont ask how I know, stupid TV capacitor

Yeah I know exactly what you mean.

When I was at MMI, the first thing the instructor did on the first day of class was toss a capacitor out of a RD350 all charged up toward an unsuspecting student. Instinctively, you'd try and catch it & get one hell of a jolt. Since I was the guy from MA with the motor mouth & the funny accent, it got tossed my way.

Then I made the same mistake when I finally bought my own RD350 in '09. I didn't recognize that same capacitor buried under the flasher relay assembly. So I got lit up twice.