Help with choosing a MOSFET from Allied for a solar panel

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
I am about to order some other things from Allied Electronics. Turns out they have an office here in Pittsburgh.

I am hoping they carry product though I suspect it is just a sales office (no product). But I am going to give them a call.

Anyway, I would appreciate some help in choosing a MOSFET from their selection.

Here is what I need: I want to switch off a solar panel with a PIC. The panel puts out about 25V and at best .6 amps. Here is the important part. I need to have some easy packing to work with like DIP or TO-220 or similar easy to use packaging. Even if it means the choice is not the best effciency wise. I just can't solder that surface mount stuff.

I am also willing to drive the MOSFET with a general purpose transistor like a 2n2222.


Oh and if there are no good choices here then I guess I can order something from Mouser or similar later. Just that now I am ordering, it would be nice to get the MOSFET.




Thanks
 
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Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
I actually have a little mini relay that I have had for years. It switches at 5 vdc. I have been looking to put it to use.

But I have always thought these were inefficient and also more prone to failure.

Since it is a solar panel, the circuit will be outdoors in a weatherproof box but will I have problems with moisture on the contacts?
 

retched

Joined Dec 5, 2009
5,207
A latching relay would be more efficient. Instead of having to continually power the relay to hold one state, you need only pulse it to change states.. like a ball point pen. a 20ms click on and 20ms click off.

As for moisture, not too much of a problem. Most automotive relays are very tough.

A standard relay will suck juice from the panel or batteries the whole time it is in the energized state. And since you are using a uC, it should be no problem to get a logic level latching relay at those specs

[ed]
5v coil latching relay rated for 30v 1A
http://uk.digikey.com/1/1/171618-relay-latching-4pdt-1a-5vdc-g6ak-434p-st-us-dc5.html
[/ed]
 
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Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
A latching relay would be more efficient. Instead of having to continually power the relay to hold one state, you need only pulse it to change states.. like a ball point pen. a 20ms click on and 20ms click off.

As for moisture, not too much of a problem. Most automotive relays are very tough.

A standard relay will suck juice from the panel or batteries the whole time it is in the energized state. And since you are using a uC, it should be no problem to get a logic level latching relay at those specs

[ed]
5v coil latching relay rated for 30v 1A
http://uk.digikey.com/1/1/171618-relay-latching-4pdt-1a-5vdc-g6ak-434p-st-us-dc5.html
[/ed]

Or like this little guy from Allied? Why do people use Mosfets in an application like mine? Cost?
 

retched

Joined Dec 5, 2009
5,207
Or like this little guy from Allied? Why do people use Mosfets in an application like mine? Cost?
That will do it. It is also epoxy sealed, so you will have little to no problems with weather.
mosfets are cheap. and reliable. The relay has a short life compared to a mosfet. Also, you can have emf problems with a relay. You will want to use a protection diode on your relay to protect the PIC.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
Yep I already figured on the diode.

What kind of life are we talking about here? I suspect it will only change states a few times a day. I figure that once the battery is charged, the relay will switch off, then I might to want to give it a little maintenance blast every hour or so.

Will I need to worry about it failing in my lifetime?


How do I know if a relay is latching? I just saw a bunch of these little guys on the cheap down at Hosfelt Electronics. How would I known if they where latching or not? No datasheet. They were in the "big bin". :)
 

retched

Joined Dec 5, 2009
5,207
The relay you linked to has a 1 x 10^8 cycle life on mechanical parts and
2 x 10^5 at 1 A, 30 VDC cycle life on electronic parts.

Thats 1,000,000,000 (a billion) mechanical clicks
and 2,000,000 (two million) electronic operations
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
The relay you linked to has a 1 x 10^8 cycle life on mechanical parts and
2 x 10^5 at 1 A, 30 VDC cycle life on electronic parts.

Thats 1,000,000,000 (a billion) mechanical clicks
and 2,000,000 (two million) electronic operations


At 10 electronic operations a day. It should last me a little over 547 years. OK that will be the route I will go. But in the 548 th year I just know I am going to regret my decision. :)
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
Naaawwww.. I think after 545 years it is time for preventive replacement.
I think I have a doctors appointment that day. Since you are the resident expert on relays, I am going to pencil you in for the replacement. Please give me at least one weeks notice, if you cannot make it so I have enough time to arrange for someone else to do it. :)


Hey should be down your way this Memorial Day. We usually sail out of Galesville but their boats are getting a bit long in the tooth. Might check out a place in Harve De Grace.
 

retched

Joined Dec 5, 2009
5,207
We launch from Lapidum - Harve De Grace. It is a much more quiet location. I love the water-free urinals there.

Its closed for renovations till mid-April.
 
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