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Why do I need a blocking diode on the output end of a solar panel? Answered

In some of the instructables dealing with solar panels, the instructions call for a 1N314 blocking diode. Is this really necessary? What happens if you don't put one in? Does the solar panel glow or something? Would a LED be just as useful? Where could I scavenge one if I had to use such a diode?


Caution, the blocking diode must exceed the output current of the solar panel or it will fry. 1n4004 is fine for the low voltage but only 1 amp capacity. Match diode to perhaps 25% greater than maximum power output for longevity.If 8 amps are needed get a 10 amp diode or parallel 10 1N4004.

Dale you are correct the blocking voltage of the diode must exceed the battery and some 25% more if you like. But it should also have a low PRV like a 1A 20V PRV low forward 0.45 voltage diode like a 1N5817 diode costing 14¢ at Jameco...

Or a 3A 40V PRV low forward 0.45 voltage diode like a 1N5822 diode costing 16¢ at Jameco..

The low forward voltage allows more power to pass through the diode increasing efficiency.

Thanks for the feedback the low PRV is true with solar - are you familiar with wind turbines and if so could a wind turbine, since inductively oriented, damage a low PRV diode. If I paralleled solar and wind gen and solar cell would that cause problems and if a blocking diode was used on both devices would that solve it?? How high could a wind turbine spike and could a capacitor filter out spikes on the DC output sufficiently? Smart power controllers might buck each other (since smart and analyzing output could screw each other up) - one for wind and one for solar.. Any suggestions how to make the devices work together in charging a battery? This is a scenario that you probably have more experience with than I and I welcome your feedback....PS loved your elaboration in your response showing your experience and technical understanding.

The first, reduces forward diode drop by another ten,

The second, mixes all power sources

(some regulation may be needed).

The third, is a unique unregulated house heat with or without wind.

The lamps generate IR under floor... the house thermostat makes up the difference....

Be sure to click the pic to see the full image !

293955-Use_a_self_powered_op_amp_to_create_a_low_leakage_rectifier_figure_1.jpgBlock charge.jpgWindGenLoop.bmp

Interesting devices/application. The power will be much greater than 1n4004 could handle but the second drawing does confirm what i surmised is true. Thanks for the feedback! The blocking diodes and capacitor bring balance to my needs. The capacitor close to the generator or at the tie point of the devices will help protect the blocking diodes from spikes that could damage other diodes. What capacitance would you recommend?

PS "The first, reduces forward diode drop by another ten," Fuzzy on Q1 - what type of FET is that? The lower forward conduction alleged (impressive with only 10mv drop) is followed, I assume by a high reverse voltage which would protect the ckt from spikes from other devices. My application is high current so I assume a larger current Q1 could be used. The diagram has a source of a/c which I assume is the generator itself minus a rectifier (which is substituted by the circuit). I also assume that the aforementioned capacitor from ckt 2 also assists in protecting Q1 from damage. Interesting circuit- am I correct in my assessment?

Thank you for your support.

I hate being right sometimes. It is a higher current than I suspected

Transistor MOSFET P-CH 20V 2.3A 3-Pin TO-263AB T/R

A higher current MOSFET, if parameters match or are acceptable might substitute the other. I found one that might be worth experimenting with.


Transistor MOSFET P-CH 30V 50A 3-Pin(3+Tab) TO-220AB Rail

Thanks for the assistance. If I need this circuit then I will substitute this and see if it works. I will use the design on the second drawing - the blocking diode with a capacitor to ground may keep the induction style Wind Generator from damaging the solar cell and visa versa - capacitor should trim the spikes and surges and only real sinusoidal wave pass - I will put it on the DC side of the diode.

Thank you much for the assistance and no BS endeavors to help me. When you venture out it is nice to have feedback and support. I speculated on some of this and as fuzzy as I am on past experience was right which re-builds my confidence. You are quite knowledgeable and supportive, thanks so much.

I use a 1N4004 on each of my 20w solar panels. You can get them new from Radio Shack much as I hate to say it. If you don't put the diodes in the batterys will discharge into the panels and you don't want that. You want a nice full battery. To scrounge one you might look in old TV sets or anything else electronic that has been junked.

iceng says:
Some glass diodes and epoxy diodes.

iceing has the right idea. The diode you use has to be able to handle the level of power your panel puts out. This is where you read the labels on the store bought stuff. You have to know your panel's specs so the label in the back of a commercially made panel will give you that info. Little cells found on the garden lights want real small voltage drops in a diode because the cell puts out so little electricity.

For better answers you need to write the size of your panel.
ie; Output Volts, Output Watts and Amps...


I cant find any online can someone help me please

Check newark, digikey, mouser, or any other electronics retailer.
You are looking for a 250ma 25Volt PRV  low forward 0.45 voltage diode
like a 1N5817 diode costing 14¢ at Jameco..


http://www.rapidonline.com/Electronic-Components/3A-Ultrafast-rectifier-diodes-29686/?sid=a86d1862-2990-4a3d-a27b-07514b1d4315 would this one be ok?

because silicon is a semiconductor, it conducts...sometimes.

This means, when there is no sun on the panel, instead of it pumping electrons 'forward', it acts like a resistor, draining power from the battery -- essentially, a solar panel with no light is a load instead of a source. The diode acts as a one-way valve, only allowing electrons to flow in one direction out of the solar panel through the completed battery circuit.

Now - LED is a 'diode', this is true. Only problem is most leds are inefficient for 'just' acting as a diode because they have a high voltage drop, 2-4 volts. That voltage drop is used to generate the light as the electrons excite the diode's molecules which then give off light. The amount of current an LED can handle is miniscule compared to most solar panels -- only .02 amps. A 1N314 diode has as small a voltage drop as possible -- and as large a maximum forward current as possible (a few amps if I recall) -- to be as efficient as possible.

So where can I salvage one of these 1N314 blocking diodes? No budget to buy them. Radio Shack and Fry's salespeople are useless...

What do you need a salesperson for? Can't you read the label on a packaged part? The sales staff at RS and other similar stores are there to point you to what they sell, not to TEACH YOU ELECTRONICS.

I know they're not there to teach me electronics, no need to be nasty. Most of those salespeople aren't even aware what's in the drawers!!

You brought up the salespersons and there was no reason to do that either.

You can find a diode that you can use probably in most computer power supplies.  If you can find one that is un needed search for one there.  There are probably 4 or more if you can find them.  They are usually a small black cylinder with some numbers and a stripe painted on them.

...from a solar panel?

There's not a lot of common applications for that diode - they are quite cheap and readily available online. Check newark, digikey, mouser, or any other electronics retailer.


6 years ago

So you don't want to kick over a solar garden light.....

Always far better then a RS and mostly outdo a Fry's counter types in my
self image, PM me and I will snail mail you some diodes because you
registered in o07 and sound broke :¬)


I have extra solar garden lights, I just don't know what the heck the diodes look like. Got a picture of one?