Interesting string so far. And a bit of good news that the load is powered by a regulated power supply. That means that any diode voltage drop will be compensated for.
As for the diode ratings, I suggest that the diode working voltage (PRV) be at least twice the maximum voltage output of the solar panel. That should be easy. For the current rating, the maximum current from a 20 watt 12volt solar panel will be less than two amps, What is unknown is the maximum current rating of the DC adapter. So my suggestion there is that the current rating of the diode should be a bit greater than the maximum output current rating of the DC adapter. That number should be on the label. So as a guess, the diodes should be rated for 50 Peak Reverse Volts, and at least 2 amps constant current. The PEAK current rating will be more, that is not important in this application.
As far as the diode polarity, if the diodes have a diode symbol printed on the case, it should point in the direction of the load. (Cathode end toward the load)

Certainly other schemes will work and some of them will provide more power or better efficiency. But the two-diode scheme meets all of the stated requirements and are by far the simplest solution so far.

 

Thank you for the clarification.

In that, case, the diodes would go between the solar panel and converter and between the 12V supply and the converter. But 12V may not be the right voltage to use.

To really design this we need to know the characteristics of the solar panel and the load.

BobTPH,

Here are the Specifications

Solar panel:

Open circuit voltage?
Short circuit current?
Max power point voltage and current?



I can use a 55 Watt Solar panel the price difference between the two is about 17 USD. Not make much difference considering the long run usage. Also I measured the current drawn by the buck converter at 12 Volts is 1.2 A so a 20 Watt solar panel is not sufficient.



Load:

Yes the current is set constant at 2.5 A. Load is a dc electromagnet.

I am using an adaptor with rating of 12 V and 5 Amperes.

When powered on and connected to the load the buck converter input voltage is 12.4 Volts and draws 1.2 A from the 12 V adaptor.

It delivers constant 2.5 A to the dc electromagnet at 4.2 Volts output. Voltage at which load draws the current is 4.2 Volts.

Buck converter:

XL4015E1 CONSTANT CURRENT/VOLTAGE 5A LITHIUM CHARGER DC-DC STEP DOWN ADJUSTABLE MODULE INPUT 5V-32V OUTPUT 0.8V -30V

https://roboticsdna.in/product/5a-c...ery-charging-module/?src=google&kwd=&adgroup={adgroup}&device=m&campaign={campaign}&adgroup={adgroup}&keyword=&matchtype=&gad_source=1&gclid=CjwKCAiAloavBhBOEiwAbtAJOw9FM2ufAvP8j-NwhdPQzl_QrO3xU-yzDK4DmwYF1xE3oLamI0OruRoCyWIQAvD_BwE

 

Great, thanks again. If you only knew how hard it is to get the necessary information from some posters….

It sounds like the 20W panel is sufficient. When stepping down from 19V to 4.5V it can multiply the current by a factor of 4, so the 1A output is sufficient. You will need a capacitor across the input because the peak current will be higher. The capacitor will charge when the buck converter switch is off, and provide the extra current needed when it is on.

Also, the specs you give make me think the two diode solution will give you nearly all of the power that a more complex solution would do. I.e. 0the voltage will be well above 12V whenever the panel can supply sufficient power.

Edited to add: It looks like the converter has a sufficient capacitor in its input, so you probably don’t need to add one.

 

From the posted data it seems that diodes with a rating of 5 amps peak, 2 amps steady state, and 50 Peak reverse volts will be entirely adequate. Those requirements should allow a good field to choose from.

 

BobTPH and MisterBill2,

Thanks a lot for your input. Please see this circuit using the diodes. Is this correct.

 

Not only is it CORRECT, the circuit is well drawn! That is exactly what I was intending to describe.

 

Not only is it CORRECT, the circuit is well drawn! That is exactly what I was intending to describe.

Great. I should have thought of drawing this first. Please see if this diode could be used for this.

SR560 60 V 5A Schottky Rectifier Diode



https://robu.in/product/sr560-schottky-rectifier-60-v-5-a-single-do-201aa-2-pins-700-mv/

Thank You.

 

That set of specifications would appear to satisfy the requirements.
Be sure to mount the diodes so that there is a bit of airs pace between them and the enclosure so that air can circulate past them, if they get warm.