A big thank you - Solar Project

[PeterDove]

Just wanted to say thanks to all the guys who helped with my solar project and explaining various electrical ideas. AudioGuru, Nigel Goodwin,JimB,ajc - thanks also to anyone I missed!

I have made an instructable from the fruits of my ignorance

**broken link removed**

Peter Dove

 

[audioguru]

Hi Peter,
I was glad to help you. I have some comments about your Instructables:

1) In your Instructabe step #3, you say to test the LED across the battery and solar cell that are in series. Nothing will limit the current and the LED will burn out if the solar cell is in full sunlight at noon on the equator. The 3V solar cell will produce its rated 150mA and the total voltage is 4.2V. The 3.6V LED has an absolute max current rating of only 30mA so a current-limiting resistor should be in series with it, or shade most of the solar panel.

2) Ni-MH cells last much longer if they are not overcharged. Energizer recommends stopping the charging at full charge, or using a trickle charge current of only the capacity divided by 40, not divided by only ten.

 

[PeterDove]

Hi Peter,
I was glad to help you. I have some comments about your Instructables:

1) In your Instructabe step #3, you say to test the LED across the battery and solar cell that are in series. Nothing will limit the current and the LED will burn out if the solar cell is in full sunlight at noon on the equator. The 3V solar cell will produce its rated 150mA and the total voltage is 4.2V. The 3.6V LED has an absolute max current rating of only 30mA so a current-limiting resistor should be in series with it, or shade most of the solar panel.

2) Ni-MH cells last much longer if they are not overcharged. Energizer recommends stopping the charging at full charge, or using a trickle charge current of only the capacity divided by 40, not divided by only ten.

Thanks again man, I'll update that.

 

[PeterDove]

Hi Peter,
I was glad to help you. I have some comments about your Instructables:

1) In your Instructabe step #3, you say to test the LED across the battery and solar cell that are in series. Nothing will limit the current and the LED will burn out if the solar cell is in full sunlight at noon on the equator. The 3V solar cell will produce its rated 150mA and the total voltage is 4.2V. The 3.6V LED has an absolute max current rating of only 30mA so a current-limiting resistor should be in series with it, or shade most of the solar panel.

2) Ni-MH cells last much longer if they are not overcharged. Energizer recommends stopping the charging at full charge, or using a trickle charge current of only the capacity divided by 40, not divided by only ten.

hmm, so how do that work - what I mean is I understand about limiting current based on : ( battery voltage - 3.6V of LED ) / Desired current = required resistor

BUT what if the current is being forced through like in this situation.. 150mA at 4.2V - what maths helps determine the resistor here?

Thanks

Peter

 

[audioguru]

Hi Peter,
Current doesn't "force through". If you use a resistor to limit the current then only the limited amount of current will flow. If your battery can supply many more Amps then the current in the circuit remains the same low amount but the battery will supply it for a longer amount of time.

Your car's battery can supply up to 200A to the starter moror. The same battery supplies only 0.5mA for the clock. What temperature would the clock be if it used 200A at 12V? That is 2400W!

Use OHM'S LAW to calculate the required resistor value:
1) The max current for the LED is 30mA so you want it to operate at 25mA to be safe.
2) The supply voltage is 3V + 1.2V= 4.2V. The LED's voltage is 3.6V (but it could be a little more or a little less).
3) The current-limiting resistor will have 25mA through it and will have 4.2V (total) - 3.6V (LED)= 0.6V across it.
4) OHM'S LAW calculates the resistor's value: 0.6V/25mA= 24 ohms. If you use 22 ohms then the current is 27mA. 27 ohms would limit the current to 22mA.