# How to limit solar current input for testing

Feb 2, 2017
111
Last edited:

#### Audioguru

Sep 24, 2016
3,656
The ebay ad has hardly any details. The converter steps down a high voltage to 5V or 12V at 250mA max. It does not say what its input current will be but the input power will probably be the output power plus a little more.

If the solar panel produces 70V and the 12V output from the converter has its current limited by you to 250mA then its output power is 12V x 250mA= 3W. Assuming an efficiency of 80% then the input power is 3W/80%= 3.75W. then the input current at 70V is 3.75W/70V= 54mA.

I do not know if the outputs from more than one of these low power converters can be paralleled.

The 12V output of the converter is too low to fully charge a 12V battery that needs 13.8V to 14.4V for charging if it is a lead-acid type.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
No, the output is limited to 250mA. The input will be something like 50mA.

You limit the input current by ensuring you don't apply a load which requires more than 250mA.

If it's a lead acid battery system, the voltage will be insufficient to charge it.

Moderator
Jan 21, 2010
25,510
SNAP

#### Audioguru

Sep 24, 2016
3,656
We said the same things at exactly the same time.

#### AprilSteel

Feb 2, 2017
111
OK so I want to put charge into a battery but only 250 mA How can I limit this ?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Well, firstly you need a higher voltage as @Audioguru suggests. Then you need a current limited voltage source.

If you can find a similar high voltage buck switch mode regulator with an output of maybe 20V, you can use that to drive an LM317 set up as a current limited voltage source.

Set up a 317 as a 13.8V regulator then place a resistor between the output and the voltage divider that will drop 1.25V at the rated current.

You will also need to protect the 317 using a diode from the output to the input. True will prevent it failing when the sun goes down.

#### Audioguru

Sep 24, 2016
3,656
A battery is usually charged with a charger circuit that limits the current.

#### AprilSteel

Feb 2, 2017
111
Perhaps i am not being clear .With the 70 volt panel i have I want to test the new bit of gear into a 12 Volt battery (I also have a 24V system) . What is the easiest way so I don't blow it up please?

Moderator
Jan 21, 2010
25,510
See post #7

Moderator
Jan 21, 2010
25,510

#### AprilSteel

Feb 2, 2017
111
Not getting as much out of that as I would like . There is a duplicate circuit in the datasheet.

I built a circuit based on the TL317 battery charger as above with a diode across the TL317 and a .2 ohm resistor on the output and a 220 ohm and variable pot on the adjust.but its so rainny here I'm only getting 30 volts from the panel so I can't test it properly. Using my lab supply which goes to 30V I adjusted it to reduce voltage to 7 volts .

The TL317 is supposed to be good for this input voltage and should drop it from 70 to just above 30V from the data sheet giving an input to output change of its max 40 volts . However as I have it set now from the 30 volt in , when input jumps to 70 again will the voltage out jump to 30? Or will it try to go to 14V with the resistors set as above . Maybe I should change the pot value to avoid the voltage change getting too high .

I don't understand what its likely to do and I only have one device on board atm HELP!

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Draw the circuit you're using and site is a picture of it. It doesn't sound like you've constructed it correctly. For a start you're only mentioning 2 resistors and I see no reason for a 0.2 ohm resistor (1.25/0.25 = 5 ohms)

#### AprilSteel

Feb 2, 2017
111

For R2 I have used a pot 10K R1 is 220 ohms with a diode from vin to Vout line on vin side.
Thats from the datasheet but its along the lines you pointed at isn't it?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
OK, but Rs must be 5 ohms (use 5.6 ohms) to limit the current to around 250mA.

You want to run this circuit from an input voltage that is about 2.5V higher than your output voltage under no-load conditions.

Do not run it from an input voltage above 30V. For one thing it will be inefficient, secondly it will cause the 317 to get hot, and thirdly it will be destroyed by high input voltages.

Make sure you have placed the diode the right way around. The cathode (usually marked with a band on the diode) should be placed toward the 317's input.

If connected correctly, after you adjust the output voltage to 13.8V with your power supply set to 16V or higher, you should be able to vary the input voltage from 16 to 28V without the output voltage changing.

Likewise, if you short the output, the current should be about 225mA and it shouldn't change as you change the input voltage.

Be careful with the second test. Set your or supply to a (say) 300mA current limit in case something goes wrong. Also beware that the 317 can get hot.

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