# Help with curent limiting

#### jombom

Joined Apr 4, 2024
3
I need to use a 3.7v battery and the input of the device is 2.8v max curent of 350mA how do I limit the curent the easy way?

#### ericgibbs

Joined Jan 29, 2010
19,005
Hi jom,
Welcome to AAC.
What is the device that requires 350mA at 2.8V.
It may be possible to insert a couple of diodes in series with the 3.7V source.
E

#### jombom

Joined Apr 4, 2024
3
I'm trying to power a laser diode it operates at about 2.6 to 2.8v at full power and it can't take more than 350mA I could do with 300mA

#### Externet

Joined Nov 29, 2005
2,235
The easy way ? A 3.7 V battery can be 4.2V after just charged. Say 4V. Use two >1A silicon diodes in series:

(+)3.7V to 4V ------------------|>|----------|>|-----------------near+2.6V ----------------laser----------------common(-)

Unless you prefer a voltage regulator IC.

#### jombom

Joined Apr 4, 2024
3
How about something more complicated to have a constant voltage output as the battery voltage drops?

#### Externet

Joined Nov 29, 2005
2,235

#### Externet

Joined Nov 29, 2005
2,235
Image :

#### crutschow

Joined Mar 14, 2008
34,692
A laser diode, like an LED, needs a current regulator, not a voltage regulator.

#### Externet

Joined Nov 29, 2005
2,235
Correct. Poster wants voltage regulation
to have a constant voltage output

#### BobTPH

Joined Jun 5, 2013
9,161
A buck-boost regulator with a current limit adjust would be ideal, giving you the longest battery life. There are inexpensive boards that do this widely available. You would set the voltage output to something higher than needed, say 3V or more, and the current limit to 350mA.

#### Michal Podmanický

Joined May 11, 2019
73
The 220 pot sets the Led current.
First time set the 220 pot to lowest voltage (viper down) to not brake the Led. Then continuously turn the viper up until you reach demanded Led current.

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#### crutschow

Joined Mar 14, 2008
34,692
Correct. Poster wants voltage regulation
He may think he wants voltage regulation, but a laser diode needs current regulation.

#### MrSoftware

Joined Oct 29, 2013
2,208
There are multiple possible solutions, are you looking for the best or most efficient solution, or the easy button? You can do something like combine the LM3102 voltage regulator with the LM8646 current limiter. There are other solutions, this was just the first to come up in a search. Here's the LMP8646, the datasheet has a complete example circuit for what you're looking for on the first page. There is an evaluation board that is a complete circuit, check it out to see if it meets your specs or can be easily modified to meet your specs.

#### DbLoud120

Joined May 26, 2014
93
I powered my laser diode with an LM317 configured as a current regulator.

#### BobTPH

Joined Jun 5, 2013
9,161
I powered my laser diode with an LM317 configured as a current regulator.
Which is fine if your voltage supply is high enough. It will not work with a LiIon battery except when it is nearly fully charged.

#### crutschow

Joined Mar 14, 2008
34,692
There's not a lot of overhead between the battery voltage as it discharges, and the Laser diode forward drop, so I would go with Michal's circuit in post #11. which requires <0.2V, with the Laser current being constant as the battery voltage drops.

#### MisterBill2

Joined Jan 23, 2018
19,009
How about something more complicated to have a constant voltage output as the battery voltage drops?
How about something much simpler like a series resistor to drop the 4.0 volts to 2.8 volts when the current is something less that 350 mA?? Then as the battery voltage drops the laser power will drop a bit also.

#### crutschow

Joined Mar 14, 2008
34,692
Below is the LTspice sim of Michal's circuit (with a few tweaks):
It shows a constant Laser Diode current (yellow trace) for a battery voltage (green trace) above about 3V.
The Laser Diode voltage is about 2.6V (red trace) at 310mA.

So the circuit with the parts shown, will give a constant-current to the Laser Diode with a battery voltage down to about 400mV above the diode voltage.

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#### MisterBill2

Joined Jan 23, 2018
19,009
Quite a few laser diode packages do include a temperature sensing diode for use in a similar sort of scheme.

#### crutschow

Joined Mar 14, 2008
34,692
Quite a few laser diode packages do include a temperature sensing diode for use in a similar sort of scheme.
What is done with the sensed temperature signal?