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# Need help designing a current limiting circuit for a DC powered device. Answered

What would the simplest and most energy efficient circuit be to limit the current from a DC transformer?  I have several 12-15VDC plug-in transformers that deliver between 800-1000mA, but a device I need to power requires 12-15VDC at 350mA MAX.  Any thoughts, help, or advice would be appreciated!

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## 4 Replies

sshuggi (author)2011-10-11

You'll want to get a three pin regulator like an LM317. Just make sure you get one that can output more than the current that you need, and just regulate it down to the correct current. Wire it up like in the attached picture. RL and VL is just your device. The way you regulate current is through R1 by the following equation:
VADJ, also called reference voltage, is typically 1.25V, but check the datasheet of the one you buy. So, for your application, you want R1 to be about 42 Ohms. The capacitors are there merely to catch noise that the wires pick up. (Since this is DC, the capacitors charge in a matter of milliseconds and then act like open circuits.)  Ignore them if you want, but they may be helpful.
Note: The regulator uses about 2V and R1 uses 1.25V. Therefore, your Vin needs to be 3.25V higher than VL.

iceng (author)2011-10-10

Both of you "well put"

Jack A Lopez (author)2011-10-10

Unless this "device" is a LED, or laser diode, then chances are it will "limit" its own current, which is to say that will draw as much current as it wants from a supply whose voltage is approximately 12-15 VDC.

That is to say most devices are expecting a constant voltage source, and the device will draw as much current as it wants... that is assuming the source can comfortably supply that much current.

A DC adapter is rated at 12 VDC, and 800 mA, it can supply any amount of current in the range from 0 to 800 mA.  Thus if connected to a device that wants 12 VDC at 350 mA, it can supply this.

Anyway, that is true for most things,

...except for laser diodes, and high powered LEDs.  Those want a source that supplies constant current, not constant voltage.  And there exist special "driver" circuits,  or "constant current" supplies for the purpose of supplying LEDs and laser diodes with constant current.    Electrolysis setups, for metal plating  or other purposes, are another example of a devices that sometimes want constant current.

Pretty much everything else out there assumes it will be connected to a source of constant voltage, and then draw as much current as it wants.

verence (author)2011-10-10

There's no need to limit it.

Your device will take as much current as it needs (but 350mA max, if your information is right).

Your transformer is able to supply 1000mA max, but that does not mean it will supply it all the time. Otherwise it would somehow (impossibly) supply them, even when nothing is connected.

So, you should be fine with your stuff. Just make sure, the supplied DC is filtered enough for your device. Otherwise an electrolytic capacitor can help.