### Author Topic: Current Limiting  (Read 3686 times)

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#### bumba000

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##### Current Limiting
« on: October 22, 2015, 11:54:07 pm »
Sorry for being such a noob guys. I want to know if it's possible to limit the amount of amps available to a load. I've converted a computer power supply to a lab power supply so I have 12v, 5v and 3.3v available. I'm missing a couple of banana jacks so it's not done yet, but I plan to use an LM317 to reduce the 5v down to 4.2v. I also want to limit the current. The 5v line has 14.8 amps on it. I want to cut this down to about 2 amps. How is this done. Please and thank you.

John
« Last Edit: October 23, 2015, 12:49:14 am by bumba000 »

#### unitedatoms

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##### Re: Current Limiting
« Reply #1 on: October 23, 2015, 01:25:21 am »
I have 2A implementation, based on Art of Electronics 3rd Edition, p.204, fig 3.106.C

http://unitedatoms.com/articles/impedance-meter-ua315-part-6/

The Q4 @ 1.04A will drop 0.824V when gate is at about 1V and input at 4.965V.
You can remove everything, keep R1, R2, R8, Q1, Q4. Ground R2.
Your input voltage can be 12V. Then the power loss @2A on Q4 will be few watt. Add the heatsink.
« Last Edit: October 23, 2015, 02:12:30 pm by unitedatoms »
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#### Vgkid

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##### Re: Current Limiting
« Reply #2 on: October 23, 2015, 01:28:55 am »
You wont be able to get 2A out of s lm317, so you will either need to parallel them upusing a .5Ohm ballast resistor(should also keep your current at about 2A), or use a pass transistor on the lm317, and use a current limiting circuit on that. If the lm317 gets you to about 1A, then the current limiting resistor will need to be .7Ohm. If no one responds with the circuit, I will put one on here in a few hours.
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#### bumba000

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##### Re: Current Limiting
« Reply #3 on: October 23, 2015, 01:51:46 am »
Apparently I wont be building this today as I'm obviously lacking some parts. I wanted to do this so as to supply a constant current and voltage to a project I'm presently working on. Hopefully I'll have my project figured out in the next day or two, likely well before I'll have these required components. I will build this though as I'll end up needing it again, and why not?! Thank you both very much for these informative responses.

John

#### w2aew

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##### Re: Current Limiting
« Reply #4 on: October 23, 2015, 02:03:57 am »
Apparently I wont be building this today as I'm obviously lacking some parts. I wanted to do this so as to supply a constant current and voltage to a project I'm presently working on. Hopefully I'll have my project figured out in the next day or two, likely well before I'll have these required components. I will build this though as I'll end up needing it again, and why not?! Thank you both very much for these informative responses.

John

Note that you can build a supply to deliver a constant voltage to a load, OR a constant current to a load. The load itself will determine the other value.
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#### blueskull

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##### Re: Current Limiting
« Reply #5 on: October 23, 2015, 02:10:13 am »
LM317 is not a low dropout regulator. It won't give you 4.2V out of 5V. You need to feed it with 12V.

#### T3sl4co1l

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##### Re: Current Limiting
« Reply #6 on: October 23, 2015, 02:19:19 am »
A one of these might be more handy, http://seventransistorlabs.com/Fuse/
It doesn't provide any more than gross current limiting (around 100A surge), but it does limit the energy delivered into a fault.  It's fast enough that you can build a circuit that accidentally explodes itself on a larger power supply, and have it turned off before damage is actually done to the transistors.

The one downside is, because it's so fast, you can't use much capacitance on the load, following the fuse.  This can be addressed by precharging it with a current-limiting resistor.

For active current limiting, a circuit like this might be more handy:

You might want to complement everything (i.e., NMOS --> PMOS, PNP --> NPN, diodes reversed..) to use it on a positive supply, so your R_L (load) can be ground referenced.

As shown, the dropout is VREF + I_L * Rds(on).  For VREF = 100mV, R1 = 0.047ohm gives 2.1A limit.  It should be a pretty flat current source, once it's out of dropout.  From a 12V source, the MOSFET will dissipate a maximum 25W when the load is shorted, so use a large heatsink.  (IRF540 would be fine here, or 9540 for PMOS.)

Tim
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#### bumba000

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##### Re: Current Limiting
« Reply #7 on: October 23, 2015, 04:41:13 am »
holy smokes guys, thank you. I was under the impression that the lm317 was a voltage regulator in general and could be used to output any voltage from 1.5 to 37 volts but the input voltage had to be a little higher than the desired output voltage.

I am interested in building this limiter because I am essentially trying to mimic a protected battery at full charge. I don't want the current or the voltage to depend on the load at all.

#### SteveLy

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##### Re: Current Limiting
« Reply #8 on: October 23, 2015, 06:37:57 am »
I don't want the current or the voltage to depend on the load at all.
You can't fix both current and voltage independent of load. Pick one or the other.
« Last Edit: October 23, 2015, 06:43:23 am by SteveLy »

#### tec5c

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##### Re: Current Limiting
« Reply #9 on: October 23, 2015, 07:10:42 am »
holy smokes guys, thank you. I was under the impression that the lm317 was a voltage regulator in general and could be used to output any voltage from 1.5 to 37 volts but the input voltage had to be a little higher than the desired output voltage.

Welcome back

I suggest you do some reading about low dropout (LDO) regulators, which essentially operate in the manner which you assumed a standard 317 to.

#### tron9000

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##### Re: Current Limiting
« Reply #10 on: October 23, 2015, 07:52:32 am »
I did a blog entry on linear power supplies - hopefully it should bring up to speed a bit and give you some useful tips.

It doesn't include anything on current limiting, but should shine some light on the matter:
http://tron9000.blogspot.co.uk/2015/05/what-supply-to-use-brief-overview.html
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#### leblanc

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##### Re: Current Limiting
« Reply #11 on: October 23, 2015, 12:49:19 pm »
If I need to limit the current (not cut it off completely after an over-current), I tend to use something like this. It's essentially the same as unitedatoms and Tim's solutions.

Tim's solution allows you to adjust the current limit without changing the sense resistor, which could certainly be nice. It also allows for a smaller sense resistor, so you can get something lower power (P=R*I^2, so 0.188W for his solution, 1.4W for mine).

Tim, is this the schematic for your electronic fuse? http://www.seventransistorlabs.com/tmoranwms/Circuits_2008/Electric_Fuse.gif

#### T3sl4co1l

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##### Re: Current Limiting
« Reply #12 on: October 23, 2015, 03:45:05 pm »
Nah, that's an older one, though it works for simple purposes.  This one has micropower comparators to run off a 9V battery for months at a time.  (The one I use regularly has been forgotten, on and off, for days at a time, and shows no sign of pooping out yet.)  And enough protection to open anything from a dead short to a  highly inductive load, at up to 40V.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!

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