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Current limiter advice

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Dirk.S

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Hello everyone

I'm trying to figure out how to implement a current limiter that will be able to limit current up to 5A. I was browsing through the web today to try and find a suitable design but I'm not too sure as to which one to use. One of the promising ones were one found on wikipedia:

**broken link removed**

and alternatively I found a current source circuit that utilizes a opamp and a mosfet.

http://www.freecircuits.net/circuit-57.html

I was thinking about using the current source circuit but just want to confirm that I understand it correctly. As the voltage applied to the opamp negative leg varies so does the output voltage effectively biasing the mosfet more or less on (By setting R1). In turn letting more or less current flow through the mosfet. If this is the case which I'm not too sure about, can't I just replace the opamp with a variable voltage to switch the mosfet more or less on?

My main idea is that I would like to use an analog output from a micro controller to bias the mosfet more or less on to limit the current more or less depending on the analog output value from the micro controller? Thank you in advance.
 
Depends by what you mean by "more or less". Directly biasing the MOSFET to limit this current is dicey because of the variation between different transistor of the same type. There's no way to determine what MOSFET gate voltage is required for a specific current without testing and calibrating the circuit. That's why the transistor or op amp circuits you reference use a sense resistor in series with the load. This gives a sense voltage proportional to the current so you know what the limit current is from the sense voltage.

So I recommend you stick with one of those (or similar) circuits to reliably limit the current to a desired value. Here's a simple example using an op amp and an N-MOSFET. (The load goes between the plus supply and the drain of the MOSFET. The limit current equals V(ISET)/R1. For a 5A limit you would want R1 to be about 0.1 ohm, thus a V(ISET) of 0.5V witll give a 5A limit).
 
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Hello Crutschow

Yes, I understand that the circuit will have to be calibrated should I drive the mosfet directly from a micro controller. I want to be able to change the limited current value to what I require automaticly. I was thinking of using Sparkfun's current sensing module to do the sensing and input the output of this module to a micro controller. I can then use a analog output to bias the mosfet more to let more current flow through or I can bias it less on so that less current will flow through.

If I implement the circuit which you have suggested, would I be able to use the current sensing module of Sparkfun in the place of R1 (Output fed into micro controller as mentioned) and then use an analog output on the positive input of the opamp to bias the mosfet in turn? I hope that I'm explaining myself correctly, if not please just let me know.
 
You can use that Sparkfun circuit but I don't see why.

The circuit with the op amp has a current limit proportional to the voltage input and the sense resistor value (V(ISET) = R1*Ilimit). So you just input a V(ISET) voltage to the op amp for the current limit you need from the micro. You don't need to feed any voltage back to the micro.
 
The sparkfun circuit would provide an isolated connection. Dunno if it would make life easier or not. You can't use it directly in crutshow's FET solution though.
 
The sparkfun circuit would provide an isolated connection. Dunno if it would make life easier or not. You can't use it directly in crutshow's FET solution though.
You could use that if you need isolation, but then you would also need to drive the FET through an opto coupler to maintain the isolation.
 
Thanks for the feedback guys.

I understood that the resistance value needed to be changed to change the current limit value. Thanks for clearing that up for me Crutschow. I was planning on optically isolating the micro controller from the rest of the circuit for protection anyways. I want to be able to sense the current as well, so I will implement the sparkfun circuit into the rest of the circuit as well. But this is all just research at this stage. I am planning to build a SLA smart batter charger. I've posted previously about another design in a different thread that made use of a opamp configured as a differential amplifier to initiate charge and to stop it, but the circuit is not a very good circuit. I want to implement a three stage charging protocol with this project. But as I said it is only research at this stage. Thank you again for the information, I might be asking about it again in the future when I start to implement these circuits.
 
I wanted "isolation" to mean, you can move the current signal reference to anywhere and it MIGHT make the circuit design easier.
 
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