# Current limiting

Discussion in 'The Projects Forum' started by Edmunds, Apr 28, 2011.

1. ### Edmunds Thread Starter Active Member

Sep 27, 2010
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0
Hi all,

I will start by saying I'm not very good with circuits yet. I'm fine with building something off a ready-made circuit diagram, even quite complex, but I must admit, I don't always understand what is each component for .

So, when the time comes to put something together myself, I have to ask for help. Here I am.

I am basically building a muscle wire actuator for a function in a model car of 1:87 size. Every millimeter of size is precious and large amounts of heat are not welcome and hardly possible. Long story short, I have a 3.6-4.2 volt power supply, depending on how happy the LiPo is in the car and I need to deliver constant current of ~50mA to the muscle wire. The resistance of the 2" section I will be using is around 24Ohm when "activated" (current flowing through).

The simplest solution would be resistor, but the size needed would not be small, the heat would be generous and it would not adjust to varying resistance of the muscle wire as it heats up/shrinks to provide the movement I need.

I have found some solutions on the net and this forum with transistors and ICs, but most of them are designed for Amps, rather than mAmps of current and start at 12V or so. The others diagrams require figuring out the values of the components yourself and I tried, but nope, not there yet .

The other thing that I maybe should ask about at the same time is that I need to switch this activator on and off from a "digital decoder". I am not sure how this matters, but I have heard people talking the fact the positive lead from the decoder is kind of common and switching leads for various functions are negative, I need to keep this in mind as not all traditional circuits can be connected that way (I was thinking optocoupler or something). If I just said something really strange, just ignore it, please . What would be the most space saving way of a switch?

Can anyone help with a complete circuit diagram? Must be a couple of components as far as I understand. Any data needed that I did not provide?

Thank you and BR,

/Edmunds in Riga, Latvia

Last edited: Apr 28, 2011
2. ### Ron H AAC Fanatic!

Apr 14, 2005
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Do you think 60 milliwatts (50mA through 24Ω) is sufficient to heat your muscle wire sufficiently? The web sites that I read said that it must reach a temperature of about 75°C.

3. ### wayneh Expert

Sep 9, 2010
11,914
2,854
Given the fairly low voltage of your power supply, I think the smallest, simplest and cheapest current limiter is simply a resistor, just as if you were lighting an LED. A bigger, better and more expensive approach would be a constant current supply, but I don't think you need all that.

If you want this to be switched by a digital signal, you need to tell us what that signal looks like, ie. the voltage high and low. I think a small logic-level FET will probably do the job. A BJT might cut current enough to eliminate the need for a current limiting resistor altogether.

4. ### Edmunds Thread Starter Active Member

Sep 27, 2010
85
0
Well, I cannot be 100% sure, but that is what the resistance/current table says here. I'm talking about 50mm of 50 micron wire. It says its resistance is 12Ohms per inch and the current required for contraction is 50mA.

I tested it with briefly applying 4.8V from three batteries and it did work, but all burned to hell within seconds. I think the outputs of the decoder are limited to about 50mA as in they shut down thinking it is a short if you draw anything more, thus that connection did not work either. So, I figure, even to get a reliable test environment, I need to be ably to control the current.

Regards,

/Edmunds

5. ### retched AAC Fanatic!

Dec 5, 2009
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Could you provide a link to that table? Possibly a link to the manufacturer of the wire?

Sep 27, 2010
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7. ### Ron H AAC Fanatic!

Apr 14, 2005
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Here are two options, depending on whether you want the muscle wire returned to ground or to vcc. If that doesn't matter, you can choose the one that has the control polarity you need.
These are switched 50mA constant current sources.
The output currents are ≈ 0.65V/R2 and ≈ 0.65V/R3.

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8. ### Edmunds Thread Starter Active Member

Sep 27, 2010
85
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Ron, this seems exactly what I need. Thank you. While I can certainly build the circuits you suggest, to learn something as well, could you please clarify what VCC stands for? I assume it is a current source input, but...

Also, what do you mean with voltage 0.65V at the resistors? Why does it matter?

And thanks' again.

/Edmunds

9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
655
Vcc is your LiPo battery's positive terminal. The negative terminal connects to ground, which is the triangular symbol.
The output current is approximately equal to the Vbe (base-emitter voltage) of the transistor Q2 or Q4, divided by the resistance of R2 or R3. Vbe of a transistor is in the range 0.6 to 0.7 volts.

10. ### wayneh Expert

Sep 9, 2010
11,914
2,854
Brilliant solution, one of those things you know in the back of your mind must be available but it would take me a very long time to come up with something so simple and elegant. Sweet.

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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655
I'm a retired geezer EE. I've had 50 years to accumulate the myriad of circuit snippets that ride around in my mental toolbox.