Current Regulator

[patpatpat]

Hi.

I am trying to limit the current draw (below 1A) of a car door actuator (this wont be used in a car). It requires 12V to fully actuate at its max speed. The problem with these actuators is that when the arm is fully extended or contracted, and 12V is still being apply, it would draw more than 1A.

Not looking for a fuse to prevent going over 1A, but a limiter to keep it drawing under 1A. Hope that makes sense.

Can anyone suggest a circuit to do such thing?

Thanks

 

[dknguyen]

A PTC or NTC might do it depending on what you want. One has high resistance and as it heats up the resistance lowers (NTC) which is good for limiting inrush startup current. The other (PTC) starts with a low resistance but as it heats up, the resistance increases which is good for limiting stall current or overcurrent.

 

[patpatpat]

Ok, I will look into these PTC and NTC. I dont know if it the PTCs would heat up fast enough to limit the current.

 

[Torben]

I take it limit switches won't work for this?


Torben

 

[stevez]

You might be able to configure a linear voltage regulator to provide current limiting or at least a constant source of current - the assumption being that it won't affect anything until the rated current is flowing. Look at datasheets for LM317 and similar linear voltage regulator circuits.

There are also current limiting circuits used in power supplies. I've seen designs for these in the amateur radio handbooks that you might adapt.

 

[patpatpat]

I dont know about the linear voltage regulator, but I will be sure to check out the radio stuff

 

[ericgibbs]

I dont know about the linear voltage regulator, but I will be sure to check out the radio stuff

hi pat,

The start up current will be greater than 1amp, make sure you have some current headroom in order to overcome the stiction etc to get the forced mass moving.

 

[patpatpat]

I found something similar also:

http://www.reuk.co.uk/Using-The-LM317T-With-LED-Lighting.htm

This might just work.

 

[ericgibbs]

I found something similar also:

http://www.reuk.co.uk/Using-The-LM317T-With-LED-Lighting.htm

This might just work.

hi,
Its the same device LM317.
Remember to calculate and allow for the voltage drop across the LM317, you are driving a 12V rated device from a 12V battery......you may also need a heatsink on the LM317.

http://www.reuk.co.uk/LM317-Current-Calculator.htm

 

[patpatpat]

I was thinking, if I can find an AC/DC adapter that has 12V/1.5A, should it regulate the current draw because of the built-in regulator inside the adapter?

 

[dknguyen]

I was thinking, if I can find an AC/DC adapter that has 12V/1.5A, should it regulate the current draw because of the built-in regulator inside the adapter?

Nope. Wouldn't count on it unless you know the exact linear regulator being used. And it it doesn't it would try and provide that extra current, then fry.

It might not even have a linear regulator inside and be an unregulated AC-DC adaptor (only smoothing out the voltage with capacitors after it has been stepped down by the transformer and rectified by the diodes bridge.

 

[patpatpat]

Thanks, I will go back to the LM317.

 

[patpatpat]

I tried the LM317 circuit, but no luck. I have a DC power supply (+- 25V and 1A) output.

The actuator is still drawing 1 A when it is bottomed out.

 

[dknguyen]

How did this lack of luck make itself known to you?

 

[patpatpat]

The positive 12V worked great (to push the actuator out). I used a 10ohm resistor and it gave me .120 amp. However, when I reversed it to -12V (to pull the actuator in), I was drawing .860A and it started to heat up and made a bad smell.

 

[dknguyen]

Oh, linaer regulators are only unipolar...they can't supply negative voltages.

If you want to use that, you need two, one on each power supply rail (so it's always positive relative to the regulator) and two diodes, one for each regulator to stop the current from flowing backwards through it when it is reversed.

(A diode in series with a linear regulator, and two of setsin parallel but reversed polarity).

 

[patpatpat]

thats what I figured too. I hate saying that when someone just gave me the answer. LOL, I will give it a shot.

 

[patpatpat]

if the LM317 is only for positive voltage, I dont see how I can give negative voltage to the actuator when I have to pass through the LM.