how to make a trip switch

[tarantulataramasalata]

anyone know how to make a trip switch? I am driving a motor using a bcx38b, and I'd like to ensure that the current does not go above the recommended 800mA. Fuses are a bit primitive but I don't know of another way. Any help appreciated.

 

[hippy]

Something like : http://www.maxim-ic.com/appnotes.cfm/appnote_number/659 ?

Or maybe an LM317T variable voltage regulator or similar set to do current limiting ?

Edited by - hippy on 13/03/2006 15:11:59

 

[Fowkc]

Depending on how critical your motor current is, you could just use a small series "current sense" resistor in line with the motor. A 1ohm resistor with 800mA through it will have a voltage across it of V=IR = 0.8v. You can monitor this voltage through any number of methods and trip the circuit. Remember that the power dissipated in the resistor is 0.8A x 0.8V = 0.64W, so use a beefier resistor than a standard 1/4watt.

 

[tarantulataramasalata]

thanks fowkc, that makes lots of sense to me. i wonder if i can get the picaxe itself to periodically check that voltage and shut down when it reaches a certain level. Then i could probably stop it at 750mA. are you a teacher, btw?

Edited by - tarantulataramasalata on 13/03/2006 20:46:22

 

[Fowkc]

Nope, I'm in the second year of a Physics Msci degree at Nottingham. Haven't had the oppourtunity to use PICs or PICAXEs yet though. Maybe in the third year when I might have to build meyown instrumentation or control hardware...

 

[tarantulataramasalata]

Also, it may seem a dumb question, but exactly how can I harness the voltage change around a current sense resistor? I was hoping to have a PIC sense the voltage drop around the resistor via an analogue input.

But how do I connect it to the PIC? Do I need two inputs; one measuring the voltage south of the resistor and one measuring the voltage north, and then comparing them? Or can I do it with one input? How?

Many, many thanks.

 

[Dippy]

Just a thought...
A possible hardware answer may be a op-amp or voltage comaprator with fowkcs resistor. You could then use a lower value res for less power loss AND by cunning use of a capacitor you could introduce an anti-surge facility so that it doesn't trip too quick.
I did one of these years ago and it made a real easy trip with auto-reset (if required).
With PICAXE: If your motor is referenced to ground could you just ADC read the res/motor junction? - perhaps again with a bit of RC in the way so that start-up surges or inductive spikes don't upset things. You could then also have software adjustable anti-surge. Depends how much spare processing time you can have.
Time to dig out the pencil and calculator.

 

[Fowkc]

If one end of your resistor is grounded, then you can just measure the voltage at the top of it as normal. If not, you'll need to measure the difference between the "top" and "bottom", either by using two ADC inputs, or using an op-amp to subtract the "bottom" voltage from the "top".

 

[Dippy]

You may also need to think about a little electrical spiky protection too.

 

[tarantulataramasalata]

"spiky protection" - you mean from the motor, ie reversed diode/decoupling capacitor?

Edited by - tarantulataramasalata on 22/03/2006 10:42:53