Hight power Current sensing device

[SkyRocketeer]

High power Current sensing device

Is there a way to monitor the current through a motor without having a negative effect on the current?

For my leccy window project, I'd like to know if the motor has been stalled mid-travel (kiddies head or something). Some form of position sensing device may work, but as I'm dealing with an electric window mech as a complete and sealed device, it's difficult to add one to it, plus the inside of a car door is a dirty place and I can't see an opto-based sensor living that long.

This leaves me with the option to monitor the current, I'm reluctant to simply stuff a resistor in series with the motor and measure the voltage from it, as a) the resistor would have to take a fairly hefty current and b) it'd be sapping power from the motor.

Is there another trick I can use?

 

[Hero999]

If you use a small enough resistor and amplifier the voltage drop will be negligible.

You could consider measuring the resistance of the cable and work about the current by measuring the voltage drop across it.

You could use a hall effect sensor which measures the magnetic field generated by the current which is totally lossless.

 

[Rolf]

It would help ........

Is there a way to monitor the current through a motor without having a negative effect on the current?

For my leccy window project, I'd like to know if the motor has been stalled mid-travel (kiddies head or something). Some form of position sensing device may work, but as I'm dealing with an electric window mech as a complete and sealed device, it's difficult to add one to it, plus the inside of a car door is a dirty place and I can't see an opto-based sensor living that long.

This leaves me with the option to monitor the current, I'm reluctant to simply stuff a resistor in series with the motor and measure the voltage from it, as a) the resistor would have to take a fairly hefty current and b) it'd be sapping power from the motor.

Is there another trick I can use?

...... if you would tell us whether the motor is AC or DC.

 

[jpanhalt]

Some approaches to this problem look at the back emf generated by a running motor. The ones I have seen have been DC on PWM. I am not sure whether that approach is applicable to AC motors. John

 

[SkyRocketeer]

Sorry, forgot to mention - motor is DC - the typical sort found in your older cars winding the windows.

I was hoping to avoid PWM to keep things simple on the drive front, I was just going to use a pair of relays for current switching. A later mod could well be PWM, but for now I'm just trying to get the basics.

The hall effect doodah sounds promising though - would it work with DC?

 

[Hero999]

Yes, a hall effect sensor will work with DC. You can buy them from all good electronic components stores, try Farnell or RS Components.

 

[SkyRocketeer]

Been doing a spot of googling an found**broken link removed** which seems to be heading in the direction of answering some of my questions.
Haven't actually found a part that says on the tin that it'll do what I want, but I'll keep on looking..

 

[Nigel Goodwin]

As already mentioned, simply measure the voltage drop over the cable - there's no extra drop whatsoever, you simply use an opamp to amplify it, and calibrate it using a known load.

 

[FRIED]

**broken link removed**

 

[Rolf]

I have seen ....

Is there a way to monitor the current through a motor without having a negative effect on the current?

For my leccy window project, I'd like to know if the motor has been stalled mid-travel (kiddies head or something). Some form of position sensing device may work, but as I'm dealing with an electric window mech as a complete and sealed device, it's difficult to add one to it, plus the inside of a car door is a dirty place and I can't see an opto-based sensor living that long.

This leaves me with the option to monitor the current, I'm reluctant to simply stuff a resistor in series with the motor and measure the voltage from it, as a) the resistor would have to take a fairly hefty current and b) it'd be sapping power from the motor.

Is there another trick I can use?

........ something on a garage door opener I think. The motor was installed so it twisted slightly when it got an abnormally high load and the mounting bracket activated the reverse switch.