current limiter

[Imignant]

need some help please,
I posted a question a few months ago about using an FSR to tell the gripper in my arm when to stop and hold. The reply I received was to use a current limiter instead.  Right now my robot is going to be Radio Controlled for the most part.  I was hoping to use either an on off switch or a three way switch (on my TX) to accomplish the function of the gripper.
I need something to tell the motor when to stop, but still enable it to hold fast on the object.
For starters can some please advise a circuit for the current limiter?  I am using a 360degree modified servo. So can I place the limiter after the servo’s circuit (I'm sure the reversal of the power flow upon the flip of my TX switch would be a factor)? The servo’s specs are: Electrical measurements done @4.8V, idle current below 20 mA, Average current 650 mA, Stall Current 1300 mA, and the power buss will be a 7.4V LIPO.  I can regulate the power if necessary.
These are some of the circuits I have found.
http://www.zen22142.zen.co.uk/Circuits/Power/add-on.html
http://www.maxim-ic.com/app-notes/index.mvp/id/298
http://electronics-diy.com/electronic_fuse.php

Thank you for your time and your past replies

[Soeren]

Hi,

I need something to tell the motor when to stop, but still enable it to hold fast on the object.
For starters can some please advise a circuit for the current limiter?  I am using a 360degree modified servo. So can I place the limiter after the servo’s circuit (I'm sure the reversal of the power flow upon the flip of my TX switch would be a factor)? The servo’s specs are: Electrical measurements done @4.8V, idle current below 20 mA, Average current 650 mA, Stall Current 1300 mA, and the power buss will be a 7.4V LIPO.  I can regulate the power if necessary.
These are some of the circuits I have found.
http://www.zen22142.zen.co.uk/Circuits/Power/add-on.html

This circuit should do if polarity reversed (i.e. using a PNP, flipping the rest upside down and regulating the high side). However, you need to open the servo and find the place where the H-bridge device gets its positive supply and place it there, as the rest of the circuit still needs the full voltage (eg. 4.8V). The servo cannot handle 7.4V btw.

I'd calculate the resistor for close to 1A and monitor the temperature of the motor for at least a couple of minutes. If the motor housing gets hot, as in burn your fingers, lower the current by eg. 10% and retry. You need to be able to put a finger on the metal housing of the motor, not just the servos outer shell.

It doesn't stop, but just keep trying to go farther, while the current limiter keeps the power dissipated in the motor windings down.

[Imignant]

Thanks Soeren
Ok, I spent an hour drawing a good picture in Power Point then I realized I could not put it into the reply box.  So this is what I got…
I looked for the BD131 and its PNP counterpart but I couldn’t find anything, so this one (BD438) is very similar as far as I can tell, but the BD131 says it is just a power transistor and the one I found is called a PNP Epitaxial Silicon Transistor, don’t know if that makes a difference. Also, the collector base voltage for the BD131 is 70V and the one I found is only 45V. everything else is really close.
__________________________
l         l                               l
l         l                              R2
l       _l                               l
l        ↑ D2                          C
l       _l_              l--------B  PNP BD438
l        ↑ D1           l               ↑
l        l__________l                E
l -     l                                 l -
        R1                              M
l +     l                                 l +
l_____l____________________ l
 I wasn’t quite clear on the flip upside down thing so please let me know if this is correct.  I have read all about this stuff but I'm pretty new to it.  As for the placement, I would bridge the + supply at the h-bridge connection point on the servo board, eg, + in from the servo board to the limit circuit and + out to the H-bridge, and the - in to the limit circuit would go to the - on the servo three wire, but where would the motor side connect to?
I hope I'm at least close with all this,  
Thanks for your time.
Also, I'm pretty new to this "Forum" stuff, if anyone can tell me how to post a picture I'd be obliged.
Thank you

[Soeren]

Hi,

As for the placement, I would bridge the + supply at the h-bridge connection point on the servo board, eg, + in from the servo board to the limit circuit and + out to the H-bridge, and the - in to the limit circuit would go to the - on the servo three wire, but where would the motor side connect to?
I hope I'm at least close with all this,  

Quite close, but the motor? (M) should still have + towards the positive side.
However, you shouldn't connect the motor to this circuit. The motor is fed from the H-bridge (that's how it is able to reverse direction),  so the circuit should connect like this:
Cut the connection to the H-bridge (post a picture if you're unsure of anything).
The positive input to the circuit should connect to the place where power comes from (to the H-bridge), the positive output goes to the other side of the break (the side going to H-bridge positive) and the 0V (ground) to a good ground connection (doesn't have to be inside the servo).

Here's a redrawn circuit (the BD438 will be fine)

Full resolution

Also, I'm pretty new to this "Forum" stuff, if anyone can tell me how to post a picture I'd be obliged.

Best way is to upload it to a web server and then use the third button in the second row above the text field (when writing a message. If you hover the mouse over it, it says "Insert Image".

[Imignant]

Awesome, thanks a lot,
I don't get to try this stuff out for a few more months but I'm trying to learn everything I can so I can get right to it when I get back to the good old US.  If I run into a problem I will be sure to send you a pic.

One last question, if you don't mind, would it be possible to replace R2 with say a 2K POT so I can easily tune it?

It's great to have people so willing to help and I will try to return the favor in the future as I do with new comers to the RC plane world.
Thanks again.