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Make a Gearhead Motor into a Monster Servo!

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Posted: 2009-10-03 02:47 
Yeah, I missed the 180 degree. Then you should be fine with a pot.
You might check for surplus relays. I'd really caution against trying to use R/C servos to push buttons!

How about using the R/C servo (if you must) to tip a pair of mercury switches back and forth? Or move a magnet to operate reed switches? Although they probably won't handle the current. Automotive relays might be a possibility, but you'd need a transistor driver.

Just ideas.

Alan KM6VV

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Posted: 2009-11-21 01:30 
Just got a hand full of the L298N H-bridge chips (below) and wondering if anybody has any experience using them in projects. I'd like to drive these chips using the board out of a standard servo. One thing I'm not familiar with in his chip is the "fast motor stop" function of the chip. Not sure if this will be an issue in my intended setup. I've noticed that the stall current of the standard frigelli linear actuators is ~4A, so this chip might make a decent driver for these when paralled. I've also noticed that the really big linear actuators (~1,000 lb.) usually have inline 10A fuses, so several L298Ns in parallel might be able to handle these.

http://www.pololu.com/catalog/product/924
http://www.pololu.com/file/0J67/L298.pdf

http://www.northerntool.com/webapp/wcs/ ... _200333245

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Posted: 2009-11-23 12:20 
I don't think one can parallel these chips. They have a bipolar output stage and if memory serves you can't parallel bipolar transistors for more current. FET's can do this though...

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Posted: 2009-11-23 13:35 
The datasheet doesn't say much about the internal workings of the chips, but there are two seperate H-bridges in each chip. Figure 7 shows the two H-bridges being put in parallel operation for higher current needs and is mentioned in the applications section. There is also a scensing pin for each H-bridge that can reportedly used to limit current flow thru the H-bridge (but no details on just how to set this up). My thinking is that if the two seperate H-bridges in each chip can be put in parallel, then more than two may be possible.

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Posted: 2009-11-23 14:10 
It's been a long time, but I think there is a big difference between paralleling two drivers on the same chip, and paralleling driver chips. I think there has to be serious monitoring of the current to ensure one chip doesn't do all the work. Good luck with it!

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Posted: 2009-11-23 15:58 
The H-bridges inside of the chip are closely matched, and are allowed to be paralleled. Outside of the chip, you'll run into problems without complicated current balancing schemes. Better to go to a bigger chip in that case.

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Posted: 2009-11-23 16:15 
KM6VV wrote:
The H-bridges inside of the chip are closely matched, and are allowed to be paralleled. Outside of the chip, you'll run into problems without complicated current balancing schemes. Better to go to a bigger chip in that case.
Alan KM6VV


I just don't know of any bigger H-bridge chips. One has to jump from the inexpensive chips to somewhat expensive H-bridge boards. I see servocity has taken a $129 linear actuator and added a $170 gizmo setup to make their $299 servo linear actuator.

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Posted: 2009-11-23 16:30 
Check out the LMD1845.

Google "LMD18245 + "DC motor" + robotics"

3A, 55V

Alan KM6VV

zoomkat wrote:
I just don't know of any bigger H-bridge chips. One has to jump from the inexpensive chips to somewhat expensive H-bridge boards. I see servocity has taken a $129 linear actuator and added a $170 gizmo setup to make their $299 servo linear actuator.

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Posted: 2009-11-23 19:57 
I looked at the LMD18245 data sheet and it has some interesting features, but it doesn't appear to be easily adaptable as a big servo control device. It also has a price tag of $16 each. I did notice that the drain side (hoping I'm getting the terms correct) transistors appear to be charge pump operated NPN MOSFETs.

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Posted: 2009-11-23 22:22 
zoomkat wrote:
I did notice that the drain side (hoping I'm getting the terms correct) transistors appear to be charge pump operated NPN MOSFETs.

er, N-channel MOSFETs. ;)
The technique of using a bootstrap diode and capacitor or an outright charge pump is pretty common with higher power H-bridges. N-channel devices are more generally efficient and cost effective than P-channel MOSFETs. This is even more important at higher currents and if you are using 4-quadrant control where both high and low side devices are chopped (vs. 2-quadrant where just the bottom side devices are chopped.)


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Posted: 2009-11-24 01:13 
I previously posted the below schematic of a high side boot strap setup that apparently is found in switched power supplies and such. My question is will it continously pass current when the driver supplies pulses greater than some minimum frequency. The N&V article mentioned conduction at greater than 1 HZ, but didn't mention if the conduction was continous or pulsed at the same rate as the driver.


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http://youtube.com/watch?v=pWjMvrkUqX0
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http://web.comporium.net/~shb/S2000video.htm


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