PWM Control of 12vdc Halogen Auto Lamps as a Test Load for PMW H-Bridge Driver - Page 1
Forum section: electronics / beginners

author: K1JOS
author_rank: Regular Contributor
timestamp: December 24, 2014, 04:01:18 am
content: I have a high current H-bridge (18vdc max, 25A) driver from Pololu Electronics (http://www.pololu.com/product/1455) that I would like to use to control a 12v low speed (2 revolution per minute) high torque DC brush motor that draws ~7A running with a stall current of about 15A. I don't want to risk damaging the expensive motor so I would like to test my PIC18F software controlling the H-Bridge driver using an alternate load. I know a resistive load cannot reproduce the inductive load of the motor but I figure its a good starting point particularly so I can test the current sensor code I am writing to shut the motor off if the stall current is exceeded more than a few secondsI was thinking to use inexpensive 12vdc auto 100W halogen lamps. Can these be dimmed using PWM? I have an Array 3711A electronic load but with PWM current to the load the constant current setting is incorrect sometimes by several fold and not in a linear manner to the PWM duty cycle. I think I understand why based on another topic here in the EEVblog forum. So my best alternative is a pure predictive resistive load.thanks in advance and Merry Christmas / Holiday Season !

author: johansen
author_rank: Super Contributor
timestamp: December 24, 2014, 04:55:30 am
content: most people's H bridge smoke problems stem from a floppy high inductance supply. that cap on the back of that board cannot handle the 5 amps or so of ripple current, and when it blows the energy stored in the inductance of both the supply and the motor ends up as over voltage avalanche.with a 7 amp running current and a 15 amp stall.. you cannot damage the motor anyways.. you might check the datasheet to make sure that a reverse polarity while the motor is at full rpm will demagnetize the magnets.. but with small motors this isn't usually a problem

author: Seekonk
author_rank: Super Contributor
timestamp: December 24, 2014, 09:24:29 am
content: Just be aware that lamps have a cold to hot resistance increase of about seven.

author: rob77
author_rank: Super Contributor
timestamp: December 24, 2014, 11:05:07 am
content: add a latching over-current protection - e.g. a high side switch with current sensing above the H bridge. set the current threshold just above the stall current and you'll be safe because full RPM one direction + changing the direction will trip the over-current protection sooner than anything wrong could happen. and as a free bonus you'll have some level of protection for your H-bridge against shorts.

author: K1JOS
author_rank: Regular Contributor
timestamp: December 24, 2014, 06:14:21 pm
content: For power source I was planning on using a deep cycle 12vdc -with about 240 A-hours reserve - this should be good for a full day of typical use for me and would have no ripple issue (I was planning on solar recharge when not in use.) Alternately I could use a high quality 13.7vdc 30A supply designed for ham radio - ripple and /or noise are Peak-to-Peak Max. <100mVpp. These are small and designed to push out 30A all day without problem. I dont know about the motor magnets but I am designing the microcontroller to not allow rapid changes in direction.The high side switch above the H-bridge I assume you mean to sense the power supply current to the H-bridge? I guess a simple high current 5vdc (for PIC18) relay would be fine? I was planning on using the ACS711 or ACS714 hall effecct high current sensors - this are bidirectional for current reversal.