> There are so many posts, replies, and whatnot to this thread at this point
> (spread over several months, too), that I am getting lost in them. Frankly,
> I'm wondering whether I even quoted the correct piece in my original post,
> earlier today. The H-Bridge is working as expected, and I'm mostly wondering
> whether I should stick with this one, which is controlled the way I prefer,
> but may have issues with overheating, or should I try to come up with a
> means to manage the control of the much higher power Sabertooth.
>
> If the H-Bridge I am using is rated to handle 1.1 amps, with a peak (or
> surge, or something like that) power of 2 amps, and I am using a motor that
> can draw 1 amp with a stall power consumption of 2amps, am I going to melt
> something? The robot will have the advantage that it will have plenty of
> thinking to do. It is a self-guided, indoor, exploration bot, so I could
> alter the behavior to sit and process information such that it is running
> for relatively brief periods (such as on for 10s, then sitting still for 10s
> recording sensor information).
>
>
Unless you are using a very healthy heat sink and/or a fan on the
unit, yes, you will let the magic smoke out or the H-bridge will thermal
very often and eventually break. The current spec'd by most chips is
when the junction temperature is at ambient (room temp, or about 25C).
As the temperature rises, the max current allowed falls. Running
current through the part causes the temperature to rise. Only a very
aggressive heat sink and airflow will allow these parts to run at their
rated maximums for extended periods of time.
DLC
>
> rtstofer wrote:
>
>> --- In oopic@yahoogroups.com, Brian Lloyd <brian-wb6rqn@...> wrote:
>>
>>> On Mar 2, 2008, at 12:58 PM, Shaggy wrote:
>>>
>>>
>>>>> What is the frequency of your PWM? The 754410 is happiest at 2KHz or
>>>>> lower. At 20KHz I've detonated these chips!
>>>>>
>>> When figuring pulse frequency and pulse width, you need to consider
>>> the inductance of the motor. As you apply voltage to a winding, the
>>> current ramps up until the core saturates and then the current is
>>> limited only by the DC resistance of the windings. (BTW, this is why
>>> "AC" motors are more efficient then "DC" motors.) By varying pulse
>>> width, pulse frequency, and applied voltage you can find the "sweet
>>> spot" for any motor.
>>>
>> I guess I don't follow this because, it seems to me, the commutator
>> and brushes apply the voltage to the windings. In fact, there is no
>> reason I couldn't hang a large capacitor after the MOSFET to integrate
>> (filter) the DC voltage.
>>
>> Of course, if I did put the capacitor in the circuit, I would have to
>> consider the peak current through the MOSFET during a smaller
>> conduction period.
>>
>> The problem I have with MOSFETs is not driving the gate hard enough
>> and allowing the rise and fall time to be somewhat long. As a result,
>> at higher PWM frequencies, the MOSFET spends a larger percentage of
>> the time in the transition region. At a high enough frequency the
>> device may never get into saturation and the heating is excessive.
>>
>> Eventually, the magic smoke leaks out.
>>
>> Richard
>>
>>
>>
>>
>>
>>
>>
>
>
--
------------------------------------------------------
Dennis Clark ooPIC Tech Support
www.oopic.com
------------------------------------------------------
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