Wouter, we have talk about this several times on this list and on others.
1N5819 has only 40V breakdown voltage. It's less than 2x24V requested
by any engineering design requirements. Also the motor load is 5A,
while your maximum supressed current is 1A (25A/8mS maximum maximorum
peak current).
Change diode with at least 100V one and check if your maximum peak
current through diodes tooks more than 2-3mS. If yes, choose a 2A
diode.

greetings,
Vasile


On 4/21/06, Wouter van Ooijen <wouter@voti.nl> wrote:
> Next problem with my motor driver hardware :(
>
> (gif of relevant part attached)
>
> My customer confronted me with a unit in which at least one of the
> supression diodes (D7, 1N5819) was totally blown. I think both FETs and
> the other diode were also dead, but with less visible damage. After
> replacing the two diodes and the two FETs the unit seems to work OK
> again.
>
> Vmotor = 24V, 5A supply, motor stall current ~ 300 mA (normal operation
> 50..100 mA) , there is stall detection that switches the motor of after
> ~ 20 ms. There is reverse-polarity protection in the circuit (P fet in
> the power line). As far as the customer could tell the problem (very bad
> smell) occured while the unit was not touched (so reverse polarity is
> unlikely, but for instance a short in the cable to the motor might be
> possible). The wiring to the motors does contain a gnd wire, but no
> Vmotor wire.
>
> I can imgaine various errors (like shorting the motor lead to ground)
> that could destroy the FETs (P fet first, when that one fails short the
> N fet will follow soon). But what failure scenario (except reverse
> power, but that is very unlikely) would blow the top (and/or bottom)
> diode?
>
> Wouter van Ooijen
>
> -- -------------------------------------------
> Van Ooijen Technische Informatica: www.voti.nl
> consultancy, development, PICmicro products
> docent Hogeschool van Utrecht: www.voti.nl/hvu
>
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>
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