On 5/31/06, Wouter van Ooijen <wouter@voti.nl> wrote:
> > What voltage and current specifications should the diode
> > across the relay coil have for a relay pulling 400 mA at
> > 15V dc?
>
> A coil tries to maintain its current, so when the driver switches off
> the current will be 400mA. When the driver is switched on the diode will
> see a reverse voltage of 15V.

And the time when diode will switch, the voltage across it could be

u = L * di/dt

As long L is constant, t is direct dependent by switching time of
diode but also by the transistor switching (that's why usually we
choose a faster diode than 1N400x and not a standard lazy darlinghton
but a switching transistor), as high will be the coil current, as big
will be the voltage across diode on the switching time. It could be
even hundred of volts with a supply of only 15V.

This behavior shows there is an important current flow and an
important voltage spike in the coil circuit. That's vhy the designer
must keep the supress circuit as short as possible (ie mounting the
diode directly on the coil ends and not far away on the PCB, -if the
coil is not mounted on the PCB-) and assure a complete separate
circuit to the coil supply (both VCC and GND) to the lowest impedance
point (the accumulator or the DC power supply at the regulator
output/ground+ESR capacitor.

In a good design, the largest current circuit will be the first on the
power supply node, then the noisy one (ie microcontroller) and the
last one will be the analogic
path (exception only comparators which are noisy and must be treated
as digital circuitry)

greetings,
Vasile


>
> So a 15V/400mA diode would do. But I would pick an 1N4004, becase that
> is my 'mid-current' yellybean 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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