I'm in the gain. Look at you old guys, 70 messges for a pour diode.
:))

On 2/7/06, Wouter van Ooijen <wouter@voti.nl> wrote:
> > I don't think that the problem is speed or current

Oh boy, again voltage without current like on the MCLR pin on 
Microchip datasheet.

What the hell is happening with more than 100V transient voltage
if the load is the coil impedance (which is also the generator) and
a poor opened (or trying to switch from off to on) diode ?

Sure,  there is no current flow in that circuit and 200mA on which
1N4148 is rated, is more than enough isn't it ?
Try to switch a 10A solenoid and see what's happening.

- the
> > problem is the voltage. A 1N4148 is rated with 100 Volts and
> > this could be less if you switch off a relay.
>
> How can you even approach 100V when switching a low-voltage relais? When
> the relais is energised there is just the power supply voltage (5, 12,
> 24V), when the transistor switches of the diode will conduct so the
> voltage is just 0.6V.

Wouter, it's the same problem like someone on the jallist has with a stepper
switch supress a time ago.

The switching time of the diode multiplied with the current flow
through diode (which is not at all neglijible, in fact is a huge spike
of current) and multiplied with the voltage drop across diode (which
is much greater than 0.6V because diode is not in direct conduction
due the switching time, it's equal with the energy requested to be
eaten by the suppressor device.

A 1N4148 could supress a small inductive load. As large is the current
through coil (and not the nominal coil voltage which could be even
12V) as large is the spike current.

As an example, back in the 80 I've measured using a volt-ampermetric
methode, the resistance of 1600KVA transformers using a 12V acid
battery (load current about 80A-100A).
The time requsted for coil charge was minutes. The flame produced when
disconect the transformer from the battery was about 50cm.
What diode could supress such energy ?An 1KA diode with 1000V
breakdown voltage. Even the measurement was done at 12V. Now I think
is clear.

best wishes,
Vasile

>
> 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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