Re: [CAD_CAM_EDM_DRO] Getting Parallel port to run relay

Scott L Golden wrote:

>
> I may be way out on a limb here but can't one put fuses in line? I admit low
> amp fuses but wouldn't that help with spikes?
>
> Now go easy on me... I am not too deep into electronics.
>
> Or isnt there a one way choke or something to limit the flow of power
> backwards?
>
> Sorry for the lack of knowledge but....my thoughts anyway. lol
>
> Scott L Golden
> Never Give Up

Hello Scott,

The coil does not actually generate a high voltage spike when the
circuit is opened. The coil tries to continue the current at the value
just before the circuit opened. Think of rushing water when you close
the dam gate. The water has momentum and will break down the gate if
there is any serious speed and volume involved.

Anyway, the current is forced to pass through a very high resistance
(open circuit) and the voltage developed across the high resistance is
current times resistance. So if you have a 1 amp current flowing and go
from 0.1 ohm to 1 kohm as the circuit opens, you instantly generate a 1
KV spark across the 1 kohm resistance. This will drop quickly as the
energy stored in the coil inductance dissipates and the current drops to
zero amps.

The open circuit resistance is usually much higher than 1kohm and the
current greater than 1 amp so the voltage spike is usually much much
higher than 1kv. The time required for a high voltage transient to
damage transistors and logic gates is a few nanoseconds.

If you use a reverse biased diode in parallel with the coil, the diode
has very little effect on normal performance. When the circuit is
opened, the 1 amp current continues, but through the diode and back into
the coil. Since there is some real resistance in the coil, the current
will drop off in a short time as the energy is dissipate in the coil
resistance and the diode forward voltage drop (0.65v for silicon
diodes).

So no high voltage spike occurs if the diode is wired reverse biased in
parallel with the dc relay coil. The current does not increase either.
So the diode must handle as much instantaneous forward current as max
current through the coil. The diode reverse voltage rating should be as
high as the max voltage applied to the coil.

I seem to recall some analysis that shows that the diode reverse voltage
rating needs to be double the greatest voltage applied top the coil. But
I may be thinking of some type of full wave rectifier. Sorry, but my EE
is a little rusty right now.

Anyway, a fuse won't help since the current is the same or less than the
normal operating current for the relay coil.

A choke in series won't help since that is just more inductance and high
inductance of the relay coil is the original problem. So a series choke
will make the spike voltage even greater and longer lasting due to extra
energy stored in the choke.

A capacitor in parallel with the switch might help. This is the method
used on older car ingnition coils. When the distributor contacts open,
the current surges through the capacitor. When the capacitor is fully
charged, the current will attempt to arc across the contacts due to high
voltage caused by current wanting to continue. By then, the contacts
have opened far enough that the high voltage spike cannot jump the gap.
I think that after that the stored energy oscillates between the coil
and the capacitor until their resistances dissipate the energy.

The traditional solution for dc switching inductive loads, like relays
and motors, is the reverse biased diode in parallel with the inductor.
It is cheap and it works.

Tom Hubin
thubin@...