The diodes in ULN2003 are there to protect the switchers in the IC from too
high voltage when connected to relay coils etc. It works well provided the
"other end" of the diodes are terminated correctly (usually to Vcc).
You usually don't need any protection diodes if you use mechanical switches
and don't switch frequently, although protection diodes extend the lifetime
of a mechanical switch.
I think you are better off if you put the protecting diode next to the
switch as you then take care of the inductance in the wires to the relay as
well.
The diode in a MOSFET (the so called body-diode) can not (or should not) be
used for this purpose as it is not in the correct position.
Normally you don't have to worry about the influence the diode has on the
timing of the relay. If you need the faster release time I suggest you connect
a resistor in series with the diode. Choose a resistor of the same value as
the DC resistance of the relay coil. That's a good start. You can be
sophisticated by adding capacitors etc. to a network around the relay but I
don't
think you gain anything substantial in this application.
73 de N2JFS, Hans
In a message dated 1/28/2009 8:35:37 P.M. Eastern Standard Time,
kip@kdream.com writes:
I had never heard of that before. The ULN2003 is a seven darlington
switch and has the diode built in.
Also MOSFETS usually have the diode built into the switch for the diode
in parallel with the switch case they show in second part of Figure 1.
They recommend the silicon transient suppressor diode to optimize for
normally open contacts.
But the diode is effective for a normally closed switch.
"Now that we have provided suggested suppression techniques based on
normally-open contact performance, we must add a qualifying comment
concerning the normally-closed contacts. When the primary load is on the
normally-closed contacts (and a small load or none on the
normally-open),
it may be desirable to use a rectifier diode alone as the relay
suppression
(or perhaps a rectifier diode and a lower value of series resistor). The
retarded armature motion that adversely impacts normally-open contact
performance will typically improve normally-closed contact performance.
The improvement results from less contact bounce during closure of the
normally-closed contacts. This results from the lower impact velocity
created by the retarded armature motion and has been utilized in the
past
improve normally-closed contact performance on certain relays."
So of course that begs the question, what to do with double throw
switches?
It seems protection for one side harms the other.
Kip
KipOn Wed, 2009-01-28 at 14:55 -0800, Jim Lux wrote:
>
> -----Original Message-----
> >From: Kipton Moravec <kip@kdream.com>
> >Sent: Jan 28, 2009 2:29 PM
> >
> >I can answer the diode question.
> >
> >It is generally good practice to put a diode across the relay because of
> >the inductive spike when turning it off. I have seen the spike go 3-4X
> >the input voltage. If you may have a very long line to the relay the
> >spike may be more.
> >
>
> Actually, the relay manufacturers (e.g. Tyco, P&B, etc.) recommend NOT
using the diode across the coil, because it reduces relay life and changes
actuation speed. Depending on whether you're suppressing across the coil or
load, the approaches are different.
>
> http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf
>
> as they say:
> "Many engineers use a rectifier diode alone to provide the transient
suppression for relay coils. While this is cost effective and fully eliminates
the
transient voltage, its impact on relay performance can be devastating."
>
> Jim, W6RMK
>
>
> _______________________________________________
>
>
>
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>
--
Kipton Moravec AE5IB
"Always do right; this will gratify some people and astonish the rest."
--Mark Twain
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