Match: #108
Message: #23392
From: "Paul Messinger" <paulm(at)olypen.com>
Subject: Diodes across relay coils
Diodes across relay coils
Date: Mar 22, 2005



For decades 99.999% of the engineering world recommended and used the common 
rectifier diode across inductors like relay coils to suppress the inductive 
spike that happens when the power to the inductor is interrupted.

Many (including Bob) have stated that while this practice extends the start 
of opening time of the relay it does not change the opening speed once the 
relay contacts start opening. Thus the only down side to use of the diode is 
a delay in the start of opening. Normally this is not important.

For example the common contactor used for starting or battery connection 
will open in 10 ms with no diode and it takes 50 ms with a diode.

A friend sent me several links including one at Potter and Brumfield (a 
major relay manufacturer) that not only had a comparison table (of 
suppression devices) but  a comment on what the use of a diode really does. 
I have extracted part of that comment below and also included at the end of 
this post the links for the interested.

""Effects of Coil Suppression on Relay Dynamics and Life

Even though the use of coil suppression is becoming more significant,

relays are normally designed without taking the dynamic impact of

suppressors into account. The optimum switching life (for normally-open

contacts) is therefore obtained with a totally unsuppressed relay and

statements of rated electrical life are usually based on this premise. The

successful "breaking" of a DC load requires that the relay contacts move

to open with a reasonably high speed.

A typical relay will have an accelerating motion of its armature toward the

unenergized rest position during drop-out. The velocity of the armature at

the instant of contact opening will play a significant role in the relay's

ability to avoid "tack welding" by providing adequate force to break any

light welds made during the "make" of a high current resistive load (or one

with a high in-rush current). It is the velocity of the armature that is 
most

affected by coil suppression. If the suppressor provides a conducting path,

thus allowing the stored energy in the relay's magnetic circuit to decay

slowly, the armature motion will be retarded and the armature may even

temporarily reverse direction. The reversing of direction and re-closing of

the contacts (particularly when combined with inductive loads) often leads

to random, intermittent "tack welding" of the contacts such that the relay

may free itself if operated again or even jarred slightly.

Based upon the impact on armature motion and optimizing for normallyopen

contacts, the best suppression method is to use a silicon transient

suppressor diode. This suppressor will have the least effect on relay 
dropout

dynamics since the relay transient will be allowed to go to a

predetermined voltage level and then permit current to flow with a low

impedance. This results in the stored energy being quickly dissipated by

the suppressor. Transient suppressor diodes are available as bi-directional

components and permit the relay to be non-polarized when installed

internally. Note that if a uni-directional transient suppressor is used, a

rectifier diode must be placed in series with it to block normal current

flow and it has little advantage over the use of a zener diode. ""


This and a table comparing different types of suppressors (see the below 
links) clearly indicates a couple of things.


First 99.99% of engineers have been using the wrong type of suppressor for 
many years.


The simple diode is perhaps the worst choice(see the table of comparisons at 
the links). In the case of power contactors the use of a simple diode 
actually slow the mechanical opening of the contacts quite a lot and 
increases the likely hood of contact welding.


I have tested this last year in the load dump testing and recorded large 
multiple contact bouncing.


I have removed all the simple diodes across relays and replaced them with by 
directional transorbs and also supplied parts to the local builders as well.


Here are a couple of links, There is also one in Europe that I cannot find 
right now with the same conclusions.


http://relays.tycoelectronics.com/application.asp

http://relays.tycoelectronics.com/kilovac/appnotes/transients.stm


Paul




End Msg: #108

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