>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 06 February 2006 06:19
>To: Microcontroller discussion list - Public.
>Subject: [EE] Transient Suppression - was Re: [ee] 3904 base resistor
>
>
>Spehro Pefhany wrote:
>
><respectful snip>
>
>>A diode across the coil has been suggested, and is the 
>simplest way of 
>>handling the coil inductance. A 1N4148 is fine for any relay you'd be 
>>likely to use.
>>  
>>
>Alas, sorry,  it is NOT fine and is no longer considered good 
>engineering practice. I see
>this idea all over the place, and this is bad information. Even relay 
>manufacturers include
>a diode, but it is not good practice.

Is the use of diodes for back EMF protection actualy documented as bad practice anywhere?  The only negative I've ever seen is that due to the low clamping voltage, the fall time of the current is quite long, delaying relay opening.  Worse case the increase in current when the armature seperates could potnentialy cause the contacts to be closed again, which is obvioulsy not acceptable in some applications.

>
>The 1N4148 is a "faster" general purpose silicon diode, used 
>at one time 
>because nothing
>any better was available. The 1N4148 cannot switch fast enough to 
>suppress the edges
>of an inductive spike, so was most successful when a cap in 
>series with 
>a resistor is in parallel with
>the coil as well as the 1N4148. Moreover, it cannot handle the high 
>current pulse of the inductive
>collapse. I've seen wholesale device failures with this setup, 
>even with 
>tiny 5V relays.

The 1N4148 is good for application up to many 10's (possibly 100's?) of MHz, is the rise time of the induced voltage really too fast? I guess that all depends on the speed of the driver but the commonly used saturated bipolar device is comparatively slow to switch off.

Fairchilds 1N4148 is rated for 1 Amp forward current for 1 second at a Vf of 1.5 volts.  I've not done the calcs but this feels like it would be adequate for the majorty of small relay applications.  Then again, I guess there may be an awfull lot of inferior devices being sold as 1N4148's that have nothing like these specs.  Still though a failure in this case could only be attributed to an under rated component.

Regards

Mike

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