Does that mean that a 12 DC relay being switched cannot produce a spike of 
more than one KV? Has it been shown that the diode was destroyed by causes 
other than the safe operating parameters having been exceeded?

----- Original Message ----- 
From: "Spehro Pefhany" <speff@interlog.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Thursday, February 09, 2006 7:37 AM
Subject: Re: [EE] Transient Suppression - was Re: [ee] 3904 base resistor


> At 01:54 PM 2/9/2006 +0200, you wrote:
>
>
>>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)
>
> IT CANNOT >>>CANNOT<<<< EXCEED THE STEADY STATE COIL CURRENT!!!!
>
> If you can switch it with a wimpy 2N3904 it's nothing to worry about.
>
>>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.
>
> Hardly. First, the energy is not 'eaten by the suppressor device'
> when you use a diode, MOST of it (95%) dissipates in the copper resistance
> of the coil. After switching off a 12V relay, there is briefly 
> (milliseconds)
> a 12V + diode drop across the coil resistance internally (opposite 
> polarity).
> Secondly, the voltage at the collector of the BJT moves rather slowly. The 
> BJT
> switches off like molasses, and there is plenty of distributed
> capacitance in the coil on top of that. There are NO terahertz or 
> gigahertz
> frequencies here where nanoseconds or picoseconds make a difference.
> No magic, this is very straightforward low-frequency analog stuff.
>
> Best regards,
>
> Spehro Pefhany --"it's the network..."            "The Journey is the 
> reward"
> speff@interlog.com             Info for manufacturers: 
> http://www.trexon.com
> Embedded software/hardware/analog  Info for designers: 
> http://www.speff.com
> ->> Inexpensive test equipment & parts 
> http://search.ebay.com/_W0QQsassZspeff
>
>
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