[sdiy] Input protection (was: asm-1/asm-2)

[Andre Majorel]

On 2004-06-03 02:33 +0200, Magnus Danielson wrote:
> From: Gene Stopp <gene at ixiacom.com>
> 
> > What happened is that the LM358 input comparator is powered between ground
> > and V+ and negative voltages drive it batty. OK in pre-patched systems
> > (where the gate can be 0 --> V+) but bad in modular systems.
> 
> Indeed. In a mixed power-supply device like the ASM-1 effectively is
> (the ADSR is GND to +15V powered) one should be aware, but general
> input protection is one of the things which we tend not to take very
> serious. Not very supprising we see things deepfried a bit into the
> design when things go terribly wrong. A resistor and a pair of
> well-placed diodes can do minor wonders to save much more expensive
> components and hard work.
> 
> So, in order to follow that general idea, add another diode to the
> +15V rail, so that over-voltage (above the +15V rail) gets directed to
> that rail. The resistor stops major current flows to that rail. If you
> end up with 230Vrms into that resistor it will for a 1k resistor
> create a current of 230mArms.  This creates a nice little heating of
> 52,9 W so it's fusing time! The power- supply only needs to keep the
> voltage stable enought for it not to cause damage. Some elaboration
> with different resistors from my finger-in-the-air- value of 1k should
> be recommended. A little higher like 4k7 just heats the resistor with
> 11,26 W, so it takes longer to burn, but on the other hand has the
> current load it dumps on the PSU-line gone down to 48,94 mA which is
> more manageable.
> 
> You don't want to make the misstake of using the diode as the fuse,
> it's there to divert the current until the chock goes away or the
> resistor fuses. The resistor is there to make sure the curren't isn't
> too high and that also to drop the voltage to a manageable level.

Coming in late but, to summarise for the benefit of dummies like
myself,

                       ^
                       |
                       |
                       -  D1
                       ^
                       |
                       |
  input  o----^v^v^v---+------>  to circuit
                R0     |
                       |
                       -  D2
                       ^
                       |
                       |
                      ---

R0 = current limiting resistor, in the 1k-4.7k range.
D1 = clamping diode, 1N4148
D2 = clamping diode, 1N4148

This is to make sure that the circuit never sees a voltage lower
than 0V minus the voltage drop of the diode or higher than the
positive rail plus the voltage drop of the diode. Is that right ?  

If you wanted to clamp it between the negative rail and the
positive rail, you'd connect the anode of D2 to the negative
rail instead of the ground.

-- 
André Majorel <amajorel at teaser.fr>
http://www.teaser.fr/~amajorel/