From: medin@catbyte.b30.ingr.comIn article <25587@oasys.dt.navy.mil>, gallant@oasys.dt.navy.mil (Robert Gallant) writes:
Subject: Re: Which Diode?
Organization: Intergraph
Date: Mon, 28 Sep 1992 20:40:53 GMT

|> Hi, I am trying to find the proper diode to combine the trigger signals
|> from two ignition coils on my car.
|> 
|> This combined signal is used to trigger a new fuel injection computer
|> (so there will be little to no current through the diodes).
|> 
|> Here is a diagram:
|> 				D1
|> 			  ------>|----------------------------|
|> 			  |                              D2   |---OUT
|> 	 (+)       (-)    |       (+)       (-)       --->|---|
|> 	 ==        ==-----|       ==        ==--------|
|>     ----------------   |     ----------------      |
|>     -              -   |     -              -      |
|>     -  Coil 1      -   |     -   Coil 2     -      |
|>     -              -   |     -              -      |
|>     ----------------   |     ----------------      |
|> 			  |                           |
|> 			 ---                         ---
|> 			 --- Condensor 1             --- Condensor 2
|> 			  |                           |
|> 			  |                           |
|> 			 GND                         GND
|> 
|> This is the stock setup except for the two diodes.
|> 
|> The voltage present at the (-) terminals should be alternating between
|> 0 and +14.1 volts at 300 Hz maximum.
|> 
|> I have tried 1N4148 diodes with a 100K ohm resistor from OUT to GND.
|> The diodes went bad immediately.
|> 
|> Any help would be appreciated.

My guess is that the voltage on the (-) terminal of the coil is not at
all limited to zero volts in its negative excursion, but rather much
higher such that the 4148's PIV rating (about 75 volts)
is exceeded and the diode breaks down. This is because the coil (-)
experiences inductive flyback and rings when the points are released,
developing up to 300 volts or so across the points.

If the OUT load is, in fact, high impedance logic, then try this
modification to your circuit (only shown from the cathode of D1 and D2):


                                  V+ logic supply (12 volts?)
                                   |
                                  -+-
                                   ^  D3
                                   |         D1
            To coil (-) ---- 10 K--+--------->|---------+OUT (combined with  
                                   |                          D2's cathode
				  -+-                         and 10 K load
                                   ^  D4                      to ground)
                                   |
                                  GND


The 10K resistor/diode clamp should be put in the D2 circuit as well. The
top diode (D3) can be omitted if the positive voltage swing at D1's cathode
(banded end) is not too high for the logic involved. With D3 in, the
logic is fully protected from positive coil spikes too.

The diode clamp lets you use cheap 4148's, which have a faster recovery time
than high voltage rectifier diodes, which would be the next choice. They're
also cheaper...

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