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Subject: Re: worried about automotive spikes in homemade relay driver
Date: Sun, 6 Oct 2002 11:47:30 +1000
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
----- Original Message -----
Sent: Sunday, October 06, 2002 8:58 AM
Subject: worried about automotive spikes in homemade relay driver
> refering to the diagram in the above page - the +5v is from the output
> of a 12-5v dc->dc converter. Will this work if the ground for the emitter
> the 2N2222 is the chassis or the battery ground?
Point 1: Check out ULN2803. 8 relay drivers in one chip.
Point 2: The automotive electrical environment is *way* harsher than
"spikes". The likely survival time of your circuit as shown could be
Expect the "+12V" line to go down to seriously negative voltages as well as
up to +60-100V with load dump and other transient effects. Do a search on
"load dump". 60-70V for tens or hundreds of mS!!! There are zener-like
devices available that will offer good protection.
Point 3: Organize your ground returns either as a star or as a gound line
where the high power stuff is closer to ground that the control circuitry.
------- Gnd Star
Control---------- Relays---------- Gnd pnt Bus
The principle is that voltages generated by high current flows from the
relays should not appear as a ground potential difference between the
control and relay circuits.
The following is bad
Relays ----------- Control ------- Gnd
Imagine 1 ohm resistance in the wire between relays and control, and 1A
ground current from the relays ... 1V of noise, enough to cause incorrect
logic levels. While the resistance in practice shouldn't be that bad, there
is always inductance, and inductance will result in voltage spikes when
currents switch on and off rapidly.
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