[Diy_efi] Triac versus Relay - Knowledge please!

[Perry Harrington]

On Sun, May 04, 2003 at 11:23:13AM -0500, Daniel R. Nicoson wrote:
> My car is a 1994 Mustang GT with T4M0 EEC.  Street driven with full EGR &
> Thermoactor operation.  I don't pretend to be able to properly design an
> electronic circuit.  I understand On-Off, Good- Ground, Bad-Ground...
> 
> I have a permanently mounted DIY-WB that is powered up via a relay any time
> the engine has "power-on" to the computer (EEC pin 57 hot, Vehicle Power -
> VPWR).

I cheated on my WB install.  I controlled the relay from the ignition hot wire.
This is just KO power anyway.

> 
> This weekend I am adding a "Remote Data Log Relay" to reconfigure the EGR
> sensor circuit for data logging at a flip of a switch.
> 
> Both relays are triggered by EEC pin 57 and I have several concerns:
> 
> 1) Can these relay coils induce a voltage spike harmful to my EEC anytime
> during their operation?   Starting up?  Turning off?

The spike is a reverse voltage spike at relay turnoff.  Trust me, the EEC
has a lot worse things to worry about.  This shouldn't present a problem
for you.  On the other hand, you might consider doing the big capacitor
mod to prevent the EEC-Tuner or equivelent from causing problems due to
power noise.  I had to do this mod on my car, a 15,000mfd cap worked great,
didn't cost $16 either.

> 
> 2) The relays are not shielded by metal boxes (both mounted in plastic
> boxes), can EMI be a factor from CB radio's, ignition or anything else?

Other things will be upset before the relays are.

> 
> 3) Would a triac be an appropriate replacement to the electromechanical
> relay's?

No.  These are intended for switching high current powerline sources.  They
*are* used in some "solid state" relays, but they do not operate like relays.
Detailed info on SCRs and Triacs can be had by searching the net.

> 
> 4) Do I risk overloading this circuit that feeds the EEC (amp load or
> voltage drawdown)?

No, 66ohms is very little.  The EEC is fed with multiple 10 or 12 gauge wires,
these flow plenty of current as long as you don't cut them.  I prefer using
3M T-taps to get juice/signal off the wiring harness.

> 
> The first relay that I have been using successfully for 6-9 months turns on
> the DIY-WB system (I think 2 amp max load?) was chosen because it was on the
> shelf at Radio Shack and seemed like it would handle the load:
> 
> RS# 275-226, SPST Contacts 30 Amp at 12 Volt DC, Coil Voltage 12volt,
> Pull-In Voltage 6volt, Coil Resistance 66 Ohms, Nominal Current 160 mAmps,
> Drop-Out Voltage 3.6Amps
> 
> Should this relay be replaced by a triac?  I have one in hand, RS# 276-1000,
> PIV 400 Volt, Forward Current 6 Amps, Gate Current 25 mAmps, Gate Voltage 3
> volt.

No.  Besides, look at the gate voltage, you are in a 12v world in the car, a 3 volt
gate voltage would get fried unless you made up a specially regulated circuit.

> 
> If I replace the relay with this triac do I need a large heat sink on the
> triac?

Triacs and SCRs, most definitely require heatsinks.  The major thing to look for
is the on resistance.  The lower the on resistance, the less heat dissipated.
MOSFET technology is used by may high current DC applications because the on
resistance is in the neighborhood of .001 ohms and such.

> 
> The second relay when un-powered allows all three wires going the EGR
> position sensor to stay in their stock connection.  When powered (activated
> by a switch for data logging) the relay disconnects two of the wires
> (circuit 359 & 351, EEC pins 46 & 26) and connects the DIY-WB to the EGR
> valve position sensor infeed to the EEC (circuit 352, EEC pin 27).
> 
> This second relay is RS# 275-214A, 12VDC PLUG-IN RELAY, continuous coil
> voltage 13.2 VDC, coil resistance 160 ohms, pull-in voltage 9 VDC, contact
> rating 5Amps at 125 VDC, nominal current 75 mAmps.
> 
> I haven't looked for a solid state replacement for this relay.
> 
> Any thoughts, suggestions?  I don't want to fry my EEC because of a voltage
> spike and the omission of some 5 cent piece that would have protected
> everything.

As long as you don't go hacking the box open and connecting to stuff inside, this
shouldn't be a problem.  Ford desined these boxes to live in an environmet a lot
tougher than your relays.

> 
> Thanks,
> 
> Dan Nicoson
> 

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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