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Help me use a zener diode

 
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FAVahalik



Joined: 23 Oct 2005
Posts: 1
Location: Fort Worth, TX

PostPosted: Sun Oct 23, 2005 8:57 pm    Post subject: Help me use a zener diode Reply with quote

I am attempting to understand how a zener diode would work in the following application:

This is for a Suzuki Hayabusa motorcycle. It has a speed restriction that will not allow the bike to exceed 10,200 rpm in 6th gear. Gears 1-5 redline at 10,800 rpm. This limits the bike to a speed of 186 mph. Without the restriction, the top speed would be 192-194 mph.

There is a wire that runs from the gear position sensing unit (GPS) that passes the following voltages to the ECU:

Gear 1 - 1.8 v
Gear 2 - 2.26 v
Gear 3 - 2.99 v
Gear 4 - 3.68 v
Gear 5 - 4.38 v
Gear 6 - 4.7 v

An author on another forum suggests using a zener diode of 4.3 volts @ 250 mw from the wire that runs to the ECU to ground. If I understand correctly, the diode will let voltages up to but not exceeding 4.3 volts through to the ECU. The voltage over 4.3 volts will be sent to ground so the ECU sees gears 1-5 normally. If I understand correctly, when the GPS sends 4.7 volts to the ECU, only 4.3 volts will be received by the ECU and the rest will be sent to ground. Thus, the ECU will see gear 6 as gear 5, thus removing the speed limitation. It is important that the ECU see gears 1-5 at the correct voltages, because different ignition maps are also involved in the selection. Gears 5 and 6 are identical except for the rpm limitation.

My question(s) are: Am I understanding how the zener diode will work? Since I am unsure of how much current is involved, is this dangerous in the sense that the .4 volts being sent to ground might constitute a short, or will the diode not pass that much current? Since the bike uses a battery similar to a car battery, a direct short would be very bad. Do I understand correctly that if the diode were connected inline that it would pass only .4 volts (when the voltage rises to 4.7 volts)?

I would appreciate any help someone would care to give. I am quite the novice when it comes to electronics, so please forgive questions that might seem obvious to you guys.
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PostPosted: Thu Oct 27, 2005 8:28 am    Post subject: What happens At 200 MPH?? Reply with quote

Hello,
Yes you have a good understanding of how a zener diode works. Usually you put a resistor in series with the power supply and a zener across the output so that 4.3 volts is across the output (zener) and the excess is dropped across the resistor. If you place a 4.3v zener from your wire to earth, the extra 0.4 volts will be dropped across something from where the power comes from (maybe the GPS). How does the GPS work, does it just switch resistors in and out of circuit or does it work by some other means? If it already has a resistor in the supply, then your zener may work well. If you are woried about burning expensive parts, you could put a small value fuse in series with the zener, then if the current flow is too much the fuse will blow, maybe saving the GPS and ECU. Zeners do not pass voltage, they pass as much current as it takes to keep the voltage constant. MAKE SURE YOU PUT THE ZENER IN THE RIGHT WAY AROUND!
Good luck and let us know if you are successful. Smile
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