avalanche diodes in automotive alternators

[brutus1955]

i have no idea where to get an answer to this one, so i come here hoping to get an explanation.

the alternator i am working with has what is called avalanche diodes in the bridge rectifier, they are supposed to go into avalanche at ~29volts or so.

from what i can tell there are some manufactures that use an avalanche diode placed in the case (ground) so that if there is a voltage spike it is clamped to ground.

what i don't fully understand is how this sort of diode would work in place of a common diode in the rectifier bridge?

if a common diode conducts in one direction, i assume so does an avalanche diode, but
what happens when the voltage breaches the avalanche voltage if it is used in place of a common diode in a rectifier bridge? it breaks down and conducts? it is already conducting as it is?

apparently the newer automotive alternators use avalanche diodes, are they extra diodes put in along side common diodes or replacements?

if so how does this work?

TIA
bob g

 

[waross]

Google Zener diodes. They are probably to reduce load dump voltage spikes. Any over voltages will be limited to 29 volts. The energy will be mostly dissipated in the resistance of the alternator windings. This over voltage is only momentary until the voltage regulator can cut back and the rotor field collapses enough to bring the voltage down. Unclamped load dump transients may reach over 100 volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[brutus1955]

how does it limit the voltage?
do they stop conducting?

this is a 3phase alternator, so if they breakover into fulltime conduction is this where effectively the excess voltage just recirculates in the stator windings?

trying to get a better understanding of how this works, so bear with me

if the alternator was excited and/or spun fast enough to exceed the 29volts, then any extra voltage would just be contained in the windings of the stator and effectively start loading down the engine (that is if the engine was small enough, if it was sufficiently large and the rpm stayed up then the alternator would eventually go up in flames?)

this example would be with a fixed excitation no regulator.

so basically when the voltage exceeds 29volts the avalanche diode conducts both ways, causing a short circuit, and the over voltage spike is dissipated in the stator. such an event would normally be short in duration (the regulator would adjust downward the excitation current)so there would be no damage to the stator?

is that the theory of operation?

bob g

 

[itsmoked]

The avalanche function essentially causes any voltage over the Zener voltage to breakdown the diode into conducting constantly in the reverse or normally blocking direction. So the output won't exceed 29V or the normally forward conducting diode would start conducting in the reverse direction also.

I have not heard that alternators were have Zeners replacing the normal diodes. I would think that one Zener across the output would do the trick but in the bizarre world of automotive where insane trade offs are made to shave a fraction of a penny, perhaps just specifying the regular diodes as having a very low PIV,Peak Inverse Voltage is cheaper than another diode.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[brutus1955]

i have found one reference to a single zener/avalanche diode being used across the output that grounds the output to clamp the higher voltage spikes.

apparently all the modern alternators are using the avalanche diodes as a method of protection on newer vehicles where computers are prolific.

for the price they are building these alternators for these days (so cheap you can't overhaul one for the price of a new one) and for what is asked of the alternator (more and more amps), its a wonder they work at all.

thanks guys, i think i have a better understanding now what the theory of operation is.

bob g

 

[waross]

Small motorcycles used to use a single Zener diode as a voltage regulator. These were very small alternators. The alternator was designed so as to be able to dissipate the heat energy in its windings and the Zener had a high enough power dissipation rating to be able to live in the breakdown range. The Zener was often mounted on a heat sink near the front of the frame so as to get the maximum passage of cooling air.
Designers of auto electronics must consider and plan for load dump events. When a load dump occurs the alternator voltage may spike to over 100 volts before the voltage regulator can respond. Limiting load dump voltage at the source would both simplify the design of on board electronics but also provide designers with a fairly low, set target for the maximum voltage to expect from load dumps.

Bill
--------------------
"Why not the best?"
Jimmy Carter