Alternator Load Dumping and Voltage Spike
Hi Jeff,
If you're trying to squelch load dump transients
there are a few things to consider:
1. The transient seeks to maintain the power
your load was drawing just before it was shut off; the higher the initial power
load the higher the transient,
2. The voltage generated during the transient will
be ain indirect function of the remaining load; the greater the remaining
load the smaller the transient.
3. 12vdc systems will have lower peak voltages
(~60vdc max) than 24vdc systems (~90vdc max)
4. Load dump transients are relatively long, many
milliseconds rather than microseconds.
5. To absorb the transient with transorbs or TVS's
requires big components or lots of them ($$).
6. To absorm the transient with capacitors requires
big capacitors (F not microF).
7. Failure of your suppression devices will cause
complete loss of the electrical system.
8. The best absorber is a battery and tight, clean
connections.
9. High voltage dropout of the alternator will
protect against load dump and regulator failure.
10. Robust connection between the alternator and
the battery with as few components and connections between them as possible is
the best protection.
11. An old battery, short on acid level, can act as
a open between the battery and alternator and lead to a load dump
transient.
12. As an alternative approach to protect a
sensitive load, you can use a three terminal linear voltage regulator (LDO
type); e.g. 7812. It will be non-regulating most of the time until the
transient occurs then it will prevent the transient from reaching the
load. One for each load will be cheaper than the necessary transorb.
The regulator will need heat sinking because the dropout voltage x the load
current will be dissipated continually. Also the devices have a grounded
substrate which eliminates any special treatment when affixing them to structure
directly. Place them on the load side of the circuit
breaker.
Jabe Luttrell
----- Original Message -----
Sent: Thursday, January 22, 2009 9:17
AM
Subject: [LML] Re: Alternator Load
Dumping and Voltage Spike
Jeffrey Liegner, MD wrote:
I recently experienced a sudden failure of my PS Engineering intercom
during the take off roll, Very interesting and timely
post. I have been having a similar experience.
My LIV-P has been
eating avionics. In 100 hours I've lost the PS Engineering audio panel,
AOA computer, encoding altimeter, one headset, and my electronic trim
controller board (Precise Flight)...The loss of trim was particularly
troublesome, it went out during decent and just before I needed to shoot an
ILS...fortunately to a high ceiling.
I have also discovered a nasty
power surge that occurs only when I shut off the low boost pump and then only
on some occasions. My first impression was that I might have a big back
EMF surge. There's no obvious surge when I have the high boost on,
though, but I've only attempted that on the ground at idle or slightly
above.
I have a couple TranZorb devices on their way from
Digi-Key. I'm concerned that they may not be nearly beefy enough to
withstand the surges that seem to be going through my avionics. And
exactly where do I put them? Across the leads to the fuel
pump?
Any thoughts about how the chase down this intermittent
gremlin?
LIV-P, Conti TSIO-550, 24V 100 Amp alternator with full size
battery, reserved and isolated backup 24V 100 Amp alternator and full size
battery.
Barry Knotts
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