Mailing List lml@lancaironline.net Message #50193
From: Jabe Luttrell <JabeLuttrell@comcast.net>
Sender: <marv@lancaironline.net>
Subject: Re: [LML] Re: Alternator Load Dumping and Voltage Spike
Date: Fri, 23 Jan 2009 17:47:10 -0500
To: <lml@lancaironline.net>
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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