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Fusing Regulators


Many boats are wired with a battery selector switch that is used to select which battery will receive alternator output. Somewhere there may be posted a warning not to turn the switch off with the engine running.

If the switch is turned off with the engine running, a high voltage transient, load dump, will result, and some electronics will fail.

As we discovered long ago, the switch doesn't need to be turned off to open circuit the alternator, which is why we recommend re-wiring them to be strictly a parallel switch. See Burnt Switch

There are other ways alternator current can be interrupted, and even momentary interruption will cause high voltage transients. Loose wires at the alternator will cause load dumps, or any loose connection at the battery, positive or negative.

Consider this situation: the alternator is putting out 100 Amps at 14 Volts. That's 1400 Watts of power being generated. When the switch is turned off, that 1400 Watts doesn't immediately stop, just the current. Say the current doesn't go to zero, but instead goes to 0.1 Amp. With 1400 Watts and 0.1 Amp, the voltage would be 14,000!

Of course it won't really go to 14,000 Volts because diodes in the alternator, and any attached electronic equipment will fail, allowing more current to flow, and thus limiting the maximum voltage. However, the transient may be a hundred Volts or more for several hundred milliseconds. See Load Dump


As the article on Wikipedia notes, some protection against load dump transients can be provided.

In the past Ample Power sold a device called a snubber that connected across the terminals of the alternator, where it would do the most good. They were labor intensive to make, and even at a break-even price, not enough were sold to justify the expense.

However, Ample Power regulators have a built-in snubber between the B+ input and the alternator ground terminal. If the regulator is wired per instructions there will be a fast-blow fuse between the power source and the B+ input. The snubber should breakdown as designed and carry current long enough for the fuse to blow. How long is that?

Fuse Information

See Fuse Data Under the section on Speed note the difference between fast-blow and slow-blow fuses. For the same current, the fast-blow may take 0.1 second versus tens of seconds.

For the NS2 regulator, an 8 Amp fast-blow fuse is required. We might expect it to blow at 16 Amps within the duration of a typical load dump, however there is more to consider.

See the section labelled $I^2T$. The time to clear, (blow) a fuse is related to the energy dissipated in the fuse, which has time as a factor, and also the square of the current.

Will the Regulator Survive a Load Dump?

Maybe. The engineers who design Ample Power regulators take load dump as a serious issue, for instance, incorporating an on-board snubber. The snubber is placed immediately after the terminal block which connects B+ and alternator ground.

If a fast-blow fuse of the correct value is connected in series with the B+ wire, there is a reasonable chance of surviving a load dump. If a slow-blow fuse is used, there is probably no chance of surviving.

Why Did a PCB Trace Fail?

There are many small PCB traces on a regulator, which will not survive an internal short circuit. Fortunately integrated circuits are very reliable and internal shorts between supply rails are virtually non-existent.

If a wide trace fails in the field drive circuits, which includes B+ and alternator ground, then a load dump has occured. A regulator does not generate energy, so there is no chance that an internal failure can cause a wide trace to blow. The energy to do that has to come from an external source, i.e. the alternator or battery.

Suppose the snubber that connects between B+ and ground develops a short circuit? That's not a problem because the external fast-blow fuse will open up very quickly. A slow-blow fuse would likely fail before a wide PCB trace would reach temperature to open up.

But because of the $I^2T$ phenomena even a fast-blow fuse may survive a load dump long enough to allow a wide trace to open up. If a wide trace has been burned open and the fuse is OK, load dump is the only plausible answer.

Prevent Load Dumps

Get rid of the battery selector switch by re-wiring it to be parallel only.

Periodically inspect all alternator and battery connections to assure they are tight.

Fasten wires at the alternator so that the connections vibrate with the alternator rather than flex separately.

Use fast-blow fuses when specified.