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ContinuousWave: Small Boat Electrical
Fuse blowing mystery
|Author||Topic: Fuse blowing mystery|
posted 08-15-2011 07:50 PM ET (US)
I have a frustrating fuse mystery I am hoping someone can help to solve.
I have a Lowrance LMF-200 fuel flow gauge that is wired to my battery switch via a dedicated on-off switch and fuse. In addition to that gauge, I also have a GPS and VHF, each direct wired to the battery switch with dedicated fuses.
This setup has worked flawlessly for five years and I have made no recent changes to the boat.
Last week, the LMF-200 fuse blew when I cranked the engine; none of my other fuses blew. I replaced the LMF-200 fuse, started the engine, and the fuse blew again. Replacing the fuse a second time, I discovered that if I turn the LMF-200 switch on *after* starting the engine the fuse does not blow.
Why am I experiencing this problem all of the sudden? What can I do?
Two other pieces of the puzzle that may be relevant: the engine is a 92 Johnson J100STLEND—I believe this outboard does not have a regulated charging circuit; my battery is a starting model and is about five years old.
posted 08-15-2011 08:47 PM ET (US)
Your problem is likely related to some sort of unanticipated circuit--a "sneak" circuit--which is occurring during engine starting where the current for engine starting is being supplied in some way by the circuit which is powering the LMF-200 gauge. This sort of sneak circuit can occur if there is a higher than normal resistance in the normal circuit that supplies current during engine starting.
Check that the battery negative circuit for engine starting is making solid, low-resistance connection to the engine. Check that the battery positive circuit to the engine solenoid is making solid, low-resistance connection.
What can happen: if there is some resistance in these normal circuits, some of the engine starting current will try to flow on other paths. There must be something odd about the wiring to the LMF-200 gauge where it is becoming part of the engine starting circuit in place of the normal conductors in the engine starting circuit.
posted 08-15-2011 10:17 PM ET (US)
Describe the appearance of the blown fuse. Is it completely disintegrated? Or just barely failed into an open circuit? This will be an indicator of how much current flowed.
posted 08-15-2011 10:35 PM ET (US)
It is an ATC fuse and it appears to be a normal 'blow'-- maybe 2mm gap.
posted 08-15-2011 10:42 PM ET (US)
If the fuse is just barely blown, this indicates only a small over-current situation. This could be caused by the following:
--the battery has a higher than normal internal resistance
--during engine starting there are voltage transients created by the inductive kick of the electric starter motor
--the battery no longer is able to effectively dampen these transients, resulting in a higher than before voltage transient occurring during engine cranking
--the higher than normal transient causes the LMF-200 to draw more current
--the current is just enough to blow the fuse.
What is the current rating of the fuse in the circuit to the LMF-200 that is blowing?
posted 08-15-2011 11:13 PM ET (US)
posted 08-16-2011 09:08 AM ET (US)
Analyze the circuitry for possible sneak circuits. You might try a slightly larger fuse, a 5-Ampere, to see if the current blowing the fuse is marginally more than 3-Amperes or much higher.
posted 08-16-2011 12:44 PM ET (US)
How about checking your connections of your LMF-200, maybe after 5 years with exposure to salt air the connections may have deterioated. Thats what I 'd check.
posted 08-16-2011 03:56 PM ET (US)
@Eagleman: I'll check that, but the curious correlation with starting the engine would seem to make that an unlikely source of the problem.
posted 08-16-2011 04:49 PM ET (US)
Bad connections due to corrosion from salt air usually tend to have higher resistance. Higher resistance reduces current flow. It is more likely that there is a high resistance connection in the engine starting circuit, and, as a result, the circuit feeding the LMF-200 is being called on to supply some current during engine starting.
posted 08-21-2011 10:27 AM ET (US)
IMHO your fuse blowing is a consequence of
In my mind, what is happening is that your old battery is no longer capable of discharging the high starting current asked of it without suffering greater voltage drop than when it was newer. This greater voltage drop applies lower voltage to whatever capacitance load may be in the LMF-200 circuit, resulting in the high initial charging current of the capacitance to be drawn for longer in order to charge the capacitance. The longer duration high current is just a little more than the fuse can stand, so it blows. Were this situation normal, a slow blowing fuse would be called for. A higher applied voltage results in quicker charging, hence the standard fuse is not heated enough to blow, for example when your battery was newer or when your engine is running before you switch the load into the circuit.
The idea of a "sneak" circuit, while attractive and even plausible, does not seem likely in this case, given that the branch circuit is
quote:Since these connection points for the branch circuit are likely at the same point electrically as the starting motor, it is difficult to see how a higher potential could exist to drive starting current through the branch circuit.
I would also agree that jimh's hypothesis is plausible:
Our common thoughts that the battery is old/has higher internal resistance, and the LMF-200 draws more current, just enough to blow the fuse, perhaps point to the root cause being that your battery has aged to the point that replacement is warranted?
Mystery solved? Maybe, maybe not, but you can always start your engine before turning on your LMF-200 if you don't want to replace your battery at this time.
Good luck and aloha.
posted 08-21-2011 04:11 PM ET (US)
Thanks to HawaiianWhaler for the added comments. The notion that a device like the LMF-200 would draw more current as the supply voltage was lowered is an interesting idea. I am not ready to agree with it. However, the notion that the fundamental problem is related to the battery health is a reasonable inference. It is also coherent with my general rule of trouble shooting with battery-operated devices:
When there is trouble with a battery-operated device, the first component to investigate and rule out as the source of the trouble is the battery.
posted 08-22-2011 11:34 PM ET (US)
Your amperage will increase in proportion to the voltage dropping AC or DC it's all the same. We see this at work many times even in high voltage. Actually both guys have given excellent and plausible ideas, I would eliminate the battery as the problem by hooking up jumpers to your car, then try starting the boat, having a test meter in the circuit will also show the voltage drop.....Jack
posted 08-23-2011 08:59 PM ET (US)
Most loads are what we call linear loads . A linear load acts like a resistor. When the applied voltage increases the current drawn by the load also increases. A load which acts in the opposite manner--which is what has been proposed here for the LMF-200 as a load--is known as a negative resistance load. That means that the current drawn by the load will increase as the applied voltage decreases. This is the inverse of a resistive load.
This sort of load is commonly seen in devices which are powered by switching power supplies. A switching power supply does not care particularly about the input voltage. If the switching power supply needs to make 200-watts of power for its output, it will draw a certain amount of power from the input in order to produce the output power. As the input voltage decreases, the switching power supply will draw more current to make up for the decreasing voltage. These negative resistance loads have become a problem for power generating utilities who can no longer shed load by decreasing voltage. Many modern devices are operated from switching power supplies that act like negative resistance loads.
However, I don't think that a LMF-200 is likely to be a negative resistance load. At least not with enough effect to cause the normal fuse to blow.
posted 08-24-2011 08:30 PM ET (US)
This thread is a good discussion which has the potential to increase the knowledge of those who choose to read and understand it. My additional comments which follow are offered in that vein and are not meant to be argumentative but rather explanatory.
My hypothesis, offered above as a possible explanation of "the mystery", is based on the possibility of there being some capacitance in the LMF-200 internal circuitry. While many circuits and devices utilize capacitors as filters, damping, or energy storage devices, none of us here knows with certainty the internal design of the LMF-200, hence all possible explanations offered for "the mystery" remain hypotheses.
So, while it is a fact of physics that V=IR, that is only absolutely true in purely resistive loads, what jimh refers to as linear loads. A capacitor, however, is a component of a non-linear sort when it comes to its charging characteristics. When voltage is first applied to a capacitor, current flow will be high, then taper off over time, as the capacitor charges up. This characteristic is, for a short while, akin to the negative resistance load that jimh refers to.
It is this charging characteristic of a capacitor which forms the basis of my hypothesis addressing "the mystery." I hypothesize that the lower than usual voltage (from the five-year old battery) must supply this higher initial charging current for a longer duration than a higher voltage (i.e. a newer battery) in order to charge the capacitor. I further hypothesize that the longer duration of high current is sufficient to just blow the fuse - a small overcurrent situation. If the duration of the initial high charging current were shorter, the fuse would not heat sufficiently to blow.
We know not the internal circuitry of the LMF-200 hence know not whether there is capacitance or to what extent, hence cannot determine initial charging current or its time duration. Consequently my hypothesis remains just that, albeit, I believe, a plausible one. For more information about the characteristics of charging capacitors and even the mathematical formulas for calculating such, visit this link (especially see the voltage vs. time curve):
When troubleshooting a newly developed problem, it's advantageous to determine what has changed from the normal. In the case of "the mystery" I believe we all can agree (or at least jimh and I do) that the battery has aged, making it a likely root cause, no matter which hypothesis may best explain its effect.
posted 08-24-2011 08:35 PM ET (US)
postscript: No disrespect to Bulldog, but I would suggest that using jumper cables from a car battery might not be the most effective way to diagnose "the mystery" given the likelihood of voltage drop in the cables replicating or exceeding the internal voltage drop of the five-year old battery. Just another of my 2¢ contributions ;).
posted 08-25-2011 11:50 AM ET (US)
Re the suggestion that a capacitive input is affecting the fuse. I don't buy it. Here is why:
The time for a capacitor to charge is defined by the time constant which is proportional to the capacitance-resistance ratio, or RC-ratio. It is not proportional to voltage. Voltage does not affect the RC time constant. If the applied voltage is lower then the voltage to which the capacitor must be charged is lower, too. The charging current will be lower.
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