P38A ALTERNATOR FAULT

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Wazzajnr said:
Higher voltage means less current flowing.......... Really.
If you're going to quote ohms law, at least understand how it works!!!
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I think you both need to forget Ohms law on Martys statement.........consider power and its transmission.

P=IE

Power is a product of Voltage and Current, Doube the Voltage, Half the current, or Double the Current and Half the Voltage to give the same power outputs.

The idea is that a different load is used with the different voltages. As a simple example:

  • on 120V: a 120V/120W lightbulb is 120 ohms (when hot) and draws 1 amp.
  • on 240V: a 240V/120w lightbulb is 480 ohms (when hot) and draws 1/2 amp.
In both cases, the load is 120 watts but by doubling the voltage, we get away with 1/2 the current, allowing smaller wires.
 
Ohms law is perfectly applicable.
If you have, let's say, a fuse box (to use the OP scenario) and you are worried that increasing voltage might damage it, then theoretically that could be true.
In other words, if the resistance stays the same (which it will unless something starts to burn out) and you then increase the voltage, then the current will increase, not decrease.
In your example, the resistance changes which is fine, but upping the alternator output on a car won't directly change the resistance so effective power of every electrical component will increase.
Saint.V8 said:
I think you both need to forget Ohms law on Martys statement.........consider power and its transmission.

P=IE

Power is a product of Voltage and Current, Doube the Voltage, Half the current, or Double the Current and Half the Voltage to give the same power outputs.

The idea is that a different load is used with the different voltages. As a simple example:

  • on 120V: a 120V/120W lightbulb is 120 ohms (when hot) and draws 1 amp.
  • on 240V: a 240V/120w lightbulb is 480 ohms (when hot) and draws 1/2 amp.
In both cases, the load is 120 watts but by doubling the voltage, we get away with 1/2 the current, allowing smaller wires.
Click to expand...
 
Both correct.

Anything drawing power should draw less current. However, bar leakages there should be no current flowing unless something switches on, other than the battery? Battery is before fusebox, I think. So, battery should get higher volts and higher amps. Anything leaking in the fusebox will, what? Leak twice as much current or draw half as much current?!
 
Grrrrrr said:
Both correct.

Anything drawing power should draw less current. However, bar leakages there should be no current flowing unless something switches on, other than the battery? Battery is before fusebox, I think. So, battery should get higher volts and higher amps. Anything leaking in the fusebox will, what? Leak twice as much current or draw half as much current?!
Click to expand...
Anything that is drawing power at all will draw more if you increase the voltage.

So, for example, a 55w headlamp bulb will draw around 4.5 amps at around 13 volts, at 14.5 volts, the same lamp will draw 4.8 amps. (my numbers might not be spot on because I can't remember if 55w is nominal at 12v.i think it is)
Anyway, the fact remains that if you increase voltage and nothing else changes, everything will draw slightly more current and hence the power dissipated will be greater.
 
Wazzajnr said:
Anything that is drawing power at all will draw more if you increase the voltage.

So, for example, a 55w headlamp bulb will draw around 4.5 amps at around 13 volts, at 14.5 volts, the same lamp will draw 4.8 amps. (my numbers might not be spot on because I can't remember if 55w is nominal at 12v.i think it is)
Anyway, the fact remains that if you increase voltage and nothing else changes, everything will draw slightly more current and hence the power dissipated will be greater.
Click to expand...

And hence heat.

Think I'll invest in a fire extinguisher. Bl**dy stupid not having one in a roving fire-trap anyway. Think it is a directive in many European countries.
 
My apologies - you are correct... I was thinking more of power, as in Saint V8's post, as most of my work involves AC power for lighting systems where a fixture is a given wattage at a specified voltage. And given that the current tour I'm on is for an American production company and they have a bunch of 120V stuff needing step down transformers, and our touring lighting gear being all local on 220V my mind has been more on that and making sure they don't blow their equipment up (which some of them still manage to do!) rather than low-voltage DC and resistance.

As a random aside - I haven't seen one P38 in the nearly 8 weeks I've been out in China either....
 
Grrrrrr said:
As in the magic dragon? I'm sure modern tours are different and it is all mineral water ... and designer drugs.
Click to expand...

Can you repair an alternator with mineral water and designer drugs? I found fitting a new voltage regulator and truing the pickup rings in the lathe sorted mine. ;):D:D
 
I ordered the voltage regulator. Yesterday I had a look to see why it hadn't arrived and only then noticed it is being posted from Lithuania! Should be here next week.
 
mine used to do that regular till it just wouldn't start at all like it had no battery found out the fault was the brass battery bolt had threaded and would work its self loose now and again new bolt on and soldered the red wire on its been great ever since
 
mutsnutz said:
mine used to do that regular till it just wouldn't start at all like it had no battery found out the fault was the brass battery bolt had threaded and would work its self loose now and again new bolt on and soldered the red wire on its been great ever since
Click to expand...

I'll watch out for that. Strangely it seems to have cured itself. No message for days. Makes me think loose connection.
 
Grrrrrr said:
I ordered the voltage regulator. Yesterday I had a look to see why it hadn't arrived and only then noticed it is being posted from Lithuania! Should be here next week.
Click to expand...
Hi,
I too want to join. From where did you purchase the v/regulator? seller's link pls.
 
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