Re: Amp Problems...Need Help

On Fri, 22 Jul 2005 17:23:04 GMT, MZ <mark@xxxxxxxxxxxxx> wrote:

>> I'm going with the idea that if you increase the supply voltage to the
>> headlamps, any voltage drop from a transient will be less-noticeable,
>> since the resulting power reduction will be smaller, percentage-wise.
>
>Nope, it's actually higher. Think of it this way: if the transients are
>an AC signal, then a higher level of AC will be transmitted to the
>headlights with lower resistance wire.
>


I agree that if some voltage is being lost between the battery and the
headlamps due to resistance, then any transient voltage drop at the
headlamp will be smaller than the transient at the battery. But I
still think that with low-resistance wiring to the headlamps, the
higher steady-state voltage will make up for the larger transient.

Here's the math I used:

low-resistance wiring -- 0.1 Ohms between the battery and the
headlamps, just to pick a number

high-resistance wiring-- 0.5 Ohms between battery and headlamps
(again, just picking a number here that's higher than the
low-resistance number).

Steady-state voltage at the battery - 15V, just to make the math
easier.

Voltage transient at the battery -- 2V.

Hot resistance of the headlamp bulbs -- 4.0 Ohms, so that they'll
draw about 55W from a 15V source voltage when they're on, assuming no
losses.

Let's look at the high-resistance circuit first:

Battery at 15V, headlamps + wiring are a 4.5 Ohm load. Current would
therefore be 3.33A, giving a voltage drop across the 0.5 Ohm wiring of
1.66V, for a final voltage at the lights of 15V-1.66V, or 13.34V.
This makes the power dissipation at the lights 44.48W.

With a 2-volt transient, supply voltage drops to 13V, current drops to
2.88A, giving a voltage drop across the wires of 1.44V, for a final
voltage at the lights of 11.55V, and makes the power 33.38W.

During the transient, power delivered to the lights would go from
44.48W to 33.38W, a reduction of 24.8%.


Now for the low-resistance wiring:

Battery at 15V, headlamps + wiring are a 4.1 Ohm load. Current would
therefore be 3.66A, giving a voltage drop across the 0.1 Ohm wiring of
..366V, for a final voltage at the lights of 15V-.366V, or 14.63V.
This makes the power dissipation at the lights 53.53W.

With a 2-volt transient, supply voltage drops to 13V, current drops to
3.17A, giving a voltage drop across the wires of .317 V, for a final
voltage at the lights of 12.68V, and makes the power 40.21W.

During the transient, power delivered to the lights would go from
53.53W to 40.21W, a reduction of 24.8%.


Conclusions:

The *percent* drop in power delivered to the lights is exactly the
same, whether you have high-or low-resistance wires. This disproves
my earlier prediction that the percentage drop would be lower with
low-resistance wires. But the percentage drop isn't any larger with
low-resistance wires, either. Turns out, the percentage drop is the
same either way.

You're correct that the *absolute* power drop is larger with the
low-resistance wiring (13.32W versus 11.1W) , but the question is,
which is more noticeable to the eye, a 25% drop in power given an
initial power of 53W, or a 25% drop in power given an initial power of
44.5W?

I admit that I made some assumptions to make the modeling easier. I
kept the resistance of the headlamps constant at 4 ohms, independent
of the power delivered to the headlamps. I'm also using power
consumed at the headlamps (Watts), rather than light output from the
headlamps (lumens).

I kept the headlamp resistance constant because I can't accurately
model the change in resistance due to temperature, and I used Watts
rather than lumens because I couldn't find a way to predict the output
in lumens based on the consumed power in Watts.

I still suspect that the 25% power drop will be less-noticeable to the
eye with the higher initial power, but without being able to
accurately predict light output given power consumed, I've gone as far
as I can go with modeling it.


--
Scott Gardner

"Never get your heart set on a peanut butter and jelly sandwich until you're sure there's jelly in the house." (Linus Van Pelt)

.

Relevant Pages

  • Re: Harddisk clicks -> System freezes/reboots
    ... No. Voltage and current are inter-related. ... If a power supply is losing ... In switching mode supplies, ... Capacitors must have high transient tolerance and yet have large ...
    (Fedora)
  • Re: Smoothing Capacitor Values
    ... That regulator doesn't have a high enough input voltage spec to survive ... power resistor to your circuit like this (view in fixed font or M$ ... The 1 ohm resistor will limit the current during ... when the battery is being charged) you won't need transient protection. ...
    (sci.electronics.basics)
  • Re: Surge Protection - Yay or Nay?
    ... A plug-in power strip and UPS protectors are shunt mode devices. ... They shunt that transient ... Protection inside computer power supplies bypassed because power ... then voltage transients inside the ...
    (uk.comp.sys.mac)
  • Re: Watson Power Supply
    ... They typically have a smaller transformer in them so that the output is ... I prefer linear power supplies.. ... You don't need a transformer to increase the voltage. ... as well as a power transformer, inductors, and filter capacitors. ...
    (uk.radio.amateur)
  • Re: Intensity and what else affects a single ligth beams temperature?
    ... now and the voltage now to compute the power now. ... you still measure or compute the current NOW and the voltage NOW. ... Then remove phi from the calculation and demonstate ... That is going to give 100 Watts at that instant in time. ...
    (sci.physics.relativity)