Capacitor in head light circuit to help with dimming?
Thread Starter
50 Watt CAFz'r
Joined: Jun 2010
Posts: 94
Stereo-dave
When I say electrical student I mean I completed a course called "Electrical Foundation". Not sure if you have it where you're from but it's basically a pre-ap to prepare students for the electrical trade. There is an electronics unit included but the majority of my electronics experience comes from the course I took in grade 11 and 12. Not saying I know a lot about this stuff but I'm just wanted to relay that I have an idea of what I'm talking about. And my prof used to be an electrician, not an electronics specialist. Also, class ended in July so I have no opportunity to ask any more.
and Dukk
Are you familiar with the formula: E=1/2CV^2?
This is the formula that I used to calculate the needed cap size.
I assume this:
E=energy in joules
C=capacitance in farads
V=voltage
Each headlight consumes 55W, so 55j/s.... say I want the cap to have enough energy stored to run the light for around 1/2 a second, let's say 30 joules.
(30)=1/2C(12)^2
Solving for C we get
C=0.417F
Is this even correct?
When I say electrical student I mean I completed a course called "Electrical Foundation". Not sure if you have it where you're from but it's basically a pre-ap to prepare students for the electrical trade. There is an electronics unit included but the majority of my electronics experience comes from the course I took in grade 11 and 12. Not saying I know a lot about this stuff but I'm just wanted to relay that I have an idea of what I'm talking about. And my prof used to be an electrician, not an electronics specialist. Also, class ended in July so I have no opportunity to ask any more.
and Dukk
Are you familiar with the formula: E=1/2CV^2?
This is the formula that I used to calculate the needed cap size.
I assume this:
E=energy in joules
C=capacitance in farads
V=voltage
Each headlight consumes 55W, so 55j/s.... say I want the cap to have enough energy stored to run the light for around 1/2 a second, let's say 30 joules.
(30)=1/2C(12)^2
Solving for C we get
C=0.417F
Is this even correct?
Administrator
Joined: Feb 2006
Posts: 16,860
That all looks right - although I have to admit that it's been 14 years since I graduated and looked at that formula so i had to do a little internet catching up 
A potential problem may be that the inrush current may be hard on whatever contacts provide energy to the circuit.
A potential problem may be that the inrush current may be hard on whatever contacts provide energy to the circuit.
, and which contacts are you referring to? The ones in the headlight relay?
Thread Starter
50 Watt CAFz'r
Joined: Jun 2010
Posts: 94
So I'm beginning to believe that the capacitors I'll need are going to be huge, considering that a 470uF cap charged to 12v doesn't even cause the light to flicker when connected.... Dukk you wouldn't have any 1/2F caps I could buy off you?
Administrator
Joined: Feb 2006
Posts: 16,860
All I have right now are a couple of 1F and a couple of 1/4F. Unfortunately I'm a plan for a rainy day kind of guy so I never sell anything I might need at some point..
There's a couple of 1/2f on ebay right now and, although I try to not promote WalMart for audio, I think they have 1/2f caps. Princess auto sometimes too.
There's a couple of 1/2f on ebay right now and, although I try to not promote WalMart for audio, I think they have 1/2f caps. Princess auto sometimes too.
Hi guys it's been awhile since I've posted on here. I have an idea of putting a capacitor in parallel with each of my head lights in order to stabilize the voltage when my subs hit. I'm a first year electrical student in college and as such I have a basic understanding of the physics going on here. I have a reasonably powerful alternator generating 150A and my calculated load for my amp is only 80A so I'm set there, but I still have momentary headlight dimming on strong kick drum hits.
Let's get this straight, these light dims are not sustained, only quick flickers. I think I can get rid of them with capacitors in the headlight circuit. The only question I really need help with is how to find the correct size of cap for this setup.
I plan on using one electrolytic cap on each light and I'm familiar with the formula for calculating energy stored in a cap.
Each light consumes 55W of power.
So using the formula for calculating capacitor energy, a .5F cap results in 51.84 joules of energy. Thats enough to sustain the light for almost 1s.
The light dimming might only be for a 10th of a second so might this be overkill? And won't a .5F cap be fairly large?
Is my thinking correct here? And has anyone else had any experience with this sort of thing?
Let's get this straight, these light dims are not sustained, only quick flickers. I think I can get rid of them with capacitors in the headlight circuit. The only question I really need help with is how to find the correct size of cap for this setup.
I plan on using one electrolytic cap on each light and I'm familiar with the formula for calculating energy stored in a cap.
Each light consumes 55W of power.
So using the formula for calculating capacitor energy, a .5F cap results in 51.84 joules of energy. Thats enough to sustain the light for almost 1s.
The light dimming might only be for a 10th of a second so might this be overkill? And won't a .5F cap be fairly large?
Is my thinking correct here? And has anyone else had any experience with this sort of thing?
estimate the R for each of these. Then you can calculate how fast the cap discharges. You will have to decide what an accepatable voltage is. Have you tried to put a scope on the battery? how low does it go?
Last edited by zoomer; Dec 15, 2012 at 10:47 AM.
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