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1 Farad Capacitor to Eliminate Headlight Flicker?

Oct 19th 2011, 13:14


Super Moderator

Joined: Apr 4th 1998, 00:00
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A member asks:

Now that I have installed a 500W amp in my mobile I'm getting headlight flicker at night (most likely in the day as well if I could see them) even though I'm loading the amp to only 300W. The mobile runs a 200A alternator and two very large batteries but apparently it cannot respond fast enough so I guess it is time for a 1 Farad cap. Any installation recommendations? I would guess it would be better for the amp if the cap were installed near the amp's power input but it would be easier to install one midway between the amp and the alternator at the positive terminal of the battery where the amp power line is connected. How about a limiting resistor for initial cap power up? What value? Anything I should avoid at all costs?
Oct 20th 2011, 01:29


Joined: Mar 6th 2008, 13:50
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The big capacity may help the situation by storing a lot of current capacity near the amplifier during the highest voice current peaks.
Just some technical comment; the alternator regulator may be poor at controlling alternator output at faster audio frequencies rates which it was not designed to do.
To get the capacitor into the circuit initally, you need to provide a series resistance to create a time constant till the capacitor plates get at least 60% charged, otherwise the battery will see it as a nearly a dead short and blow the safety fuse at the battery for the amplifier.
A standard tail lamp should be enough and you can see when the lamp begins to dim, then do the connect.
The lead from the alternator to the battery may have to much drop contributing to the issue. such that the lights and rest of the vehichle are responding to the poor system regulation.
Make sure your lead sizes is large enough to handle the peak currents, the grounds are also heavey and all connections are as heavey.
I would not expect any system to be flicker free down near or just above engine idle because the alternator just is not able to output the required current at those low rpm's.
Most of the time after cranking, a battery will accept nearly 40 amps for inital recharge before tapering down.
Your amplifier at 600 watts would be drawing near 50 amps or more on voice peaks plus the driving radio current and the rest of the vehicle needs. So you can see where the flicker might be seen at night.
Another help might be to reduce the alternator pully size so it turns faster at the lower rpm and can output more current
Good luck.
Nov 3rd 2011, 20:02


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I don't know if you've actually purchased the cap yet, and if you haven't, don't!

I do realize lots of folks use them for the same purpose, and manage to get by just fine. But, to be really safe you need to fuse the cap itself, and there really should be a protective diode across the cap. Part of the issue is voltage spikes. If you ever jump start the vehicle, it is possible to over voltage one. If you do, they have this tendency to unglue, and you can imagine what the results look like.

A nominal 500 watts amplifier (SC500) will draw about 90 amps peak, plus another 10 amps or so for the transceiver. This will lower the voltage to about 11.9 volts or so on peaks. It should obvious that the lights will flicker, and it doesn't matter how big the alternator is, or its voltage regulation strategy.

If it bothers you, then install an AGM battery, as close to the amp as you can. You'll still get some flicker, but it won't be as evident.
Nov 9th 2011, 01:50


Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
K0BG is a bit shy, it seems, so I'll post it for him. The site at is an absolute treasure of information about mobile operation. It's well worth the read.

Alan, feel free to add your URL as a sig block to any posts that you make about mobile operation and/or to refer members to your site!

73, Ed Hare, W1RFI

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