LED bulbs

..or at least be sporting enough to invite us to watch. Steve

On or around Thu, 03 Aug 2006 15:56:21 +0100 (BST), snipped-for-privacy@zhochaka.org.uk ("David G. Bell") enlightened us thusly:

22 megajoules... hmmm. joules/sec are watts, ain't they?

so you could get 22Kw for 1000 s... bit more than quarter of an hour. So you can't quite replace the engine with it.

Just been thinking back... we had a period at school of making booby-trapped not-light-bulbs using small low-voltage electrolytic capacitors wired into a

240V bayonet socket. Make a lovely bang, they do :-)

Yep.

It'll happen.

Steve

In parallel with the light clusters, there's not a heck of a lot of point doing it near the battery.

Could there be any other side effects fitting one?

Seriously, a 10 Farad one, as used by car hi-fi freaks, for similar reasons.

If you want to try per-cluster, then a 470uF would be bearable.

Steve

Must be a global phenomenon - we did the same.

I am tempted to suggest it to one of those stupid science tricks programmes on TV.

Car Hi Fi uses massive capacitors in order to get very good low frequency performance from the power amps rather than to get rid of spikes (for the same reason as using very heavy gauge wiring).

I assumed that the original contributor's contribution was meant to be

10uF, not 10F! A 10F capacitor would be a bit expensive in relation to the bulb!!!

What you do when you use a capacitor to get rid of spikes is effectively to use it as a low pass filter, where the R or L component of the filter is formed by the output impedance of the voltage source (i.e. the alternator, battery, wiring). The effect of the capacitor in removing the voltage spike thus depends on this impedance, which is very low for a car battery and associated wiring. Thus, a large capacitor is needed to get rid of spikes in car electrics.

If you want to go down the capacitor route, rather than use a massive capacitor, it would be more economical to use an inductor (coil) in series with the +12V power to the +12V on the bulb as well as the capacitor across the bulb, thus forming a conventional LC filter.

+12V --------| coil |-------------------------- | | Capacitor Bulb | | GND--------------------------------------------

Another alternative would be to look at using a semiconductor spike suppressor across the bulb. Have a look at Rapid Electronics (

) part number 26-3812. A bitcheaper, too!

Going back to the capacitor size issues, a simple capacitor consists of two conducting plates separated by an insulating material (called the dielectric). The capacitance is proportional to the area of the plates but inversely proportional to the thickness of the dielectric. Real life capacitors are made in a number of different ways (usally by rolling up the "plates" and dielectric to get a big area in a small volume). The capacitance for a given physical size thus gets higher with a thinner dielectric, but, of course, a thinner dielectric breaks down at a lower voltage. Hence the tradeoff between capacitance and maximum working voltage for a given physical size.

The capacitor is the quickest and simplest tryout. Your inductor has to carry the lamp DC without saturating, and you need one with each cluster.

Personally, I'd scope the 12V supply and see what's happening, but you have to have access to a scope....

That's a good way to do it.

2.5 V 2500 Farad is the biggest I've found so far.

Steve

On or around 4 Aug 2006 04:31:41 -0700, "ArthurC" enlightened us thusly:

newly-acquired motor here has an interesting glitch - if you turn the stereo (quite a nice JVC with 12-disc changer) up too high, it cuts out, then in again, then out again...

I speculate that the power supply into the head unit isn't up to it - too much volume = too much current from the supply, leading to voltage drop which makes the unit think the ignition's been turned off. Then the voltage recovers, and it turns back on again, voltage drops...

On or around Fri, 04 Aug 2006 13:11:25 +0100, Steve Taylor enlightened us thusly:

There was one at the school I went to some years ago (1977-1984) which was part of the emergency lighting system I think. 0.5F and presumably about

24V.

Course, have you seen these mega-amp current pulse jobbies used to shrink metal objects.

Steve

You're laughing then, because there won't be any nasty horrors coming from an alternator/engine combination.

Steve

I'm using LEDs from both the manufacturers on my boat which has no charging system other than small solar panels. So far I have had no problems with the bulbs running at 12v direct from a battery.

TonyB

Thanks for that. The transient voltage suppressors sound like the easiest idea to try. Would I need one for each bulb or could one be used for a few bulbs? I have the side lights on the Land Rover wired in three circuits, LH side, RH side and dashboard. Also I see there are Unidirectional and bidirectional ones available, would the unidirectional ones be more suited to DC voltage? Just had a thought would these suppressors be suitable to replace the regulator unit for the fuel gauge? mine is working fine, just for future referance.

Thanks Liam

Check how it's been earthed - I've seen a few that are "earthed" onto a wire that isn't an earth and that leads to this sort of problem.