Val Posted September 8, 2007 Share Posted September 8, 2007 Hi everyone, I've got the day off on Monday and I was thinking about doing some shopping. Does anyone know anywhere I can get light bulbs for the Saracen? The current bulbs (blown) are 28v even though the power supply is 24v, how does that work? Anyways, is there anywhere I can go and buy them from in London? I need headlamps Tail lights Indicators and instrument panel lights. As I've got the day off, I'd really like to get it out of the way Thankyou all Quote Link to comment Share on other sites More sharing options...
Tony B Posted September 8, 2007 Share Posted September 8, 2007 you can use a 28 volt bulb in a 24 volt system, it's like fuses. A 24 volt bulb is capable of taking more than 24 volts. The 24 volt is only a mean voltage they will charge at about 29 or 30 volts at full chat, just as a 12 volt system should charge at about 13.6. check that instrument lights aren't 12 volt. Some Saracens use a split voltage sytem. A truck or motor factor should have them. Quote Link to comment Share on other sites More sharing options...
andyroo Posted September 9, 2007 Share Posted September 9, 2007 Lord Lucas, prince of Darkness stock all the bulbs you'll ever need for a Saracen. Quote Link to comment Share on other sites More sharing options...
Val Posted September 13, 2007 Author Share Posted September 13, 2007 Habit of blowing I take it? Quote Link to comment Share on other sites More sharing options...
Tony B Posted September 13, 2007 Share Posted September 13, 2007 If they keep blowing check earths. If the circuit has extra resistance in it then heat will be craeated. Very common cause of blowing bulbs. Quote Link to comment Share on other sites More sharing options...
fv1609 Posted September 13, 2007 Share Posted September 13, 2007 If they keep blowing check earths. If the circuit has extra resistance in it then heat will be craeated. Very common cause of blowing bulbs. Not sure I understand that. With extra resistance due to a poor earth, I agree that heat will be generated at the point of the poor earth, but the extra resistance will mean less current flows through the bulb, which wouldn't make it more prone to blow. The most likely cause I would have thought would have been a fault on the generator panel. I once found mine was not regulating properly & giving an output of 30 volts! Quote Link to comment Share on other sites More sharing options...
Tony B Posted September 13, 2007 Share Posted September 13, 2007 The explanation is that Ohms law Voltage divided by ampage = restance. The more resistance, the more 'Work' the current has to do to get through, ie bad earth = more resistance. The more work the current does the greater the heat. All work in a physics sense, ie any change of energy from one state to another, ends up as heat. This results in the wire of the bulb getting to hot and blowing. It is also one reason wires overheat and burn. If the system is a 24 volt system then they should give an output of about 27/28 volt on full charge. If it's giving 30 volt I agree the system regulator is worth checking. Are they DC generated and Regulated or AC alternator, been a long time since I had anything to do with them, 30 odd years in fact, and that was mostly engines. A mechanical regulator, the springy thingy box should have an adjustment on it. Quote Link to comment Share on other sites More sharing options...
fv1609 Posted September 13, 2007 Share Posted September 13, 2007 The explanation is that Ohms law Voltage divided by ampage = restance. The more resistance, the more 'Work' the current has to do to get through, ie bad earth = more resistance. The more work the current does the greater the heat. All work in a physics sense, ie any change of energy from one state to another, ends up as heat. This results in the wire of the bulb getting to hot and blowing. It is also one reason wires overheat and burn. If the system is a 24 volt system then they should give an output of about 27/28 volt on full charge. If it's giving 30 volt I agree the system regulator is worth checking. Are they DC generated and Regulated or AC alternator, been a long time since I had anything to do with them, 30 odd years in fact, and that was mostly engines. A mechanical regulator, the springy thingy box should have an adjustment on it. Under Ohms Law if you increase the resistance & the voltage, which in this case is fixed by the regulator, then the current must fall, not increase. For example take a circuit with a resistance of 3 ohms & a voltage of 24 volts applied then it will draw a current of 8 amps. If the resistance was increased to 4 ohms with the same voltage of 24 volts then the current falls to 6 amps. So if the current falls then less heat is generated, so less likely to blow the bulb. If what you say is correct Tony, if you ran a 24 volt bulb on 12 volts it would be more likley to blow. The bulb doesn't 'know' why it has a reduced voltage whether comes from a 12 volt battery or the effect of a 24 volt supply but subjected to resistance in its circuit from a poor earth. Quote Link to comment Share on other sites More sharing options...
Tony B Posted September 13, 2007 Share Posted September 13, 2007 Agreed. Practicality wise bad earths are responsible for a lot of vehicle electrics. Trouble is when you stick electricerry in a vehicles a whole new world of hurts opens up. My first suggestion is to go over the control system from generator back to batteries and check the voltage at every connection and joint. Bloody tedious but can show up some bizarre problems. The problem is the voltage drops. Therefore bulb or whatever tries to compensate by taking more amperage, it only 'Knows' it wants to do the work. As the gadget can't cope with extra ampage it blows. For explanation to anyone whose baffled, Think of ampage as the amount of current, IE a gallon, not exact because an Amp is 1 Coulomb per second. If you want the definition of a Coulomb ask 'cause you really don't need it. Voltage is the pressure, say lbs per sq inch. throw a gallon of water over someone it will get them wet. Shove it through a pressure injector at 2000lbs per sq inch you can strip paint. If an Amp at 1 volt runs through a resistance of 1 ohm you get 1 watt of power. Haynes do a very good manual on auto electrics. Mechanically electrics on a vehicle are not difficult, you just need to know a few basic. Every one should carry a cheap multi meter its as important as a spanner. When God made electricity he didn't let us see it. having read it again. The extra ampage, is like trying to force more water through a pipe then it can take and bursts. I realise this is a ramble, but my physics teacher was a genius he got me to understand, I'm not. Quote Link to comment Share on other sites More sharing options...
fv1609 Posted September 13, 2007 Share Posted September 13, 2007 The problem is the voltage drops. Therefore bulb or whatever tries to compensate by taking more amperage, it only 'Knows' it wants to do the work. As the gadget can't cope with extra ampage it blows. I agree that with a poor earth, it adds resistance to the circuit & the voltage across the bulb changes ie goes down. By Ohms Law, as the resistance of the bulb is fixed, as the voltage across the bulb goes down, so must the current decrease, it cannot increase. But the voltage across the whole circuit (wire + bulb + poor earth) is fixed by the regulator & cannot change. I don't see how the bulb could draw more current? All 1-Ton Humbers have a 24 volt supply & of course all bulbs including turnlights are rated at & supplied by this 24 volts. However for improved reliability the turnlights on the Hornet are rated at 12 volts & supplied by a resistance fed from the flasher. Sometimes I forget this, if a turnlight bulb is damaged, I have made the mistake of fitting a 24 volt bulb. The bulb glows dimly, it doesn’t glow to full brightness because feels it should demand more current. In fact when I have left a 12 volt bulb in place for a while it would seem to be less prone to blow. This wrongly fitted 24 volt bulb doesn’t know why it has a reduced supply whether it is deliberate resistance or resistance through a poor earth. I agree that wires around a poor earth will get hot & can even melt. The heat from the poor earth like any resistance is proportional to the square of the current times the resistance. This heat is at the point of the poor earth but will be conducted thermally along the wire feeding it, getting cooler the further from the poor earth. The bulb that it is associated will be no hotter because of electrical activity from within the bulb. But as you say the practical points are to make sure you have a damn good earth & establish that you have the correct voltage from the regulator & see what happens to it after that. PS I wrote all this waffle an hour ago, went to preview, wouldn’t let me preview, try to go back then whoosh its all gawn. So did this in Word then pasted it, not getting caught out again! Computers ggrrrh :computerterror: Quote Link to comment Share on other sites More sharing options...
Dougiebarder Posted September 13, 2007 Share Posted September 13, 2007 It sounds like your next article need's to be on basic electrical theory -anything electrical seem's to be a black art to most people :whistle: Quote Link to comment Share on other sites More sharing options...
Dougiebarder Posted September 13, 2007 Share Posted September 13, 2007 I can't resist :-D adding that by that theory, when the light's were switched off (infinite resistance) You would also have infinite current-you might need to add some more batteries unless you can leave your light's on all the time! :dunno: Quote Link to comment Share on other sites More sharing options...
AlienFTM Posted September 13, 2007 Share Posted September 13, 2007 Back on topic (bear with me - I will eventually). A couple of years ago I was driving through Welsh Wales isn't it and within 19 miles of setting out, a police car flagged me down for a failed brake light. Being an honest, law-abiding man, I pulled into the first service station I found. There weren't many open that late at night. The PC had clearly hoped to get me for drunk driving at that time. Tough. None. The Police in Welsh Wales isn't it are clearly far more efficient than some I could name - without wishing to draw attention to myself. Next service station I found some. Paid, out the door, pop the boot, the sales assistant runs out. "You've got 24 volt bulbs. Anything in a green packet is 24 volt. Shall I make you a refund?" Try your nearest service station that might handle trucks. Quote Link to comment Share on other sites More sharing options...
Dougiebarder Posted September 13, 2007 Share Posted September 13, 2007 I got some from this place http://www.tools-supplies.co.uk/ And do a search for bulbs I don't know how the prices compare, but the postage is included :-o Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted September 15, 2007 Share Posted September 15, 2007 The problem is the voltage drops. Therefore bulb or whatever tries to compensate by taking more amperage, it only 'Knows' it wants to do the work. As the gadget can't cope with extra ampage it blows. Sorry Tony, have to disagree on this, regarding your comment on bulbs drawing more current if there is a bad earth or resistance in circuit, then blowing. A bad earth / resistance is akin to a dimmer switch in a light circuit....it reduces the current flowing through, the bulb will not try to glow brighter, it cannot. So bulbs do not blow due to resistance, it is normally rough treatment, ie vibration, badly mounted lights or poor quality bulbs that are the cause. This assuming the voltage has not risen, but this is a very rare fault and not a common cause of bulbs blowing on vehicles. I have to agree with Clive on his reply and suspect he is, at this moment, penning a new article for Windscreen, on fault finding in vehicle lighting systems. ;-) Quote Link to comment Share on other sites More sharing options...
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