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Power at 12v socket but nothing works..?


Neil2

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One for you electrical experts here, I'm trying to rig up a light in the cockpit where I have a 12v socket.  Tried plugging a light into it but doesn't work.  But when I tested the plug in the socket the multimeter reads a healthy 13.4v.  Two other sockets on the boat work perfectly, reading the same voltage.  I've tried plugging other 12v appliances  into the cockpit socket and none of them work.  How can I have a positive reading at the socket but no power?  The only difference between the cockpit socket and the cabin sockets is that the cockpit one is switched, so I conclude that is where the problem lies and I will try bypassing the switch but can anyone explain what is going on here?

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On further investigation it's a relay not a switch so presumably the relay allows the tiny current to power the meter but fails when I connect a 10w lamp??   I'm baffled, I guess the relay is at fault but I assumed a faulty relay would not allow any current through at all.  

 

 

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7 minutes ago, Neil2 said:

On further investigation it's a relay not a switch so presumably the relay allows the tiny current to power the meter but fails when I connect a 10w lamp??   I'm baffled, I guess the relay is at fault but I assumed a faulty relay would not allow any current through at all.  

 

 

Try removing or 'bridging across'(with a jumper lead) the relay and see if the socket then works with the light.

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It sounds as though you have a poor connection.  With effectively no load (voltmeter) it will read the full voltage, but as soon as you try and draw any current the connection breaks down.  Trace the cable back until you find the lamp works.

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1 hour ago, dor said:

It sounds as though you have a poor connection.  With effectively no load (voltmeter) it will read the full voltage, but as soon as you try and draw any current the connection breaks down.  Trace the cable back until you find the lamp works.

That's it. Volt drop across a resistance is current times resistance, the resistance is high but the current is functionally zero so the meter sees full voltage As soon as you try and draw current the voltage vanishes to reappear across the resistance. You know the old 20 bulbs in series christmas lights? take one bulb out and you can read 240V from the lampholder because in the absence of current flow the other lamps aren't dropping the voltage, fit the 12V bulb into the holder where you just measured 240V and suddenly it's 12V!

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3 hours ago, dor said:

It sounds as though you have a poor connection.  With effectively no load (voltmeter) it will read the full voltage, but as soon as you try and draw any current the connection breaks down.  Trace the cable back until you find the lamp works.

 

2 hours ago, Sir Nibble said:

That's it. Volt drop across a resistance is current times resistance, the resistance is high but the current is functionally zero so the meter sees full voltage As soon as you try and draw current the voltage vanishes to reappear across the resistance. You know the old 20 bulbs in series christmas lights? take one bulb out and you can read 240V from the lampholder because in the absence of current flow the other lamps aren't dropping the voltage, fit the 12V bulb into the holder where you just measured 240V and suddenly it's 12V!

Thanks you guys, I think I understand and I'm very grateful.   In my ignorance I just assumed the meter reading meant the current must be there.

 

I've just taken the switch panel off again and discovered the problem is with one of the connections to the relay which looked ok but was clearly making a bad contact.  

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Glad to be of assistance and, apparently, getting it right for once.   The really great thing about this forum is that there is always someone who has the answer to a problem.  The really bad thing is that there are also another nineteen people who think they have the answer, but don't.  The tricky bit is knowing which one of the twenty respondents is the one who is right.

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17 hours ago, dor said:

Glad to be of assistance and, apparently, getting it right for once.   The really great thing about this forum is that there is always someone who has the answer to a problem.  The really bad thing is that there are also another nineteen people who think they have the answer, but don't.  The tricky bit is knowing which one of the twenty respondents is the one who is right.

Yes it's nice when you get something that is an open and shut case like this even if it is less entertaining..

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