12V versus 14V conundrum

[GeorgioB]

Hello - I have just tried to install some LED reading lamps labelled as 12V DC to my motorhome. Initial installation and the first one installed didn't work. I tested power to the lamp terminal connectors and got 14.29 volt DC reading, so power from my 2 leisure batteries which seems healthy but also correct. So I removed the new lamp, jury rigged longer wires and tested it across my other vehicle car single battery (output 12.25 volts DC) and they worked. I have absolutely no technical specification data on these new lamps (supplied from Far East country) and now I am puzzled as to whether the input voltage is too high? If so anybody got any recommends on a suitable "flexible stalk light type" reading lamp that would be compatible and work on an input of 14.29V?
Many thanks George

 

[danadak]

No way of telling if 14.29 exceeded limits or why circuit failed to operate
w/o a schematic, part numbers, etc..

You could always use a LDO regulator, simple 3 pin, and a heat sink (depending
of lamp current drain) to get your 12V.

Regards, Dana.

 

[GeorgioB]

Thanks for your reply. The circuit is a common two strand (live and earth) direct DC feed from 2 leisure batteries via a lighting master switch and 15 amp fuse. so I am not sure how easy it would be to install any sort of voltage regulator + heat sink. As you say without specifications you are blind, as am I, hence why I am busy interrogating various supply options to find a compatible "stalk" lamp that will work on the approx 14 volt input - which is proving quite hard!

 

[Nigel Goodwin]

Thanks for your reply. The circuit is a common two strand (live and earth) direct DC feed from 2 leisure batteries via a lighting master switch and 15 amp fuse. so I am not sure how easy it would be to install any sort of voltage regulator + heat sink. As you say without specifications you are blind, as am I, hence why I am busy interrogating various supply options to find a compatible "stalk" lamp that will work on the approx 14 volt input - which is proving quite hard!

It sounds perfectly normal - the nominal voltage of a car battery is actually 13.8V, so 14.29V isn't outrageous, and is well within the voltage variations on a car battery. It's highly unlikely that a small voltage increase would stop it working.

However, it's a little concerning that you describe the wiring as 'live and earth' - it's no such thing, that's a mains AC term (and is wrong even then - it would be live and neutral) - for a DC lamp the correct terms are positive and negative. Are you absolutely sure you're connecting it the correct way round? - stick your meter on the lamp wires when it's connected, and make it reads positive on the positive wire (and negative on the negative wire). If it's connected the wrong way it won't work at all.

Do you have access to a variable power supply?, if so connect it to that, and try powering it from a power supply set to 14.29V and see if it works.

You might also try connecting it directly to the leisure battery, and see if it works then.

Edit: Just a quick thought?, how are you actually connecting them, is it some kind of plug and socket in the vehicle?, and is it two pin or three pin?.

 

[GeorgioB]

Hello Nigel
OK sorry misuse of terminology - positive and negative (earth) and when tested they were all correct. All the wiring worked (LED illuminated brightly) when transposed across via some fly leads to my car battery, so I was happy that I had discounted any issue with wiring connectivity and polarity etc. This is when I tested the outputs at both my motorhome and car battery (and found the slight discrepancy which I also assumed shouldn't cause an issue), although I admit I didn't try connecting them to my leisure batteries which I can try next. Unfortunately I don't have variable power supply but a friend might so I'll tap him up tomorrow. Thanks.

 

[GeorgioB]

Sunday night update and thanks to Nigel for double crossing, I meaning checking me thankfully!! I read your support message out to my wife (who is always my able assistant and she said that the multi-meter showed a negative DC reading this afternoon, to which I promptly replied (in a sexist fashion) no it didn't, what do you know? She then offered me her glasses and I am now eating quite a lot of humble pie - in the words of CJ from the Fall & Rise Reginald Perrin - I don't believe it! What a Muppet I have been today chasing my tail for the best part of 2 hours. I have now fallen on my sword publicly but thank you for your support one and all. My wife is laughing at me typing this message!

 

[Nigel Goodwin]

Sunday night update and thanks to Nigel for double crossing, I meaning checking me thankfully!! I read your support message out to my wife (who is always my able assistant and she said that the multi-meter showed a negative DC reading this afternoon, to which I promptly replied (in a sexist fashion) no it didn't, what do you know? She then offered me her glasses and I am now eating quite a lot of humble pie - in the words of CJ from the Fall & Rise Reginald Perrin - I don't believe it! What a Muppet I have been today chasing my tail for the best part of 2 hours. I have now fallen on my sword publicly but thank you for your support one and all. My wife is laughing at me typing this message!

Easy done - and congratulations to your wife