Just being curious,can you check your trailer lamps with a power probe?We do not have a towbar at the mo,as I self service,I was wondering if this can be done by powering the plug up with the probe.
How would a 9v battery work with 12v lamps?my thoughts were to use the probe into the pins to power the bulbs to check they work instead of removing each one separately.
You could do it with a cheap multi meter using the resistance or ohms range. (Might be an upside down horse shoe symbol).
Calculate the resistance of the lamp:
Formulae:
Power =P Volts = V Current ( amps) = I Resistance = R
I=P/V. V=I*R. R=V/I)
Final Formula:
R=V*V/P
or Resistance = volts times volts divide wattage
For 12 volts 5 watts.
144/5 = 28.8 ohms
For 12 volts 21 watts
144/21 = 6.86 ohms.
You then need to put the probes of the multi meter on the ohms range. With the probes separated you will get a very high reading. If you touch the probes together you should get zero or very close. May need to rub them together a bit to get a good contact.
Then to test lights, put the probes on the correct pins.
If there are more than 1 lamps on that circuit you need to add up the total wattage and recalculate the resistance which will be lower.
Lamps on the same circuit will all be in parallel so if one has failed the resistance measured will be higher.
If you power up the pins at the caravan plug, and a lamp does not light up, the fault could be the lamp, but equally a fault within the plug itself...
Its also a good idea to get a tester for the car socket, as this quickly isolates a problem to the car or the caravan, without a load of troubleshooting.
I made my own tester by mounting a car socket on a wooden box. A few switches and some batteries inside it and I can check all the lights. You can buy them though to check both the trailer and the car.
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Topic: Testing trailer lamps
via mobile 09/2/2020 at 1:17pm
