OT - Voltage Drop In Trailer Wiring

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I'm reading a voltage drop in my trailer wiring and wondering if it's normal.
Here's what I've found:
Flat four plug from vehicle reads 12.5V from ground pin to tail light pin without the trailer plugged in.
Flat four plug from vehicle drops to 10.5V from ground pin to tail light pin when the trailer is plugged in.
Light fixtures on trailer read 9.5V from when testing both "socket ground to socket hot" and "ground-screw-into-frame at rear of trailer to socket hot".
Resistance between trailer's flat four ground wire attached to trailer tongue and the ground at the sockets and the ground-screw-into-frame at rear of trailer reads about 2.5 ohms.
The side marker lights have no ground wire, using the self tapping screws that hold the markers to the trailer shell as the ground. The rear lights use a ground wire, with the ground provided by a pigtail to a self tapping screw into the trailer frame.
What would cause a 2 volt drop right at the plug when the trailer is plugged in and then another 1 volt drop at the fixtures?
Thanks!
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I would start by giving the plug a good cleaning. Without a load except for the meter, there will not be any noticable voltage drop at all. In a 12 volt system, it only takes a small ammount of resistance to have a large voltage drop.
If the lights only pull 1 amp and you have only 1 ohm of resistance, you will loose 1 volt. You mention 2.5 ohms, If you zeroed out all the resistance of the meter, that is where you are loosing most of the voltage. Going back to that 1 amp load, (which is probably not as much as you have) that 2.5 ohms will drop 2.5 volts under the load of the lights.
I would make sure that the wires going from the battery or switch to the plug are good and tight with no corrosion on them. Then clean both plug and socket and make sure the ground wires are tight and don't have any corrosion under them.
The wires should be put on with lugs, but if the wire is just wrapped around the screws, that would be a good place to start.

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On Friday, August 16, 2013 4:04:16 PM UTC-4, Ralph Mowery wrote:

That's for sure. IDK how many lights he has, but just 12 watts worth of lights would be 1 amp, giving him that 2.5 volt drop. On the other hand, if he has voltage drops without any loads on at all, then he must have some partial short somewhere.
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I had all sorts of troubles with a friends trailer used for ministeries........
I finally traced all the troubles to a poor ground on the tow vehicle....
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On 08/16/2013 11:34 AM, DerbyDad03 wrote:

[snip]
Ralph provided a pretty good answer, so there is no need to repeat what he already stated.
I will add, however, that I noticed a sizeable drop on my 5-way to 4-way adaptor, which I ascribed to a typical voltage drop due to semiconductor(s) (diode and/or transistor) in the adaptor.
The way I dealt with this was to ditch the incandescent bulbs that I previously had on the trailer for LED units, which have no problem lighting up nice and bright, despite the lower voltages (the incandescent bulbs were really dim, and I didn't feel safe driving with them in such a condition).
Jon
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Jon Danniken wrote:

Hi, Typical Si junction voltage drop in semiconductor is ~0.2 Volts.
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wrote:

Si junction drop is considered to be .6V-.7V but diodes can easily drop over 1V at higher currents. Perhaps you're thinking about Schottky diodes? They're closer to .3V at very low currents and perhaps .7V at higher currents.
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snipped-for-privacy@attt.bizz wrote:

Hi, Automotiv DC wiring is no place for Schottky diodes.
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wrote:

Wrong again. They're *often* used in automotive wiring. Battery isolators *ARE* SBDs (and will drop 1V at high currents).
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snipped-for-privacy@attt.bizz wrote:

Hmm, I wouldn't use other than SiC type.
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wrote:

You obviously know nothing about the subject.
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snipped-for-privacy@attt.bizz wrote:

Hi, Please enlighten me kindly explaining junction type, doped type and SiC type? Was Schottky German or Hungarian?
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wrote:

Good Lord, you really must prove that you're a total moron. No need to use terms you don't understand. We got it.
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Since I'm running flat four to flat four, there are no electronics involved in my situation.

The LED route is where I'm headed which is why I was checking out the wiring, just making sure everything was OK. I'm looking at LED replacement that will plug into my existing bulb socket.
This 1157 replacement bulb says it will run at 7.5 - 14 V
http://www.ledtrailerlights.com/stt/stt_1157.htm
The marker units say they'll run at 9 - 16 V
http://www.ledtrailerlights.com/cm/00-4400.htm
However this complete unit, which plugs into the 1157 socket, doesn't list a voltage range.
http://www.ledtrailerlights.com/stt/led-insert.htm
I'm guessing the complete unit will be brighter than just the bulb, and I'm guessing it'll run at the 9.5 V I have at those fixtures, but I'll have to call the vendor to find out for sure.
I'm assuming that LED's have one brightness. In other words, any voltage above the minimum produces the maximum brightness, correct?
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DerbyDad03 wrote:

Hi, LED's another benefit is it is fractional second faster coming upto full brightness. In panic braking situation it can mean a difference for avoiding the accident. LED is either on or off voltage is applied.
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On 08/16/2013 04:39 PM, DerbyDad03 wrote:

No; you can make an LED barely come on, or you can run it with too much current and it will get brighter than specified. Too much current and you burn it out (heat death), in an amount of time that depends on the excess amount of current. There is a threshold for them to begin to come on at all though, at which point they are still dim, while an incandescent has an infinite amount of dimness (down to zero).
There do exist current-limiting circuits for LEDs; I have one in my headlamp that keeps the LED at a constant state until the battery drops down below the threshold needed to maintain the illumination level.
As far as taillight assemblies go, my experience is that the unit I got works great at the 9.5VDC that I got from my original converter (the new one puts out 11.3VDC). I'm sure it would be brighter if I fed it 12VDC, but it put out more than enough light at 9.5VDC, and far more than incandescents at that voltage (which were barely discernable), which is why I switched.
Jon
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On Fri, 16 Aug 2013 23:39:29 +0000 (UTC), DerbyDad03

Not technically. The more current through a LED the more light - but good LED LAMPS use driver circuits that control the current. This makes your assumption relatively close to true - for quality lamps (but those are not terribly common on the market - you pay your money and you take your chances.
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On 8/16/2013 7:39 PM, DerbyDad03 wrote:

Better ones that have a proper driver, yes. Don't use LED replacement "bulbs" instead use dedicated LED lighting assemblies. The latter are far superior in every way, are likely available as sealed units (in case you are talking about a boat trailer where the lights may be submerged when launching) aren't significantly more expensive, and are fully legal for road use.
This or something similar should work well
https://www.truck-lite.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId 001&storeId001&productIdY003&langId=-1
Peterson, Grote, etc. are also good brands to look at - whatever your local boat supply, truck parts store, etc. has in stock. Any of those manufacturers should have a selection of surface mount, flush (snap in gasket) round, flush oval, whatever you need.
nate
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I can't use the type of tail lights that you linked to. My tail lights are built into the rear doors, which is why I provided the link to this fixture...
http://www.ledtrailerlights.com/stt/led-insert.htm
It replaces my current lens and plugs into the 1157 bulb socket. Unfortunately, it's one of the few LED products on that website that doesn't list the operating voltage range. I'll be giving them a call soon to find out.
As a side note, the kit you linked to includes those Quick Splice connectors. Those belong in the same container as back stab receptacles - the trash container.
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On 8/17/2013 1:43 PM, DerbyDad03 wrote:

oh yeah, if you're going to splice, use proper solder and heat shrink, or else proper plug in connectors... I just ASSumed that nobody actually used those things.
can you post a pic of your current taillights? I bet there is a drop in LED full fixture replacement available unless they're real odd, which is really the best way to go... they'll last you another decade at least.
nate
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