# Electrical question - i feel like a moron here

Discussion in 'Community Discussion' started by spaceballl, Jul 19, 2008.

1. ### spaceballl macrumors 68030

Joined:
Nov 2, 2003
Location:
San Francisco, CA
#1
Alright, so let me first say that my degree in college was Electrical Engineering. That is why I feel so dumb asking this basic question... But even though i feel like a retard, i'd much rather just swallow my pride and ask the basic question than fumble over it forever!

Okay so in our yard, we have a lighting system that is hooked up for a timer. Lights come on at night - go off at day. Pretty standard. I've noticed that like a third of the lights have gone out. So then I went to the transformer box where the timer for the electrical system is. I see that there's one 120VAC line that comes into the box. Then there's 3 12V DC lines that go out (a +/- terminal for each line, for a total of 6 wires).

So since about a third of our lights are off, I'm figuring that 1 of those 3 terminals isn't supplying voltage anymore. I went to radioshack to grab a multimeter. I wanted to check the voltage on each of the DC lines.

So then I racked my brain back to college... and I remembered that if I want to measure current, I need to break the circuit and reconnect it. And if I want to measure voltage, I should be able to just touch the two leads of the multimeter to the +/- terminal. So I set the multimeter to the VDC setting and I touch the leads to the +/- terminal. I'm expecting it to say something near 12V. Nothing. 0. no volts. Okay weird. I try the next one. Same thing. I try the next one. Same thing. The freaking lights are on right now so I know there can't REALLY be 0 volts on that line.

So basically, I have a feeling i'm just forgetting something basic, or missing something... Can some young undergrad who just took basic ECE help me out? I really appreciate it! Want to get these damn lights workin! But in order to fix anything, I need to diagnose the problem first!

2. ### Gray-Wolf macrumors 68030

Joined:
Apr 19, 2008
Location:
Pandora, Home Tree
#2
Do the basic process of elimination. Confirm the transformer is putting out a full 12volts. If it is, that eliminates 1 point.

The output wires lead to a light, one pair then in series. Go to the fires light, remove the bulb, and check your voltage there. **Note** Make sure you multi meter is set for DC if that is the output of the transformer. If A/C, then set for that.

You should get a reading at the first working light. Go to the next in line and check it. The likely hood, is that one wire has broken or become disconnected in the series.

My experience, is 25 years in apartment maintenance, with lots of electrical trouble shooting.

3. ### JNB macrumors 604

Joined:
Oct 7, 2004
Location:
In a Hell predominately of my own making
#3
Make sure that the multimeter leads are in the correct ports (I doubt you could screw up a RS one, but my Fluke has four ports, and I do forget sometimes), and make sure it has batteries. Switch to ACV and test a receptacle in the house to validate it.

On your DC light circuit, check the voltage by placing the (+) lead on any of the hot terminals, and the (-) lead to any ground not part of the circuit you're testing.

4. ### pianoman181 macrumors regular

Joined:
Jan 6, 2004
#4
You ought to be more careful...if you leave it hooked up for taking a current measurement, all that power goes straight through the leads. I've known many a fried tube amplifier due to 'forgetting sometimes'. I have two bench meters . One of them is set for current, always. the other is never set to measure current.

to the op: leave the negative lead on the multimeter to one of the grounds and try the positive lead on all the hot lines coming out of the transformer, they may be discrete. Also, are you sure it's outputting 12VDC? Are there any rectifiers of some form after the transformer? If not, it's more than likely 12VAC which would explain why a meter set to DC wouldn't show anything.

5. ### r1ch4rd macrumors 6502a

Joined:
Aug 5, 2005
Location:
Manchester UK
#5
Having a degree apparently means nothing. I have a computer science degree and I am often stumped by apparently simple computer queries!

6. ### 4JNA macrumors 68000

Joined:
Feb 8, 2006
Location:
looking for trash files
#6

correct as posted in the orignial post. meter in the circut to measure current, meter touching the +/- posts to measure DC voltage.

you might want to try unhooking one of the functioning sections and then try the voltage reading again. the open circut (remove the lights) would be higher than 12v, but it should show on the meter if it is working. double check the leads on the meter, as some have different ports for AC and DC.

agree with the one dead DC section guess. best of luck.

EDIT: if the the meter has an ohm/continuity setting, you might try testing the dead section from the two wires to see if the circut is complete. if it reads infinity (open), then you have a dead bulb or bad wire. or ghosts.

EDIT X2: or... how about hooking the wires for one of the two working sections to the non-working section to see if you get any lights! or the dead section to a known working section to see if you get lights! or just drink beer and blame it on the ghosts and cheap wire! 2 out of 3 is still 66%, and that is pretty good!

7. ### Abstract macrumors Penryn

Joined:
Dec 27, 2002
Location:
Location Location Location
#7
I don't know squat about circuits and such, and am not an electrical engineer, but assume you have a multimeter and you use the + lead and make contact to the +ve terminal, and use the - lead to make contact with the -ve terminal.

If it reads 0, does it read infinity (i.e. off the charts, or whatever you'd call it), when you use the +ve lead to touch the -ve terminal, and the -ve lead to touch the +ve? If it still reads 0, then nothing is happening. If it reads infinity, things are "fine".

I'm guessing, although I do hope that someone tells me how I'm wrong.