Simple wiring problem

[DiodeDave]

I have a 250watt 12v landsape transformer powering lights, LEDs and accessories on my train layout.

The total draw of all accessories is 5.57a.

At the transformer, my volt meter shows 11.8 volts.

At the end of my power wire, just before it connects to my terminal block it shows 11.8v.

When I connect the terminal block, the voltage drops to 10.2v.

Everything works properly, just slowly or dim.

If there was a short, wouldn't the current draw be greater?

What is causing the voltage drop?

 

[(*steve*)]

What is causing the voltage drop?

The resistance in the wire causes a voltage drop at the current drawn when the load is present.

The cure is to use a heavier gauge wire.

 

[DiodeDave]

WOW! I'm using 16awg stranded wire and the run is 5'.

I'll get some 14awg or 12 awg and give it a shot.

It doesn't seem like it is carrying much.

 

[duke37]

I have an old table of wire resistances.
16 awg = 18 swg 13.3 ohms/1000yd
14 awg = 16 swg 7.6 ohms/1000yd
12 awg = 14 swg 4.8 ohms /1000yd

Remember that the current has to go there and back.

 

[BobK]

So, at 10 feet the resistance of 16 AWG wire is about 0.05 Ohms.

At 5.57 A the voltage drop is 0.27V.

To drop 1.6V you would need to drawing 32 A.

Something is wrong. How did you measure the current draw. Are you sure the transformer is 250W? That is going to be a pretty big transformer.

Bob

 

[Six_Shooter]

Some pictures and circuit diagrams might help to sort out what is going on.

You can also try powering one section at a time and see if there is one section of your layout that appears to have more problem running than others.

Am I correct to assume that this transformer is simply that, a transformer? No voltage regulation?

You might also want to look at the supply to the transformer.

 

[duke37]

It is not clear what the voltage at the transformer is, it is 11.8V with no current.
The transformer will have a lot of wire in it and the terminal voltage will drop with increased load current.

 

[BobK]

I would think that a transformer rated at 12V 250W would not drop anything like 1.8V when drawing only 5.6A. I wonder what the output voltage would be at 20A.

I suspect that that 250W rating is wrong. Maybe it is 25W?

Bob

 

[DiodeDave]

Well, lots of things to look at.

I'm first going to check my transformer. It is a transformer and pretty big.

I suspect my clamp on ammeter may not have provided a good reading. I'm in a tight poorly lit location.

I estimate 3 of my light towers are drawing about 3 amps total and I have a lot of incandescent lights. I'm going to remove a few accessories and check the voltage.

I doubt I'm drawing too many amps, but anything is possible.

I will report back, thanks to all for the info.

Dave

 

[(*steve*)]

Another thing to check is the voltage at the output of the transformer without load and with load.

As has been suggested, it may be the output that is dropping, or it is something after that (cable or connections).

Does it get at all warm with the 3A load on it? If it is really rated at 250W it won't. If it's rated at 25W it should become noticeably warm after a few minutes.

 

[DiodeDave]

I read the label on the transformer last night.

It is rated at 150w.

I ran a new 12awg wire from the transformer to my terminal block. I got 11.8 volts before soldering to my terminal.

After soldering, 10.6v or so. Amps were as high as 6.2. I have several operating accessories which blink and move.

I checked the current on the 8 leads which go from the terminal block out to the layout. All were drawing less than one amp except for one which was 1.2a.

Most of the devices connected to the circuit are incandescent and LEDs. There are 2 blinking bulbs which use thermal blinkers (from the '50's) and 2 electromagnets which pulse.

All 8 leads have been disconnected as I am going to install 3a circuit breakers in 3 circuits, just for peace of mind.

I guess I'll reconnect the 8 one at a time.

 

[DiodeDave]

Another comment.

Some of this layout was built 20+ years ago and expanded 5 years ago. My knowlege of electricity has increased, but not far enough.

Last night, I found some very thin 3 conductor wire, perhaps 28awg powering 8 12v incandescent bulbs. The wire was originally designed to connect stereo cartridges in phonographs.

It has been replaced, and the bulbs are a little brighter.

Somhow I'll figure out why the voltage drop.

Could it be a weak transformer?

It has a circuit breaker (don't ask how I know) and was designed for outdoor low voltage lighting.

 

[duke37]

Transformers will give a lower voltage when loaded, this is called regulation.
A good transformer may drop 5% at full load current. Small, cheap transformers may drop 20% at full load. There are even some transformers which can stand a short on the output without burning out.

Measure the voltage at the transformer with no load and with everything connected. Check the heating of the transformer.

 

[DiodeDave]

Thanks Duke!

I would consider the transformer an inexpensive one, just designed to light small bulbs.

I've read up on regulation.

Now I can go back to running my choo choos.

 

[DiodeDave]

Last post

Last night, I re connected all of my lights and accessories to the transformer. The voltage held until a wire leading to a 2 coil, 3 incandescent bulb accessory was connected. The voltage dropped to 10.2v. The accessory draws 2+ amps and all of the other lights dimmed.

It looks like I'm using 1/2 the capacity of the transformer. For landscape lighting, it is probably ok.

Thanks again to all.