OT Electrical question

Maybe it is a.c. output.

Nope. Sticker says 12vDC output.

Steve

Grease the bearings?

Tom! It's a DC transformer! If he greases the bearings, it will spin freely alternatively back and forth creating AC, I think he needs to use directional friction grease.

cheers T.Alan

So that's the outdoor 12V lighting transformer and timer?

That click could be a relay which would not be a problem. 600 W is a lot to control with a small timer switch like that without a relay.

But if the output terminals don't have voltage, then I would say it's fried.

You have no lights connected to it right? Transformers shouldn't hum when they have no load on them. You might have a shorted winding in it.

If you have lights connected to it, disconnect them and test again. But otherwise, it sounds like you have some sort of internal short in the box.

Looks to me like it is both a timer and a DC supply. Those intermatic timers have switches that make a whacking noise when they operate, and they can easily handle 600 watts so I strongly doubt that there's a relay in there. The hum may just be the timer motor. You may just need to turn the dial to a position where it is "on".

You did say it says 12VDC, but I don't know why they would power lights with DC. You might check it with an AC voltmeter just for the heck of it.

Don sez" "> You did say it says 12VDC, but I don't know why they would power

Probably a low voltage supply for yard lights. We had a bunch of them when we moved into this house but I got tired of changing out the bulbs; seems they never had the courtest to all fail at once and I got tired of the constant maintenance, and expense. I don't remember what ours were, but Malibu is a brand name of low-voltage yard lights.

Bob (saw the light) Swinney

You NEED an AC voltmeter. A DC meter won't read anything (meaningfull anyway) on an AC cupply, and these outdoor low voltage systems are AC ALL THE WAY.

Trust me, the sticker is almost certainly wrong. Set your meter to AC Volts and check it again - hook up a scope and you'll see nice sine wave AC. And the SMACK-Hummmm... is normal as the transformer energizes and settles down to work.

I work with these every day, the Malibu Model ML600TW is a standard unit. I haven't seen a lighting transformer yet with a diode bridge and any sort of filter capacitors or chokes. They are built to a price, and AC goes further than DC on the same size and gauge wire.

If a device wants DC (like a 12V fluorescent floodlight) they have a rectifier inside the device.

-->--

Makes sense. Low voltage lightbulbs run on AC just fine, no benefit in rectifying it.

It does? I'd think that the drop in given wires would be the same for same RMS current whether AC or DC, RMS current for DC being the DC current.

I know and respect that you are a professional electrician and know what you're doing. What am I missing?

Naw ! Don. It's that pesky sine wave thing again. If you hook on at the right time you may not read anything, don't you know.

I saw someone else mention this idea recently but I've never heard it before and don't understand how it could possibly be true.

Care to explain why this is true that AC "goes further" than DC?

Anyone else know about this?

Uh, no it doesn't. For the exact same piece of wire in the exact same installation, with the same RMS current. There will be less voltage drop for the DC, than the AC. This difference will get bigger as the frequency goes up. jk

OWT - otherwise known as BS. At radio frequencies you start seeing "skin effect: which cuts back the current carrying capacity of a wire , but in the other direction, I've never seen any explanation that even comes close to making sense. Try it - explain how this (supposedly) happens. The ONLY reason AC is better for long didtance power transmission is because you can boost the voltage to transmit it, and the cut it back at the other end with a simple transformer. Higher voltage means lower current, and lower line losses. Changing DC voltage gets just a tad more complicated.

That was my assumption the first time I heard it. I heard it when I asked if there was some advantage to using AC for stick welding over DC and the guy said that AC works better in long cables than DC because there was less loss. But then he followed it with - that's why we use AC for power lines. But since I know the advantage of AC for distribution is not because it travels better, but because it works with transformers, I assumed the guy was just confused and thought AC had less voltage drop in cables than DC.

But the above post was the second time I saw the idea so I wondered if there was something I was missing. Looks like there isn't.

Well, if there is any series inductance in the cable the voltage drop could go down with frequency. But it seems to me that the capacitance or inductance in a typical power cable will be so low as to make no significant difference at 60 Hz.

"Curt Welch" wrote: Care to explain why this is true that AC "goes further" than DC? ^^^^^^^^^^^^^^^^^^ I can explain that it's NOT true. AC fluctuates sinusoidally. The RMS value is that value that produces the same heating effect as DC, and AC meters are all calibrated that way. The idea that it "goes further" probably goes all the way back to the days when Edison was in favor of DC generation and distribution. The argument that caused the widespread adoption of AC is this: Transforming AC to high voltage for distribution, and then back again for use, results in low current in the transmission lines. Power loss = I^2R, which means that AC can be transmitted for long distances with low power loss. Had Edison won the argument, power stations would have to be located close to the the users.

IOW, AC can travel farther than DC.

Edison and Steinmetz were friends, but on this they disagreed.

A transformer will hum whether it is loaded or not. Possibly hum louder under load, but they will hum irregardless.

A/C .vs. DC for transmission, it's easier to convert AC than DC for long distances. Current is the killer when it comes to transmission. The more current you are sending, the more losses there are in the lines. DC conversion to higher voltages takes a few steps, one of them basically being turning it to a pulsating DC (or ac) and running it through a transformer type device to boost it, then re rectifying it back to DC....and subsequently can have more power lossess in the process than AC conversion which requires that you simply run it through a transformer... not to mention the equipment probably will be more expensive as well. Look at the price of electricity, even 1 amp of 'lost power' can turn into a lot of $$$ over days/ weeks / years, etc etc when you are running at voltages of several hundred KV..

Another issue is, that some DC conversion equipment works in a vaccuum (ie tubes) and over 10Kv in a vaccuum can generate X rays, whether wanted or not...

Back in Tesla's days, X rays probably were not even thought of, or worried about, by todays standards though..... anyways, another 'issue' to worry about with power conversion / transmission.

With A/C you can send the power literally hundreds of miles, with DC you'd need several substations along the path.

Aaron

Simply speaking, you can step AC up to a very high voltage using transformers. So the voltage goes way up, the current goes way down. Resistance cares about current, not voltage. So your line losses are more or less proportional to the current for a given size of wire. Long story short, you can put more energy through a smaller wire and lose less of it, if the voltage is higher. It's difficult to do this with DC.

So much for the classical definition. These days, there are some very high voltage DC transmission lines which must be more efficient or they wouldn't do it. Not sure how they get there, but they exist. (and here comes that one guy to tell everyone about whateverthehell irrelevant and wrong transformer theories he has, just watch...)

Dave Hinz