Question about electricity

[HeyBub]

Hot is okay. Melt is bad.

If the latter, use bigger wire.

[Mark H]

Also remember that things like motors -both ac and dc - are basic coils of wire, also transformers - ac- are just two coils of wire next to each other.( Most have an iron core between them to help the magnetic lines of flux I think, it's been a long time here also !!!)

[mm]

To follow up on my own post, two of those relays from the Royal photocopier were latching relays. If the machine jambed, and I turned it off or unplugged it, later on it wouldn't work. Even though I'd bought the manual, it was not a narrative and I missed the bit about latching relays. So they were in the wrong position, and when the machine was plugged in again, it wouldn't work. There was even a window in the top of the relay to show it was closed, and a button to reset it, each one of them, but it took me a long time to figure out.

By the time I got that fixed, something else broke.

But I paid a dollar plust the fixer element and had it working for about 5 years, and made a few hundred copies.

[Mark Lloyd]

[snip]

BTW, there's no such thing as a DC transformer. Only a changing magnetic field (as from a changing current, AC) will generate current in the secondary. Consider that if a transformer could work on DC, you could substitute a permanent magnet for the core (and eliminate the primary). You would now be getting something secondary current) for nothing, an obvious impossibility.

An iron core is used at normal power supply frequencies. It's made of multiple thin layers shaped like the letters E and I.

[RickH]

For most applications the "last known" latched position after a power failure is the desireable one. Like if you are using a latching relay for a light switch, you get a power failure, when power is restored the light is still in its last known state. This is the same as if it were a regular wall switch. But If the power restored position of a latching relay must always be a forced-certain state, then you will need some logic that tests the position of the relay after a power up and sets it right if its not in the proper state.

[Mark Lloyd]

Peak voltage is the height of a peak above the centerline (0V). RMS is a common, but complicated measurement which is equal to .707 of this (that is, peak is 1.414 of RMS). Average (mean) is equal to .637 of peak.

Peak-to-peak is the (unsigned) sum of the heights of both positive and negative peaks. That is, it's twice peak. For one example:

12V RMS = 17V peak = 10.8V average = 34V peak-to peak.

BTW, 1.414 is also one of the square roots of 2 (the other being

-1.414 of course). For something completely OT here, how about the square roots of -2?

[mm]

Almost all wire is copper. Plus I think you're misunderstanding short circuit. A short circuit is one in which some part of the circuit, usually the load, is bypassed completely or partially.

Plainly your relay (the load) isn't being bypassed completely, because if it were, it wouldn't be hot.

Do you think there is a short circuit between windings inside the relay coil? That's rare, and it takes some abuse of the coil to make it happen. Was this a new relay, one from surplus (working correctly when it goes out of use), or one from junk which wasn't working right when it went out of use? If the relay was good when you go it and you haven't used substantially more voltage than it's rated for, the odds are very high its still good.

I think it's time you told us more about the device. What are you using for both sources of power? What are you trying to control with the relay? What determines if the relay closes or not?

[greenpjs]

Average???? Most of what you said is correct, but the average of an AC voltage is zero, not .637 of peak. That's why they came up with RMS. Using RMS voltages & currents is essentially a way to enable the use of Ohm's law on AC circuits as though they were DC. A simple average won't work because the average of the voltage and current is always zero, but RMS works because the "S" part squares the voltage to make both half cycles positive. Of course, then the "R" part (square "R"oot) is used to restore the voltage back to the correct value after having squared it. The M is mean. So RMS is the square root of the mean of the squared value. You are right... it *does* sound complicated ;-)

[Dave + Gloria]

Presuming that the initial wave form of the ac is a sine wave.

[CJT]

I don't think that was the motivation. RMS is equivalent heating value.

Using RMS voltages & currents is essentially a way to enable the

[Mark Lloyd]

I don't necessarily remember everything from college, but average voltage (.637 of peak) is a real thing. I thing it assumes ideal full wave rectification.

For one thing, analog meters respond to average. The fact they seem to read RMS is because of the calibration, and is valid for sine waves only.

Yes. 12VRMS creates the same amount of heating as 12VDC.

Voltage and current are different things. It doesn't make sense to combine them that way, and I think you didn't mean to.

[Mark Lloyd]

[stale sig snipped]

Right. Assuming sine waves is a very common simplification.

Also, considering the differences between AC and DC relays.

I seem to have forgotten something even bigger. In AC, each cycle has the OPPOSITE polarity to the preceding one. This means that the magnetic field generated by that current is opposing the residual magnetic field from the previous half cycle. This would make the AC relay less efficient.

[greenpjs]

Thanks for giving me the benefit of the doubt. I didn't mean to combine them in any way other than to say the average of each (separately) is zero. I should have chosen my words more carefully.

And, yes, as you and others have said, the whole discussion assumes a sine wave.

[Mark Lloyd]

OK. I know I've made mistakes like that before. It's too easy to do.

Interestingly, when I was writing my earlier post (mentioning RMS and average), I originally put in something about this material applying only to sine waves. I took that out in consideration for those who don't know and aren't able to understand.

[Don Young]

Since it is relatively fine the wire in the relay has quite a bit of resistance and thus does not make a "short circuit". If the relay is actually overheating then either the voltage is too high or else you have an AC relay and your "transformer" puts out DC because it includes a rectifier. Your "transformer" might also put out more voltage than it says because it is intended for a higher current load. In other words, your relay and power source are not compatible. You probably could add a resistor in your circuit to solve the problem but you have not given enough information to calculate its value. If you post the information on the relay and the "transformer" someone could help more.

Don Young

[mm]

He just means the average of AC voltage is always zero, and the average of AC current is always zero. He combined the sentences, not the voltage and current, to get what he wrote.

[bud--]

Don't think so. The magnetic field is smoothly reversed by the sine wave current.

The energy stored in the magnetic field comes back out as current returned to the source. (This produces the 90 degree phase shift between applied voltage and current through an ideal inductance.)

The magnetic pull of the coil pulses at 2x the applied frequency which can be a problem. A "shaded pole" (heavy shorted turn) is often used to smooth out the magnetic pull.

Magnetic materials have some 'magnetic resistance' to magnetic field changes which produces "hysteresis" losses.

But in a DC coil (and AC coil) there are losses in the resistance.

[AZ Nomad]

Irrelevent. The current consumed by the coil is fairly insignifigant, typically less than 200ma for a 12VAC relay with 15A contacts.

[TWayne]

Almost too hot to touch is too hot; means the voltage to it is wrong. Coil windings lifetime will be shortened considerable and it could become a fire hazard as the coil wire coatings begin to melt and short out. Wrong voltage.

[TWayne]

Very often a RS power brick is cheap and can be found that produces what the specs of the relays want. Inadvisable to add rectifiers on one's own: against code and insurance regs.