On Sun, 24 Aug 2008 12:02:21 -0500, AZ Nomad
A transformer is an AC-only device. Maybe you're (incorrectly)
referring to a power supply which contains a transformer and a
rectifier. The transformer is on the AC side.
There might be some conversion table around, something like use xVAC
for a yVDC relay. Still, I'd rather not do that.
Most of the relays I use are made for 12VDC and work with small DC
A half-wave rectifier has an even greater need for filtering. The
capacitor needs to store enough electricity to fill in the gaps. With
a half-wave circuit the gaps are half as frequent but much longer.
BTW, most wall-warts contain a full-wave rectifier that uses only 2
diodes. This is possible with a transformer with double the secondary
voltage and a center tap.
Halfwave rectification also causes a DC component to flow through the
transformer's secondary winding. That may cause the core to saturate when
the primary's "magnetizing current" is in the same direction.
I have seen fullwave ones. The 2-diode-center-tap scheme has higher
ratio of RMS to average current in its secondary windings since the
secondary windings are used only half the time. Copper has gotten really
expensive over the past 3 years and diodes are cheap.
- Don Klipstein ( firstname.lastname@example.org)
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-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?
On Sun, 24 Aug 2008 18:06:34 -0500, Mark Lloyd
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
I don't necessarily remember everything from college, but average
voltage (.637 of peak) is a real thing. I thing it assumes ideal full
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
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.
On Sun, 24 Aug 2008 20:04:05 -0500, Mark Lloyd
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
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.
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.
Don't think so. The magnetic field is smoothly reversed by the sine wave
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.
If a DC relay is used on AC it will chatter or vibrate. If a AC relay is
used on DC the coil will draw way too much current and burn up. It may be
possible to use a AC relay on DC if the voltage is cut way down to limit the
The current in a DC relay is limited by the resistance of the wire in the
coil. The current in an AC relay is limited to a small part by the
resistance of the coil but mostly by the reactance of the coil due to being
operated on AC.
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.
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.
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