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Black is "hot", white is neutral, and green is ground. These are your guides, if you expect to stay around.
32 responses total.
I'm using a Dimmer Switch at quite low power for a heater. Often if mains power fails, it stays off on the heater when power returns. Is this a property of dimmers, or is it faulty? The device would draw ca. 80 watts at full voltage, and I'm controlling it at closer to 10 watts (the dimmer is rated 600 watts).
There does tend to be an effect where you can't just power up into really low settings. You are using a lamp dimmer for an electric heater? Sounds like as long as the wattages are reasonable it should be safe. Anyway, play around with low settings on a lamp some time. You have to go to a higher setting to get the lamp to light, then you can dim it down. I'm not really sure where this comes from, though.
I have noticed that some of these devices have a different setting for "tuning down" than for "tuning up." Also, some seem to have some initial threshold setting which may indicate higher resistance until some circuit is achieved.
Yes, those effects are what I am observing. I'm hoping there is some way around them - a "hack" ? - or maybe a *low* power dimmer that doesn't have the problem.
An autotransformer dimmer wouldn't have that problem. However, one of those would give you a 10 pound lump of iron to mount somewhere... I think the effects of the SCR (solid state) dimmer are the result of thresholds where a current can be sustained, but not initiated. SCRs work by allowing a little leakage that controls things (the technical explanation requires a lot more words. ;) )
While we are at it, some of those electronic ("wave chopper") devices buzz,
get alarmingly warm, and disturb nearby radios. Can't be good!
Most dimers these days do not come on softly. As you turn them up slowly, from OFF, they suddenly flash ON to about half brightness. You can then back them off till the filament is only a red grow. Unless your dimmer is advanced beyond this turn on point, the lamp will not come back on if power is interrupted for a moment. The old dimmers, and a very few of the new ones, didn't do this. It seems to me that dimmers started misbehaving about the same time that triacs with bilateral triggers built in hit the market. It may also have something to do with how many quadratures the triac will fire in, but I doubt it. You may be able to get your current dimmer to work by replacing the combined triac / diac (bilateral trigger) with discreet units or you could find a schematic in, say, the ARRL Handbook for a dimmer circuit. If memory serves, the one in the Handbook was used to convert a fixed speed drill to a variable speed drill. It should give you nice smooth control from OFF to full brightness and come back to whatever setting even after power interuption.
I found only (unclear) descriptions of SRC and TRIAC diodes in the ARRL handbook. I've done some more studies on a dimmer, and found that the hysteresis depends upon the load resistance, getting worse as the load resistance decreases. This makes the effect very pronounced with incandescent lamps, since when they are cold, their resistance is very low. I have some data, which I will post after I've done some more calculations.
Indeed. No dimmer circuts in the handbook. Just how to use a dimmer to control soldering iron temp. I have some schematics at home. I'll try to remember to look for one. You findings r.e. load are interesting.
I put different loads (resistance R) on a dimmer, and then found the
maximum output voltage at which the unit would not come on again if power
were turned off for several minutes and then turned on again. I will call
this the Cutoff Voltage, Vc. After it did not come on again, I turned the
dial until the unit conducted again and then measured the output voltage.
This I will call the Restart Voltage, Vr.
R Vc Vr Notes
^^^^^^ ^^^^^^ ^^^^^^ ^^^^^^
108 32 40 80w heater with 7w bulb in parallel
155 19 40 80w heater
558 none - 25w soldering iron
I suspect it has to do with the gain of the triac. If there is not
enough current into the gate of the triac then it won't turn ON.
Triacs do have a minimum gate current rating to get them to turn ON.
The following light dimmer circuit is from an appication note that
came with a 6 Amp Triac I purchased from Radio Shack Several years
ago. It is typical of light dimmer circuits. When the diac and
triac are included in the same package, the resulting device is
called a Quadrac. Gate to Main Terminal 1 voltage drop is about
3 volts while the current to trigger is about 25 ma.
>---LOAD----,--100uH---,------------------------,
(LAMP) | | |
| 100K MT2
| POT (Triac)
120 vac LINE | |--15K--,--DIAC-----GATE
0.1uF | | MT1
| 0.1uF 0.1uF |
| | | |
>____________|__________|_______|________________|
The 100uH choke is to keep triac switching noise out of the line.
The capacitors should be rated to be across a 120vac line.
The triac has three terminals: MT1, MT2 and the gate. Gate current
flows between the gate and MT1. Load current forlws between MT2 and
MT1. Gate current polarity is + or - , same goes for the diac. The
diac (bilateral trigger, gate trigger diode, etc.) conducts when the
current across its terminals exceeds ~20v.
One more data point, to confuse the issue: with a 150w lamp as the load, with R = 7 ohms (cold), Vc = 18 and Vr = 39. I think I am going to abandon this line of research as it doesn't seem to have a future. It did, however, raise another issue - what voltage does my Micronta (RS) 22-185 digital VOM read on the nonsinusoidal output of the dimmer? I would guess that it reads the mean (absolute) and then multiplies that by 1.11 to report rms for a sine wave. It could calculate the true rms, though. The manual does not say.
Who knows is the more accurate answer I would guess. I'd use a true RMS meter. (Got one at the Chelsea swap for $0.75. Another $25 to get a new display. Nice Fluke 8060A will measure freq. too.) It would be interesting to look at it with a scope too.
Still not able to control that 80W resistive load at low power with a dimmer - I've started it and dialed it down to 12V (less than Vc for that load), and even left undisturbed it cuts off after a while. I am now considering trying working it through a 120VAC->25VAC transformer. I am, however, a little leery about putting a transformer on a dimmer. Phases are shifted, and if one side of the triac goes, I've got DC! What do you think?
The output of a triac is pretty messy... The transformer may have to dissipate some of the higher partials as heat?
What kind of cable would I use to run a buried line out to a lamppost?
There is a grade of cable certified for underground burial (at a specified depth). I think it is "SE" grade, but any electrical supply house would know what it is.
Rane, perhaps you should call some of the electrical shops around town and see if one of them can't set you up with a decent dimmer that doesn't have the threshold problem your seeing. Madison Electric, etc. I know they exist, though they will cost a little more. On another note: I got a copy of _Wiring Simplified_ a couple of weeks ago. Pretty good book for the money. I was, however, disapointed that they didn't cover underground service enterances and meter connections. Nor did it cover how to handle wiring in metal studs and under metal floor trusses. Oh well, what can one expect for $7. It would help scott with his question though.
I need to replace my fusebox. I've currently got a 60amp service (not the original service, it looks like) and a little 5 circuit box. I'm happy with 60 amps, since all the major appliances get their heat from gas. I do need more circuits, though. Any recommendations for an electrician to do the transplant? I'd do it myself, if it weren't for the sticky problem of how to turn off the power out at the pole, etc.
Can't you pull your meter and thus break the circuit outside your house? If not, I believe Edison will do so if you call them. In any case, I'd pull it myself and call them right away and tell them what I did and they will come back out and seal it.
Yep, that's what you do. Call your provider before you do it just to be sure it's not something they need to do. I'd pull your old box and put in a new one. I even have a book that shows one way to do it. Looks pretty simple.
Hey, can I borrow that book?
Sure. It's only a few pages out of book on general home repair and maintenance.
Years ago when I did replace a service at Six Lakes (remnember doing the wiring on the annex scott?), I ws able to take it off, and connect it back up with no problem.
This morning I had the opportunity to look into dimmers and the hysteresis
problem many of them exhibit on first turning on. I have a reading light that
we use in our bedroom that exhibits this problem. I wanted it to start up
softly instead of coming on with a flash.
After poking through my files, I came up with single time constant and
double time constant circuits. The literature states that there is a
hysteresis effect in the single time constant circuit due to the relationship
between the the diac and the R C time constant of the control circuitry.
It went on to say that this effect could be almost completely eliminated
with the double time constant circuit.
_Load___________________ _Load______________________________
| | | |
Var MT1 Var MT1
115 Res 115vac Res
vac |----Diac---Gate |- Fixed R -|- Diac -Gate
C MT2 Ct1 Ct2 MT2
_______|________________| _________|___________|_____________|
Single Time Constant Double Time Constant
I opened my reading lamp and looked at the dimming circuit. Sure enough, it
was a single time constant circuit. In my dimmer circuit, C was .086 uf.
I left this value for Ct1 and added Ct2 of about .022 uf. I added a Fixed
R of 10K. This reduced the hysteresis effects drastically! The
literature said to experiment with the value for the fixed resistor to
obtain the least amount of hysteresis. I did and wound up with a value of
33K.
Now, when going from off, when I turn up the dimmer it comes on with the
lamp's filament barely glowing and handles power interruptions without
turning off.
The circuit in No. 11 of this item is of the double time constant design so
it should work pretty well.
Interesting. My impression was that the problem was the temperature- dependent resistance of the filament in a light bulb. (A 40W bulb at hand measures about 25 ohms cold. It would be a 575W bulb (!!!) if its resistance didn't jump up to about 360 ohms when it was on. For a tiny fraction of a second when just turned on, it IS a 575W bulb - which is why bulbs tend to blow when flipped on.) Are there any brands of household dimmers that employ the superior circuitry (for those of us unmotivated enough to retrofit)?
I seem to recall Rane and Co. talking about that somewhere in the Hardward conference... (hang on a sec, OK?) (Nope, couldn't find anything)
I was trying to control a low wattage device at lower power. I could turn down the power to what I wanted, but I could not then turn it off and back on at that power by switching the mains power off and on. This is the same hysteresis effect that Klaus is addressing, but with a different objective. The improved circuit would seem to involve a change of the phase shift provided by the control circuit. In fact, this change might very well solve the problem I had. Does your circuit come on at very low power settings, at which it would not with the original circuit, when applying mains power to it?
Yes it does. r.e. #27: He was talking about it in this item ;-) (Took me a while to find it.)
My brother is getting strong electrical "hash" on his phone line when his furnace motor starts (for about one second). He suspects this is causing his modem connection to fail. I presume this comes from the capacitor-start throwout-commutator. What is an effective filter for this? (I don't know how it couples into the phone line, but probably due to proximity and failure to use twisted pairs).
Is he using a cordless phone? Some of those are really susceptible to motor starts. Dunno if thit would affect modems, though.
Good question - he didn't say, so I'll ask. I've looked in catalogs at power line filters, which are L-C passive devices. They are described as filtering line to ground and line to line low frequency noise. But I'd like to know more about the common causes and cures for this kind of interference.
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- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss