Photocell and dimmer on same circuit bad?

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mark32

mark32

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Currently in NJ
I couldn't believe my ears last week when someone told me one cannot install a photocell on the same circuit that has a dimmer on it. (It's my understanding that the dimmer in question does not control the photocell but something else and resides on the same circuit) I was about to tell him he was crazy (In fact I think I already had) but sure enough I read on the back of the photocell package the directions confirming this man's claim. Of course there is no explanation as to why so that brings me to this post. Any ideas?
 
I'm looking at an Intermatic K4221C's install instructions and it says no such thing. What make and model are you looking at?
 
mark32 said:
I couldn't believe my ears last week when someone told me one cannot install a photocell on the same circuit that has a dimmer on it. (It's my understanding that the dimmer in question does not control the photocell but something else and resides on the same circuit) I was about to tell him he was crazy (In fact I think I already had) but sure enough I read on the back of the photocell package the directions confirming this man's claim. Of course there is no explanation as to why so that brings me to this post. Any ideas?
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Are you sure you read it correct? I have seen the " Not to be used on a circuit fed through a dimmer" notice on some brands but I have never seen where a dimmer can not be used on the same circuit that has a dimmer for other loads? think about it, even if it is on a different branch circuit after the panel it will still be on the same circuit before the panel, do you think the electrons will know the difference?
 
mark32 said:
I couldn't believe my ears last week when someone told me one cannot install a photocell on the same circuit that has a dimmer on it. (It's my understanding that the dimmer in question does not control the photocell but something else and resides on the same circuit) I was about to tell him he was crazy (In fact I think I already had) but sure enough I read on the back of the photocell package the directions confirming this man's claim. Of course there is no explanation as to why so that brings me to this post. Any ideas?
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Not uncommon for instructions to be poorly written. Probably means that the photocell cannot be downstream from the dimmer.
 
rattus said:
Not uncommon for instructions to be poorly written. Probably means that the photocell cannot be downstream from the dimmer.
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That is exactly what I was thinking. If the photocell is ahead of the dimmer then there should be no problem.
 
hurk27 said:
As long as its not a thermo controlled photo cell, they will turn on if a load is not maintained to a certain level.
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you sure about that? Most I have seen simply heat up a bimetal strip which operates a dry contact. It wont care how much load there is, just the input voltage.
 
kwired said:
you sure about that? Most I have seen simply heat up a bimetal strip which operates a dry contact. It wont care how much load there is, just the input voltage.
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Very sure, ever replace a burned out lamp in a post light and have it come on when you screw the new one in? no load = bi-metal stip cooled off and contacts closed.

I also did this for a customer who wanted to dim the porch lites, ran a 14/3 to the photo cell and placed it ahead of the dimmer in the switch box at the door, all worked ok till one day he dimmed it a little to much, he called me to tell me they wouldn't turn off the next day, told him to turn the dimmer back up a little and they went out.

heater for the bi-metal strip is in series with the load, as long as there is heat the contact stays open, there was a post on here that showed the workings of a photo cell, I tried to find it, but haven't been very good at getting the new search engine to work for me.
 
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hurk27 said:
Very sure, ever replace a burned out lamp in a post light and have it come on when you screw the new one in? no load = bi-metal stip cooled off and contacts closed.

I also did this for a customer who wanted to dim the porch lites, ran a 14/3 to the photo cell and placed it ahead of the dimmer in the switch box at the door, all worked ok till one day he dimmed it a little to much, he called me to tell me they wouldn't turn off the next day, told him to turn the dimmer back up a little and they went out.

heater for the bi-metal strip is in series with the load, as long as there is heat the contact stays open, there was a post on here that showed the workings of a photo cell, I tried to find it, but haven't been very good at getting the new search engine to work for me.
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I'm having a hard time with the logic of operation you are describing. How is it going to turn off if it depends on the current of the load to heat the bimetal strip to turn on?

Doesn't light striking the photo cell make current flow in a secondary circuit which heats the bimetal strip and this causes the contact to open? As long as there is light striking the cell the heater maintains holding the contact open. If there is a power loss then the contact will eventually close and the load will be on when power is restored but will shut off again if there is light striking the cell after the heater has had time to warm up.
 
kwired said:
I'm having a hard time with the logic of operation you are describing. How is it going to turn off if it depends on the current of the load to heat the bimetal strip to turn on?

Doesn't light striking the photo cell make current flow in a secondary circuit which heats the bimetal strip and this causes the contact to open? As long as there is light striking the cell the heater maintains holding the contact open. If there is a power loss then the contact will eventually close and the load will be on when power is restored but will shut off again if there is light striking the cell after the heater has had time to warm up.
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You have to think in a reverse mode when applying a photo cell, see the bottom of this page on the right:

http://en.wikipedia.org/wiki/Photoresistor it will explain what your missing.
 
kwired said:
It describes pretty much exactly what I said, how do you understand it to be?
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I have searched for any kind of a diagram to explain what I mean, and I have come up empty, so basically the best I can do is give a sequence of operation.

first, using a ladder diagram, you have two lines between the hot and neutral, first line is the N.O contact in series with the load (lamp) the second line is the heater in series with the photo resistor, hot is connected to the contact and heater, the neutral is connected with the load and the photo resistor, now put a small wire wound resistor between the switch leg of the load and the center point between the photo resistor heater, the end result will form a H on its side.

as you will see the resistance of a fully lit photo resistor still does not have a low enough resistance to cause enough heat to be produced by the heater, so the added resistance of the load and the wire wound resistor adds enough with extra head room for different load sizes to open the contacts, the rest of the operation is as you said, (my bad) if the load is removed then the photo resistor again does not have enough to maintain the heat level and the contacts close, again replacing a bad lamp will cause the new lamp to be on for a couple minutes till the heater again heats up with the new load, now apply a very low wattage CFL or LED in place of the incandescent and the lamp wont turn off, this is only for thermo type photo cells not electronic or relay type.

I can't believe I can't find this diagram on the internet, I have dissected a couple photocells and this is how they are wired?
 
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From the packaging of Westek's SW103CT, "Do not use this control with another light control or dimmer in the same circuit.". Actually the same goes for the other two photocell's I have by Westek.
 
hurk27 said:
This ls like saying "in series" for the same load. Big differance from "on the same circuit"
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Is it really that obvious? I believe that's poor wording on their part considering it stumped a customer and myself and lost a sale to boot. Why can't they just write, "Not to be controlled by another light control or dimmer"?
 
mark32 said:
Is it really that obvious? I believe that's poor wording on their part considering it stumped a customer and myself and lost a sale to boot. Why can't they just write, "Not to be controlled by another light control or dimmer"?
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It probably could be stated more clearly but 'on the same circuit' is really vague. Depending on how you wish to interpret that you may only be allowed one of the devices per service or separately derived system, one device per branch circuit, or one device in series with a load.

More products are coming out with instructions that are not that easy to understand all the time. I hate the ones that pretty much have pictures only. Installed a soft start recently - packed in the box was a sheet with mostly pictures and very little print. Had to do some experimenting to figure out exactly how to connect the control circuit.
 
hurk27 said:
I have searched for any kind of a diagram to explain what I mean, and I have come up empty, so basically the best I can do is give a sequence of operation.

first, using a ladder diagram, you have two lines between the hot and neutral, first line is the N.O contact in series with the load (lamp) the second line is the heater in series with the photo resistor, hot is connected to the contact and heater, the neutral is connected with the load and the photo resistor, now put a small wire wound resistor between the switch leg of the load and the center point between the photo resistor heater, the end result will form a H on its side.

as you will see the resistance of a fully lit photo resistor still does not have a low enough resistance to cause enough heat to be produced by the heater, so the added resistance of the load and the wire wound resistor adds enough with extra head room for different load sizes to open the contacts, the rest of the operation is as you said, (my bad) if the load is removed then the photo resistor again does not have enough to maintain the heat level and the contacts close, again replacing a bad lamp will cause the new lamp to be on for a couple minutes till the heater again heats up with the new load, now apply a very low wattage CFL or LED in place of the incandescent and the lamp wont turn off, this is only for thermo type photo cells not electronic or relay type.

I can't believe I can't find this diagram on the internet, I have dissected a couple photocells and this is how they are wired?
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Here is a link to one such a device that I took apart:
http://forums.mikeholt.com/showthread.php/124673-Why-does-a-photoeye-stop-working?p=1185675#post1185675

see post #18.

Here is the circuit as I traced it out:
BiMetal-Photocell.jpg


I think that an upstream dimmer in series may affect the switching time since the heater power is changing.
 
ELA said:
Here is a link to one such a device that I took apart:
http://forums.mikeholt.com/showthre...a-photoeye-stop-working?p=1185675#post1185675

see post #18.

Here is the circuit as I traced it out:
BiMetal-Photocell.jpg


I think that an upstream dimmer in series may affect the switching time since the heater power is changing.
Click to expand...

Nice diagram, but I have one problem, how does the heater cool off when the load is removed with power still applied to the photo cell? I have replaced hundreds of burned out lamps in post lights (popular around here) and every time the new lamp will light and go out after about 2-5 minutes, the photo cell I took apart had a resistor between the load terminal (red wire) and the point between the heater and photo resistor, it looked kind of like another part of the heater as it was wire wound, but wasn't close enough to the bi-metal plate to be part of the heating to open the contact.

So ok to make sure my mind is not going berserk on me, I just went out to my garage and hooked up a temporary lamp holder and a button photo cell I have for a job I'm doing tomorrow, (Kid's don't try this at home):D I screw in a 60 watt lamp let it cycle off, unscrew the lamp wait 2 min's screw it back in, it stayed off, unscrew it again this time wait 10 min's screw it back in it comes on and 2 min's goes back out?:confused: all the time power wasn't removed from photo cell, next I hook up a two wire dimmer load side of photo cell, I turn it full bright lamp goes out, turn it off wait 10 min's turn it back on but leave it 80% dimmed barely glowing, lamp never goes out, last test, screw in a 4 watt (25 watt equivalent CFL) photo cell never shuts off? try a 7 watt (40 watt equivalent CFL) this one did shut off, but I have had problems with a 7 watt CFL before, 13 watt's CFL's work just fine, but if LED's only pull a few watt's they might be a problem, never tried them yet.
 
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