Strange behavior (lights dimming) when running heater on different heat settings

[Jay112]

I have an infrared heater that looks like this one: http://www.farmtek.com/farm/supplies/prod1;ft_heaters-ft_infrared_heaters;pg105806.html

except mine is 1,500 watts (120volt, USA) and has 4 different heat settings. I've opened up the circuit board before, and I think the heat level is controlled by an SCR.

I have a small light fixture connected to an outlet on the same line, which holds three 7-watt incandescent candelabra bulbs. The line is connected to the breaker box with a 20-amp breaker and 12-gauge wires. There are no other high-powered devices on the line except for the heater.

This is the strange behavior I'm seeing from the lights only when the heater is running:
-(The heat levels are from 1 to 4, with 1 being the lowest and 4 being the highest setting.)
-When the heater is set to either 2 or 4, the lights behave perfectly normal.
-When the heater is set to 1, the lights dim a lot and make bright(er) flashes every 3-5 seconds.
-When the heater is set to 3, the lights become even dimmer than the 1st setting, but they don't flash like in the 1st setting.

Do you have any ideas why the lights are behaving like this when the heater is running? Can you suggest any tests that I could do to help diagnose it? (I don't have an oscilloscope).

I tested the outlet with a multimeter, and it showed 122 volts when the heater wasn't running, and went down to 121 volts when the heater is turned on, regardless of what setting the heater is on.

Thanks!

[Jay112]

BTW I was thinking/hoping that maybe this was a common problem. If it's not a common problem, then I have some more information to give that could be potentially relevant. I didn't want to make the first post unnecessarily long if someone ended up immediately knowing the answer.

[Cubdriver]

First off, do the lights dim slightly when the heater is initially turned on in settings two and four?  Secondly, does it have an even number of heating elements?  If so, I'm going to guess that in switch settings 2 and 4 the elements are on all the time and either in series (2) or parallel (4), and thus draw a steady current.  The #1 and 3 settings would then likely also be series and parallel respectively, but with the SCR/Triac actively controlling the current.  Perhaps for some reason in setting 1 it switches off completely periodically, while in setting 3 it stays on continuously, but obviously drawing a higher current than in setting 1.

It's a shame you can't get your hands on a scope - it would be interesting to see what the line looks like.

-Pat

[Jay112]

Thanks for responding!

Yes, the lights dim a tiny bit when the heater is turned on on settings 2 or 4. The dimming is hardly noticeable at all.

As for the heating elements, it only has a single one, and it looks like a single metal rod. It becomes glowing red on the highest setting, but less red on the lower settings.

I might end up purchasing an oscilloscope sometime within the next month. I'll post the results here if I ever end up testing it.

[AmbientSix]

It would be curious to see at what point in the sine wave the SCR is turning on!