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Old 03-22-2011, 06:44 PM   #1
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Default Walker's Confounding Lamp - discuss

When I built my system, I wanted to have a lamp on the panel than would light up when there was current flowing through my heater element. I bought some 120v lamps for this.

Here's how it was connected right after installation in the in-progress panel. Red and black come through contactor. Red line is then switched by SSR and feeds lamp and heater element (heater element not plugged in at this time). Black line goes right from contactor to element. Lamp was connected to the switched 120V hot line and neutral.



After hooking it up, I flipped on the contactor. SSR was not even connected to the PID at this time. To my confusion, the lamp illuminated. Eventually I realized that this made sense. The SSR leaks a little current, and the lamp needs very little current to illuminate, so the leaked current was taking it's only route available and was coming through the lamp to neutral.

So, I think, "Well... when I have a much lower resistance path available for that leaked current to go travel through, then it should work." So, I plugged in the heater element. The SSR is still not connected to the PID, but it now looks like this:



When I turn on the contactor, the lamp still lights up.

THAT'S QUESTION #1: why is the little bit of current that leaks through the SSR going through the lamp instead of going through the much lower resistance heater element?

Screw it. I am using the system. I used it like this for a year. The lamp comes on as soon as I activated the contactor, and stays on, fully illuminated, no matter what. PID might be driving SSR, might not, but the lamp is on. Period. The only way to shut the lamp off is to turn off the contactor.

I've been adding things to my panel and tweaking stuff recently. I decided to fix this thing. I got rid of the neutral and connected the lamp fully in parallel to the heater element. With a resistor added to match the lamp so that 120V drops across the lamp and 120V drops across the resistor. This DOES behave in a sane way, but just wait for more insanity.



In this configuration above, the lamp is partly lit when the contactor is on and the heater element is not plugged in and the SSR is not being triggered. This makes sense because of the leakage current from the SSR. When the SSR is triggered, the lamp lights up fully. This all makes total sense.

So, plug the heater element in now:


Everything SEEMS to be functioning perfectly. With the heater plugged in, leakage current from the SSR goes through the element and not the lamp, and the lamp stays dark. When the SSR is triggered and the heater starts to fire, then the lamp comes on fully. Perfect and exactly what I wanted.

Happy with the fact that it seemed to be working, I shut everything off and turned out the lights in the garage.

Wait... What's that!? The lamp is on just BARELY. Hard to see with overhead lights on, but obvious when the garage is dark.

So... this is the REAL confusing part to me.

Why is the lamp on at all when I have this:



The black hot line to the lamp is alive because it's coming from a point prior to the contactor, but the red hot line is DEAD. The contactor is off. There can't be any leakage through the SSR, because there is no source attached to the SSR.



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Old 03-22-2011, 08:11 PM   #2
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I response to Q #1, the element is providing a path for the black line to "backfeed" your lamp. If you go black through contactor, element, lamp to neutral you've got a complete circuit.

As far as why the lamp was glowing when you turned the lights out, that's a good one. If you weren't using a GFCI I would suspect a VERY high Z path to ground. How long was it between when you opened the contactor and when you turned the lights out?

Also, why not just move the black line to the lamp to the load side of the contactor. That way the power's totally cut.



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Old 03-22-2011, 08:13 PM   #3
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#1: Electricity flows down ALL paths, not just the one of least resistance.

#2: I'm assuming your lamp is neon? They require very VERY little current. Under 2mA usually. Even with that 50K Ohm resistor, it's still seeing over 2mA. As for why it's glowing after everything's turned off, I'd guess the PID's internal power supply has a cap that's leaking current back over the SSR signal lines.

Incandescent indicator lamps are much less headache for projects like this...

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Old 03-22-2011, 08:21 PM   #4
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Terminator, at the point where the machines start to take over. Just wait until your Stainless starts to form different humanoid shapes.


I am expecting similar results when I finally fire up my new panel and this will be interesting to watch.

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Old 03-22-2011, 08:22 PM   #5
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Q1. Yes, I know that electricity flows on all paths, but out of the total current flowing, a prroportional amount will flow on each available path, inversely proportional to the resistance of that path.

In other words... current is being allowed to flow through the lamp AND the element in the original circuit. the vast majority of the current should flow through the element and not the lamp, because there is much much less resistance through the element.

To ask a related question, why does it behave PROPERLY now, but not in the original circuit? From the leaking current's point of view, there is little difference. There is a low resistance path through the elmement and a high resistance path through the lamp.


I do not want the lamp connected to the contactor's switched line because the lamp is on the swinging door of my control panel and the contactor is down in the belly. The swinging door has it's own complete set of wiring that is all tapped from the black incoming feed, and I power everything on the panel door with 120v. I'd like as few connections going from the belly to the door because of the long wires that are necessary when I open the door.

That's why the lamp was originally hooked to 120v. One hot line coming up and out of the belly to the lamp, but then a short connection to the neutral bus mounted on the door.

To jkarp: yes, the lamp is neon and does require very little current to light up, which is why the leakage is capable of making that happen when the SSR is off.

but, the slight illumination when the contactor is off at this time is not coming from the PID. It happens even when the SSR control lines are not connected to the PID!

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Last edited by Walker; 03-22-2011 at 08:24 PM. Reason: edit. last sentence has a "SSR" where it should have had a "PID"
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Old 03-22-2011, 08:27 PM   #6
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regarding slight illumination:

perhaps there is some slight capacitive behavior in the SSR, allowing a little charge to be built up in the SSR as the one hot line that is connected swings back and forth across the 0v line? That would result in a bit of a difference in potential between the swinging line and the point connected to the SSR, and would give a super tiny voltage drop across the lamp....

hmmmmmm

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Old 03-22-2011, 10:17 PM   #7
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Quote:
Originally Posted by Walker View Post
regarding slight illumination:

perhaps there is some slight capacitive behavior in the SSR, allowing a little charge to be built up in the SSR as the one hot line that is connected swings back and forth across the 0v line? That would result in a bit of a difference in potential between the swinging line and the point connected to the SSR, and would give a super tiny voltage drop across the lamp....

hmmmmmm
Most SSRs do have a snubber circuit on the output that contains a cap. Kinda surprised it would be enough to light neon though...

I'm unclear on the initial change from the separate neutral for the lamp. Because the SSR leakage current is fixed, adding the element effectively drains the leakage current and drops the circuit voltage below the neon's threshold.
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Old 03-22-2011, 10:30 PM   #8
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Quote:
Originally Posted by jkarp View Post
Most SSRs do have a snubber circuit on the output that contains a cap. Kinda surprised it would be enough to light neon though...
Well, this thing is barely lit. Like I mentioned, I never even noticed it until the lights were out and it was completely dark in the garage.

A cap is the only thing I can think of that would cause it to behave like this.

Quote:
Originally Posted by jkarp View Post
I'm unclear on the initial change from the separate neutral for the lamp. Because the SSR leakage current is fixed, adding the element effectively drains the leakage current and drops the circuit voltage below the neon's threshold.
By "unclear", do you mean that you also don't understand why it behaved so oddly at first?

I have puzzled and pondered on this a lot. It was strange enough that I thought others would have fun puzzling and forming their own theories.
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Old 03-22-2011, 10:47 PM   #9
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The leakage is because of the snubber internal to the SSR. You can purchase snubberless SSR's. Read the following:

http://www.galco.com/techdoc/crdm/d2450_faq.doc

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Old 03-22-2011, 10:59 PM   #10
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Walker's Confounding Lamp

I would consult the Magic Item Compendium or the Magic User's handbook



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