Author Topic: Systematic PC failure  (Read 1259 times)
sol
Member
*****
Offline

View Posts
View Gallery

Systematic PC failure « on: December 21, 2014, 02:15:09 PM » Author: sol
Does anyone know what might cause repeated photo control failure in mercury vapour fixtures ? My father-in-law has one and his mother has one, and both only lasts about 1-2 months before becoming a day burner. Both are switched from inside, and both are usually switched off at bedtime. Various brands of photo controls have been tried, all with same results. They probably have HX ballasts of 175 watts and are in 8" bucket lights (NEMA wannabe). The PC socket is in the centre top of the housing. I don't know the brand. The power comany's street lights in the area don't have this problem. There are three possible reasons that I can think of : 1. the power is not "clean" meaning spikes or over voltages (although other electrical appliances aren't affected). 2. The ballast, owing to its less quality design, feeds back an electromagnetic kickback in the line when powered up and that wears out the PC. 3. The fact the lights are switched off at about 10PM is somehow hard on the PC.

Thanks for any advice on this.
Logged
Medved
Member
*****
Offline

Gender: Male
View Posts
View Gallery

Re: Systematic PC failure « Reply #1 on: December 21, 2014, 03:57:24 PM » Author: Medved
If there is any kick back, it is when the lantern is manually switched OFF.
Definitely the photocells are not primarily designed for that type of use, so they may not like it at all.

It then could destroy the internal supply part (dropping capacitor, diode bridge, Zener).
Normally, when the input supply is ON all the time and only the photo cell switches the light OFF, the overshoot happens behind the relay in the photocell, so the control supply isn't affected.
And if the photocell uses a triac as the main switch, it switches OFF only at the current zero cross, so creates no overshoot whatsoever (or very limited energy, easy to swallow by the snubber RC or the triac itself).

For better understanding what have happened it would help to see, what is inside (include some details), hopefully it would be possible to fix, but mainly find the way, how to make that arrangement (simultaneous manual and photo control) working reliably...

I doubt any ballast replacement would help, if we do not know exactly, what happened. It could easily end up the same way...
Logged

No more selfballasted c***

ggillis
Member
***
Offline

Gender: Male
View Posts
View Gallery

AEL 175W NEMA


Re: Systematic PC failure « Reply #2 on: December 22, 2014, 04:19:24 PM » Author: ggillis
The photo control won't be damaged by switching the power off, the switching device will take the beating.  I've had many PC's that get switched off during the night by timers and the PC's lasted for years.
Logged

Red Seal Electrician

Click on the link below to view my gallery:

http://www.lighting-gallery.net/gallery/index.php?cat=11790

Medved
Member
*****
Offline

Gender: Male
View Posts
View Gallery

Re: Systematic PC failure « Reply #3 on: December 22, 2014, 04:54:49 PM » Author: Medved
The photo control won't be damaged by switching the power off, the switching device will take the beating.  I've had many PC's that get switched off during the night by timers and the PC's lasted for years.

The switching device takes the beating well, but by that exposes the line from the switch to the ballast by the high voltage overshoot. No problem for the switch, but it could be a problem for the photocell electronic.
And if the timers were using triacs, there was no overshoots - the triac can switch OFF only as a consequence of the current disappearing.

Clearly, the PC's die, when connected behind a switch, but survived when after an automatic timer (you just wrote that), so you can not blame just the photocells or ballasts alone, it is only the combination of all the three elements (switch + PC + ballast).

The photocell contact is designed for all the overvoltages the switching causes, but not the power supply for the control part. Normally that is not any problem, as the control part is  connected to the permanently present mains, where would be no overshoots at all. The overvoltages happen behind the switching element, so normally on the output side, where is nothing connected (just the ballast).
Now when you add a mechanical switch upstream of the photo cell, all behind that switch would become exposed to the overvoltages and it is a question, how that would be handled.
With the old fashioned photocells consisting just of a photo sensitive resistor and a relay (either electromagnetic or thermal), there is nothing sensitive to the overvoltages (all the control part is just the photo resistor in series with the relay coil/heater directly on the mains input), so they handle that well (500V spike is just about 4x the rated voltage for a brief moment, so not enough energy to cause any overheat).
With electronic units, the controller needs a low voltage supply (~24V for relay output, anything between 5..24V for the triac versions).
And that could be done either by dropping resistor (that would remain tolerant to the overvoltages, but it delivers only limited current, so can not feed a relay, so limited to triac output)
Or a dropping capacitor. That does not dissipate virtually any heat, so allows for higher current, so able to feed the relay output. But as  the capacitors impedance reduces at higher frequencies / fast transients, for the overshoot it becomes nearly a short circuit. So with the same 500V spike example a part designed to operate at about 24V suddenly get the full 500V, what means ~20x overvoltage. And that becomes way too much, even when there is a resistor in series (for exactly this purpose; but not designed to accept too many of such pulses)

As I wrote, without knowing what is damaged, it is not possible to tell for sure, what was the reason.

And even with cutting the electricity off upstream of the photocell, it very strongly depend on where and what cut it out. For an example when the power is cut on the utility side, the voltage just disappears, so no overshoots whatsoever (that is, what the photocells are designed for).

But if you have just a switch upstream without anything else, you easilly end up with high 100's V spike, when the ballast exhibit inductive behaviour. So the photocell could have problems.

When the ballast is of a power factor compensated series choke or HX (so has a capacitor on it's input), there will be no turn OFF overshoot. But there would be some turn ON inrush current, unless the turn ON is timed so, it happens at zero voltage across the switch. So the photocell won't have any problems at switch OFF, but it might have problems at switch ON (in case of a triac as the switching element - it tend to trigger by itself just as the response to the dV/dt from the mechanical switch operation, so out of control of the controller part, which could normally control the timing so to prevent the inrush current).
Logged

No more selfballasted c***

sol
Member
*****
Offline

View Posts
View Gallery

Re: Systematic PC failure « Reply #4 on: December 22, 2014, 11:03:12 PM » Author: sol
Thanks for all the detailed explanations. If I understand correctly, the way it is  now wired is

Mains --> wall switch --> PC --> Ballast

and to prevent the electronic components in the PC from over voltage spikes from the ballas upon switch off, a fully mechanical old style PC or this configuration is required :

Mains --> PC --> wall switch --> Ballast.

This last configuration would work but would be complicated to wire unless a remote PC is installed.

Since the PCs in question have been day burners for years, and only manually switched from inside the house, I would imagine it will stay like this for some time. It is nice to know, however, a possible explanation of why it is the case. I also have a 175 MV that is switched from inside, but the only time I switch it off manually is when I want a better look at the stars, and only happens about 2-3 times a year. The PC is still good and should be for years.

Thanks again and seasons greetings.
Logged
Medved
Member
*****
Offline

Gender: Male
View Posts
View Gallery

Re: Systematic PC failure « Reply #5 on: December 23, 2014, 10:40:21 AM » Author: Medved
With an electronic PC you ma try to disconnect it's Neutral wire:
(copy following into some fixed character width editor like Notepad or so)

Line ----------\    /------------
                Photo            |
                cell             |
                  |             Lamp
                  o /            |
                   /  switch     |
                  o              |
                  |              |
Neutral-----------o--------------


When switching OFF the Neutral, the PC looses it's power and so the relay switches OFF.
By that the overvoltage would be present just on the PC output, so not seen by it's electronic.
Compare to the "mains -> PC -> switch -> lamp" you would need just one wire extra. But try that with your PC type first, it works only, when the relay in the PC is of a NO type.

Other option would be to create a clamp behind the switch. The clamp could be either a varistor, or even just a larger capacitor.
Logged

No more selfballasted c***

ggillis
Member
***
Offline

Gender: Male
View Posts
View Gallery

AEL 175W NEMA


Re: Systematic PC failure « Reply #6 on: December 23, 2014, 11:03:49 AM » Author: ggillis
Would maybe a two pole switch work? One between the ballast and pc, and the other switching power to the whole fixture
Logged

Red Seal Electrician

Click on the link below to view my gallery:

http://www.lighting-gallery.net/gallery/index.php?cat=11790

Medved
Member
*****
Offline

Gender: Male
View Posts
View Gallery

Re: Systematic PC failure « Reply #7 on: December 23, 2014, 11:32:33 AM » Author: Medved
Switch between the ballast and PC would work the best, but it would require two additional wires from the lantern.

Two pole switch does not work, as you never know, which of the two poles disconnects first. So if the one switching the power disconnect at first, you are back where you are now.
Logged

No more selfballasted c***

Print 
© 2005-2022 Lighting-Gallery.net | SMF 2.0.19 | SMF © 2021, Simple Machines | Terms and Policies