Electrical advice for wiring new UV Exposure Unit.

[bjames]

Hello, Im not sure where the best place to post this is, but hoping someone here has some electrical know how to guide me. If this isn't the right place please suggest which place would be.
Im just about done building a UV exposure unit and figured the wiring would be pretty simple, right, just follow the diagram. Well, I got a set of tombstone terminals already wired with double wires. So my question is can I attach both wires to the corresponding color lead, or should I run the other wire around to the other side? does this do the same thing? What little I found on line talked about "pigtailing, and shunting" but I don't know what that really means. The article also talked about rapid start and instant start tubes being wired differently. Since I have the instant start kind, I thought I better double check with someone. Thanks in advance for any help
Brandon

 

[Denis K]

Instant start lamps tie both leads together and you can do this either way you mention. You could twist together the two leads in your left picture and it would be electrically identical to the one in the middle picture.

Your ballast is designed for four bulbs. Is that what your exposure unit will have? Take a look at the wiring diagram at the bottom of the following web site and tell us if there is anything that is different than your intended application, or if there is anything you don't understand. It is not a good idea to run a ballaast with fewer bulbs than it is designed for.

http://nemesis.lonestar.org/reference/electricity/fluorescent/instant.html

One thing that is not always obvious about Fluorescent lamp fixtures is that the bulbs need to be positioned next to a metal plate that is held at ground potential. If you look closely at most Fluorescent lamps you will find that they always have metal structure directly behind the bulbs. If you look at the wiring diagram I just referenced you will notice that they tie this metal structure to the green ground wire coming from the fuse box. This grounded structure provides a ground potential for the bulbs to fire against. Your exposure unit needs to provide this ground plane either by using a commercial lamp fixture or by you in your design. You could try getting by without it, but it may, or may not, work and at best it may prove to be unreliable.

The reason for the two leads coming out of each tombstone, as you call it, is that in non-instant start applications, the bulbs contain a small filament winding across these two terminals at each end of the bulb. These filament windings heat up and make it easier for the non-instant start ballasts to get the bulbs to fire across the length of the tube. After the bulbs fire, these starter filaments do nothing usuful and just set there sucking up power. Instant start ballasts solve this problem another way. They have the ability to generate a very high voltage at power on and this high voltage can fire the bulb without the need for the hot filaments. Therefore, in an instant start ballast application they just tie the two filament leads together since they don't need to run a current through them to warm up the filiment winding.

Denis K

 

[bjames]

Thanks Denis, Your input is very helpful. It is nice to get a second opinion when doing this sort of stuff. Especially since I am no expert and this stuff could be dangerous. As for the metal, being near the tubes I have read differing opinions on that. The latest being this thread which seems credible. http://www.electriciantalk.com/f8/t12-t8-3872/
The part about metal is toward the bottom of the first page. It seems the metal is not for operation of the tube per say. Grounding would be of importance when dealing with this amount of power. however from what I understand it is not absolutely necessary for function of the tubes. That being said, I have a piece of aluminum I was going to place between the tubes and the wood box, for grounding purposes and for added reflection. Is there anyone who has bought a commercially made uv wood box from bostick&sullivan or equivalent, that can tell how it is grounded and if it has a metal plate close to the bulbs.
Thanks again Denis.
Brandon

 

[Denis K]

Brandon, You could certainly go ahead with your project without a ground plane and see if the lamps fire up reliably. The ground plane is used to reduce the voltage required to fire across the length of the lamp. The ground plane acts as a capacitor, allowing the initial arc to fire between one of the ends and the nearby metal plane. The arc, once started, can then sweep across the bulb until the entire bulb is lit, thereby eliminating the need to fire across the entire bulb. It is reported that rapid start ballasts need this more than other ballast types as they rely less on higher voltages.

The down side to having no ground plane, especially in rapid start applications, is that if the ballasts have a hard time firing the bulbs the life expectancy of the bulbs is said to be shortened. Since you are using an instant start ballast, which does not heat up the filaments in the bulbs, this should not be a big factor so long as the ballast is able to light the bulbs. However, another use of the ground plane is to reduce bulb flicker. Who knows what effect bulb flicker will have on the UV emission of your bulbs, which is what you are after. At worst, it will probably just make your process timing a bit more difficult, but who knows. If you try asking the bulb manufacturer all they will tell you is to construct it properly and install the ground plane.

Denis K