Control/Dimming Building a Ghost Load Box

[lomahongva]

So this seems like something that would have been talked about before but i can't seem to find a thread that talks about it.

Im looking to build a ghost load box and I've always used ones which where nothing more then a enclosure for a lamp that was whatever wattage you needed.

But my idea was to use a 100watt 100ohm resistor similar to Power Resistor, 100 Ohm, 100 Watt, +/- 3%, Dale RH-100 at The Electrostore.com - Electronic Surplus Parts & Equipment

This way your box can be a smaller profile and the resistor is made to dissipate heat also no worries of light leakage. Is there any reason not to do this?

 

[headcrab]

It won't be NRTL listed.
Your insurance agency/AHJ might have a problem with it.
Also, A 100 ohm resistor at 120V will dissipate 144W.
I notice you linked to a surplus site, and that the resistor has been used already. I would use something like this HS150 100R F Arcol Wirewound Resistors - Chassis Mount where at least you have some assurance that it's still good.

 

[lomahongva]

Ya i just listed that one because it was the first google result . I agree with the NRTL listing but wouldn't building a box of lamps give you the same issue?

 

[JD]

Bit of a Catch 22 on that one as I don't think there is one that is listed! Maybe someone on here can point you to one but I have not come across one before. (Although I admit, I wasn't looking.)

One of the problems that face getting one listed is heat buildup. In theory, if you mounted it in a case due to the exposed wiring, you would have to insure there is sufficient air circulation through the case, which would probably mean a forced air (fan) system. Then you would have to have some safety system to insure that the unit shuts down if the air system fails. Also, there would be heat concerns regarding the case and insuring the case could not cause burns or a fire no matter what failed.

Thus, why so many drop-lights end up being hung in the closet! (unsafe)

EDIT:
The whole thing reminds me of a funny story from the 80s. A club band LD was running some low voltage transformer lights that would not run correctly unless the dimmers had a dummy load. He had built this road case that had four toasters in it!! I told him it was the most stupid dangerous thing I had ever seen. He replied "Why? The toasters are UL approved!" Lacking time to think of an appropriate comeback, I believe my comeback line was, "Yea, but only if there is toast in them!" (The 70s/80s was a very strange time on the club circuit.)

 

[lomahongva]

Bit of a Catch 22 on that one as I don't think there is one that is listed! Maybe someone on here can point you to one but I have not come across one before. (Although I admit, I wasn't looking.)

One of the problems that face getting one listed is heat buildup. In theory, if you mounted it in a case due to the exposed wiring, you would have to insure there is sufficient air circulation through the case, which would probably mean a forced air (fan) system. Then you would have to have some safety system to insure that the unit shuts down if the air system fails. Also, there would be heat concerns regarding the case and insuring the case could not cause burns or a fire no matter what failed.

Thus, why so many drop-lights end up being hung in the closet! (unsafe)

EDIT:
The whole thing reminds me of a funny story from the 80s. A club band LD was running some low voltage transformer lights that would not run correctly unless the dimmers had a dummy load. He had built this road case that had four toasters in it!! I told him it was the most stupid dangerous thing I had ever seen. He replied "Why? The toasters are UL approved!" Lacking time to think of an appropriate comeback, I believe my comeback line was, "Yea, but only if there is toast in them!" (The 70s/80s was a very strange time on the club circuit.)

HAHA GREAT STORY!!

Ya i had thought about doing a fan system but for my application i was going to build it in a electrical box with air slots. Then mount the whole box to a structure with square tubing so its basically free floating with plenty of air flow.

 

[FMEng]

It won't be NRTL listed.
Your insurance agency/AHJ might have a problem with it.
Also, A 100 ohm resistor at 120V will dissipate 144W.
I notice you linked to a surplus site, and that the resistor has been used already. I would use something like this HS150 100R F Arcol Wirewound Resistors - Chassis Mount where at least you have some assurance that it's still good.

If you want to dissipate 100 Watts, use a 150 ohm resistor. It's usually advisable to use a resistor rated for double the power actually dissipated. That will help account for a minor air flow restriction around it, too.

 

[n1ist]

I would put a normally-closed Klixon temperature switch in series with the resistor, mounted to the same heatsink as the resistor. That way if it overheats, it will disconnect the ghost load rather than causing a fire. I would also put a fuse in series to protect against shorts.

Also remember that the 150w rating is on a 1000cm3 3cm deep heatsink in 25 degree C air.
/mike