OT - Electrical repair on small transformer, halogen lamp

specfab · Dec 26, 2018

See attached pic of lamp, purchased several years back from Home Depot. I have a few of these around, and find them very useful for close work where you need some decent light. 50W halogen bulb that is supplied through a hi-low-off switch and 12V transformer in the lamp base. One of them stopped working, and after some bumbling around with a meter and replacing the switch, found that the transformer was open. I did some unwrapping of the windings, and found what appears to be a thermal fuse, which seems to be open, so I assume it's the culprit, unless the whole transformer is shorted badly. Transformer seems to have continuity on both sides, though.

Anyone out there with guidance as to how to rate a replacement thermal fuse in this application? Markings on the component are gone/indecipherable.

TDegenhart · Dec 26, 2018

The fuses you seek are not commonly available to anyone but manufacturers. They are specified by melting temperature. The following article describes how they work and applied. Note that the comments about ambient temperature are completely wrong as to increasing the fuse temperature to compensation for a high ambient. The reason is that electrical insulation ratings are based on absolute values not relative.

The U/L procedure is to measure the temperature rise of the device in a 40*C environment. The total temperature cannot exceed the temperature rating of the insulation system. The fuse is selected to be below this maximum temperature.

A blown fuse could be thermal fatigue but most likely the the device was overheating. Most likely cause is shorted turns developing in the windings.

Fuse is gone, transformer is junk. Don't try to fix it.

History of cheap thermal fuses | All About Circuits

Tom

JST · Dec 26, 2018

You CAn actually get them from Digikey, but you need to know the rating.

Those halogen transformers run hot, I have no idea what the rating ought to be. Transformers using paper insulation are generally rated for a temp that suggests a 90C switch, but that depends on the switch location and the actual switch type may have been determined by a test, so the 90C mentioned is not absolute.

They also fail, and it is rare to see a switch failure that does not have a transformer problem associated with it.

tnmgcarbide · Dec 26, 2018

THey are cheap.i'd get a few values from say...85c to 105c . and start from low to higher until they stop blowing.
( those things are/were a fire hazard in the first place! so never leave one unattended for long... )

eBay - Page Not Found

if you must keep them... a retrofit of LEDs would be a cheap and easy fix

TDegenhart · Dec 26, 2018

tnmgcarbide said:
THey are cheap.i'd get a few values from say...85c to 105c . and start from low to higher until they stop blowing.
( those things are/were a fire hazard in the first place! so never leave one unattended for long... )

eBay - Page Not Found

if you must keep them... a retrofit of LEDs would be a cheap and easy fix

The fire hazard is from the burner capsule. When I worked in lighting, all halogen lamps had to have a protect cover to prevent a shattered burner from falling on combustible material, or for the lamp to touch combustible material. When they first came out, they were a real hazard of being knocked over and starting a fire.

Selecting an assortment of fuses and trying them until they quit blowing is not safe as the fuse could have blown because the transformer is failing.

In the old days of screw in fuses, a 15 amp fuse blows, you keep putting in the next larger fuse until they quit blowing. Eventually a penny. Then a fire. Homeowner circuit breaker were a godsend because its much more difficult for a homeowner to put in a larger breaker than a fuse.

Fuses don't normally blow. They go when there is a problem. That's what they are for.

Tom

tnmgcarbide · Dec 26, 2018

TDegenhart said:
The fire hazard is from the burner capsule. When I worked in lighting, all halogen lamps had to have a protect cover to prevent a shattered burner from falling on combustible material, or for the lamp to touch combustible material. When they first came out, they were a real hazard of being knocked over and starting a fire.

Selecting an assortment of fuses and trying them until they quit blowing is not safe as the fuse could have blown because the transformer is failing.

In the old days of screw in fuses, a 15 amp fuse blows, you keep putting in the next larger fuse until they quit blowing. Eventually a penny. Then a fire. Homeowner circuit breaker were a godsend because its much more difficult for a homeowner to put in a larger breaker than a fuse.

Fuses don't normally blow. They go when there is a problem. That's what they are for.

Tom

my point was that he didn't know the actual value . up to 105c would be the limit where many devices,
like capacitors(85c-105c) are rated , so that's where i'd stop and toss the thing in the trash.

LEDs are inexpensive - he could retrofit his old lamps, or buy new. either is less costly than a new house.

specfab · Dec 26, 2018

Thanks for the commentary, all. I had already checked Digikey as a source, which led me to asking the question in the first place. I'm certainly not interested in burning my shop or house (burning up, burning down, either way), this is simply one of those projects that has been sitting around for a couple of years waiting for some "idle" time to spend on it. Looks like a mostly lost cause at this point. I looked for a replacement transformer, but finding that is much like trying to find the right thermal cutoff. Damn near impossible, it's all purpose-built for the OEM.

plastikdreams · Dec 26, 2018

It's a kamikaze lamp...just get a new one

n2zon · Dec 26, 2018

I second (or third, or whatever) the suggestion to get LED lamps. But use caution in selecting suitable LED replacements for a work environment. See: Which lamp will be optimum to eye? Incandescent, fluorescent or LED etc

Milland · Dec 26, 2018

n2zon said:
I second (or third, or whatever) the suggestion to get LED lamps. But use caution in selecting suitable LED replacements for a work environment. See: Which lamp will be optimum to eye? Incandescent, fluorescent or LED etc

Apparently that article was retracted due to a conflict of interest by the authors. Still might have good info, but you have to decide:

Retraction Notice

Lakeside53 · Dec 26, 2018

I have about 7 of these lamps from the late 80's early 90's. Switch failure is common - all of mine failed over time. 6 of them I've converted to led with a 24v ac/dc wafer insert; the last one I left as Halogen so I can use it to heat small items on my bench.

They are decent lamp - work on it but convert to led. You can undersize any thermal . Typical LED will be 2-3 watts... not the 50 of the original halogen

adama · Dec 27, 2018

You can get small ac - 12Vdc modules now, most use solid state and hence are tiny, so long as you can hollow out the existing transformer space, you should be good to go. If your happy to DIY small torodial transformers are also simply and readily available. For halogen you don't even have to rectify it to DC, they run fine on 12V RMS AC too.

n2zon · Dec 27, 2018

Milland said:
Apparently that article was retracted due to a conflict of interest by the authors. Still might have good info, but you have to decide:

Retraction Notice

Oh, I missed that. Good to know. However, the basic point remains that some emitters (halogen or otherwise) emit radiation that can be harmful to your eyes with the constant exposure you get in a work environment. It's worth a bit of research to determine whether what you propose to use is OK. For my basement, where I might make chips a few hours a week, it's not that important IMO. If I were doing it for 50 hours a week it would change the game a bit...

TDegenhart · Dec 27, 2018

n2zon said:
Oh, I missed that. Good to know. However, the basic point remains that some emitters (halogen or otherwise) emit radiation that can be harmful to your eyes with the constant exposure you get in a work environment. It's worth a bit of research to determine whether what you propose to use is OK. For my basement, where I might make chips a few hours a week, it's not that important IMO. If I were doing it for 50 hours a week it would change the game a bit...

And what would you propose doing?

Tom

9100 · Dec 27, 2018

Halogen lamps definitely radiate shorter wavelength UV than regular incandescents. When I was doing spectrophotometry, we were mostly working at 340 nanometers and regular glass cuts off at 300, which meant we were on the downside of the transmission slope. Halogen lamp filaments run at higher temperatures and the quartz envelope transmits well into the UV. These are 1965 memories so I don't recall just how far. I don't think it is an accident that the lenses in our eyes cut off sooner because UV will damage retinas and Darwin took care of the problem. That doesn't help corneas, though, so halogen lamps may damage them in long exposure.

Halogen lamps and quartz optics would go way down into the UV but heat and cost forced us to stay with glass and accept the poor performance.

BTW, when scientists were donning dark glasses to watch the Alamogordo atomic blast, Richard Feynman reasoned that glass would block all the harmful UV so he sat behind the thickest truck windshield without sunglasses. He got a super case of retinal fatigue and just saw a big orange ball for a while but had no permanent damage.

In a work environment real glass safety glasses would block the harmful radiation. So would a piece of window glass in front of the lamp. Leave a space for air circulation.

Bill

specfab · Dec 27, 2018

Interestingly, even the cheap-ass Home Depot lamps came with a decent glass filter disk in front of the halogen source. I guess they could see liability lawsuits in the future....

TDegenhart · Dec 27, 2018

9100 said:
Halogen lamps definitely radiate shorter wavelength UV than regular incandescents. When I was doing spectrophotometry, we were mostly working at 340 nanometers and regular glass cuts off at 300, which meant we were on the downside of the transmission slope. Halogen lamp filaments run at higher temperatures and the quartz envelope transmits well into the UV. These are 1965 memories so I don't recall just how far. I don't think it is an accident that the lenses in our eyes cut off sooner because UV will damage retinas and Darwin took care of the problem. That doesn't help corneas, though, so halogen lamps may damage them in long exposure.

Halogen lamps and quartz optics would go way down into the UV but heat and cost forced us to stay with glass and accept the poor performance.

BTW, when scientists were donning dark glasses to watch the Alamogordo atomic blast, Richard Feynman reasoned that glass would block all the harmful UV so he sat behind the thickest truck windshield without sunglasses. He got a super case of retinal fatigue and just saw a big orange ball for a while but had no permanent damage.

In a work environment real glass safety glasses would block the harmful radiation. So would a piece of window glass in front of the lamp. Leave a space for air circulation.

Bill

While you are correct in what you say, in very few cases do we look directly at the source of light for any length of time, i.e. the sun, filaments or arc lamps or fluorescent. Mostly we look at the illuminated object. As such, the surface properties of the illuminated object are very important in determining what spectrum reaches our eyes. The properties are, reflectance, emittance and transmission. Reflectance doesn't change the spectrum, just the directional properties of the beam. Emittance and it twin absorbance can markedly alter the spectrum. A surface with high absorbance and near zero reflectance or tranmitance will appear black. The radiation doesn't go away, it is changed into heat energy that warms the object. In thermal equilibrium, the absorbed energy can be transmitted but at a much longer wavelength and is often felt as warmth. In so doing, the short wavelength (blue) components disappear.

A case where the blue is prized in the the jewelry store. Lamps with a high blue component such as halogens are used to show off the sparkle of diamonds. Next time you are in a jewelry store, look around. General lighting is often fluorescent or incandescent but the lighting over the diamonds is halogen.

Tom

Lakeside53 · Dec 27, 2018

specfab said:
Interestingly, even the cheap-ass Home Depot lamps came with a decent glass filter disk in front of the halogen source. I guess they could see liability lawsuits in the future....

More to do with protecting fingers and paper from the hot lamp...

n2zon · Jan 7, 2019

TDegenhart said:
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And what would you propose doing?

Tom

I'd start by reading stuff shown by Google about LED spectrum and brightness: workplace lighting LEDs and eye health - Google Search

...and then be thoughtful about the lighting I'd be using 40+ hours per week.

OT - Electrical repair on small transformer, halogen lamp

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