Basic electronics question: is it bad to have a transformer that’s too big?

[Roger Feeley]

My daughter has a bunch of lighting fixtures that each have two 50 watt, 12v PAR bulbs. Inside, there is a 100w toroidal transformer. They have been swapping in 7 watt 12v LED bulbs.

I want to know if they are actually saving electricity using those low wattage bulbs with a high capacity transformer. My gut tells me Togo find a 15 watt replacement.

now a related question. They replaced the high wattage bulbs with LEDs and ran it. Now, for some reason, the transformer is bad. There isn’t much to these things. They are a doughnut shaped core wrapped with copper. Did running the LEDs lead to the failure?

[Lee Schierer]

Wattage through a transformer is governed by the load it is supporting. An oversized transformer will not appreciably use more power than several small transformers supporting the same total load.

[Roger Feeley]

Lee,
so, to be clear, there is no downside to having a 100w transformer powering a14 watt load?
what about my second question. Was the failure of the 100w transformer related to driving only 14 watts?

i am looking into 15w transformers and coming up empty.

input: 120vac
output: 12vac
I found some smaller transformers but I realized that the transformer will be completely enclosed. I have to watch out for heat. I don’t want to burn the house down.

[Jim Koepke]

Roger, How did you verified the transformer is the problem?

jtk

[Pat Barry]

Using a higher wattage rating is not an issue. The issue is using one that is underrated.

[Edwin Santos]

I agree with the others.
In fact, for LED low voltage lighting installations I have done, the recommended practice is to oversize the driver by 25-50% for a cushion.

[Roger Feeley]

Roger, How did you verified the transformer is the problem?

jtk

jim,

at first, I thought it was a bad connection inside the fixture. We have 16 of these fixtures and I’ve seen some arcing leading to a bad connection in the past. I was surprised to see no evidence of arcing or burning. Everything looked clean.

the next step was to isolate the problem. I removed the transformer. I hooked the primary up to 115vac and measured the voltage on the secondary using a digital volt-ohm meter set to measure AC. Interestingly, I got exactly 0 volts. Did I miss something?

i can take the same meter and put it across the secondary’s on any other fixture and see 12v.

[Roger Feeley]

My in-in-law just told me that he wants to replace the transformer and go back to using 50w halogens.

Here’s the one I’m looking at:
https://www.newark.com/triad-magnetics/vpt24-4170/transformer-toroidal-2-x-12v-100va/dp/11R8063

The secondary shows 12v @ 8.34A which multiplies to 100watts exactly. No cushion. Is that ok?

[Jim Falsetti]

We had flickering lights, and eventually the lights quit working, after replacing higher wattage bulbs with LEDs, and the light switch was the problem. Since replacing the switch with one rated for LEDs, no more problems. It is a dimmer switch.

YMMV

Jim

[Doug Dawson]

We had flickering lights, and eventually the lights quit working, after replacing higher wattage bulbs with LEDs, and the light switch was the problem. Since replacing the switch with one rated for LEDs, no more problems. It is a dimmer switch.


It's not commonly appreciated that when you replace lighting with LED's, on a dimmer-based circuit, the dimmer switch also has to be replaced with something rated for that.

The "bulbs" also have to be rated for it.

[Paul F Franklin]

Roger,

As others have said, an oversized transformer isn't a problem.

The LEDs may have contributed to the failure. Some poorly designed LED replacement lamps only draw current in one direction (half wave rectified DC). Transformers don't like that and will run hotter than they should, even though you are drawing what seems like less power.

Regarding your new transformer, IIWM, I wouldn't run a 100VA transformer with a 100VA load. It's not good practice. And it will get quite hot. The data sheet says 45 degree C rise and those figures generally assume some cooling air. If you stick this in a little metal box with no air flow, it will get even hotter, and heat is the biggest enemy of electronics. Shoot for 20% margin if you can.

[Jim Koepke]

I hooked the primary up to 115vac and measured the voltage on the secondary using a digital volt-ohm meter set to measure AC. Interestingly, I got exactly 0 volts. Did I miss something?

No, it seems you have it covered. Even though much of my work history is with electronics and electric things it is in my fear of what a person who doesn't have a meter might get into. It makes me reluctant to attempt giving advice to those whose electrical understanding is unknown to me.

jtk

[Roger Feeley]

No, it seems you have it covered. Even though much of my work history is with electronics and electric things it is in my fear of what a person who doesn't have a meter might get into. It makes me reluctant to attempt giving advice to those whose electrical understanding is unknown to me.

jtk
Thanks,
I was a bit puzzled at seeing exactly 0 volts ac. Usually I get some very low random values.

[Bob Grier]

If you are running a transformer that is made to be dimmed for LED lights and you use a dimmer switch foe LED use that dims the transformer then you can still have a problem with strobing or flickering if you have a a lot of lights connected to it. I run 13 can LED lights and encountered occasional strobing. Like a police car lights. I changed switch and relocated it. Switches are 3-way. I stopped strobing by replacing one LED can light bulb with an incandescent bulb.

[Dan Friedrichs]

That transformer from Newark is $43....are you sure you can't just buy a new fixture with built-in LED bulb for less? ��

[John K Jordan]

Thanks,
I was a bit puzzled at seeing exactly 0 volts ac. Usually I get some very low random values.

I assume you tested to be sure the AC volt meter is working.

I also test transformers by measuring the resistance of both the primary and secondary windings (disconnected from everything, of course!)

JKJ

[Tim Otto]

You mean impedance, right?

[John K Jordan]

You mean impedance, right?

The simple answer is I generally use the resistance scale on my Fluke digital multimeter which puts a DC potential from a battery into the probes. I read the value in ohms.

I'm not an electrical engineer. While resistance and impedance are both measured in ohms, I understand impedance is the contribution of resistance and reactance. Since this is a static DC test the resistance check is more or less a check for an open conductor. Knowing a reasonable resistance for that coil can help determine if windings are shorted, sometimes useful when debugging motors.

Before believing either an DC or AC meter reading that reads zero I like to test the meter with a known good source, perhaps a battery for DC or an AC receptacle for the AC. This proves to me the meter is not broken. With some meters, of course, it's important to use the right scale too. :)

[Jim Koepke]

Thanks,
I was a bit puzzled at seeing exactly 0 volts ac. Usually I get some very low random values.


I assume you tested to be sure the AC volt meter is working.

I also test transformers by measuring the resistance of both the primary and secondary windings (disconnected from everything, of course!)

JKJ

John brings up my second point of disconnecting everything and checking the resistance of the primary and secondary of the transformer.

That will let you know if either of the windings is open or possibly shorted. Usually a shorted transformer is first detected by the smell or smoke.

Resistance is a DC measurement, impedance is an AC measurement. For measuring the resistance of transformer windings the DC measurement is a good beginning.

jtk

[Roger Feeley]

I did an autopsy on the failed transformer and found a bad thermal cutoff. I can’t explain why it blew. These transformers are old. The last time the house was remodeled was in the seventies..

My daughter wants to just go back to halogens until she replaces all the fixtures. So I ordered a replacement. Based on advice here, I chose one that’s a bit oversize.