• I have a lamp with a 3-prong transformer.

  • I have those small 3-2 prong adaptors available to use.

  • I want to use a dimmer switch cable (in the form of an extension cord with a sliding switch at the business end that allows a lamp to be dimmed.) Unfortunately, this dimmer/extension cord has a 2-prong connector.

So, I want to plug the 3-prong lamp (transformer) into the 3-2 prong adaptor, and the 2-prong side of it into the female 2-prong connector of the dimmer cable so I can adapt the lamp (with its simple on/off switch only) to be dimmable.

However, I see that 3-prong adapters have a safety metal grounding plate that is supposed to be screwed into the wall plate(s). I understand that - but I can't possible do this on an extension cord-type (dimmer) cable!

Is it possible/safe to simply plug these three things together (the lamp transformer into the 3-2 prong adaptor, and this into the female connector of the dimmer extension cable)? I believe so because I read somewhere that modern day contraptions (like this dimmer cable) are designed to be hmmm... self-grounding?

Is that true, and would it make my idea safe to use?

Thanks for any insights,


  • There's no such thing as self-grounding. They don't get ground from the aether. (back in the day, alchemists used to believe there was a fabric of space, an "aether"). – Harper - Reinstate Monica Jan 23 '20 at 1:22
  • Can you post details about the lamp in question please? – ThreePhaseEel Jan 23 '20 at 1:48

Setting aside the issue of defeating ground wires, there's a more fundamental problem.

Actually, the lamp's ground pin is trying to protect you from this mistake.

Dimming does not work that way

The "transformer" is in fact a digital switching power supply (unless it weighs five pounds, then it really is a transformer, but I doubt it). Either way, dimming will not work with it.

The transformer will see the dimmed signal as dirty power, which it will try to clean up - getting hot as it does. Too hot.

The switching power supply will see the dimmed signal as a brownout or defective power, and work around it - doing its level best to draw rated power anyway. So the dimming won't work well, and will heat up, overcurrent and burn up the internal electronics of the power supply.

Both of these are precisely the types of loads which should not be dimmed. And they put a 3-prong outlet on there, both to safety-ground the power conversion apparatus, and to try to keep you from plugging it into one of those plug-in dimmers on the cord.

How dimming really works on this type of light

Now you haven't said boo about what exact kind it is. There are low-voltage lights which still use AC and incandescents, and this doesn't apply to them (the idea they need to be dimmed on the low-voltage side applies). I'm assuming this is a modern light, that is LED, and runs on DC voltage.

LEDs are actually, internally, run on DC power. Sometimes fairly high voltage DC, but also sometimes a low voltage in the 11-28 volt range. Now, if this is a constant-current power supply -- the label will tell, by saying a range of output voltages but one output current -- then the power supply itself needs to do the dimming, by reducing current flow. It may be possible to change it to one that does that, if the current value is respected.

Many LEDs lights, especially low-production/specialty/homebrew ones, are actually constant-voltage DC. They specify 12 or 24 volts DC at a range (or maximum) of output currents. Tear the individual LEDs are resistors to make them work right on 12 or 24 volts. These dim using a technique called PWM, and the dimming is on the DC side. So you need to look for a 12-24V PWM dimmer, and find a form-factor that will let you build it into the LED or cord.


I'm not sure but lowering the voltage via a dimmer to a transformer is not a good idea. Most are designed to operate in a given voltage range. My house had one series of switch outlets controlled via a dimmer switch (DUMB IDEA), while the lamps plugged in dimmed normally, one of my kids plugged in a laptop PC power supply into one of the switched outlets. The dimmer was not at full power and the power supply got really hot.

I know that's just one instance of anecdotal evidence, but be cautious.

Needless to say, the dimmer that controlled the switched outlets is LONG GONE!

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.