Do dimmers interfere with resistance testing

[Roy G Bivv]

I just built one of those steampunk pipe lamps. I was assembling and testing connections via resistance and a multi meter. Everything went well until I added the rotary dimmer switch. I lost all continuity. After some fussing, I loosely assembled the lamp and plugged it in. Don't worry I checked for a short by sticking my tongue to the metal pipe first.(please realize this is a joke)
To my surprise the lamp turned on and behaves as expected. Why would I not be able to confirm continuity with a dimmer in line (I turned the dimmer on all the way). I tested at 2k Ohms.

What sorcery does the dimmer switch have ?

 

[Lucien Nunes]

A dimmer switch places a triac or IGBT in series with the circuit, which only conducts when triggered. The triggering occurs at a point in each half-cycle of the AC mains waveform dependent on the control setting, in order to control the fraction of the time for which the lamp is energised and therefore its average power. If there is no AC waveform, the pass device won't conduct, and your continuity tester will see it as open-circuit.

Some types of dimmer might show a high resistance instead of complete open-circuit, due to the control circuit. Technology ranges from a simple capacitive phase shift network in old-fashioned leading-edge dimmers, to a microcontroller running a computer program in modern leading-edge and adaptive types.

The presence of electronics in the circuit is one reason that insulation testing e.g. at 500V DC should take place with line and neutral linked together, to prevent the test voltage appearing across semiconductors. In fact when an LED lamp is in series with a dimmer, the switched line conductor is actually only connected through the lamp, so that principle is kind-of voided. I would test with a tungsten lamp installed. You are going to do a 500V insulation test, I hope?

 

[Roy G Bivv]

A dimmer switch places a triac or IGBT in series with the circuit, which only conducts when triggered. The triggering occurs at a point in each half-cycle of the AC mains waveform dependent on the control setting, in order to control the fraction of the time for which the lamp is energised and therefore its average power. If there is no AC waveform, the pass device won't conduct, and your continuity tester will see it as open-circuit.

Some types of dimmer might show a high resistance instead of complete open-circuit, due to the control circuit. Technology ranges from a simple capacitive phase shift network in old-fashioned leading-edge dimmers, to a microcontroller running a computer program in modern leading-edge and adaptive types.

The presence of electronics in the circuit is one reason that insulation testing e.g. at 500V DC should take place with line and neutral linked together, to prevent the test voltage appearing across semiconductors. In fact when an LED lamp is in series with a dimmer, the switched line conductor is actually only connected through the lamp, so that principle is kind-of voided. I would test with a tungsten lamp installed. You are going to do a 500V insulation test, I hope?

 

[Roy G Bivv]

I didn't get different results using a tungsten filament bulb. I don't have the equipment to do a IR test. Am I fooling myself that this is a UL listed dimmer with 14 gauge wire surrounded by thick insulation and a confirmed ground from lamp to plug therefore there isn't much risk to be tested using a 500V IR test? Other than the mechanical rotary dimmer the circuit is super simple.

 

[littlespark]

Electronic dimmers don’t like it up ‘em

Meaning 500v dc will not only give strange readings, but can fry the components as well.

 

[Roy G Bivv]

Electronic dimmers don’t like it up ‘em

Meaning 500v dc will not only give strange readings, but can fry the components as well.

I doubt this is an electronic dimmer. It feels very mechanical and wasn’t billed as electronic dimmer. I don’t plan on pumping 500v through it. I’m confident there are no shorts. The wires are well insulated especially at splices and the ground is working as tested via continuity. I don’t fully understand why a mechanical dimmer won’t allow continuity tested as explained to me here but at least I know this is expected. The lamp is functioning formally so I may call it done. I just don’t like not fully understanding the dimmer thing.

 

[Lucien Nunes]

Ah, I didn't spot that you were in the USA until you mentioned 14ga wiring. Here in the UK, we are in the habit of doing numerous tests on everything before energising, even just changing a broken receptacle can trigger a barrage of different measurements all committed to paper for eternity. Some businesses test every appliance, power cord and fitting annually for ground continuity, insulation and leakage and record all the results for posterity. I have an official record showing the insulation resistance of my workbench lamp every year for the past 20 years. Many people believe it's required by law, although technically it isn't.

Sometimes I think we go too far, and appreciate that the conventions in the USA are different. Nonetheless, it's still good practice to test insulation with a realistic voltage in case it reveals poor insulation that a multimeter test does not show up, and manufacturers of commercial fittings and appliances do it as routine. I get that your lamp construction might be inherently low-risk.

Anyway, returning to the dimmer, all dimmers made since the mid 1960s are electronic. Mechanical rheostat dimmers were made before that, but they were large things that got as hot as the bulb itself (in the days of tungsten of course) and were usually in metal canisters with lots of cooling slots. They did not see much use for domestic lighting. Once thyristors and triacs came onto the market alongside the widespread development of silicon transistors in the 1960s, the electronic dimmer became a commercial practicality and soon they were everywhere. Although modern LED lighting requires much less power, there has been no reason to abandon electronic dimming, and it has advanced to allow superior trailing-edge control using IGBTs.

 

[Roy G Bivv]

Ah, I didn't spot that you were in the USA until you mentioned 14ga wiring. Here in the UK, we are in the habit of doing numerous tests on everything before energising, even just changing a broken receptacle can trigger a barrage of different measurements all committed to paper for eternity. Some businesses test every appliance, power cord and fitting annually for ground continuity, insulation and leakage and record all the results for posterity. I have an official record showing the insulation resistance of my workbench lamp every year for the past 20 years. Many people believe it's required by law, although technically it isn't.

Sometimes I think we go too far, and appreciate that the conventions in the USA are different. Nonetheless, it's still good practice to test insulation with a realistic voltage in case it reveals poor insulation that a multimeter test does not show up, and manufacturers of commercial fittings and appliances do it as routine. I get that your lamp construction might be inherently low-risk.

Anyway, returning to the dimmer, all dimmers made since about 1960 are electronic. They were developed in the days of tungsten lights when up to 100 or 150 watts needed to be controlled for an average room light. A mechanical rheostat dimmer to do this was a large unit that got as hot as the bulb itself, and was usually a metal canister 6 inches in diameter with lots of ventilation slots. Although now LED lighting required much less power, there has been no reason to abandon electronic dimming.

Wow. I did t realize the UK has such records for electrics. That sounds exhausting (but safe). Thanks for the dimmer info. I was laboring under false understanding of modern dimmer switches. Thanks again!