Situation:

Circuit Breaker fails and circuit seems to be dedicated to 6 or 8 Recessed Fixtures on 2 slide/preset dimmers. Breaker is clearly no good.

First course of action taken is to remove wire from breaker and test (for short) resistance (with Fluke digital MM) from circuit wire (Blk) to neutral bar with dimmers switches off and then on at full. Expect to see Lamp resistance in the teens maybe, but it is up in the Megaohm range. Why?
Must be Electronics inside? is that it?
What makes 'em tick? (How?)

Bill

Your battery powered meter has very low voltage. It may be below the breakdown voltage of the solid state components (variac) in the dimmer.

If there is a series clipper diode, you could try reversing the test leads. But again, your test voltage is too low.

In other types of circuits (not a dimmer), there could be a series capacitor which blocks the DC test voltage.

Dspark is on the right track here.
Some of my old Electronic Control projects were simple Solid State AC Lamp control circuitry [touch switches, Triac On/Off, Dimmers]. The Triac type Dimmers I made did have a Capacitor in series with the trim pot [the level control potentiometer].
On one, the tapped lead was run to the DIAC part of the gate circuit. Another simply used the pot as a rheostat [tapped lead tied to the output end of resistor].

Seems that both used a 0.22 MFD Capacitor and the pot was 500K.

It makes sense that you would get readings in the Mega Ohms area, that's the leakage across the Triac and the Capacitor. If there's an indicator lamp [like a LED], that would swing into conduction with a forward voltage above 2.0 VDC. Then you would also be reading the LED / Resistor / Incandescent Lamp series values.

Simply, the readings of >500K Ohms is normal, plus expected. If the readings were lower, that would be a sign of a failing component.
The measurements you got are from the Series-Parallel circuitry involved here. There's not a high enough voltage and current to "Fire" the Triac's gate into conduction, so the result is a very high Impedance [part pure Resistance, mostly Capacitive Reactance] to any varying power source. The DC power from the DVM simply fills the Triac up like a Capacitor, but since there's a difference in Potential on the gate and through the Anode of one part, there's a small leakage across the depletion region[s].
Also, the Capacitor / Pot trimming circuit is in Parallel with the Triac, so as tthe Capacitor is charged, it can discharge slowly through the other components [such as the Pot]. The leakage of the Capacitor would be in the Mega Ohms range too.

The only component with a relatively low resistance / impedance is the load [Incandescent lamp]. It's in series with the Triac and the trimpot circuit.

This is nearly impossible to describe with text! Best way to describe the thing would be with a schematic.
I'll draw up a few, then post later tomorrow. They are very basic, so shouldn't take a lot of time drawing.

Scott SET

>The Triac type Dimmers I made did have a Capacitor in series with the trim pot
meaning in fact that it was in series with the load? Okay, I was wrong!

I have a book showing an SCR incorporated into a dimmer and the pot (50K) regulating the delay in time that it 'fires'. The waveform vaiations shows it being cut off at the intervals dictated by said pot, thus a variation in brightness*

* not to confused with my own variation here, i AM directly referencing text...

Bill:

Like Scott said, it's hard to describe circuitry in words.

Most of our dimmers use a simple 2-wire connection, so the triac is in series with the hot side of the lamp, then across triac is wired the pot and timing capacitor in series. From the center point of these a diac is connected to the gate of the triac. (The pot/capacitor combination can also be wired directly across the supply, but of course that would require a neutral at the dimmer.)

The pot adjusts the time constant of the RC circuit so that the triac fires at a different point on the AC cycle. Once triggered into conduction, the triac passes current until the waveform passes through zero again, then it will be re-triggered on the next half-cycle.

As the others ave said, the low voltage of a typical ohmmeter isn't enough to trigger the triac into conduction so you'll just read the leakage.

There are usually one or two more chokes and capacitors included for RFI suppression.

I can't see that your basic dimmers would be much different to ours, other than different voltage ratings and slightly different componenet values to allow for the 50 and 60 Hz supplies.

Thanks guys!
Scott, If you've got a diagram that would be great, but it is enough to know that "it's in the electronics".

I was expecting somehow to get variable readings like from a potentiometer and wasn't even sure that there were electronics in these dimmers.

Bill

FYI - I posted two Schematic drawings of Triac type lamp dimmers in the Technical Reference area, which should help explain things better, along with showing some simple techniques of "Solid State" AC Lamp dimming control.

Look under subject heading:
AC Incandescent Lamp Dimmer Circuits 1

Hope they are helpful to everyone!

Scott SET 10/02/2001 06:52:00

Scott:

Do many of your dimmers use a transformer coupling to the triac?

Only the "luxury" models (read "more expensive") do that here. Most are the straight diac coupling.

Scott,

I just wanted to thank you for the dimmer schematic. My HS Electronics is way too rusty to be able follow right now, But maybe I can get into it slowly. I've got to start way back at the individual component function/operation level again. I can see though that there is no Simple path through.

What is the Round thing in the middle on the second diagram?

Bill

Bill,

The round component in the center of the second drawing is an SBS [Silicon Bidirectional Switch]
This switch can control / pass AC, like the Triac does, except the Triac is a better amplifier for this type of job.

The SBS can be thought to be "madeup" of two SUS [Silicon Unidirectional Switch], in order to pass currents in both directions.
Similar to the way a Triac can be thought of as "madeup" of two SCR [Silicon Controlled Rectifier], in order to pass currents in both directions.

Scott SET

Bill:

Don't let all the different terminology get too confusing. The SBS name Scott used is what I referred to as a diac. Just a different name.

An SCR (Silicon Controlled Rectifier) is also called a thyristor, a solid-state equivalent to the old thyratron vacuum tubes.