When Is A Dimmer Not A Dimmer?

When Is A Dimmer Not A Dimmer?

With the industry moving towards more and more fixtures that require direct power, it is essential that we evolve our perception of what a dimmer rack is and what it can achieve for us. These days a dimmer has to do more for us than just dim conventional fixtures, we need our dimmers to be a one stop shop for all of our fixture's power needs. Today's dimmers have to cater for all scenarios, such as high fidelity dimming, bypass relay switch circuits and bypass relay constant power circuits.  This need is especially obvious in installed systems where the limitations of space, power feeds and cost is never more apparent plus the luxury of never having to repatch again. The HPC dimmer with Air-gap technology enables this one product solution that cuts down on space, cost and time. The biggest hurdle to overcome however is the mental one, removing the mental barrier of plugging a constant power fixture into a "dimmer circuit", because we have all been burnt by this in the past. The best way to remove this barrier is to really know what is going on inside the unit so that you can be sure the circuit is providing true constant AC waveform to your fixtures.

So how does the HPC provide constant power?
To provide the constant power to your fixtures the HPC dimmer utilises our own AirGap technology. Instead of modifying the AC wavelength of the dimmed power to try and synthesise a true constant power, we utilise a very simple but effective bypass relay concept.

In previous HUB’s we have demystified the process of theatrical dimming, but in order to explain why the AirGap circuit is the only way to allow constant power, let's go over the effect of dimming an AC waveform.

A digital dimmer works by artificially chopping up the incoming AC voltage so that, instead of the load drawing its full rating, its only drawing that full rating for some of the time, and nothing the rest of the time. This happens fast enough that you can average it out, so a dimmer that’s chopping out half of the wave is dimming its load to 50%.

Most dimmers in use today use either triacs or back-to-back SCRs, which chop up the wave form in sync with the sine-wave cycle of the source AC waveform.

Here we can see a normal AC waveform, as we’d expect to see when powering a standard incandescent lamp:


Figure 1: A Normal 50Hz Mains Sine Wave

Here is what the same waveform with the same load looks like when passed through a dimmer running at 50%:


Figure 2: The Same 50Hz Wave Dimmed at 50Hz

While the physics of the real world mean you don’t get completely clean and flat “off” and “on” parts to the slope, you can still clearly see the half of the wave-form that is allowed to pass through normally, and the other half which has been gated (or, more correctly, the two quarters of each, as the switching happens twice per full cycle).

In traditional dimmers all of the outlets are constantly being passed through this dimming filtration circuit this means that even at 100% the AC waveform will still be slightly distorted as it is still required to pass through this circuit. The waveform can be altered with software and high fidelity dimming circuits but there is no way to guarantee a constant AC waveform while it is passing through the dimming circuitry. This creates no issue with incandescent lamps or robust electronics, but for sensitive electronics and other inductive loads this can spell the death of the equipment. These days all LED fixtures and Moving Lights contain these sensitive electronics.

Therefore the only way to guarantee a constant AC waveform is to separate the Input and the Output of the dimmer by the use of contactor bypass. The AirGap technology in the HPC allows by employing the use of a pair of inbuilt contactors to bypass the triac and filtering circuits when switched to full and open the circuit properly when switched to zero, these contactors also allow the constant bypass of the dimmer circuitry to allow for a true AC waveform constant power circuit.

The diagram below shows this circuitry:


Fig 3: AirGap Contactor Circuitry

We can see from this diagram how the three modes of operation within the HPC will utilise the Contactor Bypass Circuitry.

  1. Standard Dimmed Circuit
    When the dimmer is at anything over 0% the Isolate Relay closes allowing current to pass  through the dimmer circuitry and out to the fixtures. The Bypass Relay stays open so only one path is allowed through the dimmer.
  2. DMX Controlled Switch Circuit
     When in Switch mode, and driven to DMX 255 the Bypass Relay is closed allowing the circuit to provide clean undistorted power output. The Isolate Relay stays open ensuring that no current is passed through the Dimmer Circuitry.
  3. Constant ON Circuit
    When in constant power mode the AirGap latches closed the Bypass Relay to allow for constant undistorted output, the isolate relay stays open ensuring no current is passed through the dimmer circuitry.

With the AirGap technology you can be sure that your equipment is receiving a constant supply of clean undistorted power. As you can see from the diagram the connection between your equipment and the mains is just a direct copper line through the Bypass Relay, and will only pass through the dimming circuitry if the dimmer circuit mode is set.

The Advantage of an Isolate Relay on the Dimmer Circuit?
It is sometimes erroneously believed that a dimmer at 0% is isolated from the mains with the same level of safety and certainty that a switch or relay performs the isolation. This is not the case, however, as there is always some “trickle” output to a dimmer, even when it’s dimmed all the way to 0%.

 It is important to remember that the outlet of a dimmer is always live. In most dimmers, the only way you can isolate an output, in the same manner a wall socket is, is to flick the breaker on that channel off, assuming it has a circuit breaker on each channel.

There is very little energy in a fade curve dimmed to 0%, so as soon as you put on any normal load, the waveform will collapse and you will see what looks like a flat line. But, some very low-power devices will continue to work even when you drive the dimmer to 0%. From a safety point of view, you should never consider a circuit “off” just because its attached dimmer has been dimmed to 0%.

The advantage with an Isolated Relay on the dimmer circuit is that the relay can open to cut off all power to the output completely. When the HPC isn't driving a circuit on the dimmer for more than one minute in the dimmed mode it automatically opens the Isolate Relay therefore removing all trickle current from the output and completely killing the circuit. Therefore you can be assured there is no output from the circuit and you will have no more issues with low power devices.

Once DMX value is detected to the circuit the HPC responds by closing the relay very quickly to ensure that there is no control response time from input to output greater than 50 milliseconds. Ensuring no perceptible lag from the closing of the relay.

Click here for more information on Jands’ HPC Dimmer.


Bookmark and Share


Please comment using the form below (email will not be displayed)





Please note that your comment will only appear after it has been moderated.


Security key