triacs

[icarus]

Hi there, I'm doing some research into Dimmers, but I have a question which I can not find the answer to. How many triacs are there in a dimmer, is it 1 per unit or 1 per channel? I'd really appreciate any help anyone could give. Also If anyone could tell me any pros and cons they have found of working with phase control dimmers and reverse phase control dimmers would be great. Thank you

 

Moderation: Topic moved to Lighting

[misterbassman]

Dimmer packs use one per channel because they offer completly independent control over all channels

[Tomo]

Here's some info you might find vaguely useful.

Analogue Dimmer

 

This design is not necessarily safe or workable, but should give you some idea of the theory.

[PH_Zone]

You need one Triac's per channel of control.

But Triacs only work in straight phase control.

 

eg normally off until they get a pulse on their gate input when they lock on until there is no voltage across them ( Zero mains cross)

 

The problem with Triac's is the interference they cause when you suddenly connect a large load to the power in the middle of the phase cycle. and you need filters on the mains in the dimmer pack to try and minimise this.

 

There are some newer electronic devices that can be used the other way around. eg starting with the load connect after the zero cross, then disconnect it during the mains cycle.

 

The devices used in this arrangement are IGBT's.

 

 

Paul

[Mush]

Triac is a bi directional device can switch in positive and negative going half cycle of mains, so only one per channel, they`re cheap, run cool and easy to mount. They can also be a bit fragile and tend to blow with the lamp and fuse unless heavily overated.

 

Thyristor or Silicon Controlled Rectifier, SCR is a uni directional device so two of them are used per dimmer for each half cycle.They run HOT needing bigger heatsinks and can come in big threaded stud type packages. They tend to bve much more robust than a triac and will tolerate lamps blowing without problem.Only used in high end dimmers because of cost and weight .

 

Some dimmers use Solid State Relay, SSR modules which are all the power switching buits in a handy easy to mount package, these can be either Triac or SCR output.

 

IGBT dimmers can be reverse phase as already explained or sine wave which varies the amplitude of the mains. Look for IES Dimmers and Rosco Capio? and few others.Article in Lighting Dimensions by Mats Karlsson called `Silence of the Lamps`

[Brian]

Thyristor or Silicon Controlled Rectifier, SCR is a uni directional device so two of them are used per dimmer for each half cycle.They run HOT needing bigger heatsinks ...

For the same load, the power dissipated by a pair of thyristors in inverse-parallel will be the same (as near as makes no odds) as a single triac. In both cases the device is heated because they have a residual voltage drop across them when turned on. The on-voltage, VTM, of a typical device is around 1.5 volts, so with 10 amps flowing you will generate 15 watts of heat in the device when it is fully turned on.

[Mush]

Won`t pretend my semiconductor physics is any good but....

 

Believe something to do with 2 devices controlling and blocking load with an SCR contributes higher heat load than a single device in a triac controller.

 

Dimmers run hotter and noisier at low check levels where the load is mainly being held off.

[Mush]

Cheers to Spehro, sorry to Brian, I was talking bol..er..nonsense

 

"Similarly rated (eg. triacs rated at double the SCR rating) will have

fairly similar power dissipations (around 1W/A), maybe a bit higher

for the triacs than for the SCRs, but the SCR dissipation is split

between two devices so they may be easier to cool. "

 

So there you go, SCRs are still tougher but more expensive though.