NSI Dimmer Pack Question

[Herr_Sprecker]

I was wondering if anyone has had experience with NSI Dimmer equipment. I do not recall the exact model of NSI that we have, they are 8-channel rack mount packs, 1200w per channel. Dimmer modules are still available for these units, and it is a good thing because these modules seem to have the lifespan of a dayfly.

Each channel has a safety breaker, but the module is kind enough to save the safety breaker by blowing out before the safety breaker can save it in the event of a short. And it never takes much. We've also got a couple channels that are always stuck at 100% regardless of what the control board tells them they are supposed to be at.

I was wondering if this is normal behavior for NSI equipment, or if we are overlooking a serious issue as we repair these units. Any info would be helpful.

[JD]

For some reason, a number of manufacturers seem to have taken to using 15 amp triacs, such as the BTA16-600 on 1200 watt dimmers, with the results you speak of. Strange, as the BTA24-600 is only $1.78 for a 25 amp triac, which is only two cents more! Check your warranty status, and if all are out, have your local qualified servicer take a look at the pack. If they are using the 15 amp triacs, I would steer clear of buying anymore of that unit. As a rule of thumb, Triacs on any dimmer should have a 200 to 400% safety factor above the rated load. (Half that amount if they are SCRs, as the "pair" gives you the total current.)

[waynehoskins]

The ones that are stuck on have bad triacs too and should be replaced. A triac (or SCR or SSR) achieves dimming by turning on and off really fast, and it's the ratio of on time to off time that determines intensity. When such a device fails, it simply no longer switches, so it sticks in either the always-on condition ("latched on") or the always-off condition "latched off".

Because of the PWM dimming technique, and the nonlinear resistance of a lamp filament, it's very easy to overload a dimmer if you think (as we all did before we learned) that you can put double the rated load on a dimmer if you only turn it up to half. It actually draws effectively full current, because of the peak-response nature of power cubes and the nonlinear resistance of filaments (their resistance increases as they heat up, so a cold filament is nearly a dead short while a hot filament isn't, so the relatively cold filament at, say, 10 percent intensity, draws the same current as a hot full-intensity filament would.

All that to say it's very possible to overload dimmers without thinking you're overloading them.

[TimOlson]

some of those racks also have a switch that latches the dimmer on at full. does yours?

peace, Tim O