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Question about panel light dimmers

 
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harvey4(at)earthlink.net
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PostPosted: Wed Feb 27, 2008 6:23 pm    Post subject: Question about panel light dimmers Reply with quote

The panel dimmer I selected can handle 1.5 amps.
I have 9 gauges needing 0.2 amps each.
So at max dimming, the dimmer is being asked to dissipate (0.2 x 9 =)
1.8 amps - a bit more than it's rated capacity.

I figured that was close enough - until I realized I forgot to
include the radio back light, which takes me WAY over the 1.5 amp capacity.

Theory; this humble M.Eng assumed the dimmer uses a
silicon-controlled rectifier that gets hotter the more it's being
asked to dim (so that for zero dimming = full brightness, there's no
heat being dissipated by the SCR's heat sink). But then I recalled
that SCR's work by clipping the tops of the current wave - except we
use 12v DC. No current curve to clip. So much for that theory!

So....how does a DC dimmer work?
Can I safely go over the rated capacity of my dimmer with no
expectation of disaster?
If a disaster were to occur, what form would it take?

Many thanks to any EE who'll take a moment to set this tyro straight.

Neil


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schaefer(at)rts-services.
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PostPosted: Thu Feb 28, 2008 5:56 am    Post subject: Question about panel light dimmers Reply with quote

Some DC Dimmers are pulse width modulators. So the transistors are used in
the switching mode witch makes them quite efficient. (i.e. not much heat
in normal operation). They do not dissipate the load current, they switches
it. But internal losses increase as the load current increases.
The percentage of "ON" to "OFF" determines the brightness of the lights.

The MAX current rating is likely to be limited my the maximum current the
transistor can pass. Applying more current can cause thermal run away at the
Junction ... then it will die. The transistor junction will cool during the
Off cycle. So the worst case is when you have full brightness.
r.t.s

--


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echristley(at)nc.rr.com
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PostPosted: Thu Feb 28, 2008 7:01 am    Post subject: Question about panel light dimmers Reply with quote

Neil Clayton wrote:
Quote:

<harvey4(at)earthlink.net>

The panel dimmer I selected can handle 1.5 amps.
I have 9 gauges needing 0.2 amps each.
So at max dimming, the dimmer is being asked to dissipate (0.2 x 9 =)
1.8 amps - a bit more than it's rated capacity.

I figured that was close enough - until I realized I forgot to include
the radio back light, which takes me WAY over the 1.5 amp capacity.

Theory; this humble M.Eng assumed the dimmer uses a silicon-controlled
rectifier that gets hotter the more it's being asked to dim (so that
for zero dimming = full brightness, there's no heat being dissipated
by the SCR's heat sink). But then I recalled that SCR's work by
clipping the tops of the current wave - except we use 12v DC. No
current curve to clip. So much for that theory!

So....how does a DC dimmer work?
Can I safely go over the rated capacity of my dimmer with no
expectation of disaster?
If a disaster were to occur, what form would it take?

The dimmer I have works by switching the DC off and on at about 4kHz.

Way faster than your eye could ever detect. With it off, there is not
current flow...hence, no heat. When it is on, the transistor is in a
very low resistance state...hence, very little heat.

If yours works this way, overloading it may make the switcher stop
switching, or the "very little heat" when it is on may turn into "too
much heat" and then it just goes off and stays there.


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rv-9a-online(at)telus.net
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PostPosted: Thu Feb 28, 2008 10:10 am    Post subject: Question about panel light dimmers Reply with quote

Hi Neil. From the specs you gave, I am assuming that your dimmer is using a
common LM317 regulator.

This regulator has a built in current limiter that will (normally) prevent
it from being damaged from overload, but it will not perform properly in
your application with much larger loading. Typical of this type of dimmer
is the Van's dimmer module.

In addition to the maximum current limit, the LM317 has a thermal operating
limit (for reliability). The power dissipation of the device is equal to
the voltage drop times the load current. The maximum power dissipation
occurs at less than full brightness.

In my case, my maximum current load was 0.9 amps (measured at full
brightness, 11 volts output), but peak power dissipation of 5.1 Watts was
when the output was about 5 volts.

Based on this number (5.1 Watts), I decided to install a much larger heat
sink on the Van's module in order to keep the maximum junction temperature
of the regulator below 125 degrees C (with a 40C ambient).

What does this all mean? My recommendation is that the Van's dimmer
(unmodified), should only be used with about 700 mA of full-intensity load.
Higher loads need bigger heatsinks. There are other dimmer modules out
there that can be used, or you can split the load using multiple regulators.

Thanks, Vern Little.
--


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Eric M. Jones



Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

PostPosted: Fri Feb 29, 2008 7:55 am    Post subject: Re: Question about panel light dimmers Reply with quote

A very good explanation of how voltage regulators, both switching and linear, work is contained in:

http://www.national.com/appinfo/power/files/f4.pdf

But it should be said that 1.5A does not mean there is any margin above than at all--NONE. Otherwise it would be called a 1.6 Amp (or whatever)regulator. There are protection circuits onboard, but they DO DEPEND on some operating conditions being met. This is not always clear to the designer.

I sell tons of 1.5A LM317-based voltage regulators (EGPAVR) on my website. I have also made these in slightly larger 2A and 3A versions.

Above a few amps the game goes to switching regulators. BUT!!, in my humble opinion, there is a large zone where a larger, heavier, linear regulator is still preferable, since it is more reliable AND is electrically quiet. I have abandoned the use of switching regulators in some of my applications for this reason. Yes, squelching the RFI/EMI is possible, but why bother?

I have seen some LED dimmers that are just horrendous. They use switching regulators with variable frequencies. They are guaranteed to induce wild hallucinatory visions in high-vibration environments.... Like chewing on hard candy while looking at a computer monitor.


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www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
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