Today's Message Index:
----------------------
1. 05:56 AM - Re: Question about panel light dimmers (Richard T. Schaefer)
2. 07:01 AM - Re: Question about panel light dimmers (Ernest Christley)
3. 10:10 AM - Re: Question about panel light dimmers (Vernon Little)
4. 02:22 PM - Re: Low voltage indicator for LR3 (Jeff Page)
Message 1
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| Subject: | Question about panel light dimmers |
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
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Neil
Clayton
Sent: Wednesday, February 27, 2008 8:19 PM
Subject: AeroElectric-List: Question about panel light dimmers
<harvey4@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?
Many thanks to any EE who'll take a moment to set this tyro straight.
Neil
Message 2
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| Subject: | Re: Question about panel light dimmers |
Neil Clayton wrote:
> <harvey4@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.
Message 3
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| Subject: | Question about panel light dimmers |
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.
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Neil
Clayton
Sent: February 27, 2008 6:19 PM
Subject: AeroElectric-List: Question about panel light dimmers
--> <harvey4@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?
Many thanks to any EE who'll take a moment to set this tyro straight.
Neil
Message 4
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| Subject: | Re: Low voltage indicator for LR3 |
At 06:46 PM 2/24/2008 -0500, you wrote:
> Hmmm. I am planning a circuit that will light that same LED when the
> master or auxiliary contactor is engaged when the engine isn't running
> (eg. left on after flight). They neglected to mention this bias
> resistor when I called to ask about the circuitry inside. This will
> change my design somewhat.
> Do you know what the resistor value is ?
It's big . . . but keep in mind that the current
through the resistor is multiplied by the gain
of the transistor.
If the LR-3 is getting its power from the main bus
the light will flash any time any battery is feeding
the bus -AND- the alternator is off. What's the need
for alternative "design"?
Bob . . .
I am wiring per Z-13/8.
If I have a main alternator failure, the LR-3 indicator will tell me.
I switch on the Endurance bus and switch off the main bus, the
indicator will go out, since there is no power there at all.
At this point, I need to switch on the aux alternator. As drawn,
there is no low voltage indicator to show if this alternator is
working. In an emergency, I might forget to do this. I usually good
with checklists, but an indicator would be helpful.
This also means there are 3 switches I could accidentally leave on
after a flight or maintenance that would ensure a depleted battery
before the next flight.
I could have 3 indicators for the switches and 2 low voltage
indicators. However, I really only need 1 indicator that indicates
low voltage in flight while running on either alternator, acting as an
idiot light after shutdown.
By adding another pole to the endurance bus and aux alternator
switches (main bus already has a usable switched to ground pole on its
switch), I can easily sense the switch positions. However, to avoid
an indicator continuously lit in flight, these contacts need to
activate a low voltage detector with a threshold abow the normal
battery voltage, but below the charging voltage.
So depending on the value of the internal resistor driving the gate of
the FET, it could turn on my LED, which is essentially pulled to
ground from battery voltage through a 680 ohm resistor.
Jeff Page
Dream Aircraft Tundra #10
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