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Science Forum Index » Electronics - Components Forum » 95 VAC to 120 VAC Treshold Detector
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| Lostgallifreyan |
 Posted: Thu Feb 22, 2007 4:44 pm |
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"Homer J Simpson" <nobody@nowhere.com> wrote in
news:qUkDh.129942$Oa.28738@edtnps82:
Quote: BTW, why are you using LED lamps for this? Seems odd.
Not to me. I remember when I was eight, that there was a big similarity
between stage lighting, and traffic lights. Anything that makes an
efficient narrow projection is useful, and LED's are particularly good at
that, as their use in traffic lights shows. |
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| Homer J Simpson |
| Posted: Thu Feb 22, 2007 4:55 pm |
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"Dr. Phibes" <none@your.biz> wrote in message
news:icmDh.115$MV7.57@newssvr23.news.prodigy.net...
Quote: BTW, why are you using LED lamps for this? Seems odd.
I'm trying to replace my conventional stage lights with the much lighter,
much more power efficient LED lamps.
But are they? ISTM that they still produce fewer lumens/watt than
incandescents - or so I've been told.
Do they really dim as well? And if so, isn't the curve very different?
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| Mark Aitchison |
| Posted: Fri Feb 23, 2007 1:10 am |
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Dr. Phibes wrote:
Quote: I'm replacing regular incendescent stage lights with home made LED lamps. I
have a light controller that controls the various color lamps. Since the
LEDs are much more sensitive to voltage and current than regular lights are,
I've found out that the controller actually puts out 90-something volts when
the lamp should be "off". This works fine with regular lights. But with
LED lamps, the lamps go dim rather than switch completely off. Thus, I'm
trying to come up with some sort of a gate circuit that will detect the
undervoltage and electrically (or mechanically) cut the LED string off.
I see. The answer may be a lot simpler than I suggested before. The
problem seems to be that the controller puts out a bit of leakage
current when it is supposed to be off - enough to get a glow from the
sensitive (and very non-linear current/voltage curve) LEDs yet not
enough to light a normal bulb.
There are two solutions (a mixture of the two might be the best for
efficiency). One is to put some load ACROSS the LEDs to soak up some of
that extra current, enough to bring the voltage down to less than the
threshold for light emission (about 2.something per LED in series);
ideally a load resistor that has a lower resistance at low voltages -
such as a PTC thermistor of the correct value (expensive/rare) or a
light bulb (cheap, common, and somebody else suggested that).
The bulb in parallel will drop the efficiency somewhat. Another
solution is to put a zener diode in series, and I suggested this before
with a whole lot of other circuitry that would have been needed if the
load wasn't LEDs. As it is, you will probably need two zeners in
series, back-to-back, because I presume your controller is putting out
AC and your rectification for the LEDs is taking place within the LED
unit? (If we are talking about DC then you only need one zener). So
long as the current to your LEDs is under about 0.2A RMS I'd start by
trying a pair of 3 Watt 15 Volt zeners back-to-back in series with the
LEDs, and add a low power 120V bulb (say a 5 Watt pilot bulb) in
parallel if it still doesn't switch off enough. (Just the bulb by
itself might work, and would be cheaper, but less energy-efficient).
Mark A |
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| Franc Zabkar |
| Posted: Sat Feb 24, 2007 1:24 am |
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On Tue, 20 Feb 2007 16:36:39 GMT, "Dr. Phibes" <none@your.biz> put
finger to keyboard and composed:
Quote: It seems to me that your controller is the problem though since it doesn't
really switch off. Is there any way to modify that ?
Yes, it's definitely the problem. It's a DMX light controller, very commonly
used for stage light controlling. Definitely not made to drive LED lights.
I don't know anything about this subject so I did some reading:
http://en.wikipedia.org/wiki/Digital_Multiplex
It seems that your controller uses RS-485 to send a digital value to
an intelligent dimmer. If the output of this dimmer is merely chopping
the AC by means of a TRIAC, then the idea of a 95VAC threshold makes
no sense.
- Franc Zabkar
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Please remove one 'i' from my address when replying by email. |
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| Franc Zabkar |
| Posted: Sat Feb 24, 2007 1:24 am |
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On Thu, 22 Feb 2007 13:31:25 -0600, "Dr. Phibes" <none@your.biz> put
finger to keyboard and composed:
Quote: Are these back to back LEDs or do you use a diode in series?
They're LEDs wired in series with each other.
--|<---|<---|<---... and so on.
Your suggested solution seems very ass backwards - there must be a better
way.
BTW, why are you using LED lamps for this? Seems odd.
I'm trying to replace my conventional stage lights with the much lighter,
much more power efficient LED lamps.
You need to wire the LEDs in anti-parallel, otherwise they will only
light during each alternate half cycle.
|--|>|--| |--|>|--|
+--| |--- / ---| |--- R ---+
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AC o--| +-- C --o AC
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+--| |--- / ---| |--- R ---+
|--|<|--| |--|<|--|
- Franc Zabkar
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Please remove one 'i' from my address when replying by email. |
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| Lostgallifreyan |
| Posted: Sat Feb 24, 2007 1:46 am |
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Franc Zabkar <fzabkar@iinternode.on.net> wrote in
news:1ngvt2p0mh0kn4je162vk89ufv977jk0sg@4ax.com:
Quote: On Tue, 20 Feb 2007 16:36:39 GMT, "Dr. Phibes" <none@your.biz> put
finger to keyboard and composed:
It seems to me that your controller is the problem though since it
doesn't really switch off. Is there any way to modify that ?
Yes, it's definitely the problem. It's a DMX light controller, very
commonly used for stage light controlling. Definitely not made to
drive LED lights. ;)
I don't know anything about this subject so I did some reading:
http://en.wikipedia.org/wiki/Digital_Multiplex
It seems that your controller uses RS-485 to send a digital value to
an intelligent dimmer. If the output of this dimmer is merely chopping
the AC by means of a TRIAC, then the idea of a 95VAC threshold makes
no sense.
Good point. Also, LED's have a largely constant forward voltage drop, so
it's not the voltage that must be controlled, but the current. |
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| Dr. Phibes |
| Posted: Mon Feb 26, 2007 12:15 pm |
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Quote: You need to wire the LEDs in anti-parallel, otherwise they will only
light during each alternate half cycle.
I have a full wave bridge rectifier as the first component of this circuit.
Also, it does seem like the controller needs to "see" a load out there. I
plugged a regular stage light in the DMX controller along with one of my LED
lights in paralell on the same DMX outlet, and both the stage light and LED
light click on and off like they should. So it's now clear to me that the
DMX is looking for the same type of load to function like it should.
Which sucks, because the entire point of me making the LED lamps was to save
on power consumption, as well as other perks that LEDs have to offer. |
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