95 VAC to 120 VAC Treshold Detector

"Dr. Phibes" wrote in news:bznCh.64309$ snipped-for-privacy@newssvr25.news.prodigy.net:

I've not thought through the control system, but two ways you can detect that voltage (or something close) are a chain of three diacs, or a small neon lamp. In either case you'll need a gain stage, probably a triac, to do the switching. As either the diacs or the neon will conduct at beyond threshold, no other comapator method is needed. You'll need to limit the current through either the diacs or the neon.

I might have missed something, so don't rely on this post.

Lostgallifreyan wrote in news:Xns98DCDF960D4A7zoodlewurdle@140.99.99.130:

Comparator...

Does the switched load have to be via a mechanical relay ? Is the load even connected to the same ac supply ?

Graham

1. If you are new to high voltage wiring get some help from a local expert, if only to check over the wiring before plugging it in.
2. The approach you need to take is to rectify the AC with a current-limiting resistor in series, smooth it, shove it through a 150 volt zener, and drive something like the base of a high voltage transistor via a safe variable resistor (you really need expert help in knowing what variable resistors are safe in this context), then use that transistor to trigger an opto-isolator or sensitive relay.

even

No, it doesn't have to use a mechanical switch like a relay. Yes, the load is connected to the same power supply. Here's the jyst of it

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.

The LED string simply consists of a string of LEDs connected in series with a resistor in front and in back of the circuit, then connected directly to the light controller that puts out 120 VAC when "on". And anywhere from

75-90VAC when it's supposed to be "off".

I decided to put a switching transistor in the circuit, that would switch the LEDs on and off. But the problem I ran into with that was I couldn't figure out how to bias the base of the transistor in relation to the voltage change between 90 VAC and 120 VAC.

I really appreciate everyone's help! I love this kind of stuff, and I know I can learn a lot too!

That's not tricky ! Can do that fine.

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 ?

Graham

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. ;)

Sorry, I didn't fully answer your question. No, there's no way I know of to modify the DMX controller.

doesn't

commonly

lights.

I wonder if the light controller is getting confused by the characteristics of the LED light. Have you tried putting a normal incandescent lamp in parallel with the LED lamp? If things work that way, you could then use a suitable resistor in place of the incandescent lamp (although this would increase the power consumption somewhat.)

--
Peter Bennett, VE7CEI  Vancouver BC, Canada
peterbb4 (at) interchange.ubc.ca  
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

You have a VERY good point. I'm going to try that. Hopefully, it's really that simple.

I made safety the top priority for this circuit, so it uses a very small transformer and an opto triac driver. This will do it for you if you have room on your PC board for the parts. The transformer needs a bit less than 1" sq and 1" high. The opto is a 6 pin dip, the TL431 is a TO-92. The trimpot is 5K.

Opto = Sharp PC3SD12NTZAF Mouser # 852-PC3SD12NTZAF TL431 - Mouser #512-LM431ACZ Transformer Mouser # 823-BV020-5384

+------------------------------------------------+ | | | ----- D1+ | AC-+-|Trans|-->|--+----+------+---[1.5K]---+ | | | |+ |+ | | 1 | 4 | 12V | [D2][470uF] | [Opto] [OptoOut] | | | | | | 2 | 6 AC-+-| |------+----+ P --- | | ----- | 5K O

THIS IS AWESOME! I can't tell you how much I appreciate this! Especially in that you took the time to explain it.

Again, I appreciate EVERYONE'S input. I've already learned a lot!

I def> > I want to trigger a relay so that when the input voltage is at 95 VAC

(134 V

(170 V

energized.

small PC

or

seperate

Are these back to back LEDs or do you use a diode in series?

Your suggested solution seems very ass backwards - there must be a better way.

BTW, why are you using LED lamps for this? Seems odd.

--
.

They're LEDs wired in series with each other.

--|

I'm trying to replace my conventional stage lights with the much lighter, much more power efficient LED lamps.

"Homer J Simpson" wrote in news:qUkDh.129942$Oa.28738@edtnps82:

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.

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?

--
.

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

On Tue, 20 Feb 2007 16:36:39 GMT, "Dr. Phibes" put finger to keyboard and composed:

I don't know anything about this subject so I did some reading:

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

--
Please remove one 'i' from my address when replying by email.

On Thu, 22 Feb 2007 13:31:25 -0600, "Dr. Phibes" put finger to keyboard and composed:

You need to wire the LEDs in anti-parallel, otherwise they will only light during each alternate half cycle.

|--|>|--| |--|>|--| +--| |--- / ---| |--- R ---+ | |--||--| | +--| |--- / ---| |--- R ---+ |--|