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LOR Units and Transformers


Roger Leon Forbes

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I have built a unit that is a 120 volt to 24 volt AC transformer to power two 40 amp break contact relays to control my static display lighing. I turn the channel on dureing each song and then off dureing voice over or just music. I remember reading or someone told me that LOR units might have a problem with transformers, anyone have any input about this please. Roger

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Hi Roger,

There is always a chance that the counter EMF spike from an inductive load could punch a hole through a silicon controlling device such as a Triac. To be on the safe side, a surge or spike suppressor would be wired in parallel with the transformer, relay or motor. A cap. / resistor in series and then the combo in parallel should do the trick.

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Last year I used several Intermatic 120 volt to 12 volt AC 300 watt transformers on a LOR controller ,to power a bunch of 50 watt incandescent MR-16s. They worked flawlessly with some pretty aggressive sequencing and some slow fades as well.

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I'm using 3 transformers on 3 channels for my Rainbow floods - no issues yet.

I have used relays in the past with OTHER controllers and had issues - I would NOT use a relay without a MOV.

It probably would not be a bad idea to have a MOV on the transformer. I have no idea what the value of the MOV was ( I ripped them off some sort of old multi line-phone office surge protector) I put them parallel to the relay coil with a diode.

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gizmomkr wrote:

I have no idea what the value of the MOV was...

Just using a random MOV may not have done anything. MOV's are rated by the voltage at which they become a low value resistor. Even if you did have the right voltage, MOV's not a forever device. They're meant as just in case device but they will eventually stop working. If you sent a lot of high voltage spikes through a good MOV, it probably isn't anymore.

Probably the simplest protection is to parallel a couple of C7 bulbs across the input. Two bulbs because one might fail but less likely two. The TRIAC's in all LOR controllers are a snubber-less type so it's less likely that you would cause any damage with a small transformer anyway.
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Erine,

Please refresh my memory. For some reason I am thinking that a snubber in a Triac would be a good thing. The name or term snubber in my mind is something that would help dampen any kind of voltage spikes. Been away from some of this stuff to long and need to start reading again to refreshen my memory.

Thanks

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My old Controller used reed relays to fire its outputs. I connected the n.o. contacts on the reed relay to a slightly larger relay to fire my lights. The kickback from the coil of the larger relay would immediately cause the contacts on the small relay to arc and freeze. I tried various combinations of cap's and resistors, nothing would work.

As soon as I threw a MOV on it, the problem instantly and perminantly went away. I still dont know the value on the MOV, dont use it any more - but it was rated to be a phone line sruge protector. The MOV is about the size of a quarter. The voltage on the larger relay coil was 5vdc

I dusted it off to bring for show an tell at the christmas carolina conv this summer - It still works.

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Max-Paul wrote:

Please refresh my memory. For some reason I am thinking that a snubber in a Triac would be a good thing. The name or term snubber in my mind is something that would help dampen any kind of voltage spikes. Been away from some of this stuff to long and need to start reading again to refreshen my memory.


There's a little confusion here and you're not alone on this.

You remember when you apply voltage to a coil it charges up and when you open the circuit and the field collapses, a large voltage in the reverse direction is induced on the wires. If you were to apply AC to the coil and open the circuits while there was still a voltage potential present, you'd a get a variable voltage depending on where in the AC wave the circuit was opened.

This is true for a switch but a TRIAC doesn't work that way. A TRIAC can be turned on by a voltage on its gate pin at any time in the AC wave as long as the current passing through it is high enough (higher than the holding current) the TRIAC will stay on even if you try to turn it off by setting the gate voltage to zero. When the voltage drops near zero (determined by the current) the TRIAC will turn off and stay off unless externally turned on again.

So you see there is never an instant off condition that would cause a coil to output a high voltage spike.

Here's where the non-technical group will start to roll their eyes, look at the ceiling, fall asleep and maybe fall off their chair if there even still reading this.

If a purely resistive load is connected to a power supply, current and voltage will change polarity in step. Inductive loads such as transformers and motors (any type of wound coil) the current waveform will lag the voltage. The timing for the TRIAC’s is based off of the AC line voltage and where it passes through zero. With a transformer connected, the TRIAC turns on later in the cycle. With the channel output set to maximum, you start to get into the next cycle. The result is that the output voltage is less than expected but if you back off the output to about 80%, you get the full output.

LED’s have the exact opposite effect. They tend to act like a capacitor which means that the current is leading the voltage. In this case, the TRIAC turns on earlier and the LED’s are on even when the channel is off.

What will cause a problem is using an AC motor that has brushes such as a vacuum cleaner motor. You could over voltage a TRIAC with this type of load but the real problem is the rise times of those spikes can actually cause the TRAIC to stay on. A snubberless TRIAC doesn’t mean that it has built-in snubbers. It means that it can deal with the fast rise times of these voltage spikes though they still have to be within the TRIAC’s maximum voltage spec’s.

With LED’s, you can put a 47K resistor across the channel output and minimize the capacitance. With a transformer you can parallel a C7 type load and minimize the inductance. With an AC motor with brushes the standard capacitor in series with a resistor (I’ve seen .1µF and 33Ω as typical) should work though LOR (and typically DIY) use a snuberless type so it’s probably not an issue anyway.

Max, if you passed on while reading this long post, I'm sorry but you did ask.
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