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AC Current Limiter For Smart Dimmer

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ZachL

New Member
Hi There,
I've recently had a Control4 (http://capaletydigitalmedia.com/docs/WirelessAdaptivePhaseDimmer-InstallationGuide.pdf) smart lighting system installed in my 100 year old house. The house does not have a neutral wired to the switches and it will be too expensive to change that.
The dimmer switches can be wired without neutral, but they apparently suck power directly from the load to power their microprocessor. This means that the lights are noticeably dimmer and make a humming sound.
To compensate for the dimness (and to be more energy efficient), I am trying to use LED bulbs. Technically, this is not supported by the switch (without a neutral) because, to steal the power from the load, they need a certain amount of current passing through at all times and they take advantage of the fact that incandescent bulbs allow a low level of current to pass through without creating any light. LEDs require a minimum amount of power before they pass any current.
I have a workaround, but it needs to be improved. If I put in a 10kohm, 5w resistor, in parallel with the bulbs, it allows enough current through that the switch does not freak out. The only problem is, when the lights switch is turned all the way up, the resistor gets hot and I'd like this to be safe enough to install in a lighting junction box.
First question: When the lights are on, is the 1.44 watts generating enough heat to be dangerous if installed in the junction box?
Second question: Is there a better way to trickle some power through - this only needs to be when the switch is "off"? Once the LEDs are on, at any level, the switch can steal power from the current flowing through them.
Third question: Is there an easy circuit that will allow the power through when the the switch is off, but not once the power increases?

Thank you so much!!
-Zach
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
When the lights are on, is the 1.44 watts generating enough heat to be dangerous if installed in the junction box?
In my view, yes. Heat will build up in a standard enclosed box and could melt the PVC insulation of the wiring.
 

Tony Stewart

Well-Known Member
Most Helpful Member
Standard Triac dimmers require a minimum holding current to latch. It is often >1% of rated load power.

This can be achieved with an 8W bulb as the nonlinear thermal resistance 10:1, satisfies the holding current at low phase angle when cool acting as a somewhat constant current sink over a 10:1 range.

This can be used anywhere in parallel with the LEDs. Also when the triac is off or min. level the leakage current equivalent resistance compared to the bulb, cold resistance of 120^2V /8W *1/10 = 180 compared to 100k to 1M leakage R in the triac shunts the off voltage to safe levels.
 
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ZachL

New Member
That was my first iteration - I was using a 7w appliance bulb. But I would like to put this in the ceiling junction box. The problem is that some of the fixtures can't fit a y-splitter with a small incandescent bulb (and the lighting becomes non-uniform).
Also, my main issue is that my dining room has a candelabra and there is no place to run a small bulb in parallel.
 

Tony Stewart

Well-Known Member
Most Helpful Member
The. bulb will last much less in an enclosed space. about 1/2 time for every 10'C rise above max temp.

find an open space for it.
 

ZachL

New Member
Agreed - I'm really hoping someone can give me a circuit for an enclosed space that doesn't involve a bulb. Or maybe one that limits current to the bulb over the required current sink.
 

Tony Stewart

Well-Known Member
Most Helpful Member
They make power resistor that can bolt to the outlet box for heatsinking, see mouser, digikey
 

ZachL

New Member
Is there a special name for that? Is that kind of heat safe if bolted rather than loose? The original responder (alec_t) seemed to think that heat might be too much for the box...
 

spec

Well-Known Member
Most Helpful Member
Hi ZachL,

Which State are you at, presumably your mains supply is 120V, 60Hz.

By the way, if you fill in 'Location' on your user page it will display in the box at the left of your posts.

Hmm, 100 year-old house. that makes it 1916.

1.44W is not an awful amount of power, but as the other members say, you need to watch the heating effect. As they also say, a bolt-down resistor would be able to handle the power no trouble providing you had a decent heatsink to bolt the resistor to. Can you show a picture, and indicate dimensions of the junction box. Is the junction pox made of heat conducting material.

Thinking out loud, another approach may be to use an X rated capacitor (not an ordinary capacitor) to provide some current to the dimmer switch. The capacitor would not get significantly hot.

spec

PS: if you look here you will see an image of a bolt-down power resistor: http://www.electro-tech-online.com/articles/core-components.782/
 

Les Jones

Well-Known Member
Most Helpful Member
Hi spec,
I have use the capacitor idea to prevent the dim glow you get from some mains voltage LED lamps when the capacitance between the live feed wire and the switch wire is enough to cause the effect. I normally use a contact supressor (Cap + resistor) such as this as it has insulated wires coming out of it so it can be just connected easily in parallel with the lamp. The OP may require a larger value capacitor paticularly if his mains voltage is 110V.

Les.
 

ZachL

New Member
I love the idea of the contact suppressor - I have no reason to assume my mains voltage is anything other than the standard US residential. Isn't that 120v/60Hz? Which unit would be best in that case?
What are the chances that this is the solution I'm looking for?
Regarding the bolt-down resistor possibility, should I be concerned that the terminals aren't insulated? Seems like I'll be dangerously close to shorting my load to the junction box.

Thanks all for the continued assistance!
 

Les Jones

Well-Known Member
Most Helpful Member
Hi ZachL,
I would try one that yses a 0.47 uF capacitor. The value of the resistor in it is not very important as it will be much lower than the reactance of the capacitor. You may find one on ebay but here in the UK CPC seems cheaper than ebay for these.

Les.
 

Les Jones

Well-Known Member
Most Helpful Member
Use it in place of your 10K resistor. A 0.47 uF capacitor will have a reactance of about 5.6K at 60HZ.

Les.
 

ZachL

New Member
Not sure what I'm doing wrong, but could someone point me to where I can buy one of these in the US? I've looked in mouser.com and digikey.com for a 120VAC 0.47uF "contact suppressor". I can't seem to find anything :(

Thanks,
Zach
 

spec

Well-Known Member
Most Helpful Member
Hey Guys, Any chance some could show me one I can purchase online for my 120V mains? Thanks!
Hi ZL,

yes, RC contact suppressors are difficult to find. Ampohm is only a small Brit company too.

As long as the voltage is 120V or over that will be fine. Do not worry about the frequency either: it is the capacitance and resistance that counts.

spec

Links
(1) ebay.com, list: http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313.TR1.TRC0.A0.H0.XAMPOHM.TRS0&_nkw=AMPOHM&_sacat=0
(2) ebay.com, 0.47uF/100R with mounting tab: http://www.ebay.co.uk/itm/Contact-S...UND-PRODUCT-/361268057982?hash=item541d41177e
(3) ebay.com, 0.47uF/100R without mounting tab: http://www.ebay.com/itm/FE-SP-HDR28...328038?hash=item2edd1f8ce6:g:hccAAOSwYGFUztze
(4) ebay.com, 0.1uF/22R without mounting tab: http://www.ebay.co.uk/itm/Contact-S...ND-PRODUCTS-/251912927017?hash=item3aa72e3729
(5) Newark (Element14 [Farnell]): http://www.newark.com/webapp/wcs/stores/servlet/Search?catalogId=15003&langId=-1&storeId=10194&eq=N=422+2203&Ntk=desc_pn&Ntt=ampohm&Ntx=rel+gensearch_store_specific%2cstatic%28P_STORE_MARKETING_RANK_NEWARK_US%2cascending%29+mode+matchpartialmax&Nty=1&Ntpc=1&Ntpr=1&D=ampohm*&Dn=2203+2203&Dx=mode+matchall&st=ampohm&pageSize=25&showResults=true&aa=true&vw=
 
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ZachL

New Member
Christmas came early for me today - the contact suppressor got here from the UK and it worked perfectly! Now I'm going to order another 20...

Thank you all so much for your help!
 

Les Jones

Well-Known Member
Most Helpful Member
Hi Zach,
I'm glad it solved your problem. It is normally us having to buy things from the US rather than the other way around. I hope you did not have to pay a lot of import duty and charges as we do when we buy from the US.

Les.
 
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