Newbie needs help: converting range hood lighting from halogen to LED

wayneh

Joined Sep 9, 2010
17,160
I metered at the transformer's output direct and it' now clear that it crapped out. I get 0.5 - 0.4V AC. No damage or dark spots are visible anywhere.

At this point, since this is a simple circuit board, I'm wondering if I could just replace everything except the coils...and start almost from scratch? I sincerely don't know what crapped out and maybe I could source better components than those that are on that PCB...? Could that be a viable option?

I'm including a pic of the board, again.
Ah, just as I suspected. But I’m a little confused. Why are you concerned about the PCB when you have identified the transformer as dead. How do you know there’s anything wrong besides the transformer?
 

Thread Starter

lightpsycho

Joined Mar 21, 2018
18
Ah, just as I suspected. But I’m a little confused. Why are you concerned about the PCB when you have identified the transformer as dead. How do you know there’s anything wrong besides the transformer?
I think I didn't express myself correctly...

Something on that board crapped out...I don't know much about the components on that board apart from the resistors and toroidal coil with a "center" tap. I metered at a few points near the mains and got some "expected" voltages. So I'm thinking something on that board between the mains and the coil has to be replaced...
 
These are just comments, because I don;t have the time to help effectively.

If we assumed that the thing is working, it may have selected the 9.5 or 12 V AC with triacs. They would not be happy if they both were on at the same time. That would likely blow the transformer.

I think the TS basicallly said the dimming was done by sending 9.5 or 12 V AC to the halogen lamps. Halogen lamps don;t last as long if not operated at full voltage.

What i would have proposed would not be simple. 9.5 VAC and 12 VAC are too close for some simple detection. I belive AVAGO, now Broadcom makes some optoisolator IC's that can be set for specific voltages with external parts. Triacs require a minimum load (probably 50 mA although that may have already been provided.

Probably from this https://www.broadcom.com/products/o...ic-function/isolated-voltage-current-detector table of parts.

Reverse engineering might have also been able to find the likely DC signals necessary to select dimming.

Once this interface was created, a LED driver that either used a voltage or resisor for brightness programming (resistor preferred) would change the brightness levels. I'd probably use an OPTOMOS relay (an IC) to select the brightness levels.

It would likely require a PCB and you might have trouble making all of the parts fit. The LED driver being the largest part.
 

sghioto

Joined Dec 31, 2017
2,691
I metered at the transformer's output direct and it' now clear that it crapped out. I get 0.5 - 0.4V AC.
The transformer must have a load connected to get an accurate reading. Have you tried connecting a halogen bulb to see if it still works? If the halogen bulb works then try connecting one halogen and one LED bulb.
SG
 
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Thread Starter

lightpsycho

Joined Mar 21, 2018
18
The transformer must have a load connected to get an accurate reading. Have you tried connecting a halogen bulb to see if it still works? If the halogen bulb works then try connecting one halogen and one LED bulb.
SG
I didn't try any halogen bulbs yet, but bought some this weekend so I guess I'll try them out this evening.
 

Thread Starter

lightpsycho

Joined Mar 21, 2018
18
The transformer must have a load connected to get an accurate reading. Have you tried connecting a halogen bulb to see if it still works? If the halogen bulb works then try connecting one halogen and one LED bulb.
SG
Well I'll be damned. It works.

It also works with one halogen and one led bulb, which confirms my initial theory that the led must draw too little current. Something happens, I don't know why, but the load is too low to fully light the leds when two are connected.
 

sghioto

Joined Dec 31, 2017
2,691
That's the way some of those electronic transformers work. Some are designed to be on with no load and others like the one you have require a minimum load which in this case is maybe 10 watts. One option would be to add a load resistor across the 12 volt output. Assuming the circuit requires 10 watts you would need a 36 ohm resistor rated 10 watts. That 's 4 watts from the resistor plus the two 3 watt LEDs
SG
 
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it's a very typical problem because the TRIAC (the control device) requires a minimum current. Usually, if you measure the connections to say a lamp controlled by a TRIAC, you show the full line voltage for both on and off states. 20-100 mA is the typical range of minimum currents.

I have a UPB module that required both a small light and a LED light to work. Otherwise, it's always on.
 

Thread Starter

lightpsycho

Joined Mar 21, 2018
18
That's the way some of those electronic transformers work. Some are designed to be on with no load and others like the one you have require a minimum load which in this case is probably 10 watts.
SG
So what would be the best solution while being able to maintain hi and low functions? There is no such thing as a led driver with center tap...

What if I would connect both yellow and green to the + output on the driver, and use a resistor inline for one of the two output wires and connect that to the "low" terminal on the processor board?
 

Thread Starter

lightpsycho

Joined Mar 21, 2018
18
Check the option I suggested back in post #27
SG
Whoops I missed that one.

Considering the diagram (3-wire transformer output because of CT), I think it would be better to add the resistor at the socket output on the processor board...sorry for the newbie question but I should put the resistor on the live side (it's AC), not neutral, right?

Also, I will try to add the lowest-rated resistor possible...starting by 36Ohm rated 10w, as you mentionned, but keeping in mind that originally the bulbs were 20w. So I might have to get one rated at 20w too...? I gues there will be a bit of trial and error...
 

sghioto

Joined Dec 31, 2017
2,691
Yes a bit of trial an error. Across the socket is good, that way no power is consumed when the lights are off. Just need to size the resistor so it works on high and low settings.
SG
 
My guess, around 100 ohm 1/2 to 1 W metal oxide resistor.

R=9.5/0.1
P>= =9.5/0.1 ; Note that isn't even 0.1 W, but 1W metal oxides are common.

e.g https://www.digikey.com/product-detail/en/panasonic-electronic-components/ERG-1SJ101V/P100W-1CT-ND

Metal oxide is flameproof. You could also use a chassis mount WW resistor like I did for my car trunk to accept LED's. Just because of location and not free air. The resistor cannot be on continuous anyway in my application. The car turns the lights off after a certain amount of time if accidentally left on.

Without the resistor, the trunk lamp turns off and opening/closing the car doors makes the trunk lamp dim. Initially, the trunk lamp is full bright.
yep odd. This resistor was sized closer to a real lamp,
 

Thread Starter

lightpsycho

Joined Mar 21, 2018
18
My guess, around 100 ohm 1/2 to 1 W metal oxide resistor.

R=9.5/0.1
P>= =9.5/0.1 ; Note that isn't even 0.1 W, but 1W metal oxides are common.

e.g https://www.digikey.com/product-detail/en/panasonic-electronic-components/ERG-1SJ101V/P100W-1CT-ND

Metal oxide is flameproof. You could also use a chassis mount WW resistor like I did for my car trunk to accept LED's. Just because of location and not free air. The resistor cannot be on continuous anyway in my application. The car turns the lights off after a certain amount of time if accidentally left on.

Without the resistor, the trunk lamp turns off and opening/closing the car doors makes the trunk lamp dim. Initially, the trunk lamp is full bright.
yep odd. This resistor was sized closer to a real lamp,
This scenario with the same transformer thats in use right now, or with a led driver? I'm a little confused...
 
This scenario with the same transformer
Example datasheet: http://www.nteinc.com/specs/5600to5699/pdf/nte5621_27.pdf

The spec of "holding current" is listed as 50 mA. 100 mA is about 2x that. Calculate for the lower voltage of 9.5 V @ 100 mA.
That's a value of R with a lot of slop and not knowing all of the particulars. Just "typical' ones.

if you don;t like thru-hole. This https://www.digikey.com/products/en?keywords=696-1477-ND could work too. Wattage is high, but it's based on availability.
 

sghioto

Joined Dec 31, 2017
2,691
I don't think those are triacs, just power transistors. Two 3 watt LED lamps would draw .5 amp at 12 volts, certainly more then the minimum holding current for any triac.
SG
 
I don't think those are triacs, just power transistors. Two 3 watt LED lamps would draw .5 amp at 12 volts, certainly more then the minimum holding current for any triac.
SG
\

When the LED's are off, there is 50 mA available to light the LED's. Without the minimum load the output voltage of a triac is full. e.g. 50 maA at 12V. That's enough to turn on a 3 W LED.

In contrast a 20 W 12 V lamp draws about 2 Amps AND the resistance when cold is 15x smaller than when hot. A triac would be happy.

There are a lot of instances in EE where you have to have a place for currents to go. Even pico-amps of current. A few mV across across .001 ohms (A PCB trace) turns out to be significant current.
 
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