Xmas light string controller

[John12345]

Hi. I want to control 5 Xmas light strings using one of the light string controllers. One controller is not powerful enough to run many of them at once so I would like to amplify the variable signal. One controller puts out a variable voltage from 0 to 110 VDC. Each string has a resistance of 105 ohms. I would need a circuit or transistor or Mosfet that would be capable of 110VDC and an amperage of about 5 amps (for 5 strings). I tried to measure the amperage of each string and read about 0.15 amps so I must be doing something wrong here or my meter is bad. I bought a Mouser mosfet p/n FCPF099N65S3 which I thought would work but I can’t figure out how to put together the circuit. Any help would be appreciated.

 

[dknguyen]

Are you sure it's a variable voltage between 0 and 110VDC that is used (for example to fade the lights)? If it is, it won't be so simple. and you're better off getting more controllers. It would be much simpler if it the controller achieved fading by by blinking the lights faster than the human eye can perceive for periods of time or shorter/longer duty cycles.

 

[John12345]

I am not sure that the voltage is variable. I can only say that when measured with a simple meter it seems to be. I don't have an oscilloscope. Here are some pictures of the controller guts. Using more controllers would be simple but then all of the strings of lights would not be synchronized.

 

[dknguyen]

Is the 110VDC being supplied by that board? Is it offboard?

 

[John12345]

Supply voltage is 120vac and converted to dc.

 

[dknguyen]

Supply voltage is 120vac and converted to dc.

But the conversion is not done on this board right? It's done by a brick somewhere else along the wire? If that's the case, it seems to be chopping the signal so a MOSFET should work. Does this light string just blink the lights or walk the lights or can it also fade them in and out too?

You will also need:

1. A 110VDC@5A supply
2. A circuit to convert 110VDC from the controller to a ~20V signal that can drive the MOSFET (the MOFSET gate can accept up to 30V).

The current you measured might be smaller because if you are using bulbs and not LEDs, the resistance of the bulb increases as the bulb heats up so you would get a lower current than if you went I = 110VDC/[Cold resistance]

 

[John12345]

Thanks. I have all of the above items except I don't know how to wire up the circuit. The ac is converted on board with the diode rectifier.

 

[dknguyen]

The circuit here is the basic idea. I assume there's two strings of lights and each string has a positive and ground wire? You would need two of them if that's the case.

EDIT: Typo on the schematic. The 4.7K resistor doesn't need to be 5W. It can be pretty much any resistor wattage (it's 80mW dissipation).

 

[John12345]

Thank you very much. You have been a very big help.

 

[tcmtech]

I have worked on similar light controllers and they are not exactly as the seem. Some used triac control of the AC power and others use a SCR output which I suspect yours is the second design and not using a mosfet or transistor output.

I could be wrong being without a part number to identify what the three TO-92 case output devices are, there is no easy way to tell but logically for simplicity of control from a tiny uC that is under the black blob on the vertical circuit board SCR's would be the simplest and easiest to control given the rectified and unfiltered power available.

Which if so modifying that controller to drive larger external SSR's (Solid State Relays) would not be all that hard.

 

[John12345]

Yes. Mine is the second design. They use 3 thyristors - XL1225. I'll look into getting larger ones. Thanks for all your help.

 

[dknguyen]

I was wondering why there were 3 TO cases and not just two.

 

[John12345]

I replaced one XL1225 SCR with a bigger one of the 3 on the board. I used a TN1610H-6T SCR. The light string comes on fine but it does not get brighter and dimmer with the other 2 color strings as it should do. It stays on all the time at the full illumination. Do you know of another SCR that I should use? Maybe I need to use more resistance somewhere in the circuit. I don’t know. Any more help would be appreciated. Thanks.

 

[tcmtech]

Have a look at the two SCR's operating specs, specifically the IGT (Gate trigger current) The tiny one is ~200 microamps and the bigger one is ~ 4.5 milliamps a ~23:1 difference.

More than likely you will have to reconfigure the circuit a bit to use the smaller SCR's to trigger the gates of the larger one though a few tens of ohms resistor to get them to work right.

Beyond that if you are putting a higher overall load on the same circuit you will need to change out the input diodes to a much heavier set as well to balance things out.

With a bit of clever circuit modification you should be able to mount the larger SCR's and diode sets/bridge rectifier on a independant heatsink (aluminum strip of some sort) so that all the circuit board does is work as a remote brain for the current carrying muscle components which conceivably in theory if carried out far enough you could mount this all in a modified Power Stip and have each SCR's output running a separate outlet or set of outlets.

 

[John12345]

Thanks. I'll let you know how it works.

 

[John12345]

Would this be the proper way to try your recommendation?

 

[tcmtech]

The concept I had is basically this circuit but your existing SCR's stand in for the optocoupler SCR device.

 

[John12345]

Wow. Not sure how that fits into my existing circuit. I think I'll go with your first suggestion. I assume that will work with the varying voltage output? Thanks again for all of your help.

 

[tcmtech]

The circuit just show that a smaller SCR can be used to control a larger one. (the opto isolation concept is irrelevant) Basically you are using the tiny stock SCR's to overcome the lack of adequate gate drive current issue the larger units have.

 

[John12345]

I understand what you are saying but with my limited electronic background I can't figure out how that works in my circuit. Your example has the load (lights) attached to the anode. My existing circuit has the lights attached to the gate (see pdf file). What replaces the lights in the new circuit? I'm too ignorant to know how to use your suggestion. Thanks again for all of your help.