Capacitive Touch Switch Capacity

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IBStrider
 
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Capacitive Touch Switch Capacity

Post by IBStrider »

I’m a metallurgical engineer with a limited knowledge of electronics, and I need a little help/advice. I’m trying to add touch control (on/off) to a couple of LED gooseneck reading lamps. The 110V wall plug for each lamp contains the DC power supply, marked 100-240VAC, 50~60Hz, 4.2V @500mA, and the two wires from the plug power the lamp. Measured voltage across the lamp leads is 3.58 VDC when the lamp is lit. From the website, the lamp wattage is listed as 3 watts and the amperage capacity (?) is listed as 350 microamps. Attempts to measure current with digital or analog meters result in no meter reading and the lamp itself flickers dimly when either meter is connected.

To try to accomplish this touch control, I purchased two capacitive touch switches (Item #1375), but I’m uncertain if this switch can actually switch the full load of the lamp, or do I need to get auxiliary relays to actually switch the lamp? If I need the relays, any suggestions as to which I should get?

Thanks for any help/advice anyone can provide.

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zener
 
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Re: Capacitive Touch Switch Capacity

Post by zener »

OK.. I will take a stab at this. There are a few different issues. First of all you say it is 3W but the supply is 2.1W (4.2V x .5A) so that is a little odd. You actually measure 3.6V so the current is maybe around .75A. You try to measure it but that doesn't work. I will address that first. Measuring voltage with a multimeter is very straightforward, but measuring current is a little more involved usually. A few things have to happen. You have to set the meter to current of course. Sometimes you have to push a button to set it to DC current, sometimes not. So far so good. Next issue is usually you have to move the red lead to a different hole in the meter. Now the the one that is probably getting you: You need to put the current THROUGH the meter, which is to say the meter has to be "in series" with the circuit. I will guess you put the probes across the voltage and shorted out the supply which is why the LED's dimmed and flickered.

Now, to the actual question: can the breakout board switch that load? No. It only has a logic level output that is probably around whatever supply voltage you are giving it and can only drive about 2mA.

So you need a relay or a transistor. Only problem with a relay is I don't know of any that work with only 2mA. They may exist but I can't think of any. You can use a mosfet such as https://www.adafruit.com/product/355

You would need to package that up with the board somehow.

Maybe someone else has another idea.

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IBStrider
 
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Re: Capacitive Touch Switch Capacity

Post by IBStrider »

Zener, thank you so much for your reply. To clarify a few things, the 3 watt rating for the lamp is the description on the website, not something that I determined.
Second, I actually did connect the multimeter in series with the lamp. Both multimeters (one digital, one analog) produced the same effect—the lamp was significantly dimmer and it pulsated rapidly when the meter was in the circuit. I suspect that this is because the power supply produces a square-wave rectified output (?) which is somehow altered (reduced?) when passing through the multimeter.

As to your suggestion, I went to the datasheet and noted that the MOSFET pins are labeled G (Gate), D (Drain), and S (Source). How would I connect this to my capacitive switch and the lamp? I’m thinking that the ‘Out’ terminal of the capacitive switch is used to somehow ‘trigger’ the MOSFET to have it send current to the lamp, but it’s not clear to me what pins do what.

Finally, could I go to my local Radio Shack and purchase an acceptable substitute MOSFET? I ask this because adafruit would charge me $12 to ship $3.50 worth or parts.
Thanks for your help, I really appreciate your time and effort.

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adafruit_support_bill
 
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Re: Capacitive Touch Switch Capacity

Post by adafruit_support_bill »

Not sure what is going on with your current measurement. If you have access to an oscilloscope, it might tell you more about what is coming out of the power supply. It would be best to understand that before connecting any other circuitry to it.

The MOSFET linked by Zener is an N-channel device, best suited to low-side switching - i.e. between your LED and the ground.. The source goes to ground, the drain to the negative terminal of the LED and the gate is controlled by the logic level output of the cap-touch breakout.

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zener
 
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Re: Capacitive Touch Switch Capacity

Post by zener »

Multimeters generally measure current by passing it through a resistance. If the meter has two ranges for current, the lower range will present a higher resistance. You might try using the higher range if you have one, usually a 10A range. The resistance might be the problem.

When using the N channel mosfet you have to make sure the ground of your mosfet (the source) and the ground of your breakout board are connected together. How are you going to power the board?

I think you can get this at Radio Shack: IRF510, 276-2072. Mouser has cheap shipping if you specify USPS (you might have to call your order in to do that.)

IRF510 is technically not a logic level part but if your current is below 1/2A (you still need to measure this) then it is probably OK as long as your gate voltage is at least 5V. Gate voltage will depend on what the breakout board supply is, and the max is 5.5V I think.

Hope that helps.

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