120v AC SSR resistance question

[smssk]

How do I go about calculating the required resistance to drop 120v AC down to 1.4v DC to drive the SSR in the attached circuit at approximately 15ma?

[Konkedout]

1) Think current instead of voltage.  The LED operates at a relatively constant voltage which is small compared to the incoming voltage.   It wants to be fed by current; not voltage.  How much current do you want?  Maybe 10 - 20 mA??  Well OK 15 mA...read below:

2) Thinking that way; use a simulator!!  LTSpice would work, or I could even simulate it with Excel if I wanted to spend the time.  Use the simulator to determine how much current you get through the LED, depending upon input voltage and series resistance.

3) I think that the zener D3 will do no good and may well interfere with circuit operation.

4) Finally...it may be better to use only one dropping resistor in series with the LED.    Use a capacitor at C2 that is rated at 200 or 250 VDC.  You might be able to use a 1 uF film type for example.  If you do this than you would need one series resistor of about 10K or 12K.  But this needs to be (conservative) a 10W rated resistor.  Best to use a step down transformer or low voltage power supply to drive the LED.  Did you throw away that power adapter from your old cordless phone?

[johansen]

On the order of a .1uf capacitor in series with a bridge rectifier will provide tens of milliamps at constant current. Use the capacitive reactance calculator, put a resistor in series, and a zener to protect the led from power surges.

If you just use a resistor then you have to dissipate 10ma times 120vac.


You can probably buy 120vac input SSR relays for less than the time to figure out how to build your own.

[Andy Chee]

How do I go about calculating the required resistance to drop 120v AC down to 1.4v DC to drive the SSR in the attached circuit at approximately 15ma?

Have a read here, particularly the capacitive supply:

https://ww1.microchip.com/downloads/en/appnotes/00954a.pdf

[BillyO]

I would consider a capacitive dropper instead. You will avoid the the dissipation issue.