I want to use a capacitor for an alarm clock but am not completey sure on what size to use?

Question

This is a little project I am undertaking for the fun of it. My alarm clock doesn't have battery backup so I wanted to install a capacitor in the event of short brownouts of a minimum of 3 seconds. I am not sure of the secondary voltage of the alarm clock but it is no more than10 volts as their are other capacitors rated for 10 volts. I measured the amperage during normal operation at .2 amps. So the total resistance should be around 50 ohms. I am thinking I will need a fairly large capacitor like in the milifarad range. Thanks!

This is a little project I am undertaking for the fun of it. My alarm clock doesn't have battery backup so I wanted to install a capacitor in the event of short brownouts of a minimum of 3 seconds. I am not sure of the secondary voltage of the alarm clock but it is no more than10 volts as their are other capacitors rated for 10 volts. I measured the amperage during normal operation at .02 amps. So the total resistance should be around 50 ohms. I am thinking I will need a fairly large capacitor like in the milifarad range. Thanks!

This is a little project I am undertaking for the fun of it. My alarm clock doesn't have battery backup so I wanted to install a capacitor in the event of short brownouts of a minimum of 3 seconds. I am not sure of the secondary voltage of the alarm clock but it is no more than10 volts as their are other capacitors rated for 10 volts. I measured the amperage during normal operation at .02 amps. So the total resistance should be around 500 ohms. I am thinking I will need a fairly large capacitor like in the milifarad range. Thanks!

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Answer 1

well, charge in cap = voltage x value of cap, so value of cap = charge / voltage. You want the charge to last 3 sec, and charge = current x time, so cap value = (0.2Ax3sec)/10v=0.06F, which is a large cap. Remember that you can use smaller caps in parallel if this is too large.

Answer 2

Better check your math first. If e= 120v and r = 50, you then have a 2.5 amp alarm clock....don't think so. Once you know the true current consumption, you just need to calculate the capacitive time constant.

Answer 3

use yor RC time constant formula to determine how long the cap's charge need to last to keep the clock powered and if a series resistor is needed to control the discharge