I'm using ATMEGA644 in a measurement circuit. I'm providing the primary supply from a battery + boost regulator and if/when the voltage (the one that I have to measure) is available I want the supply to switch to that voltage. Switching will be done using a relay. So I figured I need to hold 5V till the release time of the relay which is nearly 15ms.

I know I'll have to hold 5V at the supply pin of ATMEGA644. I think I can use a capacitor to hold 5V for 15ms but I'm not sure.

My questions are:

  1. Can I use a capacitor to hold 5V for a current flow of 50mA? If yes, then what will be the circuit and how will I be able to calculate the capacitance?
  2. If not then what circuit should I consider following? or what other option should I be looking into?
  • \$\begingroup\$ "Switching will be done using a relay." I'm really curious to know why you need to use a relay in this application. \$\endgroup\$ Dec 5, 2015 at 10:19
  • \$\begingroup\$ No special reason. I figured I only need a few amps at 5V \$\endgroup\$ Dec 5, 2015 at 10:47
  • \$\begingroup\$ So then why not use one of the several power switching schemes that don't have a cutover delay? \$\endgroup\$ Dec 5, 2015 at 10:49
  • \$\begingroup\$ Um. Is using relay a bad option? I have some in spare I don't want to spend more \$\endgroup\$ Dec 5, 2015 at 10:50
  • \$\begingroup\$ When the secondary vltage is available why not use it to charge the primary battery then you don't need to switch. \$\endgroup\$
    – Andy aka
    Dec 5, 2015 at 11:10

1 Answer 1


.If you must use a 15ms relay then you need a cap just like you said .The tolerable voltage drop which occurs while the cap discharges slightly during the 15ms outage,so I will assume that 250mV drop is OK .Then to the nearest preferred Value we get 3300 microfarad .

  • \$\begingroup\$ Could you please tell me how you calculated that value ? \$\endgroup\$ Dec 6, 2015 at 13:40
  • \$\begingroup\$ Could you please explain to me how you calculated this value? \$\endgroup\$ Dec 6, 2015 at 14:57

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