I’m building a circuit that involves the use of latching relays. In this circuit, the relay will switch between power supply A or power supply B, to my load.

To illustrate my case, let's say that power supply A is currently supplying current to my load, but I want to trigger a switch to power supply B. There are two possible implementations that I have thought of. I can use either capacitors or diode to ensure that my load continue to function in the event of a switch triggered by the relay. In the former, during the switching process, power supply B turns on, and both power supply A and power supply B will briefly overlap (thus the need for Schottky diodes for each, otherwise problems cause by competing DC power supplies ensue) before one is cut off by a relay, thus ensuring a smooth transition of power supply. The load remains unaffected. In the latter case, I envision a design where the (?)microsecond switch would be covered by a capacitor (or multiple capacitors) that ensures continual current delivery to my load, before power supply B takes over.

Given these two options, which should I use? I am thinking that in terms of power loss, diodes may cause more wastage than capacitors? Also, there might be abit of voltage drop if diode is used. Thus, I am inclined to use the capacitor approach. However, I am not very knowledgeable about this. I wanted to ask which would be the recommended option?


1 Answer 1


Diodes don't cost that much and neither do capacitors so I would implement a decent capacitor hold-up AND diodes for when the relay switches over. You don't have to fit the diodes if you find that the prototype is OK without them.

BTW, the relay switching might be in the several milli second region so I urge you to read the data sheet so that your capacitor value can be correctly estimated thus minimizing voltage droop when switching from A to B power supplies.

  • \$\begingroup\$ Thanks for your input. So it sounds like you would favour the capacitor implementation first and if necessary, then add on the diodes? Certainly cost is not a factor for me, but I thought of minimising any power wastages and keeping the circuit less complicated where possible, hence the decision between the two before I begin soldering. \$\endgroup\$
    – Craver2000
    Jan 23, 2018 at 10:28
  • \$\begingroup\$ It comes down to knowing the real numbers and if you are unsure about them then a level of redundency is probably appropriate. \$\endgroup\$
    – Andy aka
    Jan 23, 2018 at 10:32

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