# microsecond power loss durring relay switch over

Discussion in 'General Electronics Discussion' started by kerpap, Feb 11, 2014.

1. ### kerpap

10
0
Jan 20, 2014
I have a relay that switches between 2 power sources.
I noticed that there is a microsecond loss of power to the load during the NC - NO switchover going from power source 1 to 2.

im scratching my head. how to compensate for that micro second loss. I need continuous power to the load. the microsecond power loss causes the device receiving power to reboot.

2. ### Harald KappModeratorModerator

11,439
2,627
Nov 17, 2011
Add a big capacitor across the output of the relay circuit to buffer the load while the relay is switching.

by the way, you're lucky if you see only microseconds of interruption. A few milliseconds are far more typical for a relay.

What are the voltages and currents were talking about?

3. ### kerpap

10
0
Jan 20, 2014
That is what I figured. I am not sure though where to position the cap.
12 volt.
I also am planning to build a 9V variant.
it is a solar to battery switch over.

4. ### Harald KappModeratorModerator

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2,627
Nov 17, 2011
The capacitor goes on the load side of the relays.

5. ### kerpap

10
0
Jan 20, 2014
so, if I understand correctly it should look something like this?
the line in red is there because I am not sure if it is needed. if I just put the cap, wont it block current? shouldn't it have an alternate path (red line) then once current stops the cap discharges.

maybe im not understanding capacitors as well as I should.

6. ### mursal

75
0
Dec 13, 2013
Connect the capacitor across the load, one leg on the supply to the load from the relay, one leg on the return from the load (as if you were connecting a battery to drive the load). In other words, connect it in parallel with the load
The capacitor may need to be connected in a certain way (polarity), so if one leg is marked negative (-) it must go to the return from the motor.

Last edited: Feb 11, 2014
7. ### Harald KappModeratorModerator

11,439
2,627
Nov 17, 2011
No, like this:

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