I know what I want but not sure how to achieve it! I have car dash cam supplied by 5 volts. I've set up a 5volt power bank to run the dash cam when parked and wired up a relay operated by an Accessory circuit to switch from car battery power (via a 5v usb adapter) to power bank battery when the car is parked. Very rough circuit diagram below. Problem is that the fraction of a second it takes for the relay contacts to switch over causes the camera to switch off and back on, causing data loss and/or having to reformat the SD card. What I need is a 'simple' capacitor circuit to maintain the 5v on the camera for that split second! I'm no expert so would really appreciate some advice on what components to use, and suggested values.Apologies for the very rough diagram of what I currently have!

  • \$\begingroup\$ Is there any reason you can't just do (12V Acc) ---> (Power Bank) ---> (DashCam) and skip all the relay switchery completely, or will your power bank device not allow that? \$\endgroup\$ – brhans Dec 5 '17 at 20:00
  • \$\begingroup\$ Can you use a Make Before Break Relay? \$\endgroup\$ – Tyler Dec 5 '17 at 20:23
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    \$\begingroup\$ There is a schematic tool in the toolbar, you should use that, it would be easier than trying to follow the hand drawn diagram. \$\endgroup\$ – Ron Beyer Dec 5 '17 at 20:38
  • \$\begingroup\$ I'm confused why you need both too. USB adapter hooked straight to the car battery should be enough... or just leave it wired from the power bank and let it charge while it drives the camera. \$\endgroup\$ – Trevor_G Dec 5 '17 at 20:40
  • \$\begingroup\$ Thanks for the replies so far. The power bank doesn't support "pass through" charging, and I understand in any case it can be dangerous to charge lithium batteries whilst devices are connected. The idea of a "make before break" relay appeals though, any suggestions as to the specification of this please? \$\endgroup\$ – JimCM Dec 5 '17 at 21:16

You do not need a relay on the ground side.

You may get away with a single relay and two SCHOTTKY 60V 5A DO201AD diodes acting as bypass while the relay moves as shown below. It will depend if the camera can handle the voltage dropping to about 4.7V briefly. They are cheap though so a quick test is easy enough. NOTE: You may not even need the relay with this circuit.

However, your issue may be a sequencing one. If the battery pack shuts off the outputs before the USB adaptor completely comes up it may still be an issue. If so you will need to add a large capacitor in after the diodes.


simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Really appreciate your advice Trevor, and I shall certainly give this a go. Could you just explain please how the diodes would work, connected across the relay terminals as indicated? Thanks again! \$\endgroup\$ – JimCM Dec 5 '17 at 22:29
  • \$\begingroup\$ @JimCM It's basically an OR gate. Whichever power supply is available will power the camera while the relay is between poles. If your battery pack cuts off when the charger is on, all the better, you can ditch the relay. If not leave it in, it ensures power is coming from the right supply. \$\endgroup\$ – Trevor_G Dec 5 '17 at 22:33
  • \$\begingroup\$ @JimCM again. assuming the camera can handle the diode drop \$\endgroup\$ – Trevor_G Dec 5 '17 at 22:35
  • \$\begingroup\$ Cheers Trevor, really appreciate your help. Just need to track a couple of these diodes down now! \$\endgroup\$ – JimCM Dec 5 '17 at 22:40

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