[SOLVED] Charge capacitor before supplying power to my circuit?

[yellowfever13]

I have searched on this for a couple hours with no luck, perhaps I don't know what to search for!
Wondering how I can charge a capacitor before supplying power to my circuit. Basically I have a DC power source, and the devices it's powering on experience a power dip upon turning on and they sense inadequate voltage so they don't turn on. A capacitor fixes this, but I need it to be charged beforehand. Is there a way I can have my DC power source charge the cap and then let current flow to my devices? This way the cap can take care of the dip, since my DC power source obviously can't. I'm open to other ideas as well, thank you for your time.

 

[betwixt]

Connect the capacitor to the power source side of the switch ??

Brian.

 

[yellowfever13]

That's a good idea, currently there is no switch. So that will be my last resort. But ideally I need this to not have human interaction. So it can reboot upon power outages. I was hoping for something automatic that closes the circuit once charged. Perhaps I will have to use a timer and a mosfet that is longer than charging time.

 

[betwixt]

It would be difficult to pre-charge a capacitor if the power had failed! I think you need to tell us exactly what the circuit does, it may be easier to delay it's start up than to control the power to it.

Brian.

 

[yellowfever13]

No problem, let me explain. I am using raspberry pi computers, many of them. They have USB devices on them, that when booted either don't start or the raspberry pi's don't start due to power dip. Powered hubs are not an option. Probably the main problem is the cheap 5v 10A power supplies I bought, seems this problem should not exist in the first place.

I can't separate the power of the USB and the computer because they are tied together on the board. So I am left with needing to charge the cap before the computer starts. I thought there might be an easy trick for this, but it seems I will need to just hook up a Tiny85 with a mosfet and just hard code a time that allows for charging.

 

[betwixt]

I haven't got a Pi to experiment with and I'm not sure how the USB ports are connected to it but most USB controllers have a power enable switch in them. Is it possible to disable the USB power in software for a short while?

It does sound like a PSU problem though, USB devices are not supposed to draw more than 500mA and most draw far less so they shouldn't cause a significant dip.

Brian.

 

[yellowfever13]

I may try that Brian, although sometimes it prevents the computer from booting so that might not be an option.

I'm going to mark this as solved I guess. I was hoping for a more elegant solution, but I'm just going to use a Tiny85 and mosfet to give me a couple seconds to charge the cap, then turn on the computers. Basically a switch like was mentioned, but this will need to be automatic.

Thanks