Problems with MicroView +5V and USB Programmer

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Schema
Posts: 23
Joined: Wed Apr 13, 2011 11:33 am

Problems with MicroView +5V and USB Programmer

Post by Schema » Thu Sep 10, 2015 8:26 am

I have a MicroView project I'm working on, where the MicroView is typically powered by a host computer (a Commodore 64 if it's relevant :wink:).

I've been powering the MicroView off of the regulated +5V supply provided by the computer, hooking it directly to Pin 15 (+5V) and this works perfectly.

However, I would also like to be able to program the MicroView in-situ with the USB programmer. Unfortunately, this causes +5V to be output from Pin 15 of the MicroView, this feeds back into the computer and causes issues (A C64 needs way more current than the MicroView can source, and it also needs 9VAC, and so on).

What are my options to work around this problem? (A similar issue exists when the MicroView is powered externally though VIN).

I've considered putting a diode in between, but this will drop the 5V to 4.3V when it reaches the MicroView. Will this be sufficient to power the Microview? Or should I just switch to VIN and take the inefficiency hit through the regulator? Or is there a better solution?

Thanks!

scotta
Posts: 44
Joined: Thu Jul 24, 2014 1:31 pm
Location: Toronto, Canada

Re: Problems with MicroView +5V and USB Programmer

Post by scotta » Thu Sep 10, 2015 5:08 pm

Use a Schottky diode and you'll only get about a 0.3V drop. The resulting 4.7V should be sufficient to power the MicroView.

Schema
Posts: 23
Joined: Wed Apr 13, 2011 11:33 am

Re: Problems with MicroView +5V and USB Programmer

Post by Schema » Fri Sep 11, 2015 12:21 pm

Thanks for the reply, I'll test and see. Any recommendations for a common part#? (11,271 matches on Digikey ;-))

scotta
Posts: 44
Joined: Thu Jul 24, 2014 1:31 pm
Location: Toronto, Canada

Re: Problems with MicroView +5V and USB Programmer

Post by scotta » Fri Sep 11, 2015 5:23 pm

Schema wrote:Any recommendations for a common part#?
How much current will the diode have to handle?
Do you want through-hole or surface mount?

For around 500mA or less with through-hole, a 1N5817 would probably do.

For surface mount, you'd probably want to look at forward current capability, low forward voltage, size and cost.

Note that usually a diode with higher than required current capability will have a lower voltage drop at low currents. Look at the forward voltage graph in the datasheet.

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