Adding protection to my circuits

Thread Starter

narkeleptk

Joined Mar 11, 2019
548
I am a bit new to designing circuits so bare with me please. I'm trying to improve on one of my older projects I made to assist in my daily work. Currently my original version works great stand alone but its lacking a few features that would be nice.

I will be adding a usb connector and ftdi ic to access the mcu's UART instead of icsp header I use currently. I also want to try and add a dc/dc step up from the usb to about 12v to supply the units I'm working on. Is this feasible? Typically 12V 1A is needed. I was thinking of the LM2623. I have just started on the schematics so all I have on the power side so far is below but this got me thinking I better start learning about protection since I will be attempting to add different power sources before I continue.

My guess would be to add some protection diodes between vbus coming in from usb and on 14vdc coming in from j1 and maybe on a2 and b2 on sw so b2 doesnt feed back through a2 when jacks not being used? Any advice?

supply.jpg
 
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Wolframore

Joined Jan 21, 2019
2,577
USB "power supply" (not USB downstream device) can supply up to about 2 A @5V max... but stepping up to 12V check [edit] pwoer law... see what the relationship is when P = I x V. as currrent and voltage has inverse relationship holding P constant... also consider efficiency boost converters are lower... This looks like the standard Arduino power supply which I don't like because of the lack of reverse protection at VCC. So easy to blow out the regulator.

It would be better to have a separate 12V supply instead of in essence taking 120V converting to 5V (USB supply) then boost to 12V. Do you need the USB connection for some reason?
 
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Thread Starter

narkeleptk

Joined Mar 11, 2019
548
Thank you for the advice. For some silly reason I did not even think to consider the current from the usb side tho I expected it to be around 2a. I will read up on it for a better understanding but already sounds like the step up is not a good idea for my needs. There is really no reason for me to try and force it from the usb anyway it was just kind of an after thought when planning to add the usb.

Most of the time I run the unit stand alone with 13.5V 2A supply through a barrel jack. This takes care of most of the functions needed and also feeds the modules off board that I am working on. For the mcu I currently use a lm7805 to supply it 5v. I think now I will just keep my original setup and simply add in the usb and ftdi separate from that.
 

MrChips

Joined Oct 2, 2009
27,128
... but stepping up to 12V check your ohms law... see what the relationship is when V = IR.. as voltage goes up current goes down holding R constant...
Not correct. As voltage goes up, so does current.

You meant the power formula:

P = I x V

As voltage goes up, current goes down for the same amount of power, exactly like an AC power transformer.
With 100% efficiency, power is conserved. You cannot get out more power than what you put in. With less than 100% efficiency, the power out is going to be lower than the input power.
 
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