Pro Mini 5v, Ftdi breakout and external 5v powersupply.

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1

Hello fellow Sparkfun enthusiasts…

I had a question about external-power-supplies (5v) , an arduino pro mini 5v, ( https://www.sparkfun.com/products/11113 ),

and the 5V ftdi breakout board ( https://www.sparkfun.com/products/9716 ).

In my present setup, I use the ftdi-breakout to both program and monitor the output from the pro-mini. During my

development phase, I rely on the ftdi-breakout to provide power to the pro mini.

Now that I have a full-blown circuit/project with various sub-components, and an external 5v powersupply; I have

concerns about hooking up the ftdi-break-out board to the pro-mini, while the external powersupply is

providing power to the pro-mini.

Part of why I want to hook it up, while everything is live, is to monitor for any error-messages.

Addionally there have a been a few times where I’ve needed to do a field-program/upgrade on the pro-mini. With the

external supply off, I’ve noticed that the ftdi does illiuminate the power-led on the external-supply.

Is there any recommended way (or practice) in wiring up the arduino-pro-mini into an externally powered circuit, and

still be able to safetly use the ftdi breakout for both monitoring and programming??

Hopefully this makes sense.

Thanks. :smiley:

–Mike

2

You are right to be concerned. Generally wiring two power supplies together is not good. They will fight possibly causing damage to one or both. To generate suggestions for a solution, please provide more information. Is the external supply wired to VCC or to RAW? I’m guessing it must be VCC else the external supply LED would not light. What voltage is the external supply? What other constraints (size, mechanical interference, accessibility, other voltages, current draw, …) are there on the project? Do we need to think about what the pro mini is hooked to? How is that part of the circuit powered?

Thanks,

  • Chip
3

Hi Chip,

Thank you for the reply…

Here are answers to your questions…

  • Is the external supply wired to VCC or to RAW?

The external supply is wired to the VCC pin.

  • What voltage is the external supply?

The external supply voltage is 5 Volts.

  • What other constraints (size, mechanical interference, accessibility,

  • other voltages, current draw, …) are there on the project?

All in all, there, there is the mentioned-pro-mini, a L293D drive a slow-rpm motor, again at 5v.

A potentiometer is used as a feedback element to the motor. (It’s also tied to vcc)

Finally, a Sparkfun 7-seg serial display is tied to the pro-mini, over i2c.

(https://www.sparkfun.com/products/11442)

There are a handful of ttl-logic chips and some led emitters (transistor driven) and several ir-sensors.

Do we need to think about what the pro mini is hooked to?

See previous answer.

How is that part of the circuit powered?

Pretty much everything is being driving from the external 5v supply.

I don’t have solid-numbers on hand (at my day-job) in terms of current draw, however

the motor runs intermittently and doesn’t put much load on the entire

circuit.

Hope that helps.

–Mike

4

Okay, I see a couple of different solutions depending on what scenarios you want to support.

If it is important to be able to power the circuit from the FTDI after deployment:

  • You could build a switch into the wire feeding power into the Pro Mini from the external supply. Just turn the switch off before you plug in the FTDI.

  • You could build an extender to sit between the FTDI and the Pro Mini. This would be a connector that connects to the FTDI and one that connects to the Pro Mini. All signals pass straight thru except FTDI power. That one has a switch in it. When external power is off the FTDI extender switch should be on and visa versa.

  • You could automate the previous solution by using a MOSFET instead of a switch. Another wire would need to connect to the external supply to detect when that power is on and turn the MOSFET off.

**** If you have the possibility to have the external power on to the motor control and display and the Pro Mini power off, you need to be aware of potential power bleed back from the rest of the circuit back to the Pro Mini. This is usually not a problem but it’s possible it could cause damage. You need to look to your circuit to evaluate.

If, after deployment, the circuit is always powered from the external supply and you just need to connect the FTDI to reprogram or pull stats or whatever, then the simplest solution is to clip the power pin from the FTDI connector on the Pro Mini as the last step before deployment. That means that if you should ever have to go back into development mode, you would still need to power from the external circuit. That is, you would not be able to power from the FTDI after that deployment point.

I hope that one of these solutions will work for you. Good luck with your project.

  • Chip
5

Hi Chip,

I apologize for the delayed response… Work has gotten in the way of life…and my little projects. :\

Anyways thank you very much for the suggestions; they make perfect sense.

I suspect, for now, I’ll go the route of adding a switch to the FTDI, though I like the idea of the mosfet and will investigate that further.

Thanks again.

–Mike