What would happen if there was a voltage at the V-OUT pins of a boost converter?

or, i suppose the “grown up” question would be - is the Boost converter output high impedance " ?

i have soldered a boost converter to a ProMini (for a permanent project) to provide it 5V and now am wondering what would happen if i needed to update the sketch and try to upload via a “FTDI programmer”.

something like this;

would the boost converter be affected, or maybe even the uploading process ?

it goes without saying the boost converter V IN pins will be left ‘OFF’ (a 3xAA batt.pack with switch).

Probably okay. I usually put a jumper on my FTDI adapter to disconnect VCC. Sometimes I add a diode to prevent reverse flow. Sometimes I forget and nothing bad happens. Often my projects run off a five volt supply and then when I plug in the FTDI I have power coming from both sources. If both voltages are nearly the same there is no problem, but the voltage regulators will be fighting each other. Usually not a problem for short term use, but a bad design if kept that way.

Some linear regulators can be damaged by reverse current flow (output has power, the input is off), but I don't see a buck converter having a problem with that.

Long story short, I don't think it is anything to worry about for short term, and probably is not a problem even when left that way. Do you have a data sheet on that converter? If it is a problem it will usually be called out, recommending a diode being placed from the input to the output (so that it normally will not conduct).

I think it is likely the converter will survive if its input is left unconnected. It is less likely if + and - of input is shorted together. But to be sure you should know the converter's circuit. Some ICs have feature to short output when enable pin is low (to discharge output cap to prevent prolonged brown out of the load). And possibly there may be other problems.

amdkt7:
Long story short, I don't think it is anything to worry about for short term, and probably is not a problem even when left that way. Do you have a data sheet on that converter? If it is a problem it will usually be called out, recommending a diode being placed from the input to the output (so that it normally will not conduct).

the product was listed as "MT3608 Step Up 2A Boost Regulator DC Mini XL6009 Power Booster Supply" but the chip on it is a B6285i - all the datasheets are in Chinese :frowning:

i am guessing it's meant to be analternative to an XL6009 (like how many sellers list a "L298 H-Bridge Motor driver" but that chip is not on it !) - but the common info is the "MT3608" which seems to be the module identification.

amdkt7:
Probably okay. I usually put a jumper on my FTDI adapter to disconnect VCC.

sorry for this dumb question, but how does that work - you short VCC to the next pin ?
isn't VCC needed when uploading a sketch ?

amdkt7:
Sometimes I add a diode to prevent reverse flow.

ahh yes, that would've been a nice and simple solution !!
just adjust the boost output to 5.7V and then drop a diode between V-OUT+ and Vcc(on the ProMini).

amdkt7:
...
Usually not a problem for short term use, but a bad design if kept that way.

yes, i admit it's not a good design - i don't intend to update sketches after the project is "deployed" - so it really is just a temporary stop-gap for uploading any fine-tuning that may be needed for the sketch.

Smajdalf:
I think it is likely the converter will survive if its input is left unconnected. It is less likely if + and - of input is shorted together.
...

hmm, interesting - how would that be "better" than left unconnected ?

Is there a reason why you can’t power the Mini from the boost converter while flashing through the FTDI adapter? You would just not connect the adapter’s 5V output (desolder the header pin or whatever). Then there would be no conflict, and no need for a diode. Of course the diode would work, but then you are “wasting” your battery power because of the diode’s voltage drop. And you waste it all the time your Mini is running, not just when flashing. I think flashing should work fine with the Mini powered from the converter as long as the FTDI’s ground pin is connected.

The Arduino does need power to be flashed. When I use a jumper I am breaking the connection to VCC, not shorting it to anything. I have modified several FTDI adapters so that I can do that. That prevents power from the USB reaching the Arduino when I have it already powered from some other source. If I use a diode I place it between the FTDI VCC and the Arduino VCC.

Don't count on a diode to drop .7 volts. It varies depending on how much current flow there is. Sometimes it is as little as .1volts, but usually more in the range of .4 to .6volts.

Also, the Arduino will flash and operate just fine on 4.5V, and sometimes as low as 3.3V. Lower voltages could have problems depending on what the Arduino is connected to, but to program it works just fine.

amdkt7:
The Arduino does need power to be flashed. When I use a jumper I am breaking the connection to VCC, not shorting it to anything. I have modified several FTDI adapters so that I can do that. That prevents power from the USB reaching the Arduino when I have it already powered from some other source.

ohh, i see - that's quite different from what i had in mind, i was more concerned about what was going on between the Arduino/FTDI and the Boost converter (when off).

but if you are saying i can actually power the Arduino (via the Boost converter), and just have the TX/RX/DTR(+GND) pins connected, then that would be rather more straight-forward - ie. just disconnect the Vcc jumper wire to Arduino (from FTDI/USB-to-TTL serial adaptor).

amdkt7:
If I use a diode I place it between the FTDI VCC and the Arduino VCC.

ahh, you mean in reverse bias - so current does not flow from the FTDI Vcc to Arduino Vcc.

amdkt7:
Don't count on a diode to drop .7 volts. It varies depending on how much current flow there is. Sometimes it is as little as .1volts, but usually more in the range of .4 to .6volts.

i see, not a good solution then - as well as the wasted power mentioned by ShermanP.

amdkt7:
Also, the Arduino will flash and operate just fine on 4.5V, and sometimes as low as 3.3V. Lower voltages could have problems depending on what the Arduino is connected to, but to program it works just fine.

yes, i "discovered" that when i absent-mindedly powered an Arduino (via Vcc pin) with a 4xAA batt.pack of Ni-Mh (ie. was only 4.8 V)

ShermanP:
Is there a reason why you can't power the Mini from the boost converter while flashing through the FTDI adapter? You would just not connect the adapter's 5V output (desolder the header pin or whatever). Then there would be no conflict, and no need for a diode. Of course the diode would work, but then you are "wasting" your battery power because of the diode's voltage drop. And you waste it all the time your Mini is running, not just when flashing. I think flashing should work fine with the Mini powered from the converter as long as the FTDI's ground pin is connected.

Thanks, that makes it quite a straightforward solution then - i was under the impression that "it" comes as a package-deal - meaning power and data (for flashing) all coming from the same device (the FTDI) - i didn't realize the 5V power could come from anywhere.

Yes, as long as the grounds are all connected, it should work fine. In fact, it may be possible to flash without any Vcc power at all. I once flashed a Texas Instruments MSP430 processor, but forgot to turn on the power. But it flashed successfully anyway. It turned out that enough current flowed from the other lines, when they were high, through the protection diodes to Vcc, and charged up the capacitor there, so flashing could be completed successfully. Not a recommended practice, but something to keep in mind when connecting active lines to a device that's powered down. Dave Jones (EEVBlog on Youtube) has a video on this:

ShermanP:
Yes, as long as the grounds are all connected, it should work fine.

ok, thanks again :slight_smile:

ShermanP:

but something to keep in mind when connecting active lines to a device that’s powered down. Dave Jones (EEVBlog on Youtube) has a video on this:

EEVblog #831 - Power A Micro With No Power Pin! - YouTube

thanks for that - i’d not seen that before - very interesting !