So confused on power with the 3.3 Pro Mini

Projects General Guidance
1

Super confused.

I wired a relay to the VCC, GND and the GPIO that's shared with the internal LED. I uploaded the example blink sketch and it worked exactly as expected. Everytime the LED flashed, the relay clicked on.

Yay, right?

That's with it powered by the FTDI via USB.

EVERY other power method I've tried fails.

The specs say the pro mini can take up to 12V on the RAW pin. So I tried a 3S lipo that measured at 11.7. The lights shine BRIGHTLY but don't blink and the relay is simply ON.

Whoa. What happened to voltage regulation? Hmm.

So I try 7.2V... the light blinks, but the relay doesn't click. What??!?! The light ON the relay also shines... but again, no click.

OK, maybe I blew the relay with that 3S somehow... so I connect the FTDI back up and it works perfectly again.

The only non-FTDI setup that I can make work is a 1S (3.7V) directly to VCC.

What am I doing wrong? Why can't I power it with a 3S (as long as the battery isn't charged above 12V)?

Thanks.

2

And with the external supplys on the RAW pin the VCC voltage on the Pro Mini is ?

What type of diode do you have across the relay coil ?

3

srnet:
And with the external supplys on the RAW pin the VCC voltage on the Pro Mini is ?

Shame on me for assuming it'd be 3.3V, cause... you know... regulator. I'll go check.

srnet:
What type of diode do you have across the relay coil ?

The absent type. I haven't really used relays much and didn't realize it was a good idea. I did some quick Googling and I see it is. But it obviously isn't an absolute requirement. I've used this same relay with my rPi in essentially the same way and it works fine. In that case, the output is 5v.

And this is a 5v relay... I didn't know if it would work at 3.3v or not. I was pleasantly surprised that it did - when powered via the FTDI. But it doesn't otherwise. That's where I'm confused.

4

bcsteeve:
The specs say the pro mini can take up to 12V on the RAW pin.

Sales talk.
They forgot to mention that you can't draw ANY current from ANY pin at that voltage (including the 5volt pin).
Leo..

5

OK, so with the external supply on RAW, the VCC measures 3.3V... except for the 11.7V input it measures 9V. So obviously it can't handle that.

At 7.2V and 6.6V it comes out 3.3V... and I guess that's not enough to trigger the relay.

So why does the FTDI? Well, with that connected, VCC measures 3.5V. Apparently that IS enough to trigger the relay.

At this point I'm not sure what the best way to proceed is... but I think it will begin with not using this Pro Mini. I've got other tools to work with.

Thanks for responding anyway.

6

What relay. Post a picture/link.
5volt relay modules must be powered by 5volt,
but some can be controlled with a 3.3volt pin.
Leo..

7

But it obviously isn't an absolute requirement.

Just about any coil, without a kickback diode, will eventually destroy the circuitry to which it is attached. There may already be a diode on your module.

8

Cheapo relay module from Aliexpress and yes, probably has built-in diode. Relay is something like this: https://www.aliexpress.com/item/Free-Shipping-5V-low-level-trigger-One-1-Channel-Relay-Module-interface-Board-Shield-DC-AC/32801762863.html?spm=2114.search0104.3.23.4ab91f08jgGDlL&ws_ab_test=searchweb0_0,searchweb201602_3_10152_10151_10065_10344_10068_10130_10342_10547_10343_10340_10548_10341_10696_10084_5722520_10083_10618_10139_10307_10059_5722620_5722920_306_5722720_5722820_100031_10103_10624_10623_10622_10621_10620,searchweb201603_1,ppcSwitch_4&algo_expid=1f0240b1-291d-4826-b171-3c722315c2bc-3&algo_pvid=1f0240b1-291d-4826-b171-3c722315c2bc&transAbTest=ae803_2&priceBeautifyAB=0

9

That's AFAIK a 5volt only relay module.
Can't be used wth 3.3volt processors.

An Uno can power one relay module from it's 5volt pin when the Uno is powered by ~12volt on the DC socket.
Two relay modules (2*~75mA) could already overheat the onboard 5volt regulator.
Leo..

10

3.5V works (ftdi). 3.3 doesn't (reg). So I was close, and that's what threw me off. I understand now, thank you.