Step-down voltage regulator - what happens with low input voltage? (Solved)

I have been using a “LM2596S Adjustable DC-DC Module” (can be found at 2pcs LM2596S Adjustable DC-DC Module | Open ImpulseOpen Impulse) to power an Uno. The converter is connected to a 12v motorcycle battery that is periodically trickle-charged (two times per day for 30 minutes each charge) by a 110vAC battery trickle charger. The regulator is adjusted to output 10.0vdc.

It works very well, but the web page says the following:
“Note: The input voltage must be at least 1.5V higher than the output voltage.”

What happens to the output voltage if I have the regulator’s output set at 10.00v, and the input voltage drops to, say, 11 volts or 10.5 volts between charging cycles? I’ve seen the battery drop to just slightly below 11.5vdc (like 11.4vdc), and there wasn’t any ill effect on the output. But what if the input voltage drops lower?

Thanks.

ugfrog

The output voltage will drop, and there may be a limitation in the current it can supply.

Why not adjust it to a lower output voltage ? - the uno can tolerate 7V on it's Vin pin, or you could directly feed the +5V rail.

This will give you better battery life because the linear regulator on the Uno is less efficient than a buck regulator.

Allan

Thanks for the quick response, Allan. I will try a lower voltage and see what comes of it. The Uno I'm powering, though, has one (of 3) DS18B20 temperature sensor that is on a fairly long oneWire bus (51 feet), and I've had problems with that sensor not responding to temperature queries when trying to power the Uno with a 9v wall wart. I went back to a 12v wall wart, and things got better for the sensor, but I started getting Uno resets like crazy, average of 46 resets per night over the 26 nights I monitored the resets. Those resets seem to be mostly hardware/power resets, although some are apparently watchdog resets. (That is a whole other subject that I'm documenting for another post here in ArduinoLand.) But now that the Uno is 'dining' on 10.0vdc through the regulator, it has been stable for 2 nights. Yippeeeeee!

I'll knock the output voltage down to 9vdc and see what happens. Thanks again.

ugfrog

Wallwarts are ( of course) reliant on the mains supply - which can have many 'blips' - eg a pulse when a fridge starts up. A largish ( 2000uF ? ) capacitor on it's output could help cover these.

regards

Allan

The resets, though, were happening only in a 5-hour window... between about 2:AM and 7:AM. Nothing I can think of in our house accounts for that. I don't want to go into a lot of detail here that will be duplicated in my next post, though. What 'flavor' of 2000uF capacitor would you recommend? Ceramic? Electrolytic? Other? (I'm NOT an electronics expert - just a retired programmer who's hooked on Arduino.)

Electrolytic with a voltage rating at least 1.5 times the voltage it will see. They're not expensive.

eBay etc will give you lots of choice.

Get the polarity right!

Allan

ps since you have sensors on long leads it may be worth considering screened cable with it's screen earthed to keep out interference.

Yeah, the voltage itself shouldn’t be a problem. The Arduino’s onboard voltage regulator should be holding the board-voltage to 5V.

…If you have a lot of “extra stuff” powered through the Arduino’s regulator or if you have a high temperature environment, the regulator could overheat and shut down causing a reset. But, the temperature sensors don’t use significant current.

The more voltage you “drop across” the regulator and the more current through it, the hotter it gets. So in that case 12V would be worse than 9V.

Thanks to both Allanhurst and to DVDdoug. As mentioned earlier, I plan to post a detailed description of the resets - completely separate from this post. I didn't know where to post it, though - there are several areas of the forum where I could post it (where it would fit), but I think I'll post it here, in the electronics section. It'll probably be a day or two from now. Please stay tuned, and let me know what you think of the 2nd post when it comes.

Again - THANK YOU for your help!

ugfrog

ugfrog:
I have been using a "LM2596S Adjustable DC-DC Module" (can be found at 2pcs LM2596S Adjustable DC-DC Module | Open ImpulseOpen Impulse) to power an UNO. The converter is connected to a 12v motorcycle battery that is periodically trickle-charged (two times per day for 30 minutes each charge) by a 110vAC battery trickle charger. The regulator is adjusted to output 10.0vdc.

It makes no sense at all to power an Arduino via "Vin" or the "barrel jack" when you have a proper source of well-regulated 5 V with adequate current capability - such as a LM2596 switchmode converter.

The on-board regulator is not specified to power more than the processor chip itself and a few LEDs. If you are powering anything else - that draws more than a few tens of milliamps - you need a proper 5 V power supply. You apparently have one :astonished: stop fooling about and set the regulator to 5 V and feed it into the 5 V pin on the Arduino. :roll_eyes:

ugfrog:
Thanks for the quick response, Allan. I will try a lower voltage and see what comes of it. The Uno I'm powering, though, has one (of 3) DS18B20 temperature sensor that is on a fairly long oneWire bus (51 feet), and I've had problems with that sensor not responding to temperature queries when trying to power the Uno with a 9v wall wart. I went back to a 12v wall wart, and things got better for the sensor, but I started getting Uno resets like crazy, average of 46 resets per night over the 26 nights I monitored the resets. Those resets seem to be mostly hardware/power resets, although some are apparently watchdog resets. (That is a whole other subject that I'm documenting for another post here in ArduinoLand.) But now that the Uno is 'dining' on 10.0vdc through the regulator, it has been stable for 2 nights. Yippeeeeee!

I'll knock the output voltage down to 9vdc and see what happens. Thanks again.

ugfrog

The input voltage to the regulator doesn't affect the 5V output at all(*), that's the whole point of a regulator.

(*) assuming the input voltage is 7V or more, and actually there is a small effect, a handful of millivolts
which won't make any discernable different to the one-wire setup.

Paul__B:
It makes no sense at all to power an Arduino via “Vin” or the “barrel jack” when you have a proper source of well-regulated 5 V with adequate current capability - such as a LM2596 switchmode converter.

The on-board regulator is not specified to power more than the processor chip itself and a few LEDs. If you are powering anything else - that draws more than a few tens of milliamps - you need a proper 5 V power supply. You apparently have one :astonished: stop fooling about and set the regulator to 5 V and feed it into the 5 V pin on the Arduino. :roll_eyes:

Hi Paul, it´s an old thread but maybe you can help me. I´m removing power LED from a Nano in order to gain more battery-powered time. If the Nano is powered via 5V pin (not Vin), is if of any use to remove the built-in regulator (1117)? or as it´s not used, it draws no current?

Thanks a lot!

It probably will be draining some current, you could try removing it and measuring the difference. The 1117 is not a micropower device so it could easily be draining a few mA.

For a lot lower power consumption a lower supply voltage and lower clock speed will make big difference. For
instance the 3.3V 8MHz Pro Mini uses far less power than the 5V 16MHz Pro Mini.

The rule with clocked CMOS chips that usually works well for this is that power consumption is proportional
to the clock speed times the supply voltage squared. 5V 16MHz is about 4.5 times the consumption of 3.3V
8MHz. Note this is power, not current.

For 3.3V operation from a battery I've sometimes used LiFePO4 cells, which are nominally 3.2V. I've used
1Ah cells in CR123A format. The Pro Mini 3.3V is about 5mA consumption, 200 hours (basically a week) from
such a supply.

Whether you can pull the same trick with the Nano I don't know, there's the USB chip to consider too.

Thanks MartT.

I am planning to move to Pro Mini, because I need a long battery life.

Maybe I´ll have to stick to the 5V version, because I have some 5V peripherals (7-segment display, 433Mhz receiver, etc).

I will check a 9V standard alkaline battery with a DC/DC converter, fixed at 5V output.

Because of project features. maybe a rechargeable battery won´t be not an option. Changing the battery, if it takes a couple of weeks to die, could be OK.

Bet you find the 7-segment display eats up your battery pretty quick if it is used a lot.

If you want to operate from batteries, you really do not want to use a Nano as whatever you do to shut down the mega328, the USB interface chip - whatever it is in a clone - will be wasting substantially more current than the regulator might.

So you use a Pro Mini, remove the regulator and the resistor for the pilot LED, and power it from three alkaline “AA” cells at about 4.5 V. It should operate until the cells are down to about 1.3 V each and they start at 1.6 V.

Using a 9 V “PP3” battery is never advised, except in the most terrible of “tutorials”. :cold_sweat:

CrossRoads:
Bet you find the 7-segment display eats up your battery pretty quick if it is used a lot.

What’s a 7-segment display got to do with it? :astonished: