4.8 V input to the 5V pin, help!

Hi there,

I understand that its possible to externally power the arduino by bypassing Vin, and passing a regulated 5V line (in my case from a battery + regulator) to the 5V Pin. I am wondering what would happen if my '5v' was instead a (consistent) 4.8 volts? I expect to have some voltage drop from my regulator to my arduino (because I am using a radio frequency relay between these components for remote on/off) so i'm hoping someone here could please advise- will ~4.8V fail to power my arduino properly?

Many thanks for any help anyone can provide!

4.8V should be fine.

Hi @willfaust .
Please post a schematic of your project and let me know which arduino you are using.

RV mineirin

Hi, sorry for the late reply I had to get the schematic up to a presentable state haha. Please see attached

Ultimately, what im worried about is the RF relay. THe Adafruit Powerboost outputs a steady 5.2 V, but from some rough calculations, I should expect about 400mv drop across the relay, which brings me a smidge bellow the 5v for the arduino's 5v pin. Im using an Arduino Mega

Thanks for the schematic.

Please explain about the RF relay. Why would you feed DC through a relay designed for radio frequencies? On what basis do you think it would drop 400mV? I can't think how that could possibly be the case. In any case, if a relay is dropping 400mV then it's probably the wrong relay for the job.

Hi @PerryBebbington

Thank you for your response- my appologies I wasnt clear about the relay. The relay is triggered by a RF switch, so the signal for the relay to switch from on to off (and thus, power my arduino and external loads) is provided by a RF signal from a little keyfob I have. It is this unit here: https://www.amazon.co.uk/gp/product/B07DNVC5DS/ref=ask_ql_qh_dp_hza#HLCXComparisonWidget_feature_div

Unfortunately I dont have a datasheet, but the 400mv is just a guess based off a question I asked on the listing page for the product- they dropped 300mv using it for leds (I estimated a 'worse case' of 600mA load from those LEDs), which means for my application of (worse case) 800mA peak, I should expect 400mV.

Right. I suggest that an RF relay is a relay designed to switch RF, not a relay controlled by a remote control.

If the relay is really dropping several hundred millivolts then it is no use for the job. Any voltage drop will be as a result of contact resistance so will depend on current (Ohms law, if you don't know it then please do some homework and learn it, Ohms law is fundamental to electrical circuits). A relay correctly specified for the job should not be dropping that much, especially at the voltages and currents you are working with.

Right I see sorry about the miscommunication with the Relay. the 800mA I worked out before was using Ohms law, but you're right that 400mV is allot- unfortunately I couldn't find many options online for RF controlled relay break out boards. I will test practically if the system can power the Arduino- if it cannot, would it be feesable to put a step-up converter between arduino and the 4.8V line?

Your 400mV assumes constant current, which is certainly not going to be the case. What is the maximum load you need to supply? The specification for that thing says 1A max, and suggests not more than 700mA.

The maximum load is realistically around 700mA I have worked out, this is considering the external load at max and the arduino's draw as well as the draw from the relay and powerboost itself. This is the worst case scenario, I set to 800mA prior just to include a factor of safety but even at 700 I end up bellow 5V post-relay.

Your schematic shows motors. Motors like lots of current, especially when the are first powered up. It does not seem likely that your 700mA is going to be enough.

So these are coin motors- I have measured myself their current draw at max voltage. At 5V they draw 100mA, at most 4 will be active at one time. I budget a further 100mA for the PWM lines that activate these coin motors (generated by the adafruit PWM expanders). 30mA for the relay, then another 100mA for anything I miss, then an extra 10% factor of safety- brings me to 700mA.

I expect because these are coin motors they draw less current that one might expect with other motors.

I do not know what a coin motor is.

You can't just measure the current of a motor, a motor has a 'stall current', which can be 10 x the normal run current. The stall current is what it draws when it is not turning, so when you first connect power. You could measure the current while you hold the motor to stop it turning, but be sure to do this very quickly so it does not burn out. Alternatively measure its resistance while you turn it slowly, calculate the current from the lowest value of resistance you measure and the voltage.

Sorry perhaps I should have sent this link before: https://www.precisionmicrodrives.com/wp-content/uploads/2016/04/310-101-datasheet.pdf (this isnt my datasheet but one I found online for basically the same motor) . I considered the stall current, rated as 110mA on my datasheet. So coin motors just vibrate, they have no shaft and they just vibrate at high frequency. When I took the measurement of 100mA, I had completely clamped the motor down (as coin motors draw more current when they are arrested in their vibration). Even if they draw 150mA at start, which is more than I ever expect they will, I still fall bellow 1A even with all the factors of safety ect- so I dont expect my current draw to be an issue. And going back to the problem, the only time I would be able to achieve 5V exactly post-relay would be at 400mV, so I can almost guarantee my arduino would recieve 4.8V, maybe lower. Would a step up be nessesary in this case?

I would not think so, but you need flyback diodes across the motors (see my tutorial in the general electronics section) and be generous with capacitors across the supply.

Also, your schematic doesn't say anything about how you are controlling the motors, I suspect there are problems hiding there.

Hi Perry,

Yes I have a whole PCB I produced attached to the motors with diodes, caps ect, its the PWM that controlls the voltage recieved by the coin motor. Sorry I didn't want to say much more just because i'm not sure what the rules are around IP for the project im working on!

Thank you for the advice about the step up, I will try to power my arduino on the lower voltage and hopefully it works!

If there is IP that prevents you providing information then you should not be asking in a public forum. Ideally you should ask in the paid section and get agreement with someone to help you for a fee and have a confidentiality agreement with them.

Sorry I thought because my origional question was (in my mind) a simple stand alone question about powering arduino though 4.8V rather than 5V, it wouldnt involve anything else about my project. I'l do it the other way next time!

If this is a commercial project you need to consider that you have taken advice from a complete stranger with whom you have no commercial agreement. My advice might be wrong, you might be about to build something that won't work very well, and, if that happens, you will have no contractual redress for my bad advice. This is a hobby forum.

I understand- sorry I should say this isnt commercial its still hobby, I just have hopes that with time it could be non-commercial and pre-disclosure always affects patent applications. Again though, this is not a guarantee I should say