# Arduino, output voltage dropping

hey guys, so im working on using a LM34 temp sensor to give a read out to two seven segment displays being driven by two DM7447 ICs. I check stuff along the way and made sure the sensor was giving good readouts to the serial monitor, but it seems when powered up the ICs and seven segment make it so the 5v output drops to about 3.6, and that messes up my temp sensor code, why is that supply voltage dropping so much just from 2 seven segments? also i tried with a power source too, drop to about 4.6, its giving me bad numbers. When i disconnect the ICs and check the serial monitor i get good data. Thanks for your time

If you use LCD display, it will normally draw less power that LED displays.

What power supply are you using? Are you running the LED displays from the arduino onboard regulator?

If you are working with a low power source, it does not take much to overload it.

im not sure about the specifics of my supply, but is two 7 segments, too much to handle for going from USB to the arduinos regulated 5v?

I dont have resistors from segments to output pins. Could there be too little resistance causing the current to spike?

I see in the specs for that LED driver,

Each segment output is guaranteed to sink 24 mA in the ON (LOW) state

So if you do the math.. it is a lot of current. Did you say your power source is the usb cable ? hooked to your computer ?

Yes, adding some resistors to the LED ckt, should reduce the power used.

You could be damaging the pins...

The quick fix is to connect a 300ohm resistor to the common (cathode/anode)

I had a guy here in Holland with the same problem… put the LEDs directly on the pin… Gave great lights… for a while…
But people forget/don’t know that the leds and 7Segs oalso have a working voltage where in this case the 24mA apply. Suppose the voltage is 2.4V.
Then the theoretical resistance = 2.4 / 0.024 = 100 ohms.
No if you apply 5V on that same circuit, then the current will rise to 5/ 100 = 50mA. More than the Arduino pin can deliver. Thus the output voltage will drop because of internal circuitry

so its working decently know when powered by USB to arduino. It holds 4.8 volts well. Why not five? i put 560 ohm resistors on the common anodes. But heres the odd thing, when i hook it up to a power source 5v, the numbers displayed go crazy, i have no idea why.

amp625:
so its working decently know when powered by USB to arduino. It holds 4.8 volts well. Why not five?

USB spec is 4.75 to 5.25V. The Arduino doesn’t regulate when powered by USB, that’s up to the computer.

amp625:
i put 560 ohm resistors on the common anodes.

This will be a temporary fix, at best. You’ll quickly find that it causes the segment brightness to change depending on how many segments are turned on. You really want a resistor per segment.

understood, but when put on 5v from power supply it goes completely wacky

What power supply? Where is it connected? Is the Arduino also connected?

You probably should help us and provide a diagram of what you're doing.

so im going back and forth between USB connection, and power supply. I have the power supply 5v connected to my rails, and i have a lead from my plus rail going to 5v on the arduino, and i have the grounds connected, other than that its the sam setup i used with usb

got it working, thanks everyone

So, what was the solution. Others may have a similar case, and your solution could be valuable. Also, please mark your subject line as (SOLVED).

Thanks, Jack

Well, I am glad it is now working, even tho we don't really know what fixed it.

Have a great day, Jack

unfortunatly, it isnt working anymore, anywhere i give it power that isnt my computers usb, it gives odd values... i cant figure it out

when its up to my computer the reading is 4.8 volts, which gives consistent serial readout, and the systems works good. When its up to 5.03v from the power source heres a list of the serial temp reading, it never stop fluctuating

55
66
68
66
64
60
55
53
53
66
60
60
53
55
55
55
53
51
55
66
53
59
57
48
66
53
48
59

If it works, then doesn't , and then does again, etc. It can be a lose connection. Check all the connections, especially the power ones.

that was my first instinct, but after i checked them all, i started to notice the pattern of how it only worked on my computer, the second i unplug the external 5v it starts going strong again

okay, i might have got it. I was keep the pin from the power rail in 5v of the arduino instead of putting it into Vin when i switched to the power source