So I’m using the 5V pro mini in one of my designs. The project has +/- 8V rails and I wish to power the pro mini from these rails. The rails are created from a single regulated 16V and split using resistive dividers to create a virtual ground at the midpoint.
Anyway I plugged in +8V to RAW and ground to GND. The red LED lights up but the rail voltages are now around 3.3V. Plugging the arduino actually changes my voltages. Every works fine when powering from an FTDI basic. Right now the whole bit is on a breadboard.
Sounds normal. You’re using a voltage divider made of resistors to get your target 8v. It would be normal for the voltage to drop as soon as you hook up something that actually draws current.
Why not hook up a 9v voltage regulator between your +8 and -8 rails to give you a good, clean 9v?
Well I was hoping to conserve space by using the arduino’s onboard regulator.
I actually have a complete circuit set up off the power rails. Theres an op-amp based headphone amp, a digital pot for volume control, a mux for source selection. So its a bit strange to be that connecting the arduino caused the 8V rails to drop to ~3.3V. I already have a regulator for the rails, and 2 TL431s to regulate +/- 2.5V for the digital pot. So not looking to add another regulator if possible, though I do have one on hand luckily.
Okay if I use a 5V regulator and put +8V on input, -8V on GND, and connect output to the arduino’s VIN. Then I’d have to put the arduino’s GND to -8Vs as well. Wouldnt that be problematic when I try to connect the digital pins to the other ICs in my circuit that are powered off the +/-8V rails. Now that 5V logic level high is referenced to GND rather than -8V.
This is really frustrating. The project has working amplification, source selection, and digital volume control. I have the code written for most of that including IR control but I can’t get the two together lol
jtavrisov:
Okay if I use a 5V regulator and put +8V on input, -8V on GND, and connect output to the arduino’s VIN. Then I’d have to put the arduino’s GND to -8Vs as well. Wouldnt that be problematic when I try to connect the digital pins to the other ICs in my circuit that are powered off the +/-8V rails. Now that 5V logic level high is referenced to GND rather than -8V.
You are correct, it would be problematic. Can you post a block diagram (or schematic) of what you have ? It would be a lot easier to visualize your setup with one. I’m curious why you haven’t had problems before you added the Pro Mini, though I might guess it’s because what was present drew (roughly) equal current from each rail. Obviously the Pro Mini does not.
One potential solution is to use opto-isolators to connect digital signals from the Mini to whatever other circuitry (mux, digital pot ?) you have. I’m not sure I’m in love with this idea but it’s an idea …
Thats the schematic to the cmoy. Mine is near identical except instead of the battery I use a AC-AC wallwart thats rectified and regulated to +16V. I also have a CD4052 mux thats powered from the +/-8Vs. The gain on that amp is at 11 while I have mine configured for 4 I think. Just 1 resistor value change.
A DS1802 digital pot is used instead of a mechanical one. Currently I have it connected as the schematic show below. I hope to interface it using the arduino instead of the push buttons. The same goes for the mux controls. I will also have an IR receiver but thats unconnected at this point (though the code is written ^_^).
Hopefully thats enough for you to go on. I have a schematic but I’ve changed stuff so much the original plan no longer resembles what I have on board. This is my first project after all lol.
You can’t use a resistive divider to make a voltage regulator. The current you draw through the resistors will change the voltage drop. V=IR.
You can use a resistive divider to make a reference voltage for an opamp since an ideal opamp draws zero current into its inputs. In practice this is true most of the time and almost always for fet-input opamps.
jtavrisov:
Mine is near identical except instead of the battery I use a AC-AC wallwart thats rectified and regulated to +16V.
I meant to ask … is that an AC-AC wallwart that you’ve rectified (or a typo) ? If so, how wedded to it areyou ? Would you be open to using a different transformer, one that’s center tapped and gives a true + and - set of rails ? Or perhaps adding another transformer to the output of your wallwart to get the same end result ?
Like I said above, for a few digital signals you could use opto-couplers to interface your micro to the other circuitry. But then you’d still need another DC-DC regulator as you’ll have 16V to feed into your Pro Mini … and it only likes up to 12V. Replacing the 16V with true +/- 8V rails eliminates the need for that regulator at the cost of another rectifier circuit. Not sure what SF has in this regard but Digikey has some choices I think.
Yea its an AC-AC wallwart and I have a rectifier, filter caps, regulator etc on board.
I thought about a center tapped transformer, but it’d be costly and I dont exactly have room for one. If I did that then I’d need a another enclosure to put all that in.
Well thanks guys but I solved it by using another op-amp as a buffer. I havent really tested anything but I plugged everything and the circuit only gets unbalanced by like <10mV. Perfectly livable since the audio op-amp doesnt care.
