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Basic Electronics » Problem with Voltage Regulator?

December 25, 2010
by textor
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I had been having a lot of problems with my MCU lately, specifically not wanting to let me flash the chip and unrecognizable data to the LCD screen. After checking the connections, I figured it must be the battery as the culprit and switched to powering through USB.

Today I put in a new wall wart adapter. Multimeter reads output of the 9v adapter at 12v DC, which I believe is well within the range of the volt regulator included with the kit (7805). After hooking it up and switching it on, I started getting the same issues with A/C power as when I had it on a 9v battery. Multimeter read the output as 4.85v from the regulator, a bit less than the 4.93v that I was getting from the 9v battery originally. The only difference between my circuit on the wart and my circuit on the USB is:

USB Setup:
USB black: blue rail
USB red: i-27
SPDT switch: G-26, 27, 28
Cable: I-26 to red rail

Wall Wart setup:
WW Black: blue rail
WW Red: I-27
SPDT switch: G-26, 27, 28
Cable: I-26 to B-28
Regulator: D-28,29,30
GND: A-29 to Blue rail
Cable: A-30 to Red rail

I do run the LCD backlight, and when I switched the power on, the backlight had lit up as normal. However, the LCD displayed the two lines of blocks indicating bad input, rather than the data the code is programmed to display for me.

I switched it off again, unhooked the adapter, and put the USB power back onto the board. After switching the board back on, it responded as it had been since I originally put it on USB power, with the multimeter reading the voltage as 4.97v.

As a note, the wall wart was running off of conditioned power (plugged into my computer's UPS) and was steadily reading as 12.01v on my multimeter.

As I am still rather new at electronics, I was curious if this could mean my voltage regulator is faulty or damaged?

December 25, 2010
by BobaMosfet
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Disconnect the LM7805 and plug it into a part of the breadboard that isn't connected to anything else. Put a 150 Ohm Resistor and an LED in series across the 5V output and Ground terminals- this constitutes a "load", and you get feedback to see it's working.

Connect your upstream power-supply to the appropriate input and ground terminals on the LM7805 and measure the voltage across those two points with your meter. Should be between 7 and 18 volts. I think you said you had a 12V supply.

Now connect your meter between the 5V output and Ground pins and see what that reads. Should be registering about 4.8 to 5.2 volts. Within that range.

Is the voltage in spec? In voltage and out voltage? Is the LED lit okay? If these things are true, the regulator is okay.

BM

December 25, 2010
by textor
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I did everything except the LED part, and it seemed to work fine (I didn't have a 150ohm resistor available, and being Christmas, I couldn't jog down to the local RadioShack and pick one up). The multimeter said everything is within normal range: The 9v wall wart (it is 9v according to all the markings on it, despite running at 12v) is outputting at 11.02v, the output of the regulator is 4.90v, and the red rail is running at 4.90 on the left (where it is hooked in) and 4.89v on the right (bridged with a cable)

However, despite this the MCU still isn't responding correctly. The LCD lights up as it should, and the display comes on (an artifact of a previous success is displayed), but none of the LEDs are lighting up like they should, and if I unplug the wall wart and flick on my USB power switch, the unit springs to life and does everything it is supposed to. Seeing as I just moved the regulator over to a new section of the board and reconnected it (middle to blue rail, output to red rail), I do not believe it is a connection issue (especially because using USB power works perfectly).

December 25, 2010
by mongo
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To get the backlight to work, try connecting it to the incoming side of the regulator with about a 1K resistor instead of the output of the regulator. Leave the 5 volts to run the important part.

Also, add a capacitor to the input power to help cut noise on the line. Something like a 220uF to 470uF @ 16 V should suffice.

December 25, 2010
by mrobbins
(NerdKits Staff)

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Hi textor,

Welcome to the forums!

I do think that BobaMosfet and mongo are on the right track suggesting that you check inputs and outputs, and that some serious filtering capacitance is needed. You might want to try placing that on the output of the 7805 voltage regulator and see how that compares to having it on the input.

Out of curiosity, are you sure that the wall wart specifies a DC output, and not AC? I only mention this because I once had similar strange issues when I accidentally used an AC wall wart (really just a step-down transformer) when I meant to use a DC one!

One more long shot: can you try using the wall wart after removing the USB cable completely from the board (remove black/green/yellow/red wires totally from breadboard)? In some rare cases with multiple power sources, I have seen some strange issues that have to deal with transient signals triggering the bootloader to pause.

Mike

December 25, 2010
by textor
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OK, I removed the backlight (disconnected the +5v lead from the nerdkit) and the program started working partially. The LCD is cutting in and out and the LED lights are shining but clearly underpowered. Multimeter testing of the rails indicate that it's at 4.93v.

December 25, 2010
by textor
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Testing with a multimeter, I always pick up between 11v and 12v and no A/C current.

For reference, here is a picture of the wall wart I am using:

Adapter

December 25, 2010
by textor
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Forgot to mention: As per Mike, removed USB completely from breadboard. Same results: LCD is "flickering" (turning on and off, less than a second each time it alternates) and LEDs are underpowered.

December 25, 2010
by mrobbins
(NerdKits Staff)

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Hi textor,

Two thoughts:

Is the 7805 voltage regulator getting very hot? (Careful -- don't get burned!) If so, it might be reaching its thermal cutoff point. That's a slow enough effect where you might see it resetting every second or so. If so, try removing the LEDs as well, and see if the LCD stabilizes. By driving the system from a higher voltage (11V), the 7805 has to dissipate more power as heat. Remove all non-essential current consumption (LEDs, backlight) and see if you can get it to stabilize.

The LCD "flickering" might actually be the chip resetting or otherwise re-initializing the LCD. Do you happen to have an LED present on microcontroller pin 28? Are you running the factory pre-loaded program, or something else? In our pre-programmed code, we look on that pin for a button and use it to re-initialize the LCD. If there were an LED there, it might think that this button was continually pressed, and would keep re-initializing the LCD. That leads to exactly the symptom you're talking about, and it's something I've seen before because we use the same pre-programmed MCUs we send to customers in order to test LCDs before they're shipped. We connect a fresh LCD and then press a button on pin 28 (pulling the voltage on that pin to ground) to instruct the program to re-initialize the LCD. Still, I admit that this wouldn't explain why it works on the USB power.

Mike

December 25, 2010
by textor
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I do have an LED on pin 28.

Ok, removed the LED and reprogrammed the chip to ignore pin 28. No effect. Going to programming mode results in "programmer did not respond to command: chip erase" error that I had gotten back when I was trying to get the 9v to work when I was trying to flash program 3 onto the chip. Again, switching over to USB power allows me to flash with no issue and the program runs properly.

The 7805 is warm, but not hot enough to burn me. It seems to be as hot as it was when the 9v battery used to work properly.

December 27, 2010
by textor
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Got some resistors today, so I put the LED setup through as instructed:

(+) - 7805 (in)

(-) - 7805 (GND) - LED Cathode

--- - (unused row) - 150 Resist - LED Anode

See below: Circuit

LED lit up fine.

Row 1: 11.40v (Input)
Row 2: 0v (GND)
Row 3: 4.86v (Output)
Row 6: 2.17v (LED Anode)

December 27, 2010
by mongo
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Hey guys, have a look at how some of us put the kit together. It makes troubleshooting much easier if you follow some of the examples in the following thread. Here

December 28, 2010
by BobaMosfet
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LM7805 : 4.8V to 5.2V -- So your 4.86V is within Spec.

Looks like a 100 Ohm resister, so you're running about 26mA through the LED. Since it lit fine for you, it means the regulator is working, and it's in spec, with a load.

Based on your description, I'd check:

  • How much current will the wallwart deliver, and
  • How much current are you drawing (the "load") <- make sure nothing is shorted, fried, reversed, or the regulator will collapse repeatedly.

BM

December 28, 2010
by BobaMosfet
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I see the wallwart is rated at 9V out at 600mA.... that's about equivalent (slightly better than) and average battery. If you're using this as your input, the wallwart SHOULD be outputting 9V. If it's output 12V, it's failed, and should not be used.

Your Regulator may be protecting your circuit from the wallwart by repetitively shutting down due to current-draw problem.

BM

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