PCB problem with voltage

Thread Starter

camerart

Joined Feb 25, 2013
2,436
Hi,
I'm making PCBs which have two PIC plus 3x voltage regulators.
When I switch on, the PICs sometimes program, but there's obviously something wrong.
I checked the input voltage (8x AA cells battery). When off it shows app 10V but when switched on I get this:
Can anyone shed light on what I've done wrong please? Not my best photography.
Camerart

Mod:Cleaned up the image.E


OSC.jpg
 
Last edited by a moderator:

DickCappels

Joined Aug 21, 2008
7,288
From where in the circuit did you grab this waveform? I am going to guess across the power supply.

Do you have bypass capacitors across the power supply (both battery pack and output of regulator)? You might need to increase the capacitance.
 

Thread Starter

camerart

Joined Feb 25, 2013
2,436
From where in the circuit did you grab this waveform? I am going to guess across the power supply.

Do you have bypass capacitors across the power supply (both battery pack and output of regulator)? You might need to increase the capacitance.
Hi D,
Yes, at the Battery connection.
100uF and 100nF after each regulator.

I'm running an all port ON/OFF test, so the LEDs are flashing, but just got 'smoke' from one of the PICs. Switched off, and a quick test, and the LEDs are still flashing. OOoops!

EDIT: I've been trying to program the 18F4431 blank, incase the program is heating it up. While doing so, I saw PIN 7 VVD, sparking, so this is my 3rd 18F4431, but apart from your suggestion, I haven't found anything wrong yet. Can a lack of capacitors make them get so hot?

C.
 

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DickCappels

Joined Aug 21, 2008
7,288
If you have inductors in the circuit there is a chance of excess voltage in places where it can do harm. I know very little about PIC controllers and can only suggest double-checking your connections, especially to the chips that get warm.

Depending upon the regulator(s) a large capacitor on the battery side of the regulator might be needed.

Let's see who comes along with superior knowledge of the controllers of which you write.

I think this is an excellent time to post a schematic of your board.
 

jjw

Joined Dec 24, 2013
605
How many leds
Hi D,
Yes, at the Battery connection.
100uF and 100nF after each regulator.

I'm running an all port ON/OFF test, so the LEDs are flashing, but just got 'smoke' from one of the PICs. Switched off, and a quick test, and the LEDs are still flashing. OOoops!

EDIT: I've been trying to program the 18F4431 blank, incase the program is heating it up. While doing so, I saw PIN 7 VVD, sparking, so this is my 3rd 18F4431, but apart from your suggestion, I haven't found anything wrong yet. Can a lack of capacitors make them get so hot?

C.
How many leds or other loads there are on port pins?
Do the leds have a series resistor?
 

MrChips

Joined Oct 2, 2009
23,257
From what I can see it looks like a 125kHz square wave ripple of about 2.5V on top of about 10V.

You need to set up the oscilloscope controls better in order to get a clearer picture.

At the top of the screen where it shows M Pos: 248.0μs, adjust the HORIZONTAL POSITION to bring this to 0μs.
You should then see a small arrow at the top in the middle of the screen.

At the bottom right corner where it shows CH1 1.28V, adjust the TRIGGER LEVEL to bring this to 9V or at least so that the arrow on the right hand side edge of the window is within the ripple voltage.
 

Thread Starter

camerart

Joined Feb 25, 2013
2,436
From what I can see it looks like a 125kHz square wave ripple of about 2.5V on top of about 10V.

You need to set up the oscilloscope controls better in order to get a clearer picture.

At the top of the screen where it shows M Pos: 248.0μs, adjust the HORIZONTAL POSITION to bring this to 0μs.
You should then see a small arrow at the top in the middle of the screen.

At the bottom right corner where it shows CH1 1.28V, adjust the TRIGGER LEVEL to bring this to 9V or at least so that the arrow on the right hand side edge of the window is within the ripple voltage.
Hi,
I took a quick picture, but will change the settings tomorrow, and take another if needed.
C
 

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BobaMosfet

Joined Jul 1, 2009
1,653
@camerart
Looks like your using LM1117 voltage regulators for fixed 3.3V regulation. Couple of things- You shouldn't be dropping 12V across them, you need to drop that to 5V beforehand- 12V is just running the junctions way, way too hot - see the _test_ conditions on the datasheet for where the mfg really expects you to run it. Secondly, I don't see bypass caps on the 12V side of the regulator. You're supposed to have tantalum (polarized) caps on both sides of the regulator, usually 10uF or so.

Your problem isn't the PIC, it's your your layout. You're not managing your thermal energies correctly, so things are overheating.
 

Thread Starter

camerart

Joined Feb 25, 2013
2,436
@camerart
Looks like your using LM1117 voltage regulators for fixed 3.3V regulation. Couple of things- You shouldn't be dropping 12V across them, you need to drop that to 5V beforehand- 12V is just running the junctions way, way too hot - see the _test_ conditions on the datasheet for where the mfg really expects you to run it. Secondly, I don't see bypass caps on the 12V side of the regulator. You're supposed to have tantalum (polarized) caps on both sides of the regulator, usually 10uF or so.

Your problem isn't the PIC, it's your your layout. You're not managing your thermal energies correctly, so things are overheating.
NOTE: I've updated the Oscilloscope image in #10, is this ok?
Hi B,
Ok, I'll move things around a bit.

To clarify:
For each voltage regulator a 10uF Tantalum (definately not simple ceramic) each side of each regulator?

Regarding the 100nF caps. 1x as near to each PIC VDD VSS as possible.
Do I need any more around the regulators?

I should remove the existing 100uF ceramic caps?

EDITED.

C.
 
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BobaMosfet

Joined Jul 1, 2009
1,653
@camerart - yeah, if you look at the datasheet for the 1117, they specify tantalum caps, which are polarized, and these should be on both sides of the regulator. Digital circuitry is busy, and the caps not only filter, but also provide additional current for the regulator when it needs it faster than the power-supply can provide it.

The 100uF caps probably aren't helping you. They just store a lot more energy, but because of their size, they may not be doing any useful filtering based on frequency of energy on the PCB. You could try replacing them with a 1uF, perhaps.
 

BobaMosfet

Joined Jul 1, 2009
1,653
@camerart You'e got a TekTronix TDS220 (One of my oldies but goodies is a TekTronix TDS360)-- Can you take a larger picture of the front of your scope, so we can see the controls?

You should calibrate your probes using the calibration ability before doing anything.
Your scope should have a STOP button on it (bottom left of scope?) and if you press it, it can freeze the screen.
You should have measurement bars, but maybe not.

Does your scope have a menu for printing or saving screenshots to perhaps 3.5" floppy? If so, I may have a couple of good 3.5" floppies I can send you. Uf you have an external 3.5" floppy drive for your computer, you could then read them in. I know my scope can save in .TIF or .EPS formats.

Just wondering if we can help you use your scope better.
 

sparky 1

Joined Nov 3, 2018
497
Because I had ordered a bunch of 3.3V LM1117's I will no doubt be facing simlar transient issues.
Does the problem originate before the regulator section ? If so, is the post supply filter adequate?
It is not uncommon to have inadequate supply filtering but is that the problem and how so ? (a useful topic thanks)

I use a circuit simulator on supply regulators trying different values capacitors. I could probably improve in simulating faulty supplies.
It is helpful that you know the unwanted frequency is about 100 to 200 kHz refering to the scope shot picture 2.

Scope shot picture 2 shows slow rise in voltage at which point an oscillation of 210 kHz occurs then also 105 kHz

If power supply has low ripple and adequate current the pcb and components can still make some improvement. A wall wort driving uC may be adequate but the regulator may be compromised with two uC there may be a question if it is necessary to improve heat sinking and use two regulators in parallel to distribute the load.

LM1117 fixed 3.3V , decouplng within needed ESR range must be between 0.3Ω - 22Ω . capacitor suggestions.
On page 27, 28, 29 shows another voltage regulator setup load transient, PCB design.
https://www.infineon.com/dgdl/Intro...4.pdf?fileId=5546d46146d18cb40147440c4569288d
 
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Thread Starter

camerart

Joined Feb 25, 2013
2,436
@camerart You'e got a TekTronix TDS220 (One of my oldies but goodies is a TekTronix TDS360)-- Can you take a larger picture of the front of your scope, so we can see the controls?

You should calibrate your probes using the calibration ability before doing anything.
Your scope should have a STOP button on it (bottom left of scope?) and if you press it, it can freeze the screen.
You should have measurement bars, but maybe not.

Does your scope have a menu for printing or saving screenshots to perhaps 3.5" floppy? If so, I may have a couple of good 3.5" floppies I can send you. Uf you have an external 3.5" floppy drive for your computer, you could then read them in. I know my scope can save in .TIF or .EPS formats.

Just wondering if we can help you use your scope better.
Hi B,
How do I calibrate the probes?
EDIT: Found and did a calibration.

For the images in #10, I stopped the Osc, then it automatically started when I switched ON the voltage.

I don't think mine has the add on, for printing, as there are no ports.
3.5" floppies! You remind me of loading the 16? floppies for Windows 3.1 and looking forward to seeing the paper clip :)
EDIT: Mine doesn't have the output extension, so no prinint etc

C.
 

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jjw

Joined Dec 24, 2013
605
What kind of batteries you have?
Alkaline, rechargeable Nicd, Nimh?
8 alkaline in series might have fairly large internal resistance.
You could use two 18650 Lipo batteries in series.

Could you measure with the scope the output of the 3.3V regulators.
 

Thread Starter

camerart

Joined Feb 25, 2013
2,436
Because I had ordered a bunch of 3.3V LM1117's I will no doubt be facing simlar transient issues.
Does the problem originate before the regulator section ? If so, is the post supply filter adequate?
It is not uncommon to have inadequate supply filtering but is that the problem and how so ? (a useful topic thanks)

I use a circuit simulator on supply regulators trying different values capacitors. I could probably improve in simulating faulty supplies.
It is helpful that you know the unwanted frequency is about 100 to 200 kHz refering to the scope shot picture 2.

Scope shot picture 2 shows slow rise in voltage at which point an oscillation of 210 kHz occurs then also 105 kHz

If power supply has low ripple and adequate current the pcb and components can still make some improvement. A wall wort driving uC may be adequate but the regulator may be compromised with two uC there may be a question if it is necessary to improve heat sinking and use two regulators in parallel to distribute the load.

LM1117 fixed 3.3V , decouplng within needed ESR range must be between 0.3Ω - 22Ω . capacitor suggestions.
On page 27, 28, 29 shows another voltage regulator setup load transient, PCB design.
https://www.infineon.com/dgdl/Intro...4.pdf?fileId=5546d46146d18cb40147440c4569288d
Hi S,
This is the 6th set of PCBs I've made with almost the same set-up, and the first to have this problem. Having said that, there have been lots of mysterious problems along the way (3ish years), that were never solved, and I wonder if they are related to this?

Your data sheet, is quite complicated, and I would like to know how you get on with your PCBs, regarding this.
C.
 
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Thread Starter

camerart

Joined Feb 25, 2013
2,436
Hi B and S,
Regarding the choice of Cap: I saw this when I was looking through the D/S: and wondered how important the Tantalum type is?
C.
 

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Thread Starter

camerart

Joined Feb 25, 2013
2,436
What kind of batteries you have?
Alkaline, rechargeable Nicd, Nimh?
8 alkaline in series might have fairly large internal resistance.
You could use two 18650 Lipo batteries in series.

Could you measure with the scope the output of the 3.3V regulators.
Hi J,
8x NI-MH, but ultimately 3x LI-ON will be used 11.1V+
Bearing in mind, earlier comments about the regulators not being happy with >5V supply, it's got me wondering?

After calibration, and finding the START/STOP button, here are all 3x regulator outputs, + the battery +.

Low quality jpgs = less bandwidth
C.
 

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