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PK2 - PICkit 2 Clone Vpp Error

7/11/2014

1 Comment

 
I recently received a message from a reader of this blog that had tried to build a version of my PK2 PICkit 2 clone programmer around a PIC18F4550 (40-pin) instead of the PIC18F2550 (28-pin). They were getting a Vpp Voltage Error when powered up and connected to the PICkit 2 software. Turns out I had the same issue on one of my custom designs that had the PK2 built into it. I had tried to figure out the cause of the error and found I had grabbed the wrong resistors for the base of the Q2 and Q4 transistors. What should have been 10k (brown-black-orange) were actually 300 ohms (orange-black-brown). So I replaced them but not before replacing the L1 inductor and the C1 47uf capacitor. 
Picture
The resistors were definitely wrong but after replacing them I still couldn't get it to work so in frustration, I set it aside.

Then when I got the message through my contact form asking me about the Vpp Voltage Error I decided to pull mine out and do some debugging again. Only this time I compared it to a working unit. I probed the various points of the working programmer with a probe from my oscilloscope and saw that the Vpp voltage on both sides of the L1 inductor were 5 volts at idle. 

I remembered seeing a lower voltage on one side of the inductor in previous tests of the failed unit so I probed it again and found that the L1 inductor had 5 volts on only one side. So even though I had replaced it, it was not working as expected. I disconnected everything connected to the dead side and still the voltage was zero. This told me nothing was shorting the inductor to ground as I thought may have been happening. Could I have replaced an inductor with a bad inductor and created a new problem in addition to the wrong resistors?
I decided to replace it again only this time I didn't fully bend the leads to fit the tight space I had created in the board design. I think I mentioned in the PK2 build blog post that I didn't allow enough spacing for the inductor leads and maybe I bent the leads too much and broke something internal trying to make it fit flat on the board.

I removed the old one and then carefully bent the leads of a new inductor to fit the hole spacing in the board but with a relief bend in the lead instead of 90 degrees. This left the inductor sitting off the board but at least the leads weren't bent 90 degrees. 
Picture
A quick plug into the computer USB port and the PICkit 2 software reported everything was working as expected. No more Vpp Voltage  Error. I now had voltage on both sides of the inductor. 

So take this as a lesson if you build the PK2 design, make sure the inductor is not bent too tight to break the inner wire. Apparently some of them are very fragile. I will fix the layout in the future so this problem doesn't happen again. I sent the reader an email describing this but have yet to hear if it was their problem as well. I decided to share this with everybody in case anybody else has one of these sitting around with a Vpp Voltage Error.
1 Comment
Filipino New Mexico link
2/21/2021 03:29:37 pm

Very thougghtful blog

Reply



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    About Chuck

    Chuck has been programming with PIC Microcontrollers since there were only five devices. Now there are over 700 and growing. He also has a lot of fun 3D printing designs using his Davinci 3D printer and TinkerCad software. In this series of blog posts and occasional videos on his YouTube Channel he tries to help you get started with electronics and 3D printing.

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