I think I killed my PicKit3, but how? - Page 1
Forum section: electronics / general

author: SmokyMcToad
author_rank: Contributor
timestamp: January 17, 2018, 08:44:52 pm
content: A couple of days ago I was making some changes to a circuit. This included updating the firmware of a PIC chip using my PicKit3. But somehow during this process I managed to kill my programmer; the V+ and GND pins that should be connected to the target circuit are now causing a direct short(around 8.3Ohm resistance).(Un)fortunately my PSU clamped the current to 0.3A so nothing popped and I am having problems figuring out the cause(and how to prevent this in the future).Sofar I can think of 2 possible causes:1- Circuit was changedI changed the circuit and replaced 2 switches with a single pot. Attached are diagrams showing the connection to the PIC. The old situation using the switches worked as a charm but the pot allows a direct connection to GND to happen. This pin was shared with the ICSP_CLCK pin of the programmer.I checked the PTC(marked PTC3 on the PicKit3 schematic)for this line and it is showing a resistance around 11Ohm, which I guess this is about normal for a 50mA fuse. The Zener(marked TR2)also seems to work when running a diode-tester, although supply-voltage never exceeded 5V so I don't think these conducted much anyway when the issue occured.Is it possible to fry the PicKit3 by just connecting ICSP_CLK to GND like this? I would guess the PTC catches the current before it gets too high but that did not happen at 0.3A it seems; perhaps it did not get hot quickly enough.2- Connecting the PicKit3 to a live circuitInitially the PicKit3 was disconnected while I wanted to flash the new firmware I created. I then connected the PicKit3 but forgot to switch of the circuit first. The programming itself worked fine but after that, when I stopped and restarted the PSU, the PicKit3 would short the circuit.As programming itself worked after connecting the pins, I am hoping this was not what caused the issue(but my gut fears this is the probable cause).I've been probing around the PCB but sofar I had no luck in finding what exactly happened. The weird part is also that everything else seems to work fine; when I connect the PicKit3 to USB the LEDS light up the same as before(if I remember correctly)and MPLAB seems to communicate with it normally too. Currently I am tempted to connect it to the PSU again and let it rip with 0.5A or something to see which component gets hot. But I am hoping someone has a better idea that might perhaps even allow me to repair the kit.

author: JPortici
author_rank: Super Contributor
timestamp: January 17, 2018, 10:51:03 pm
content: Is it a genuine Pickit3?Open a ticket. Microchip will probably send you a new one, doesn't matter if you are an individual or a business.

author: thm_w
author_rank: Super Contributor
timestamp: January 18, 2018, 12:44:19 am
content: The lowest I see here across the pickit is 860 ohms when off. It looks like the PTC and zener are the main protection. I can't see how anything would short <100 ohms though.Schematic here (pg67):http://ww1.microchip.com/downloads/en/DeviceDoc/52116A.pdfhttps://www.eevblog.com/forum/repair/blown-pickit-3schematicwhat-part-is-this/Quote from: SmokyMcToad on January 17, 2018, 08:44:52 pmI do this regularly.

author: SmokyMcToad
author_rank: Contributor
timestamp: January 18, 2018, 06:36:36 pm
content: Thanks for the replies!@Jportici:Yep it's genuine; but I bought it around 2 years ago so I think my warranty is expired anyway.@Thm_w:That is a reliefgood to know that connecting to a live circuit should not have killed it.So assuming the changed connection on ICSP_CLK was the cause, it would mean the U9 chip(an 74lvc1t45 transceiver IC)had +5V on the input while the output was set to GND. Perhaps this IC(marked 'CT1F' which matches a TI chip:http://www.ti.com/lit/ds/symlink/sn74lvc1t45.pdf)was not handling the load properly and 'melted' into a short.But if I read the sheet correctly, it should be able to clamp the current down to 50mA. Also; when i measure the resistance between the Vccb and B pins it shows 4.7K Ohm(from R7 probably)so that connection seems fine.

author: hans
author_rank: Super Contributor
timestamp: January 18, 2018, 10:55:24 pm
content: In my experience, the onboard target supply of the PICKIT is not that rocksolid.Perhaps not applicable for PICKIT3: but I was able to fry a PICKIT2 by shorting the onboard target supply while in logic analyzer mode. Normally it would trigger a soft overcurrent condition and shut the supply down, but when in logic analyzer mode the firmware did not support this protection.I was able to fix the PICKIT2 by replacing a diode (D1) and N+P combined MOSFET (U6). I think the power rail generation is pretty similar in the PICKIT3, but unfortunately the failure modes do not sound very similar.Looking at the schematic of the PICKIT3 output translators, I'm not completely convinced that a 50mA PTC will protect the 74LVC1T45 chip with a max continuous current of 50 to 100mA.PTC's take an awful long time to trip:https://www.mouser.com/ds/2/240/Littelfuse_PTC_MICROSMD005F_2_Product_Specificatio-1092638.pdf1.5 seconds at 250mA (!) in 20C ambient.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 19, 2018, 07:16:28 am
content: Quote from: SmokyMcToad on January 18, 2018, 06:36:36 pmSorry but I blew mine up years ago doing exactly that, I think it's luck depending on the exact sequence of the pins connecting so maybe what angle you plug it in or unplug it. Anyway since that experience I double check everything (I thought I was before but not well enough) for example I always ensure usb is disconnected before unplugging/plugging in the PK3 as well as the target unpowered.I wish I could remember what blew up, I think it was the signal driver chip.

author: SmokyMcToad
author_rank: Contributor
timestamp: January 19, 2018, 10:37:12 pm
content: @Hans:I am not powering the circuit from the PicKit, so that might be why the failure mode is a bit different. But thanks for the advice on the PTC; I hoped these would offer sufficient protection but 1.5sec is way longer as I had hoped for.@FourtyTwo42:Hmm, good point on the angle; i think it got connected bit 'skewed'(if that is the right word)due to my cable; during soldering the pins got a bit out of alignment. I guess pin 1(Vpp)and pin 2(Vtarget)could have been connected first by accident. The programming voltage for PIC is around 12V right?

author: SmokyMcToad
author_rank: Contributor
timestamp: January 20, 2018, 09:32:20 pm
content: Ok; was out of ideas and just hooked it up again at 0.2A and let it run for a few secs. U11 was heating up quickly so I guess I finally found the cause.This IC is on the Vprog line so I think FourtyTwo42 and Hans were indeed right and I plugged it in at a weird angle that only got pin 1 and 2 connected at first. So Vdd acted as ground for Vpp and U11 popped before PTC4 was able to restrict the currentflow at 50mA.Thanks for the help!Small followup question:If I connect 6 schottky diodes to the pins of the PicKit, would this be able to prevent these issues? Or would U11 still get fryed before any protection kicks in?

author: jpanhalt
author_rank: Super Contributor
timestamp: January 20, 2018, 09:57:09 pm
content: Quote from: SmokyMcToad on January 18, 2018, 06:36:36 pmA widely know secret is that Microchip is interested in selling microcontrollers, not programmers. It is very liberal in interpreting the warranty for its programmers. I got an ICD3 at discount plus full credit for the PK3 that failed. It even replaced the PK3 too.

author: SmokyMcToad
author_rank: Contributor
timestamp: January 20, 2018, 11:27:49 pm
content: Quote from: jpanhalt on January 20, 2018, 09:57:09 pmEven in case of clear user-abuse?

author: jpanhalt
author_rank: Super Contributor
timestamp: January 20, 2018, 11:38:27 pm
content: Quote from: SmokyMcToad on January 20, 2018, 11:27:49 pmWhy don't you call Microchip in Chandler, AZ and find out? My guess is that MCP doesn't really care. It wants to sell/promote its MCU's.