[Solved] PICkit 2 clone Vdd output ? - Page 1
Forum section: electronics / repair

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 02, 2020, 03:32:14 pm
content: I have this PICkit 2 "SURE Electronics" knock-off that doesn't seem to work as intended...If I believe the PICkit 2.61.00 software the Vdd output should be variable depending on the device to program.On this programmer it outputs 4.8V whatever the setting.The Vdd generation section of the attached schematic appears identical to the layout on my programmer.I've probed around and the filtered PWM output at OP-AMP's inverting input (LM358 for me) does vary when adjusting Vdd on the software.I don't see how the OP-AMP and Q1 can produce a variable output. What am I missing?Vdd output is pin 2 on header, Vpp on pin 1.

author: Ian.M
author_rank: Super Contributor
timestamp: October 02, 2020, 03:49:29 pm
content: Q1 provides inversion, so the overall feedback via the R4,R3 potential divider to the +in of the OPAMP is actually negative. As a whole, IC2 and Q1 act as a buffer with a gain of two and an output that can source as much as Q1 can withstand but only sink a fraction of a mA via R6.Check the output of IC2. If its up near 5V, Q1 has failed shorted. If its down near 0V, either IC2 is bad or R4 has failed open. I'd put my money on Q1 having shorted. On most clones there isn't a lot of copper area to dissipate heat from its drain and its easy to cook it powering a lower voltage target that draws too much Vdd current.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 03, 2020, 12:16:36 am
content: Thanks for the explanation, I overlooked the P-MOS...I had replaced the op-amp just in case, no effect.Filtered PWM varies from 1.4V to 2.8V for target Vdd settings from 2.5V to 5V.The Non-Inv. input sits at 2.5V with output at 3.9V, if Inv. input exceeds 2.5V output goes low, so that's good.However Q1 is always fully conducting so there is no possible linear region. The problem is I bought this programmer used and never saw its normal operation... I'm only assuming it has been damaged.Q1 tests good out of circuit... But looking at the FDN302 datasheet the VGSth looks much too close to the 1.1V drop from +5V when op-amp output is high.I gave Q1 a try with a higher VGSth model BSS84 and it works as intended.So the conclusion is that it likely never worked as it should straight from the factory.There was also a problem with the Vpp (different setup from schematic) failing after a few seconds active.Crap design!

author: jaromir
author_rank: Unknown
timestamp: October 03, 2020, 12:28:47 am
content: Quote from: shakalnokturn on October 03, 2020, 12:16:36 amMCP601 (as per schematics) is rail-to-rail output and it should go higher than 3,9V. I wonder if the opamp isn't some "price optimized alternative" instead.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 04, 2020, 12:28:29 am
content: The schematic found on internet wherever it came from does appear to have better component choices.The hardware I have definitely is price optimised rubbish, hell to troubleshoot too as the PCB is black making the tracks very hard to see. Solders are below average, flux residue all over the place.