Mk3 Dev Board oddity?

[Cruster]

Hi all.

I'm having an odd problem with my Mk3 development board. I wonder if anyone can shed some light?

On my board, U3 (the CD4052 analogue mux) will on most occassions become very hot immediately after applying external power through the PSU-jack socket. J29 is correctly set to PSU operation and I am applying +12V (regulated) DC. When this occurs the +12V supply sags due to the current being drawn by U3. The +12V power supply is working correctly.

It made no difference whether USB (to my PC) was connected or not and there is no problem running the board solely from USB. Only when external power is supplied through the jack would U3 (on most occassions) start to become very hot.

Thinking U3 to be just somehow faulty at this point, I desoldered it and replaced it with an identical part. Exact same issue.

I have checked the surrounding circuitry around U3 and all is in order. Q1, Q2 function correctly and U3 is connected correctly. On the few occassions that U3 did not become hot on power up, all voltages around it were as they should be and PICs could be read and written to.

I'm only guessing at this point.. but I'm wondering whether U3 on my board is exhibiting CMOS latch up type behaviour on power up?

Here's the curious thing.... if I power the board instead through J27, with the identical power supply, all is in order.... the only difference now being is that the board is no longer being powered through BR1 (which checked out ok)

Thinking the diode drops (introduced by BR1) may have something to do with it, I powered the board through a bench variable PSU and tried different voltages but was unable to get U3 to exhibit the same behaviour! But, connecting the bench PSU to the jack socket - bingo. Same problem reoccurs.

Any ideas?

Kind regards,

Chris

[JohnCrow]

Hi Chris

Hmm Ive not come across that before. Certainly not had it on my board, which Ive had for over 4 years.
There is a sticky on the e-blocks section about the regulator getting hot.

Have you see this?

http://www.matrixmultimedia.com/mmforum ... p?f=2&t=76

[Cruster]

Hi John,

I hadn't seen that thread. Interesting! Thank you.

On my board it's the 4052 which first gets hot followed by the regulator (due to the excessive current drawn by the 4052)

I've found that removing BR1 (the full bridge) and replacing it instead with one diode has completely eliminated
the problem. I'd have guessed if the problem was down to excessive supply ripple then that should have made it worse...

I have a hunch the 4052 is being driven into SCR lock up on power on, but I'm unsure why...

[JohnCrow]

Thats a strange one

I know the 3pin regulators get very warm on all of my programmers, but not hot enough to be of any concern. I have noticed certain e-blocks tend to draw more current than others and hence the regularor gets hotter.

[Steve001]

Hi,

I have a v6 board which is doing something similar i now only use it from 5 volts, and it's working fine and use my v7 board to program .

I would also like to know whats happenining. My v6 board did however get exposed to some large magnetic fields which i thought may have damaged it.

steve