Kit problem, possibly a 1602 LCD issue? Keeps blowing up! - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 09, 2014, 02:24:51 pm
content: I successfully built a U3 TX kit, details here:http://www.gatesgarth.com/u3.pdfand here :http://www.hanssummers.com/ultimate3.htmlIt ran fine for a few minutes, on 10 meters, then it died. I found the ATmega328PU was dead and the DDS module, too. I got a new ATmega328PU and programmed it seemingly successfully using Extreme Burner, and replaced the DS module as well, but the 1602A display showed random lit areas, and no text at all. I then found that the supply was giving nearly1.5 amps and the ATmega328 had died again, with the DDS module chip also mad hot. The only other addition is a small 5V GPS module giving 1PPS and NMEA outputs, this is still working fine. I run it off a 0 to 30V Farnell dual section bench supply which has never given any issues I know of before. A multimeter shows correct volts out as per the volts control knob, but I have not looked for spikes. I am not sure what is killing what, but when the LCD display is unplugged it never seems to draw abnormal current. Can anything happen to the LCD display that would cause the ATmega328 to fail, *AND* the DDS module chip to fail, or is it likely to be a power supply issue? I was thinking of buffering the PSU with a 5V voltage regulator fed at say 9V and giving a fixed 5V output, to protect downstream stuff from a spike, in case the PSU is iffy. Is there anything else I should check, it's now eaten 2 DDS modules and 2 ATmega328PU's. I have not changed the LCD display module, but if I had to guess I would suspect that, or just maybe my PSU. Not sure if the LCD module can be tested stand alone though? The 5V backlight diodes, fed alone through the on board resistor don't seem to be doing anything odd. I don't want to keep blowing things up!So looking for a testing regime that will isolate the issue, thanks.

author: mij59
author_rank: Frequent Contributor
timestamp: December 09, 2014, 02:49:37 pm
content: Hi,Did you check the BS170 mosfets ?If they fail B+ could be supplied to the inputs of the MCU and DDS.

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 09, 2014, 02:52:11 pm
content: I did pull the PA ones, not the driver, when it failed the first time, but they are a bear to get off the PCB as the through holes are very tight. Is there a way of checking them for shorts in circuit? I am loathe to try and de solder needlessly as I may knacker the board. Cheers for the (very) fast reply!

author: mij59
author_rank: Frequent Contributor
timestamp: December 09, 2014, 03:17:15 pm
content: In-circuit measuring can be tricky, but in this case if there's a short between gate and drain or source the BS170 is toast.Start by testing with only the +5V connected.Photo's of the setup could help.

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 09, 2014, 06:30:24 pm
content: OK, thanks, I am thinking a gate to drain short would back feed both the DDS module and LCD module? Can't see that doing either a power of goodWould cutting the tracks to the gates and bridging the cuts with say 4K7 0805 surface mount resistors offer future protection? As I see it the PA section will kill most of the rest of the stuff if a gate to drain short occurs. Thanks for your input. Will try and get the darned things out and test them.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 09, 2014, 07:32:58 pm
content: A 4k7 will kill any RF signal, it might work if you bridge it with a capacitor too. But that won't prevent you blowing the BS170 again!What voltage PA B+ ?What loads point X? I realise L1 is relatively small but I will appeal to Murphy's law and suggest there will be sufficient energy to produce a fet-fatal inductive flyback voltage without any load to dampen it.What determines the DC current drawn from the PA B+ supply? When it's producing RF you've got on reckon those PA fets being "on" for 50% duty cycle - the average DC level will limited by the PA B+ voltage and the on resistance of the fets.