Oh this is an easy answer. At 12V, the AC2117 is dropping about 8 of those volts. 8V*400mA=3.2W. Thermal resistance to the heatsink on this part is 100C/W. So with a room temperatureish heatsink at 30*C, you're going to end up with a junction temperature of 100C/W * 3.6W = 320C + 30*C = 350C. This is above the absolute maximum operating temperature range of 150*C.
The most you can draw through this with an 8V drop and an appropriate heatsink not exceeding 150*C (I'm assuming it gets a bit warm) is 150C - 50C = 100C. 100C/W means this part can dissipate at most 1W with an adequate heatsink. This equates to 1W/8V = 125mA max with a 12V input voltage. If you don't have an adequate heatsink or the temperature gets higher this is even lower, since the rating without a heatsink is 125C/W. Note that this isn't a problem off of USB because the AZ2117 doesn't drop much voltage, so even at 1V of drop, you'd only have 1V*400mA=400mW of power dissipation, and the unit should be able to handle this even without a heatsink. ALSO: Could you clarify here if the USB is the preferred input? If not, one could rip out IC2, R26, R3 and Q1 and replace it with another diode. ALSO: Not sure why the indicator leds are done the way they are - they probably work, but seems like overkill. On Fri, Mar 1, 2019 at 12:14 PM can...@believewireless.net <p...@believewireless.net> wrote: > > Schematics attached. The main issue I'm having is when powered via USB, > everything works fine. When powered via a 12V car battery, the > chips seem to not have enough power. There are two comparator circuits, one > to monitor USB/Batt and report how the unit is being powered. > The 2nd actually provides the power. One chip will occasionally draw > ~150-400ma of power causing the board to lock up. Chips are powered > via Vin. > > What I don't understand is why everything works perfectly on USB but has > issues when powered via 12V. > > On Tue, Feb 26, 2019 at 11:53 AM <ch...@wbmfg.com> wrote: >> >> Circle of stability, miller capacitance all good stuff. >> >> From: Mark Radabaugh >> Sent: Tuesday, February 26, 2019 9:48 AM >> To: AnimalFarm Microwave Users Group >> Subject: Re: [AFMUG] OT: EE Consultant >> >> Oh sure, now you probably want me to believe transistors have states other >> than off and on. >> >> Mark >> >> On Feb 26, 2019, at 11:20 AM, Ken Hohhof <af...@kwisp.com> wrote: >> >> > And good luck finding an analog circuit guy anymore ;-) >> >> That’s what I was. Not much demand, that’s why I’ve been doing other things >> for over 20 years. Actually made the switch to engineering management in >> the 80’s but still did some design work for another 10 years because all the >> engineers wanted to do digital and processor designs. I still have the >> books, the brain is rusty though. >> >> I tried to convince the new engineers that high speed digital design meant >> knowing analog, RF and microstrip techniques, but they weren’t buying it. >> Getting their products through regulatory emissions testing did give them a >> bit of religion. >> >> I still remember laying out PCBs using red and blue tape. >> >> From: AF <af-boun...@af.afmug.com> On Behalf Of Mark Radabaugh >> Sent: Tuesday, February 26, 2019 9:09 AM >> To: AnimalFarm Microwave Users Group <af@af.afmug.com> >> Subject: Re: [AFMUG] OT: EE Consultant >> >> Yep - I’m qualified to answer some things and would be totally lost on >> others. EE has become a huge field with many many disciplines. >> >> And good luck finding an analog circuit guy anymore ;-) >> >> Mark >> >> >> On Feb 26, 2019, at 9:56 AM, Ken Hohhof <af...@kwisp.com> wrote: >> >> Analog? Digital? Even with those there tend to be specialties like >> emissions, safety, PLD/ASIC design, microprocessor/microcontroller, RF, etc. >> Or do you need a generalist? >> >> From: AF <af-boun...@af.afmug.com> On Behalf Of can...@believewireless.net >> Sent: Tuesday, February 26, 2019 8:33 AM >> To: AnimalFarm Microwave Users Group <af@af.afmug.com> >> Subject: [AFMUG] OT: EE Consultant >> >> Having an issue with a circuit design, anyone know a good electrical >> engineer they can >> recommend? >> -- >> AF mailing list >> AF@af.afmug.com >> http://af.afmug.com/mailman/listinfo/af_af.afmug.com >> >> >> -- >> AF mailing list >> AF@af.afmug.com >> http://af.afmug.com/mailman/listinfo/af_af.afmug.com >> >> >> >> ________________________________ >> -- >> AF mailing list >> AF@af.afmug.com >> http://af.afmug.com/mailman/listinfo/af_af.afmug.com >> >> -- >> AF mailing list >> AF@af.afmug.com >> http://af.afmug.com/mailman/listinfo/af_af.afmug.com > > -- > AF mailing list > AF@af.afmug.com > http://af.afmug.com/mailman/listinfo/af_af.afmug.com -- - Forrest -- AF mailing list AF@af.afmug.com http://af.afmug.com/mailman/listinfo/af_af.afmug.com
