I had some fun with a 78M05 (the TO220 kind) last week. My circuit was designed to provide about 35V to the input of a 78M05. That is about the limit of the input voltage as well. The chip works fine up to about 38 volts input, but near 40V input (I was testing what happened at higher input voltages) the 5V end of it rapidly loses it's ability to supply current. I was trying to figure out whether the chip was shutting itself down because it was outside of it's SOA (I think these things have a thermal shutdown ) or just failing. Now, the problem would occur just milliseconds after the power was turned on, so I decided it couldn't be a thermal shutdown issue. As soon as I turned on a few LEDs and sucked up some current the power supply would dive to 4V and reboot my PIC. Also, the problem seemed to get worse as the overvoltage was applied more times, so I decided it was a failure mode instead of a self-protection mode. After a few hundred cycles on an automatic test machine the unit would not work at all. I replaced the 7805 with a fresh one and added a 33V zener to clip those peaks. This design also has a resistor in front of the 7805 to reduce power dissipation. This morning my automatic test machine shows 1100 cycles without a hitch. I haven't had to use a pre-regulator before my regulator in anything but a precision benchtop power supply. I wonder if there is a cheap automotive 5V regulator that will handle 48V DC input voltage without complaining? -- Lawrence Lile Russell McMahon Sent by: pic microcontroller discussion list 08/25/02 07:28 AM Please respond to pic microcontroller discussion list To: PICLIST@MITVMA.MIT.EDU cc: Subject: Re: [EE]: Crowbar protection > Since the failure in shorted mode is so rare, my detective instincts lead me to suspect something else - Someone else noted the need for reverse voltage protection diodes. These are covered in the data sheet. The need arises when the input voltage falls much more rapidly than the output and the output cap discharges "backwards" into the regulator. This can occur if the input is shorted suddenly OR if the output load is small relative to the output cap provided while the input can discharge rapidly when turned off - either because there is little or n input cap or because some other load rapidly discharges the input cap. In these cases and others the LM293x family are specifically designed to survive. Their forte is automotive situations replete with alternators, car batteries, inductive loads and numerous somewhat silly people wielding jumper leads and reverse connected batteries. I strongly recommend this regulator family. (Standard disclaimer, no shares in NatSemi or other financial etc interest yada yada). You said 7805 - hopefully NOT a 78L05 - the TO92 miniature version. These do die for reasons that I have never bothered to determine - not using them is easier. The TO220 version costs less usually and the TO220 dissipates more power when unheatsunk than the TO92 pkg. RM -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics