Why does this happen when I regulate 9v to 5v? - Page 1
Forum section: electronics / beginners

author: makerimages
author_rank: Regular Contributor
timestamp: November 13, 2013, 08:21:36 pm
content: I got this setup for power regulation: a 78M05A regulator, and 2 electrolytic 10uF caps- one 25V one btw input and gnd and one 50V one btw output and gnd: Had a 8.7V battery, ran it trough the regulator, got 5V as expected, connected that 5V to an osscillator and a z80 and the system twitched a bit.. when investigating I found out this:The battery had dropped to around 7VI was now getting 3V from the regulatorwhat could be the cause of this drop when regulating 7V? Is it just that the regulator cant properly regulate that voltage to 5V?

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 13, 2013, 08:34:15 pm
content: How much current were you pulling from the regulator? Check the data sheet. The first 78M05A data sheet I found showed a limit of around 240mA to 300mA short circuit current (varying a bit depending on the precise part number).If the current is within limits, how fresh was your battery? If the battery voltage was truly pulled from 8.7V down to 7V with a current load of less than 300mA, then the battery is probably not good.

author: Izrun
author_rank: Newbie
timestamp: November 13, 2013, 08:35:37 pm
content: It looks like 78M05A regulators have a 2 V typical voltage dropout, meaning you need at least ~7V to get 5V. I'm not sure what the tolerance is of your actual regulator, but you are probably out of range. You can get low dropout regulators which would probably be a better option for you.

author: Stonent
author_rank: Super Contributor
timestamp: November 14, 2013, 12:48:19 am
content: Definitely if your regulator is a TO-92 part (looks like a plain transistor) it won't have a very high current rating. 7805 regulators with a metal backing and screw hole typically have ratings over 500mA and normally at least 1 Amp.

author: mariush
author_rank: Super Contributor
timestamp: November 14, 2013, 01:12:03 am
content: TO-92 7805 linear regulators usually can do maximum 100mA (0.1A).TO-220 7805 regulators can do 1-1.5A WITH HEATSINK. Without, they can do about 200mA9v batteries can't do a lot of current... maybe 100-250mA for short periods of time, see for example :http://www.adafruit.com/datasheets/MN1604_US_CT.pdf9v , 100ohm ... v= i x r ... you can see the battery drop the voltage quite fast.So figure out how much current that z80 draws and then see if your 7805 regulator and battery can handle that.

author: gotvolts
author_rank: Contributor
timestamp: November 14, 2013, 01:30:34 am
content: Well the regulator he is speaking about can handle 0.5A according to the part number.But, seems like too much current and the device datasheet doesn't indicate any short circuit protection or thermal overload.

author: Dago
author_rank: Frequent Contributor
timestamp: November 14, 2013, 06:36:46 am
content: Sounds like you have a short circuit and it is overloading your regulator and this pulling all the voltages down. You can check with a multimeter if the power rails on your Z80 thingie are shorted (even if they're not directly shorted there might be something else wrong with it like an IO pin shorted and pulled high or something).

author: amyk
author_rank: Super Contributor
timestamp: November 14, 2013, 08:55:37 am
content: Quote from: mariush on November 14, 2013, 01:12:03 amIf it's the NMOS version, a standard 9V battery will basically be unable to power a Z80 - it consumes around 200mA at 5V / 4MHz or 1W.