Voltage question

scherzo · Post by **scherzo** » Mon Oct 18, 2004 11:14 am

OK, I understand that batteries in parallel increase amps and will provide power longer and batteries in series increase voltage. What I'm curious about is providing more voltage than is needed in a circuit. I notice many of you say "provide at least 7.2 volts" or something like that and, in turn, say that hooking up a 9 volt battery would be good. Does the voltage regulator (7808, etc.) burn off the extra voltage through heat? Is 9v battery used because the voltage drops slowly as the battery dies so the higher initial voltage is to account for that. I just am looking for more general understanding. Thanks.

Post by **SpongeBuell** » Mon Oct 18, 2004 1:40 pm

The most common voltage regulator is the 7805. This takes any voltage between 7.2 an 36 and converts the rest into heat, though that doesn't provide the most efficiency (not terrible, either). A good altenative is the LDO (low dropout) which requires less volts (needs at least 6) and doesn't get as hot.

Post by **Gamelver** » Mon Oct 18, 2004 1:46 pm

yeah too many times have I gotten burned from 7805s

Post by **SpongeBuell** » Mon Oct 18, 2004 2:02 pm

that's right, my electronics teacher gave me a heatsink for the 7805 when I showed him my atari today

stereth · Post by **stereth** » Mon Oct 18, 2004 3:05 pm

In the same circuit, LDOs produce the same amount of heat as normal regulators. They allow you to use a lower input voltage, which would mean less heat.

Alkaline 9V batteries start at about 9.8-10V when they're new. The voltage drops linearly to 5.5V for a dead battery. Keep that in mind. If you need at least 7.2V, the 9V battery fails when it still has 25-30% of its energy remaining.