Can I get rid of heat from a 78XX chip by...

MM007 · Post by **MM007** » Wed Oct 20, 2004 1:32 pm

I realize that a 7805 takes in at least 7.2V and puts out 5V, but generates heat. If I am only using one battery voltage(7.2V), can i decrease heat by putting in a specific voltage LED and maybe current-limiting resistor instead of a 7805? It may work better since they give off some of that energy as light instead of just heat like the 7805. Of course, this means a much smaller voltage range can be used...

I ask this because I have some 2.1V LEDs, which would take 7.2V down to 5.1V. ^^

Will this work, or am I overlooking something?

HK-47 · Post by **HK-47** » Wed Oct 20, 2004 1:40 pm

Sorry, not sure. But I think some types of diodes do different things to the voltage. Im almost positive that theres a diode the bring the voltage down a bit at a time. Sorry I cant be more help.

Post by **gannon** » Wed Oct 20, 2004 1:54 pm

Have you ever thought of the amp draw? I know most leds only have a forward amperage of less than 100mA

stereth · Post by **stereth** » Wed Oct 20, 2004 2:45 pm

No good. The 78XX chips are meant to provide a constant output voltage, something those alternatives cannot do. Resistors and diodes can't deal with changing input voltages and changing output loads.

Say you need 5V for a load that draws, on average, 1A. So you put a big 2 ohm resistor between your 7V battery and your system. If the system suddenly needs 2A, the voltage drops to 3V. Besides, your 7.2V NiMH pack starts at about 8.5-9V when fully charged.

Look into DC-DC converters if you need them - I can give you some part numbers - but those'll cost you $12+.

MM007 · Post by **MM007** » Wed Oct 20, 2004 5:04 pm

$12+, huh? Well, What DC-DC converters can give me 12, 5, and 3.3 V? It is either that or an ITX.

stereth wrote:No good. The 78XX chips are meant to provide a constant output voltage, something those alternatives cannot do. Resistors and diodes can't deal with changing input voltages and changing output loads.

Say you need 5V for a load that draws, on average, 1A. So you put a big 2 ohm resistor between your 7V battery and your system. If the system suddenly needs 2A, the voltage drops to 3V. Besides, your 7.2V NiMH pack starts at about 8.5-9V when fully charged.

Look into DC-DC converters if you need them - I can give you some part numbers - but those'll cost you $12+.

stereth · Post by **stereth** » Wed Oct 20, 2004 5:37 pm

Depends what you're looking for. I'm talking about ones I've used at work, which we used as drop-in replacements for regulators, to convert 12V to 5V.

MM007 · Post by **MM007** » Thu Oct 21, 2004 10:35 am

It's a good start! ^^ Please fill me in! Also, are they just 12 to 5, or is there a range?

stereth wrote:Depends what you're looking for. I'm talking about ones I've used at work, which we used as drop-in replacements for regulators, to convert 12V to 5V.

stereth · Post by **stereth** » Fri Oct 22, 2004 6:29 pm

The input range is 8-30V, but we only used them for 12 to 5. They're a slightly larger drop-in replacement for a 7805. I can't find them anymore on Digikey, but I remember them being $12-15 for 1A of output. I can email my former job to find the model number if you guys are interested.