For a long going project of mine, meant for a car, I have a 7812 regulator leading to a 7805 regulator, which so far seems to be working fine in my prototype
however for the finished product I was wondering if its worth it to use a switching power supply instead of those 2 regulators
I know the battery is usually 13.8 when running so I dropped it to 12 then to 5 hoping it wont heat up too much for my draw of about 350-450 ma
I didn't give much thought to it before but now im wondering if I should or not, heat is the main factor not efficiency although I know they go together
I know the 5v regulator should dissapate around 3w of heat in the 12-5 drop
is that alot to warrant a switching power supply?
Also I have the 7812 just powering a mega2560, so would a 5v +/-5% switching regulator be steady enough to power the mega as well?
The answer to your question depends on whether a large enough heat sink can be accommodated, and ventilated properly. If not, then a switching supply may be warranted. Else the linear regulators are probably the easier and cheaper route. Is the purpose of the 7812 just to spread the power dissipation over two devices, or does the circuit actually need 12V in addition to 5V? If it only needs 5V, I might consider just the one regulator and a larger heat sink, as it will then dissipate ~4W instead of ~3W. There is no free lunch, the total power dissipation is the same with one regulator or with two. The 7812 will dissipate nearly a watt, so it probably needs a heat sink too.
winner10920:
Also I have the 7812 just powering a mega2560, so would a 5v +/-5% switching regulator be steady enough to power the mega as well?
Should be.
I guess il go with a switched power supply, found some decently priced ones on ebay,
Id rather not have a heatsink as its in an enclosed space in a car whre temperature can vary a lot, and while original design had a fan i didnt mind but i couldn't find one quiet enough : /
If I don't give power to vin on the arduino mega board does that mean ill lose power on the 3v3 pin?
Obviously a switched regulator is the way forward for maximum efficiency and minimum heat dissipation.
However, if you have a reasonably constant load and are not too worried about heat and efficiency one simple way of minimising regulator loading is to drop some of the "unwanted" volts across a power resistor.
In you particular example, assume maximum current is 500ma. With a nominal 13 volt supply, you want say 7 volts at the 5 volt regulator input. Therefore you can dissipate 13 - 7 = 6 volts across a power resistor of 6/0.5 = 12 ohms. Resistor power dissipation will be 3 watts, so use a 7 watt (minimum) resistor. Resultant heat dissipation on the regulator is now only 2x0.5 = 1 watt. Whilst this still requires a heat sink, it will be substantially less than if it were dissipating the total power dissipation of 1+3 = 4 watts.
That's an interesting idea I haven't thought of before
unfortunately heat is important, and the draw varies something like 200ma, lcd backlight, bluetooth, on board leds,
Just thinking, the 3v3 works of usb so it must be an ldo regulator coming off the 5v right?
I've used little 12V -> 5V switching modules before, seem to work nicely apart from generating significant noise on the analog inputs (no doubt more filtering would cure that). Just something to look out for.
Yeah I've deiced to just replace the board space with some caps to help filter it coming in
and I checked the shematic and confirmed the 3v3 reg comes off the 5v supply so no need for anything on the vin
Another option is the 1A switchmode power supply from Dimension engineering that drops into standard TO220 holes. It's a bit taller, wider, etc. but the holes line up and the efficiency is much higher, hence less heat.
I was looking at one of those but I think the solution is an off board supply, then I have plenty of 5v supply left for some usb charging dedicated ports