Hello,
I built an independent +5 voltage system for my bot. I am powering it with a 3cell LiPO battery. At full charge it measures at approximately 12.5 volts. It works fine, and powers the motor driver circuit and arduino really well, however under nearly any load it gets quite hot.
I built something very close to this (I added a network of diodes to the input for polarity protection) - but I am surprised at just how hot this chip gets. I added a heatsink, and from the spec sheet, it has thermal shutdowns - and it hasnt done so. So I am assuming that perhaps it is supposed to run this hot? If it is, is there a more efficient manner to build this? I am quite surprised at the heat, and if it is generating all of that heat (you cant really touch it) then that is a decent amount of power being wasted to dump down to 5v.
So at any rate - thats my general question. I started reading about low drop 5v chips, but those seem to need a fair amount of heat dissipation as well. Also many of those that I saw do not like the 12.5v source range.
LDOs are still linear devices, so they're not gonna help with efficiency.
For better efficiency, you could use a switching type regulator. Sorry, I don't have one to recommend off the top of my head. Better efficiency means less energy turned into heat.
You could also consider switching to 2 Lipos instead of 3 - that would mean less overhead voltage for the 7805 to dump as heat. If you need 3 Lipos elsewhere in your system, just tap the packs at the second one to feed the 7805.
I would be very careful tapping a 3S liPo to use two of the cells to drive the Arduino. If the regulator is too hot to touch even with a heat sink, the drain on two of the three cells could unbalance the battery enough to damage it.
a switching regulator or 2s for everything (perhaps with a ldo regulator) are preferred solutions.
Edit:- the link is too long search the above site for part number 152-524
All low drop out regulators (LDRs) do is allow them to work at a voltage closer than the one you want to regulate to. For example a typical 5V LDR might be able to work down to 5.8V where as your circuit would probably need about 7V, that means you can actually burn less power by arranging a lower input voltage.
It's only with switching regulators like the one above that you don't have to burn so much excess power.
I would be very careful tapping a 3S liPo to use two of the cells to drive the Arduino. If the regulator is too hot to touch even with a heat sink, the drain on two of the three cells could unbalance the battery enough to damage it.
I would agree mem, except I made the assumption that, if he's running motors, the power consumed by the 7805 and stuff downstream of it is negligible compared to that of the motors. If that assumption is wrong then, yeah, watch out for exploding Lipos.
I made the assumption that, if he's running motors, the power consumed by the 7805 and stuff downstream of it is negligible compared to that of the motors.
I was considering the case where the device was left switched on but not moving for many hours, the Arduino would be draining two of the three cells.
I looks like I should have left more details - however even without this I got many good directions to go with.
To answer some of them, I am currently drawing a max of roughly 650ma - that includes the motors, servo, arduino, ultrasound, LED's etc... What struck me as odd is that just attaching the arduino and running that, it drew about 165ma, and the 7805 would still get quite warm. I replaced the cap's thinking perhaps one might be shorting, but no change with that. And while powered up without a load, perfectly cool to the touch.
Thank you for the clarification of the low drop as well. That explanation makes perfect sense to me, and removes the last bit of my desire to go with that.
It sounds as if all of these style "bleed off" the excess voltage in the form of heat/etc... in order to do the voltage drop. Granted my understanding is not perfect, but it does not sound too much more efficient than a resistive voltage divider. (albeit one with voltage clamping.. but still...)
I think that I am going to get my hands on that switching regulator mentioned - it sounds to be more on target of what I was wanting.
I have a couple of them and they are pretty great little boards for the price. Shipping may take a couple of weeks since the products ship straight from China but the cost savings is worth the wait.
The way LDO's work is that the input voltage above the input (12.5 - 5 = 7.5v) has to be dissipated as heat. The little regulator will probably be dissipating something around 5 Watts of heat in your circuit not counting the current used by the device itself.
A motor can require a huge amount of current at startup and especially at stall where the only resistance is that of the internal coils. Are you taking that into account with your current calculations?
ut it does not sound too much more efficient than a resistive voltage divider.
It's exactly the same as a resistive dropper only you can change the current without changing the resistor. That's the technique they used in old TV sets
A potential divider is even less efficient as you have to also supply current down the other leg and that generates heat as well.
What struck me as odd is that just attaching the arduino and running that, it drew about 165ma, and the 7805 would still get quite warm.
12.5V going down to 5V is a drop of 12.5 - 5 = 7.5V
at 165mA this is a power of 7.5 * 0.165 = 1.2375 Watts that the regulator is burning. That is quite a bit of heat so it will get hot.
Thanks jes1510 - I am limited to a max draw of roughly 650ma (total) due to the motor driver circuit I built (at peak, 400ma constant), so I know it is below that. I am looking at building a new motor driver circuit using the SN754410, which is rated at 1A for more motor power, which subsequently drove me to build this new power supply circuit.
Thanks Grumpy_Mike - I have been out of electronics for many years now, but that made perfect sense. I love learning, even re-learning what I once knew many moons ago
