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I'm currently working on long-term remote-sensing & data-acquisition. Thus, I'm planning on using 12V 45Ah wet cell battery (accumulator) for the supply.

The system consists of microcontroller unit, sensors, DC water pumps, communication module, and UI. As we all know, DC water pumps require a lot of current to operate. That's why I'm using a huge supply (45Ah), so the system can operate for a month.

But I wonder, how can I regulate the supply voltage into 5V and 3.3V? As far as I know, LM78XX voltage regulator IC has limited input currents rating. Looking forward for your respond.

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    \$\begingroup\$ All voltage regulators have limited current handling capacity so, based on this fact, what is your new and specific question? \$\endgroup\$ – Andy aka Nov 24 '18 at 12:21
  • \$\begingroup\$ Are the water pumps 12v? \$\endgroup\$ – rdtsc Nov 24 '18 at 13:15
  • \$\begingroup\$ @Andyaka the question is, can i regulate a supply with high-current input through a commonly-used voltage regulator LM78xx? If it's not possible, is there any particular way to regulate it? \$\endgroup\$ – Marchio Kevin Nov 24 '18 at 13:45
  • \$\begingroup\$ @rdtsc yes, it is. \$\endgroup\$ – Marchio Kevin Nov 24 '18 at 13:45
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The regulators don't give a snap how much current your battery can supply.

They only care how much current you pull through them.

If your low voltage parts (presumably a microprocessor of some sort) only draws 100mA, then that's all you have to worry about when selecting the regulator.

Just because your battery could deliver 300A into a short circuit doesn't mean you need to size the regulator to handle 300A (unless you need 5V and 300A for some scary reason.)


All that said, you will probably want to look into a switching regulator rather than using a linear regulator like the 7805.

Linear regulators control their output voltage by acting like a big variable resistor. They waste power.

Take your 5V regulator and assume 100mA.

To get from 12V to 5V, it has to drop 7V. At 100mA, that's .7W.

Your battery supplies something like 540 watt hours.

The wasted power in the 7805 would be like (0.7w * 24h * 30 days=504 watt hours) 500 watt hours in a month.

So, never mind powering the pump. You would be hard pressed to run the low voltage stuff for a month.

Switching regulators waste far less energy.

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  • \$\begingroup\$ Alright, so that's how regulator works. Thank you so much for your clear explanation! One more question, you said that: "They only care how much current you pull through them." @JRE Then, do you know how much current that, for example LM7805, could withstand before the IC burn? From the datasheet, i could only find Absolute Maximum Power Dissipation of 15W. If the maximum power dissipation is 15W and it's running on 5V, the maximum current is 3A. Is this correct? \$\endgroup\$ – Marchio Kevin Nov 24 '18 at 13:52
  • \$\begingroup\$ Nope. You power it from 12V. It puts out 5V. So the voltage is has to get rid of is 7V. 15W/7V= 2.1A. Using a 7805 on 12V, you could draw about 2A. But, only if you have a good heat sink on it. That 15 watts turns into heat, and you have to get rid of it. \$\endgroup\$ – JRE Nov 24 '18 at 14:06
  • \$\begingroup\$ I see. Thank you so much, you've been such a huge help :) \$\endgroup\$ – Marchio Kevin Nov 24 '18 at 14:13
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    \$\begingroup\$ @JRE Nope, a 7805 has a max current spec as well as a max dissipation spec. The popular TO220 packaged device has a nominal max current of 1A, with a limit above which it starts cycling of 1.5A IIRC. The L version is 100mA, and I think there once was a TO3 version with a higher current. \$\endgroup\$ – Neil_UK Nov 24 '18 at 14:31
  • \$\begingroup\$ @Neil_UK by Nominal max current, did you mean maximum input (flowing) current? \$\endgroup\$ – Marchio Kevin Nov 24 '18 at 15:09

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