Can I draw 1A 3.3V from a 0.5A 5V regulator? - Page 1
Forum section: electronics / beginners

author: amin08
author_rank: Contributor
timestamp: January 09, 2018, 01:17:35 am
content: Hi all!I'm a noob in electronics, and the question is if I can draw 1A 3.3V from a 0.5A 5V regulator. It's for powering an ESP-07I have two regulators, one is 7805LDO at 5V 0.5A and the other is an AMS1117-3.3 at 3.3V 1A for powering the esp.Is it a good idea?Thaanks!

author: james_s
author_rank: Super Contributor
timestamp: January 09, 2018, 01:24:47 am
content: No, 1A at 3.3V is 3.3W, and 0.5A at 5V is 2.5W, you can't get something for nothing. Additionally assuming these are linear regulators the input current has to equal the output current so if you pull 1A from a 3.3V regulator with 5V going in, it will be drawing 1A (plus a small bit for powering the regulator itself) from the 5V source. The "extra" power will be converted to waste heat in the regulator.

author: Signal32
author_rank: Frequent Contributor
timestamp: January 09, 2018, 01:43:17 am
content: Quote from: amin08 on January 09, 2018, 01:17:35 amjames_s's answer is technically correct, but I don't think it applies to your scenario ( how you describe it is somewhat confusing ).Just because the AMS1117 has the capability to handle 1A that doesn't mean you will actually use 1A.For powering an ESP your AMS1117 will draw somewhere around 0.1A, which means you probably can use the 7805LDO + AMS1117 in your scenario.To get a sure answer, tell us what you are connecting to the 7805 and to the AMS1117.

author: amin08
author_rank: Contributor
timestamp: January 09, 2018, 02:01:02 am
content: the main problem is when I power up the ESP07 the blue led blinks, but after 2 seconds, when I unplug power and plug again the esp07 module does not blink the blue led, that means that it has not been initialized.My 7805 regulator has two electrolytic 47uf capacitors, and the AMS1117 has two ceramic 10 uf

author: mariush
author_rank: Super Contributor
timestamp: January 09, 2018, 02:25:24 am
content: 1117 regulators can oscillate or just not work right if the capacitor on the output has too low ESR... most 1117 manufactured expect at least 0.1 ohm ESR from the output capacitor, and you're using a ceramic capacitor which has much lower esr value.You should figure out if the ESP-07 uses so much power all the time, or if it averages much lower power consumption but only uses up to 1A (3.3w) when sending data or receiving data, for just a few ms at a time ... or in periodic pulsesAlso, does that need to run 24/7, or you could turn it off and on , as needed?Have a look, measure the actual curent used, maybe for 80% or more of the time, the thing actually uses much less than 1A, or maybe see the documentation and check if you could reduce the transmission power or receive power in order to make the chip uses less peak power.Because if that's the case, you could have the 5v regulator charge a big capacitor or a super capacitor with energy, and then you could have a very efficient switching (step-down) converter produce 3.3v at up to 1A using the energy stored in that capacitor or super capacitor - the capacitor could hold enough charge to provide 3.3v up to 1A for let's say 1 to 10 seconds, or could power the thing indefinitely if the average power consumption is something like 80-200mA.Basically, the capacitor or super capacitor in front of the switching regulator would act like a "reservoir" , an energy tank from which the regulator could take energy when the esp-07 demands more energy than what the 5v regulator can provide.Downside of this is that when you first power up the device, it may take a few seconds until that capacitor or super capacitor charges with energy enough to be above 3.3v and stay above 3.3v

author: Teledog
author_rank: Regular Contributor
timestamp: January 09, 2018, 04:02:33 am
content: This was discussed previously.With an ESP-xx unit, the 1117 is going to get screaming hot!Easiest /cheapest route is getting an "LM2596 DC-DC" adjustable module on fleabay/Asian suppliers ~$1 eachJust set them to 3.3V & headaches are gone

author: amin08
author_rank: Contributor
timestamp: January 09, 2018, 09:22:08 pm
content: i just found something very curious. When I unplug ESP07 tx line, all seems to be working, but when I plug it back, I see the problem again (esp07 not starting correctly / blue led not flashing). Please any help is appreciated

author: james_s
author_rank: Super Contributor
timestamp: January 09, 2018, 09:32:36 pm
content: If you unplug the tx line doesn't that prevent the device from being initialized? It may not be coming out of sleep mode, current draw is going to depend on what it is actually doing.

author: amin08
author_rank: Contributor
timestamp: January 09, 2018, 09:43:05 pm
content: I don't think it has something to do with current draw. The ams1117 is supplying enough current and the esp07 has it's capacitors and resistors connected. When I unplug esp's tx and power up the board, the blue led flashes, but when I plug it back it doesn't flash.