Power Raspberry with DC/DC converter - Page 1
Forum section: electronics / beginners

author: german77
author_rank: Regular Contributor
timestamp: April 26, 2019, 07:48:55 pm
content: I want to power a raspberry pi3 from two car batteries in series. The maximum voltage will be 30v and minimum 21v. Right now I have a XL4005 DC-DC converter that lowers the input voltage to 5.2V. It should deliver up to 5A witch should be more than enough to power the raspberry through its microusb power conector.At first the power LED on the raspberry stays on. Then turns off while the ac Led still blinks. After some investigation I found that the voltage drops slowly to 4v with a lot of noise on the 5v rail of the raspberry pi. And the output of the DC-DC converter stays always at 5.2v and the current consumption hardly goes up from 300mA.I added some decoupling capacitors but the results where the same. The weird thing is that I have the same DC-DC converter on another raspberry that has been working since December without problems. The raspberry works fine with a 5.3v 2A usb charger.Anyone knows why the DC-DC converter is not working properly. I make sure that the input voltage is solid as a rock, and everything is soldered .

author: mariush
author_rank: Super Contributor
timestamp: April 26, 2019, 08:41:32 pm
content: The microUSB connector can barely a bit more than 2A of current. They claim their connector is up to 2.5A and supply the pi with a 2.1A wallwart adapter.Your USB cable must also have thick wires inside otherwise the higher the current, the more voltage drop.It's not realistic to do 5A through the usb connector and cable.

author: maginnovision
author_rank: Super Contributor
timestamp: April 26, 2019, 08:56:35 pm
content: What happens if you connect some wires to the 5V header pins instead. That's how I've done external supplies.

author: Bratster
author_rank: Frequent Contributor
timestamp: April 26, 2019, 09:39:02 pm
content: That sounds like crappy micro USB cable.There are some test pads on the bottom of the pi where the micro USB connector is, what voltage do you measure on those two pads when you're seeing the 5.2 volts on your DCDC converter?Sent from my Pixel 2 XL using Tapatalk

author: german77
author_rank: Regular Contributor
timestamp: April 26, 2019, 11:03:17 pm
content: Quote from: mariush on April 26, 2019, 08:41:32 pmThe raspberry is rated at 5W roughly 1A continuous. No one said that I will draw the 5A that the converter is able to deliverQuote from: maginnovision on April 26, 2019, 08:56:35 pmI'm worried about lack of some sort of protection. That will be my last resource.Quote from: Bratster on April 26, 2019, 09:39:02 pmI use the same cable to power the raspberry with the phone charger. It will be a little hard to test on the pads. But after some test I found that with the DC-DC the voltage does drop to 4v, while with the with the phone charger stays always at 5v. It's the same cable on both test.I'm starting to think that the problem resides on the usb conector of the DC-DC converter. I will solder a thick cable and clean the contacts to see if that makes a difference

author: Ian.M
author_rank: Super Contributor
timestamp: April 26, 2019, 11:23:00 pm
content: See if you can reproduce the problem without the Pi, i.e. replace it with a USB2 micro-AB socket breakout and an approximately four ohm resistive load (which will draw 1.3A @5.2V dissipating a bit under 7W).

author: german77
author_rank: Regular Contributor
timestamp: April 27, 2019, 01:15:20 am
content: I did test with some restive loads. The DC-DC converter was set to 5.1V. With a load of 1.12A the voltage went down 4mV. After the USB socket I saw 5v. On the micro-USB 4.46v. This time I used a shitty USB cable because I din't had a female micro-USB socket on hand. and I directly connected the resistive load to the cable.With the phone charger the voltage went from 5.3v to 4.81v with a 1.3A load.After some seconds I saw the voltage getting lower and lower until it reached 4.5v on the DC-DC converter. Even without load, I adjusted the voltage again to 5.1V and put again the 1.12A load. It stayed there without problems. I think this happen because the converter got hotter.Then I decided to connect the raspberry again. The power LED did turn off some times. But the raspberry did boot and worked normally, the LED was always on after boot. Leaved it doing something for an hour and worked fine also the voltage was kind of stable at 5v on the header pins.I'm not happy on how this is working. The cable loss can be fixed. I only need a 10 cm cable. But the voltage drop after the converter has heated up is way to much, also the converter is hot to touch even with heat sink at an idle load.

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2019, 02:14:38 am
content: Hmm. According to theXL4005 datasheetit should be good for up to 5A output current, and that the peak switch current is 8A. At 300mA in and a 5:1 buck ratio the peak switch current shouldn't be over 3A so plenty of margin.Assuming you've got the buck module in free air at room temperature, there's either something shady about it or the XL4005 IC is damaged. Can you tell which of the module's IC, diode or inductor is heating up the most? I suspect its a fake, possibly a remarked lower current rated buck IC.

author: floobydust
author_rank: Super Contributor
timestamp: April 27, 2019, 05:03:50 pm
content: A common problem with Raspberry Pi is 5V sagging due to voltage drop in the polyfuse at the microUSB power input jack. You can get stuck in a reboot loop if Vcc sags too much when the CPU current drain goes up.MF-MSMF250/16X is 2.5A hold, 5A trip and 0.015-0.1 ohms. I just short the PTC out with a jumper and run them off a ~2A adapter. I would not run 5A through a Pi...Raspberry Pi 3 model B, B+ hasMF-MSMF250/Xat the USB power jack.Raspberry Pi 2 model A+, B has MF-MSMF200/X at the USB power jack. These have big voltage drops.There is no polyfuse on the Pi Zero.Raspberry Pi schematicsThere are 5V buck converters good for over 5A which use TPS40057 and synchronous rectification. Much better thanXL4005.Lately I use QSKJ 8A four port USB charger boards, from Ali or eBay etc. These have TPS40057, two DPAK mosfets and 8-35V input, fuse, TVS etc. and output ~5.2Vhttps://www.electrodragon.com/product/dc-dc-buck-4-usb-charger-module-8aKIM-055L heats up past 4A.