8v to 5v: Regulator not working (PS1 power supply) - Page 1
Forum section: electronics / beginners

author: Oxodao
author_rank: Contributor
timestamp: January 06, 2016, 12:49:01 pm
content: Hi,I am on my way toward my PS-Pi which is like you'd expect a standard PS1 modified to be used with EmulationStation as a PS1.On my first attempt, I cut the power board to have just the small portion on the top to keep the original plug, and the other side to keep the buttons and the led. Then in the middle, I tore down a USB charger and soldered through the mains and the button. It was working nicely, but pretty ugly on the inside.For the second attempt, i'd like to keep the power board safe, and using it's 8v output and reduce it to 5v to have my Raspberry Pi and other components working.My main idea was to use a regulator since it's not much difference in the voltage, it would not heat that much.I have at home an L7805CV which is acting wierdly: I use it on the 8v supply and it was outputting 5v as expected, but then I plugged my mobile phone on it and.. surprise, it barely detected it, it started to charge, and after a second or two it stops...So I tried to measure the ampere but with my poor 12€ multimeter I do not know how to read it correctly, my guess would be that it outputs ~1mA and it's fluctuating a lot between .6 and 1.2 mA (That's what is written on the screen when I am on "20m".Then I read the datasheet and it was written that the minimum supply needs to be "10v" so I tried on my ATX psu on the 12v and it does the same thing..I would like to use the one I have if possible, and if not I would like to know which do you recommand me...I found this on ebay and it looks like enough (my guess would be that I need 1A, for the Raspberry, the mouse/keyboard eventually, the wireless key and the gamepad controller):http://www.ebay.fr/itm/5-L7805ABV-ST-MICROELECTRONICS-/151758550869?hash=item235583b755:g:szkAAOSwHnFVtQ4tIt still looks like pricy for me, and I don't want to waste money for nothing...There is also a 3.5v on the PS1, if I can bind them together or somewhat I could have a 10+v input for the regulatorThanks in advance

author: Oxodao
author_rank: Contributor
timestamp: January 10, 2016, 03:45:57 pm
content: Bump please

author: Audioguru
author_rank: Super Contributor
timestamp: January 10, 2016, 04:54:10 pm
content: Many electronic components bought on ebay are fakes. Buy from a creditable parts distributor that is probably local.

author: Oxodao
author_rank: Contributor
timestamp: January 10, 2016, 05:25:18 pm
content: Actually the one I got did not came from ebay but it was salvaged from an tv RCA transmitterI now use it for powering an arduino in a led-band driver... And it is working nice, so I still have no clue how to do it in my PS1

author: Audioguru
author_rank: Super Contributor
timestamp: January 10, 2016, 06:47:20 pm
content: The datasheet for a 7805 is available online for free from many manufacturers. The datasheet shows the recommended voltages and capacitors.

author: mariush
author_rank: Super Contributor
timestamp: January 10, 2016, 07:44:34 pm
content: 7805 needs about 1.5v above the output voltage to output a stable 5v, so you need at least 6.5v.The difference between input and output voltage is dissipated on the regulator as heat. So if you have 8v at input and 5v at output and a device like your Arduino uses 100mA, then there's (8v-5v)x0.1A = 0.3 watts dissipated as heat. A 7805 can probably handle about 0.6-0.8w of constant power dissipation (so about 0.25A maximum I would say) if it doesn't have a heatsink installed. A tiny to-220 heatsink would raise this value up to around 2-3 watts, making it possible to output up to around 1A without turning itself off due to overheating.Most 7805 are built to be able to dissipate up to 10-15 watts but to do that would need huge heatsinks and possibly fans blowing air over them.Some phones have a bit of "brains" in their battery charging system, they don't just use the 5v coming from the usb jack. Some phones expect some voltage to come on the two data lines (rx and tx) and depending on the voltages on those two wires, the phone will know how much power can it pull from the charger safely. If your data wires aren't connected anywhere, the phone may refuse to charge and that may be what happens here.Seehttps://learn.adafruit.com/minty-boost/ichargingto learn about how to use resistors to send some voltage through the data lines and signal the phone how much current can your "charger" supply the phone with.There's one more thing... you say your PS1 gives you 8v but you're not saying how much power is that power supply capable of giving on 8v. If you want the 7805 to output 5v at 0.5A or 1A, then the power supply also has to be capable of providing 8v at 0.5A or 1A.Here's a thought: If that power supply is only capable of 0.25A or some low value, and at the moment the phone is plugged in it may try to take up to 0.5-1A from the 7805, the ps1 power supply may be unable to provide that much power all of the sudden and instead of 8v you may see much less voltage (5-6v) there and in turn, the 7805 regulator will no longer be able to output 5v (because the input is lower than 5v+1.5v it needs), and in turn the phone sees less than 5v on the usb connector and turns off.

author: Oxodao
author_rank: Contributor
timestamp: January 18, 2016, 04:47:31 pm
content: Thanks,Actually I have no clue how much it can give, I did not found any documentation on the internet, I just hope it will be enough for what I need :p