5v cc source - Page 1
Forum section: electronics / projects

author: quadzillatech
author_rank: Contributor
timestamp: March 23, 2019, 03:15:37 pm
content: hi all,im trying to find a simple way to get 5v at 1300ma from a 7805 any ideas?,cheers m3vuv.

author: Benta
author_rank: Super Contributor
timestamp: March 23, 2019, 04:17:05 pm
content: Use a 78S05 (ST).

author: quadzillatech
author_rank: Contributor
timestamp: March 23, 2019, 04:54:59 pm
content: maybe i should of said,i can get 1.5 amp from a 7805,the issue is it cant go over 1.3amp max,its how to limit the current to that im stuggling with.

author: Zero999
author_rank: Super Contributor
timestamp: March 23, 2019, 06:21:25 pm
content: There's no way to get 5V at 1.3A into any load, due to Ohm's law.You need to have two modes: constant current and constant voltage.The LM7805 isn't guaranteed to output any more than 1A, but will most likely give 2.2A peak.http://ee-classes.usc.edu/ee459/library/datasheets/LM7805.pdfThe only way to set the current something else is to add transistor and a sense resistor before the LM7805. If a very accurate current limit is required, then you'll need a voltage reference and op-amp, as well. The downsides are it increases the drop-out voltage, as well as requiring more components.

author: jeroen79
author_rank: Frequent Contributor
timestamp: March 23, 2019, 08:05:49 pm
content: Quote from: quadzillatech on March 23, 2019, 04:54:59 pmWhat exactly are you trying to do? What is it that you want to accomplish?

author: ebastler
author_rank: Super Contributor
timestamp: March 23, 2019, 09:11:30 pm
content: Quote from: quadzillatech on March 23, 2019, 03:15:37 pmConnect a 3.85 Ohm resistor as your load.

author: quadzillatech
author_rank: Contributor
timestamp: March 24, 2019, 12:27:18 am
content: its to drive a m140-a laser diode.

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2019, 10:30:54 am
content: Quote from: quadzillatech on March 24, 2019, 12:27:18 amThen you don't need to regulate the voltage, only the current. The laser diode is type of LED. Limit the current to below the maximum rating and provide adequate heatsinking.

author: quadzillatech
author_rank: Contributor
timestamp: March 24, 2019, 11:59:25 pm
content: i wasnt asking how to regulate the votage!,i need a way of trying to get a 1300ma max from a 7805!,is it not clear now??,regards Paul.

author: hamster_nz
author_rank: Super Contributor
timestamp: March 25, 2019, 12:29:59 am
content: Wire a it up as a current source, with a 3.9 Ohm 10W resistor for R1. That will limit in the load to ~ 1.28A.Will need careful attention to heatsinking.. if Rload is a dead short then the 7805 will be dissipating ~10W.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 25, 2019, 12:56:35 am
content: Quote from: quadzillatech on March 24, 2019, 11:59:25 pmIf you weren't asking how to regulate the voltage, maybe your first post shouldn't state that you want 5V.

author: jeroen79
author_rank: Frequent Contributor
timestamp: March 25, 2019, 12:00:26 pm
content: Quote from: hamster_nz on March 25, 2019, 12:29:59 amBut then an LM317 will be more useful.That will drop only 1.2V across the resistor instead of 5V with the 7805.

author: ogden
author_rank: Super Contributor
timestamp: March 25, 2019, 12:14:20 pm
content: Quote from: Nerull on March 25, 2019, 12:56:35 amRight. Initial post seemingly asked for 5V **and** 1300mA regulation meaning 5V supply with 1.3A current limit. Using 7805 to build constant current supply is bad idea - because it does not regulate current but voltage, 5V. To get constant 1.3A out of 5V voltage regulator, you need 5/1.3 ohms resistor which will dissipate 1.3*5 = 6.5 watts of waste power. Better just buy LED driver for < 2$, eBay search: "LM2596 LED Driver DC-DC Step-down Adjustable CC/CV"

author: Zero999
author_rank: Super Contributor
timestamp: March 25, 2019, 12:37:24 pm
content: Quote from: quadzillatech on March 24, 2019, 11:59:25 pmWhy do you want to use the LM7805? It's most likely not needed!Anyway, to answer the question. A current regulator needs to be connected in series, before with the LM7805.Purely for educational purposes. Attached are two solutions: an LM317 current regulator and an op-amp controlling a Darlington pair, with the TL431 used as a voltage reference.The laser is just an LED. Scrap the LM7805 and use either of the circuits I posted previously, with the laser connected between where the LM7805's input pin was connected and 0V.https://jtechphotonics.com/wp-content/uploads/2013/05/Instruction-Manual-2W-445nm-Laser-Diode-Component-V6.pdf