Using a cellphone charger as power supply on breadboard - Page 1
Forum section: electronics / beginners

author: samintheoc
author_rank: Newbie
timestamp: February 15, 2015, 07:17:43 am
content: Hello,I am taking this logic gate class and I want to work on and test my circuits at home. The power supply that we usein lab is a constant 5V.I have this samsung phone charger that I stripped the wires. It says it outputs 5V and 0.7A. Am I alright in using thischarger to power my bread board without burning my logic gates?I have 1k ohm resistor in series with the power supply, logic gates, and LED 7 segment display. I can't quite figure what is the max current for the logic gates and LED display from their data sheets. Im using 74LS08, 32, 04, 86...etc.Am I being reckless? Please help thanks.

author: SeanB
author_rank: Super Contributor
timestamp: February 15, 2015, 07:26:59 am
content: Would not hurt to measure the actual voltage as well. Most cell phone chargers deliver around 7-9V unloaded, but yours might actually deliver 5V unloaded, especially if it is a micro USB one. Anything from 4.5V to 5.5V will work for standard TTL, with about 30mA per chip with those SSI chips. If the voltage out of the charger is over 6V you probably will want to use a LDO 5V regulator to make a 5V supply for the breadboard or buy a simple buck converter board off eBay that has a 6-15V input and a 5V output. That will be easiest if it is a poorly regulated supply.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2015, 07:40:40 am
content: I think a standard USB power supply of the kind used for charging modern phones will provide a regulated 5 V and will be suitable to use. All you would need to do is cut a USB cable in half, strip the power conductors, and plug them into the breadboard. Use a decent quality USB charger like an Apple one and not a dollar store kind. The cheap ones can have really bad mains isolation.

author: mentaldemise
author_rank: Regular Contributor
timestamp: February 15, 2015, 07:54:21 am
content: Quote from: IanB on February 15, 2015, 07:40:40 amI wouldn't trust that. I took apart a cheap charger not long ago with no regulation at all.

author: Seekonk
author_rank: Super Contributor
timestamp: February 15, 2015, 10:51:11 am
content: I use them for everything. Any major brand cell phone will have a UL listed charger. A very few will have multiple wires for different charging modes. Those can be tricky to sort out. Others will have two black and two red, those are just parallel wires fro more flexibility.

author: rob77
author_rank: Super Contributor
timestamp: February 15, 2015, 12:26:01 pm
content: all my chargers with USB connector are regulated and within specs 5V +- 5%... and i have branded ones and no-names as well (some of the no-name ones got some ripple , but output voltage still within specs).if a charger is not regulated then it CAN'T HAVE a USB connector ! anything with a USB connector must be 5V regulated and within specs.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 15, 2015, 12:53:37 pm
content: If you have access to an Apple power cube, those put out a well-regulated 5.1V.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: February 15, 2015, 01:41:14 pm
content: any modern phone charger will do the job ... 5V 0.7A i think can be good for any kind of logic application you put on bread board, even Chinese one are not that bad when everything is designed well, cause you can forget about any kind of protection circuit. but yeah i guess for learning purpose ... phone charger is a good choice to start with

author: dannyf
author_rank: Super Contributor
timestamp: February 15, 2015, 02:21:46 pm
content: I have done that many times, without no problem.The wimpier chargers are actually better because they don't pose a fire hazard if shorted.

author: zapta
author_rank: Super Contributor
timestamp: February 15, 2015, 02:22:24 pm
content: You don't need the series resistor, it actually makes things worth (voltage drop). Measure the voltage and if it's close enough to 5v the you are OK. The ICs will take only the current they need, not more. Don't use cheap Chinese chargers. They are often unsafe.I have a bench power supply but use those chargers most of the time. Compact and handy. Only problem is that they don't have low current limit to bring up new boards.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 15, 2015, 05:18:33 pm
content: Any cheap Chinese phone charger will not do. I'm sure there are good ones, but the bad ones are atrocious.

author: Monkeh
author_rank: Super Contributor
timestamp: February 15, 2015, 05:20:06 pm
content: Quote from: rob77 on February 15, 2015, 12:26:01 pmYou think China cares? There's lots of cheap garbage out there.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 15, 2015, 07:57:49 pm
content: I have a wall wart here that I am using for odd tasks. Rating on it is 6V @ 500 mA. The unloaded output is more like 9V. With a 220 uF filter cap and a 20 mA load, voltage drops to ~8V. You may be able to use something like this, but for a solid 5V, you might need a regulator in the chain.

author: samintheoc
author_rank: Newbie
timestamp: February 15, 2015, 08:57:47 pm
content: Thanks all for your insight.When my power supply says it outputs current of 0.7A what does that mean? Cause one gets different currents through a circuit depending on what resistors or components are in series with the power supply.Quote from: zapta on February 15, 2015, 02:22:24 pmReally? I also have red LEDs that have a max current of 20mA which turn on when I flip my switches. These are in series with a 330 ohm resistor. They won't burn out if I take out the 330 ohm resistor? Also the 1k ohm resistor? Taking that out my circuit might put the components in danger no? Thank you.Do LEDS have an internal resistance?I'm waiting on a multimeter so I can't take any direct measurements. Just trying to infer if I am in the proper range by seeing whether or not my an LED burns out.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2015, 09:56:08 pm
content: Quote from: samintheoc on February 15, 2015, 08:57:47 pmIt means it can supply up to 0.7 A while maintaining correct operation. The exact current used by the circuit depends on how much the circuit needs. Devices like logic gates are designed to take just as much current as they need and no more.LEDs are different from normal circuits. The can't control how much current they consume and so you need a resistor in series with them to limit the current. The LED will definitely burn out if you take the resistor away.As for the phone charger, its suitability depends on whether it is a USB cube or some older and proprietary kind. As long as you use a proper USB phone charger cube from a respected manufacturer it will have a regulated 5 V on the output. As mentioned above, you can use one of them by making a flying lead USB cable to plug into your breadboard.When you tell us about your Samsung phone charger we don't know exactly what it is, so it is hard to say whether it is suitable or not. Some older phones used an unregulated charger and put all the regulation electronics inside the phone.

author: zapta
author_rank: Super Contributor
timestamp: February 15, 2015, 11:42:35 pm
content: If your components are rated for 5 volts than you don't need a resistor. Regular Leeds are not rated for 5v and this requires a current limiting resistor.