Electrical Engineering Stack Exchange is a question and answer site for electronics and electricalengineering professionals, students, and enthusiasts. It's 100% free, no registration required. log in Power supplies are available in a wide range of voltage and current ratings. If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to specify? What if I don't know the device's specs, but am replacing a previous power supply with particular ratings? Is it OK to go lower voltage, or should it always be higher? What about current? I don't want a 10 A supply to damage my 1 A device. voltage current edited Jul 7 '12 at 0:02 Ryan Edwards 103 2 asked Jun 28 '12 at 21:54 Olin Lathrop 136k 14 144 326 29 I think I've seen this question before ;) . All in favor of making this the canonical "consumer electronics power supply question" vote +1! – W5VO ♦ Jun 28 '12 at 23:05 3 Not sure. I wonder if the wording might not be clear to the kind of person who needs to ask this kind of question. – Rocketmagnet Jun 28 '12 at 23:53 3 Wording might need adjusting, but I agree with the idea I think we could definitely do with a Community Wiki on choosing the right power supply. – Oli Glaser Jun 29 '12 at 1:29 I have a tricky one that you didn't think of. Model:ELP03PS1; PRI:220240V~50/60 Hz 0,06A; SEC:Constant Current: DC350mA 0,5V...7,2V; Constant Voltage: DC8.5V I <= 300mA; tc:max. 75°C; – jippie Jun 29 '12 at 17:55 4 Whoever downvoted this question: What exactly do you think is poorly asked, unclear, ambiguous, etc? We get questions closely related to this regularly, so the purpose was to write a detailed answer once that we can point to in the future. This question and answer was essentially solicited in the meta post meta.electronics.stackexchange.com/a/1268/4512 – Olin Lathrop Jul 7 '12 at 20:30 tour help Take the 2minute tour × Choosing power supply, how to get the voltage and current ratings? powersupply sign up A 9 volt 5 amp supply is a superset of a 9 volt 2 amp supply. does. unlike with voltage. you don't know what might happen. he's only going to eat two whether you put 2. but the higher than intended current can damage something. Replacing Existing Supply . Both lower and higher could be bad. not what it will always force thru the load somehow. In that sense. When you violate required specs. but damage can't be ruled out unless you know something about the device. so it just sits there getting hot. A constantvoltage supply doesn't determine the current: the load. lower voltage will just make a device not work. Electrical components all have voltages above which they fail. However. or 20 A. A device that wants 2 A of current works the same way. then you have to assume it needs that voltage. If the device has a motor. with lower voltage the device will not operate correctly in a obvious way. so picking the right voltage tolerance for the components in the device probably got significant design attention. At best. If Johnny wants to eat two apples. Take what a device says on its nameplate seriously and don't give it more voltage than that. Most of the time. Some devices might draw more current to compensate for the lower voltage. 5. which in this case is the device. It will draw 2 A whether the power supply can only provide the 2 A. for example. then the motor might not be able to develop enough torque to turn. Some devices can even be damaged by too low a voltage for extended periods of time.4 Answers Voltage Rating If a device says it needs a particular voltage. some devices might appear to operate correctly. for example. Applying too much voltage violates the design assumptions. 5. 3. The current rating of a supply is what it can deliver. but you don't know where that level is. Current Rating Current is a bit different. Higher than specified voltage is definitely bad. the current rating of a power supply must be at least what the device wants but there is no harm in it being higher. or whether it could have supplied 3. Components rated for higher voltage generally cost more or have less desirable characteristics. Some level of too much voltage will damage something. then fail in unexpected ways under just the right circumstances. or 20 apples on the table. If you are replacing a previous power supply and don't know the device's requirements. such a "12 V" DC supply might make 12 V at 110 VAC in. Another issue with unregulated supplies is that the output voltage not only is a function of the input voltage. if a unlabeled device was powered from a 9 V and 1 A supply. and then higher yet at higher input voltage. just step down the input AC (generally the DC you want is at a much lower voltage than the wall power you plug the supply into). for example. and the filter cap. then consider that power supply's rating to be the device's requirements. it's probably better to quit now. you can replace it with a 9 V and 1 or more amp supply. In most cases that is all you need to know to go to a store or on line and buy a power supply. Such a supply could easily put out 15 V. Years ago. but will also fluctuate with how much current is being drawn from the supply. Regulated versus Unregulated Unregulated Very basic DC power supplies. For example. like 110 V. For example. under some conditions. called unregulated. and call it a day. rectify it to produce DC. Advanced Concepts The above gives the basics of how to pick a power supply for some device. add a output cap to reduce ripple. but then would make over 13 V at 120 VAC in. Regulated Modern power supplies don't work that way anymore. A unregulated "12 volt 1 amp" supply is probably designed to provide the rated 12 V at full output current and the lowest valid AC input voltage. You can still get unregulated . Pretty much anything you can buy as consumer electronics will be a regulated power supply. and it assumes some basic understanding of electronics. the output voltage is dictated by the turns ratio of the transformer. This is fixed. so instead of making a fixed output voltage their output is mostly proportional to the input AC voltage. In these kinds of supplies. many power supplies were like that. If you're still a bit hazy on what exactly voltage and current are. so that was fine. Devices that needed the "12 V" were designed to handle that. They were little more than a transformer. It could be over 13 V at 110 V in at no load (0 amps out) alone. four diodes making a full wave bridge (takes the absolute value of voltage electronically). This section goes into more power supply details that generally don't matter at the consumer level. meaning the supply is designed and sold to be embedded into someone else's equipment where the right kind of engineer will consider this issue carefully. Jameco has wide selection of power supplies. For example. Current Limit All supplies have some maximum current they can provide and still stick to the remaining specs. These contain additional circuitry that can tweak the output voltage up and down. and are called universal input. that means all is fine as long as you don't try to draw more than the rated 1 A. Try asking for a unregulated wall wart at a consumer store that sells other stuff too. A regulated supply actively controls its output voltage. have a minimum load requirement. This generally means they can run from 90240 V AC. Their wall warts are specifically divided into regulated and unregulated types. There are various things a supply can do if you try to exceed the 1 A rating. This is done continuously to compensate for input voltage variations and variations in the current the load is drawing. and they probably won't even know what you're talking about. More and more supplies are being made like that. A regulated 1 amp 12 volt power supply. For example. it's not much harder to make the valid input voltage range wider and cover any valid wall power found anywhere in the world.supplies from more specialized electronics suppliers aimed at manufacturers. a 12 volt 2 amp supply with a minimum load requirement of 10% isn't guaranteed to work right unless you load it with at least 200 mA. you won't likely run into unregulated supplies. This restriction is something you're only going to find in OEM models. professionals. is going to put out pretty close to 12 V over its full AC input voltage range and as long as you don't draw more than 1 A from it. for example. However. This is usually 10% of full rated output current. and that can be 50 or 60 Hz. or at least hobbyists that should know the difference. For a "12 volt 1 amp" supply. Minimum Load Some power supplies. Universal input Since there is circuitry in the supply to tolerate some input voltage fluctuations. It could simply . I won't go into this more since this isn't going to come up on a consumer power supply. generally older switchers. unless you go poking around where the average consumer shouldn't be. blow a fuse. edited Jul 27 '14 at 1:50 Ricardo 3. the ripple is a direct function of the input AC. – Thomas O Jun 29 '12 at 7:58 4 @clabacchio and others: Yes. there will be some frequency at which the output oscillates a little. regulation versus not. However. Often the current limit is set a little higher than the rating to provide some margin. not catch fire. even a regulated one. To abuse the 12 volt 1 amp example again. the most likely response is that the supply will drop its output voltage to whatever is necessary to not exceed the output current. but everything should stay safe. and recover nicely once the excessive load is removed. The "12 V 1 A" supply might limit the current to 1.. can keep its output voltage exactly at the rating. This is called current limiting. there is a lot more that can be said about power supplies. otherwise the output voltage increases. Regulated supplies are usually switchers and therefore ripple at the switching frequency.293 5 17 46 answered Jun 28 '12 at 21:54 Olin Lathrop 136k 14 144 326 23 Personally. A device that is trying to draw the excessive current probably won't function correctly. or ripples. With unregulated supplies. ripple. The maximum ripple might not be at maximum output current. This question is aimed at people that are . for example. And. the ripple could easily be a volt or two at full load (1 A output current). A regulated 12 V 1 A switcher might ripple ±50 mV at 250 kHz. etc. Basic transformer unregulated supplies fed from 60 Hz AC will generally ripple at 120 Hz. Ripple No supply. some SMPS can have minimum load currents.1 A. I think Johnny shouldn't be so wasteful with his apples. The ripple of unregulated supplies can be fairly large. nowadays. etc. Usually due to the way the supply works. – Oli Glaser Jun 29 '12 at 0:25 1 +1! What about some tips on current limiting protection? – clabacchio ♦ Jun 29 '12 at 7:53 1 One thing to be aware of is old magnetic transformer plug packs have an optimal load range. for example. like current limiting. for example. Specialty OEM supplies that are stripped down for cost could catch fire or vanish into a greasy cloud of black smoke. minimum load issues. low load issues. so let's keep it simple here. Olin explained with fruit. it doesn't mention . If the device says 500 mA. even if you only need 500 mA. The right voltage and type of voltage are important: a too high voltage may damage your device. From a 1000 mA supply.worried their 10 A supply will kill their 2 A device. Start another question with power supply nuances like those mentioned and more. – jippie Jun 29 '12 at 20:43 The three main parameters for a power supply are voltage type of voltage: AC or DC current If your device or broken wall wart says 9 V DC. On several electronics sites I see enthousiasts like us overwhelming visitors with so much information and details. – Olin Lathrop Jun 29 '12 at 11:44 3 @variousResponders: I think that it is important to realize that people asking a question like this obviously don't know what current is. The faucet still can supply 10 liter per minute. That's the 1000 mA the wall wart specifies. I'll try to explain with another flow: water. it is a pitfall of my own too. let alone current protection. A 1000 mA is safe.com/questions/34754/… says 12V(AC) and 35VA. Same goes for 'old magnetic' vs. If my faucet can fill a bucket in a minute that's its maximum flow. a too low voltage too. I know all too well. If I want only a few drops to fill a thimble I'll be a 1 mA device. 'SMPS' and 'ripple'. or current. but will supply less if I ask for less. A 100 mA wall wart may overheat and set your house on fire if not properly protected. I can ask for anything as long as it isn't more than the 10 liter per minute. get a power supply that can deliver at least that. and I'll be happy to upvote. Yet I can open the faucet partly to fill my glass. But at a too low voltage it may not work properly. Get a power supply at minimum the rated current. and then I'm the 500 mA device. answered Jun 29 '12 at 15:07 stevenvh 110k 9 335 552 The transformer in electronics. so much that they leave and probably never dare to return.stackexchange. though that's less common. get a 9 V DC replacement. . In this case. The original monitor PSU (PA165068) was fixed at 19V and 3. but for the sake of this discussion what if it was? – jippie Jun 29 '12 at 17:46 A related question came up recently: I'd like to power my new LG M2450D monitor with a basicXL BXLNBTU02 universal AC DC power adapter. AC/DC: Power adapters can output either alternating current or direct current.75A.current at all. a "=" with a dashed line means DC. and sometimes it is the opposite.32A.5k 6 58 110 If you are replacing a "wall wart" with a salvaged one then there are a couple more things to be aware of in addition to what the other commenters have already said. what is "safety margin" i should assume for the real max amperage? To be conservative in the face of ambiguous specifications like these. so you should be good to go with the output set at 19V with this amount of current. You must make sure it matches what the device expects. Now. you should assume that the maximum power rating of the universal adapter occurs at the maximum output voltage. You can tell by looking at the symbols: a "~" symbol means AC. I know it isn't an adapter. answered Sep 23 '12 at 0:48 Dave Tweed ♦ 54. Polarity: If you are using an adapter with the round barrel jacks. and that all lower output voltages are limited to the same current.74A Of course this is an ideal value. doing the math: 90W / 19V = 4. This PSU is rated 90W max and supports 1524V output voltage. Most adapters and devices are DC. then be aware that sometimes the centre is positive and the outside is negative. 90W/24V = 3. just cut the jack off and swap the wires around. answered Aug 3 '14 at 9:44 MrZebra 248 1 6 protected by Community ♦ Sep 20 '13 at 5:58 Thank you for your interest in this question. Usually it's "centre positive". If it doesn't match then the fix is simple. posting an answer now requires 10 reputation on this site. Would you like to answer one of these unanswered questions instead? . Because it has attracted lowquality answers. There will be a symbol on the adapter showing which is positive and negative.This must match what the device expects. or it could damage the device.