Choosing an LM317 Adjustable Voltage Regulator - Page 1
Forum section: electronics / projects

author: aidan2496
author_rank: Contributor
timestamp: April 30, 2015, 11:37:24 am
content: Hey,I'm a beginner and I'm really confused about what Potentiometer to choose to decrease power from 12v?I want to get a 10 turn Potentiometer, so i searched it up on ebay and that was all good, until I came across that you had to choose the one you wanted by ohms.My choices were:1K 2K 5K 10K 20K 50K 100KSo the power that will be going into it will be 12 volts (MAX-18A) and all I want to do, is be able to vary the voltage from 0-12v within 10 turnsSo if anyone can find out or help me choose what one to get, it would be great!http://www.ebay.com.au/itm/1-2-4-5pcs-3590S-500-1K-2K-5K-10K-20K-50K-100K-Rotary-Potentiometer-Pot-10Turn-/151400715289?pt=LH_DefaultDomain_15&var=&hash=item23402f9419Thanks,Aidan

author: Vgkid
author_rank: Super Contributor
timestamp: April 30, 2015, 11:45:14 am
content: How much power does your load need to draw.

author: mikerj
author_rank: Super Contributor
timestamp: April 30, 2015, 11:59:59 am
content: Quote from: aidan2496 on April 30, 2015, 11:37:24 amIf you mean a maximum of 18 amps, then you can not possible use a little 10 turn potentiometer to do this. It sounds like you are looking for a variable voltage regulator, but something capable of handling 18 amps will not be cheap.

author: aidan2496
author_rank: Contributor
timestamp: April 30, 2015, 12:26:13 pm
content: Quote from: mikerj on April 30, 2015, 11:59:59 amOkay . If i were to put 12v into one of those Potentiometers what one would let me vary the voltage from 0 to 12v and what would be the limit for how much power i could put through it?sorry if i've got this all wrong(by the way this will be used as a power supply for small things)

author: mariush
author_rank: Super Contributor
timestamp: April 30, 2015, 12:29:23 pm
content: I think you intend to use a potentiometer as a simple voltage divider to get an output voltage depending on where the potentiometer is set.For lots of reasons, that's not going to work. In addition, it wouldn't work for more than a few mA of current, because the potentiometer is not built to handle lots of energy going through it.The way you would do this is by using an adjustable linear regulator and replace one of the resistors that sets the output voltage of the linear regulator with a potentiometer.Have a look at a datasheet for a linear regulator, for example have a look at LM317 and here's the datasheet for the one made by Texas Instruments :http://www.ti.com/lit/ds/slvs044w/slvs044w.pdfLook at section 9, "Application and Information", there you can see a circuit showing how to use a potentiometer to adjust the output voltage. If you replace R1 in that example with a 100 ohm resistor (anything reasonable below 240 ohm will work) then you could probably use a 1 kohm potentiometer to get a wide range of voltages at the output.For 1000 ohm , you'd have a maximum output voltage from the linear regulator equal to 1.25v ( (1 + R2 / R1) = 1.25 ( 1 + 1000 / 100) = 1.25 x 11 = 13.75vNow of course, the linear regulator won't output 13.75v unless the input voltage is high enough, about 15v. With just 12v at the input, your range is about 1.25v - 11v.As for how much current, a linear regulator like LM317 is capable of 1-1.5A which should be enough for most experiments. If you really want up to 18A, it's not a smart idea to use linear regulators but it is possible with other regulator chips installed in parallel, or using some tricks which are not the point of this reply.

author: Ian.M
author_rank: Super Contributor
timestamp: April 30, 2015, 12:35:29 pm
content: A potentiometer is only any use on its own for supplying a DC voltage if the load current is small in comparison to the current through it. For an ordinary 270 deg rotation pot, small would be at least an order of magnitude less, e.g. from your 12V supply, 1mA from a 1K pot, but for a multi-turn pot, your maximum load current is several orders of magnitude lower. e.g. 0.1mA or less, otherwise the voltage variation due to any changes in the load current will be excessive.The only way of using a multiturn pot to do what you are asking is to use it to set the output voltage of a regulator.If you use a simple linear regulator it will dissipate over 100W at half output voltage if your load is simply resistive and draws 18A at the full output voltage of 12V. That's going to need a big heatsink and a cooling fan and wont be a simple one chip solution.The commonly available switching regulator modules (as found on EBAY + various 'maker' suppliers) tend to be in the 2A to 5A range, although a few 10A ones are available, and certainly wont handle an 18A load.In all cases, the regulator circuit will have a limited range of pot values it will work with so either you need to get the same value to replace the on-board preset on a pre-built module or if building a regulator from scratch you need to do all the design calculations before ordering.If you tell us what your 18A max load actually is, and what you hope to gain by varying the voltage to it, we may be able to suggest an easier and more economical method of getting a similar result.

author: aidan2496
author_rank: Contributor
timestamp: April 30, 2015, 12:50:14 pm
content: Thanks!The 18 amp max is just the maximum power output from my 12v transformer. What would you suggest would be enough amps to power small little things like circuits or leds or things like that?And I dont have the type of skills to create something, so is there anything out there that I can buy pre built to vary the power on little things like circuits and leds with a max amp rate?If I have this all wrong just point it out because I have another simple design that I can use instead of varying the voltageBut thankyou so much for the help!It's really goodThanks,Aidan

author: Ian.M
author_rank: Super Contributor
timestamp: April 30, 2015, 02:09:28 pm
content: I certainly wouldn't start with a 18A 12V transformer. It would be useful if you were into mobile HAM or CB radio or large 12V lighting projects, but for typical breadboarding its got far too much current capability and its difficult to add adjustable current limiting without either a fairly complex circuit or a significant voltage 'headroom' above the desired maximum output voltage.If I were you and I couldn't afford a proper bench PSU, I'd probably get a module like this one:http://www.ebay.co.uk/itm/DC-Buck-Step-Down-Converter-DC-5A-Regulator-Voltage-Current-Adjustable-DE-/321439418999, remove the presets and replace them with front panel pots. Power it from an old laptop PSU that outputs between 18V and 22V. A digital panel meter for output voltage and current would be a useful addition. Avoid the modules with built in meters as they are normally fairly crummy. You should add a series resistor to the high end of the set current pot to limit the maximum output current to no more than 90% of the laptop PSU's rating to prevent expensive mistakes.If you are working with logic circuits, another module set to give a fixed 5V (or 3.3V) could be added in the same box, but drop the max current output of the adjustable module to 70% of the laptop PSU rating to avoid overloading the PSU if you are running both the 5V and adjustable outputs at max current.A linear regulator like the LM317 suggested earlier is better for small signal analog circuits as the output of a switcher module can be rather noisy, but there is no reason you cant hang one off the output of the switcher module when you need a smoother low current supply. Set the switcher to about 2V to 3V more than you want out of the linear regulator.

author: aidan2496
author_rank: Contributor
timestamp: April 30, 2015, 09:23:02 pm
content: Thanks for you're help! Ill figure something out using what you've said. Great help!Thanks,Aidan

author: aargee
author_rank: Frequent Contributor
timestamp: May 01, 2015, 01:22:12 am
content: I'm assuming you have little electronics knowledge but you're eager to learn!I would suggest you have a read of Ohm's Law (https://en.wikipedia.org/wiki/Ohm%27s_law)Just have a read and try to understand the Voltage-Current-Resistance relationship a bit better, once you do these sorts of problems will be a lot clearer to understand.

author: ez24
author_rank: Super Contributor
timestamp: May 01, 2015, 02:31:14 am
content: I am going to start a new topic onQuotetitle will beDC Buck Step Down Converter DC 5A Regulator Voltage Current Adjustable DEso I hope we can get more info on thie Buck Step Down Converter

author: aidan2496
author_rank: Contributor
timestamp: May 01, 2015, 03:49:33 am
content: Quote from: aargee on May 01, 2015, 01:22:12 amThankyou! You're right im new into ohms and currents law so that will be helpfulThanks!

author: Fungus
author_rank: Super Contributor
timestamp: May 01, 2015, 01:28:29 pm
content: Quote from: aidan2496 on April 30, 2015, 12:26:13 pmYes.Quote from: aidan2496 on April 30, 2015, 12:26:13 pmNot very much at all.Quote from: aidan2496 on April 30, 2015, 12:26:13 pmNo.What you really need is an adjustable voltage regulator, eg. the LM317. This will let you power small circuits with varying voltage using just a few extra components. They can get them for about 25 cents from most electronics shops (be sure to get a heatsink as well!)eg.http://pcbheaven.com/opendir/index.php?show=238fv1024rc6f8c2d63

author: aidan2496
author_rank: Contributor
timestamp: May 02, 2015, 04:49:27 am
content: Hi guys,Thanks for your help! I've been looking at videos and info about the LM317 and I found this on ebay:http://www.ebay.com.au/itm/LM317-AC-DC-to-DC-Adjustable-Voltage-Regulator-Step-down-Power-Supply-Module-/141599806445?pt=LH_DefaultDomain_15&hash=item20f80193edI'm happy with the specs and I'm also happy with having a max of 2.2 amps out and I don't really mind about the drop out voltages.So is this the type of circuit I could use to vary the 12v power?Thanks,Aidan

author: Rerouter
author_rank: Super Contributor
timestamp: May 02, 2015, 05:13:28 am
content: You could, but not at 2.2A for very long if you are dropping more than about 3 Volt, (LM317 has thermal limiting, if it gets too hot it reduces the current on the output)This comes back to how a linear regulator works, it gives off the voltage dropped by it as heat, say going from 12V to 5V, it would be dropping 7 Volt, 7 Volt times 2.2 Amp is about 15 Watts of heat being given off by the regulator, the heatsink used may not be large enough to get rid of this wattage causing its thermal limit,For a few hundred milliamp, it would be fine, say a project with 10 led's a micro and a lcd,

author: aidan2496
author_rank: Contributor
timestamp: May 02, 2015, 05:58:08 am
content: Quote from: Rerouter on May 02, 2015, 05:13:28 amOkay thanks. Sounds pretty good! I'll do some more research and I'll see what happens!

author: ez24
author_rank: Super Contributor
timestamp: May 02, 2015, 06:45:34 pm
content: I took Ian.M advice and bought a few of these:http://www.ebay.com/itm/400714751657?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITand for more power thesehttp://www.ebay.com/itm/111299051606?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AITI believe these will give you more optionsBUT do not believe any specs you see on ebay. Ask and if no answer you will have to do your own testing.Look at the specs for the above : one has 5 amps with no heat sink and one has 2 amps with a heat sinkI am going to use an infrared thermometer and check the heat of the components against their datasheets.The 2 amp model has LEDs that can help a beginner like us.I lived in Taiwan and went to a electronics company and asked them how much power an amp had that they made. They said whatever I want, ie they will put anything on itSo you can get an audio amp that you can put in your pocket that they say puts out 1,000 watts ( ie a 1 watter)

author: atferrari
author_rank: Frequent Contributor
timestamp: May 03, 2015, 01:23:03 am
content: Quote from: aidan2496 on April 30, 2015, 11:37:24 amThe title of the thread and the above do not match.

author: paulie
author_rank: Frequent Contributor
timestamp: May 03, 2015, 03:56:09 am
content: LOL. And it was kinda funny to see comments about 18amps and using a pot to regulate.If low cost is a factor the basic adjustable 2596 buck converters are available for less than a buck unlike some of the suggestions here. Not only orders of magnitude more efficient but wider voltage range and actually cheaper than those LM317 modules. Current may not be fine tune adjustable but is inherently limited.BTW I would like to see a photo of the 18a "transformer".

author: Vgkid
author_rank: Super Contributor
timestamp: May 03, 2015, 04:57:57 am
content: Quote from: paulie on May 03, 2015, 03:56:09 amhere you go, 12v@20Ahttps://system.netsuite.com/core/media/media.nl?id=12583&c=ACCT126831&h=53ea07482f4b0858bea7&_xt=.pdf

author: paulie
author_rank: Frequent Contributor
timestamp: May 03, 2015, 05:24:26 am
content: Thanks but aside from the fact that's 20a not 18a, what I was really curious about was the one from OP. I don't think 18a transformers are not possible, there are a few dozen 160a in my basement. Mostly it would be interesting to see if it actually was a transformer and not a PSU and if so what type.

author: paulie
author_rank: Frequent Contributor
timestamp: May 03, 2015, 05:30:20 am
content: Judging from subsequent posts it wouldn't be a surprise if it turned out to be an 18va power supply and not 18a transformer at all. Big difference.BIGdifference!

author: atferrari
author_rank: Frequent Contributor
timestamp: May 03, 2015, 12:29:09 pm
content: Quote from: aidan2496 on April 30, 2015, 11:37:24 amHola Aidan,Sorry for posting now (maybe late).I am not sure if you undesrtand that whatever you do (build or buy) a common potentiometer, by itself would not be useful to drop some voltage if more thanvery fewmilliampers are involved.You mentioned the LM317. There, the pot could control the beast doing that.I suggest you trying to learn three things related to this:Ohm's law.What is voltageWhat is powerI am affraid you are confusing the last two.Do not worry. I still recall a pot on my bench and lot of smoke around it. That day I learnt something. (And that pot still (somewhat) worked after that!)