LM337 Voltage Regulator - Page 1
Forum section: electronics / projects

author: otpowell
author_rank: Regular Contributor
timestamp: May 13, 2015, 10:25:50 pm
content: Hi all, I bought an LM337 and I was under the impression that I could input a positive DC voltage and get a negative DC voltage out. I hooked up this circuit many times but always get a positive voltage out. I'm hooking the circuit up exactly like the schematic and made sure my capacitor polarities are correct. Am I using this wrong? Am I really supposed to be feeding a negative DC voltage in? That's what the data sheet illustrates. I guess I'll have to use a bridge rectifier and feed an AC voltage in to get a negative DC voltage out..O.T.Electrical & Electronics EngineeringSent from my iPhone using Tapatalk

author: kjs
author_rank: Regular Contributor
timestamp: May 13, 2015, 10:43:26 pm
content: this is a linear regulator and not a switchmode inverting one.You input a negative voltage and it outputs a regulated negative voltage.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 13, 2015, 10:44:56 pm
content: Yes, absolutely wrong. It's a linear regulator, it can't do anything about the direction of the current. And your capacitors will surely fail if they're reversed electrolytics.What's a "beige" rectifier?

author: cpt.charlie
author_rank: Contributor
timestamp: May 13, 2015, 10:56:58 pm
content: Note that the schematic says-VinnotVinit can't change the polarity.

author: AG6QR
author_rank: Frequent Contributor
timestamp: May 13, 2015, 11:10:54 pm
content: Others have it correct, as the datasheet says. It may be worth pointing out that the LM337 is exactly analogous to the more popular LM317. The LM317 takes a positive voltage as input and outputs a lower magnitude, but regulated, positive voltage. The LM337 takes a negative voltage as input and outputs a lower magnitude, but regulated, negative voltage.Both of them behave as though they are "smart" variable resistors between Vin and Vout, as though there's a magical elf inside watching a voltmeter and quickly dialing in just the right amount of resistance to drop the voltage to exactly the desired value. I don't think they really put elves inside, but the point is that, as a linear device, the current in is equal to the current out (with a small correction factor due to the current through the ADJ pin), and there is no voltage boosting nor polarity reversing.

author: otpowell
author_rank: Regular Contributor
timestamp: May 13, 2015, 11:11:05 pm
content: Quote from: kjs on May 13, 2015, 10:43:26 pmGreat, thank you! I'll get a switch mode regulator. !O.T.Electrical & Electronics EngineeringSent from my iPhone using Tapatalk

author: otpowell
author_rank: Regular Contributor
timestamp: May 13, 2015, 11:12:07 pm
content: Quote from: cpt.charlie on May 13, 2015, 10:56:58 pmI noticed this later, I'm not sure why I thought I could input a positive voltage and get a negative out which he data sheet clearly states. Thanks for the info!O.T.Electrical & Electronics EngineeringSent from my iPhone using Tapatalk

author: otpowell
author_rank: Regular Contributor
timestamp: May 13, 2015, 11:13:16 pm
content: Quote from: Alex Eisenhut on May 13, 2015, 10:44:56 pmI meant bridge rectifier. I found this circuit whereI can use AC and get negative DC out, or so I think.O.T.Electrical & Electronics EngineeringSent from my iPhone using Tapatalk

author: otpowell
author_rank: Regular Contributor
timestamp: May 13, 2015, 11:19:08 pm
content: Quote from: AG6QR on May 13, 2015, 11:10:54 pmThank you very much! I actually just learned that in class about linear devices. Guess I should go back and read that chapter a bit. Thanks guys I really appreciate it.O.T.Electrical & Electronics EngineeringSent from my iPhone using Tapatalk

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 13, 2015, 11:40:10 pm
content: Quote from: otpowell on May 13, 2015, 11:13:16 pmOh of course, bridge. Well, you could do it with just one diode too, it depends on what you need.

author: paulie
author_rank: Frequent Contributor
timestamp: May 13, 2015, 11:46:27 pm
content: Quote from: Alex Eisenhut on May 13, 2015, 10:44:56 pmMuch lower Vf compared to teal. Chartreuse are worst of all in that respect.

author: otpowell
author_rank: Regular Contributor
timestamp: May 14, 2015, 12:32:27 pm
content: Well guys, what's really the easiest way to get a negative voltage? By using AC, a rectifier, linear regulator and some caps and resistors or the voltage divider method, or using a inverting regulator? Which method uses the least amount of components? I'm surprised that not many DC power supplies provide a negative voltage. I know if you switch your polarities with a positive voltage the multimeter will read negative but that isn't the same as a negative voltage supply, is it?

author: cpt.charlie
author_rank: Contributor
timestamp: May 14, 2015, 03:16:04 pm
content: The polarity depends of what point you consider as ground, that's all. The best method to get a negative voltage depends of your requirements (current, voltage...)

author: paulie
author_rank: Frequent Contributor
timestamp: May 14, 2015, 03:39:49 pm
content: Quote from: otpowell on May 14, 2015, 12:32:27 pmBy far the easiest and simplest with an AC source is a single diode and a single cap. "Bridge rectifier" and "voltage divider" are not capable of generating negative voltage. You should also be aware most internet suggestions will involve the most expensive and complicated method, not the simplest.

author: otpowell
author_rank: Regular Contributor
timestamp: May 14, 2015, 03:43:43 pm
content: Quote from: cpt.charlie on May 14, 2015, 03:16:04 pmMostly using it for labs and biasing op amps. Usually just minimal current.O.T.Electrical & Electronics EngineeringSent from my iPhone using Tapatalk

author: Kalvin
author_rank: Super Contributor
timestamp: May 14, 2015, 03:51:42 pm
content: Quote from: otpowell on May 14, 2015, 12:32:27 pmI guess the easiest way is to create virtual ground between the positive and negative rails. This is quite handy if the current requirements are modest or the efficiency is of no concern.With a transformer you can create positive and negative rails quite easity either with center tapped transformer or using a half-wave rectifier circuit. If the negative voltage requires only modest current, a simple negative charge pump using diodes and capacitor may do the trick.In microcontroller systems one may even use a PWM output of an MCU to create needed negative voltage for op amp circuit from the positive voltage.

author: otpowell
author_rank: Regular Contributor
timestamp: May 14, 2015, 04:56:33 pm
content: Quote from: Kalvin on May 14, 2015, 03:51:42 pmMay I ask, How would one use PWM to create a negative voltage? I have an Arduino and a UDOO, so I have PWM capability. I also have a function generator that supplies up to -10 volts. I would need to know how to amplify that -10 in case I need -15 for an op amp circuit.

author: Kalvin
author_rank: Super Contributor
timestamp: May 14, 2015, 05:56:28 pm
content: Quote from: otpowell on May 14, 2015, 04:56:33 pmYou can create a simple negative (or positive) charge pump using capacitors and diodes. And, using the MCU's A/D-input to measure the output voltage, you can even regulate the output by varying the PWM duty cycle.With a single stage, you should be able to get -4V from the +5V PWM output. With a dual stage, you should get -5V quite easily. Best of all, you can hook the charge pump to the -10V power supply and get the -15V of few milliamps.Here's a few circuits using 555 to generate positive an negative voltages, but the idea is similar when using Arduino:http://www.eleccircuit.com/the-many-dc-to-dc-converters-using-ic-555/See this Dave's video (dealing with the positive voltage pump, but it is applicable also to the negative charge pumps as shown in the 555 circuit collection) for further explanation:Here's Dave's more fundamental video talking about the practical limitations:

author: otpowell
author_rank: Regular Contributor
timestamp: May 14, 2015, 06:24:50 pm
content: Quote from: Kalvin on May 14, 2015, 05:56:28 pmThat's awesome, thank you so much! I have a bunch of the 556 chips too!

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2015, 06:41:36 pm
content: Quote from: otpowell on May 14, 2015, 12:32:27 pmThere's no such thing as a "negative" voltage. That's why you don't find any DC power supplies that output a negative voltage (and also why you don't find any left-handed screwdrivers).Voltage is measured (and defined) as the difference between two potentials. If you have two potentials "A" and "B", then "A - B" has one sign, and "B - A" has the opposite sign. The difference has the same magnitude either way.It just so happens that for convenience people often choose to assign a value of 0 to B and reference everything to that. If you have a voltage:V = A - BThen by assuming B = 0 you have:V = A - 0 = AThis may lead you to think that "A" is a voltage, but it is not really. A voltage V is still a difference from some reference and you have to keep straight what your reference is.Always just remember that in circuits, voltages are differences and currents are flows.

author: otpowell
author_rank: Regular Contributor
timestamp: May 14, 2015, 07:35:25 pm
content: Quote from: IanB on May 14, 2015, 06:41:36 pmThank you for your explanation. I understand the potential between two points concept. So what is it then you are reading when you get a reading like -15 V when referenced to zero ground? Also, what am I to do if a op amp circuit requires +15 volts on one DC input and -15 on the other? Would that be the same as 30 V on the positive input and zero on the negative DC input? Thanks IanB I appreciate it.

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2015, 08:46:08 pm
content: Quote from: otpowell on May 14, 2015, 07:35:25 pmWell zero ground doesn't have any real existence; it is just a choice made by you or the circuit designer when deciding where to measure voltages from. It's a bit like "putting a stake in the ground". If you put a stake in the ground and take your tape measure in one direction you might go a distance of 100 ft. If you went the same distance in the opposite direction you could call it -100 ft. But always your distances are relative to where you put the stake. You could put the stake anywhere...(It turns out the power companies like to put a real stake in the ground and attach their transformers to it, making the earth beneath our feet their chosen ground reference. But this has little to do with signal processing and electronic circuits.)So for the op amp, it doesn't know the difference between +15 V on one power rail and -15 V on the other. It could as well be 30 V and 0 V. This is because (+15) - (-15) = (+30) - (0).The reason for using +/- supplies with op amps is not for the op amp, it is for your signal. Op amps are signal amplifiers, and signals are usually arranged to be small differences around a reference point. The op amp will work much better if the reference point is somewhere in the middle of its range and not out near the edges. So if you arrange a power supply as -15 -- 0 -- +15 V you get a nice convenient 0 right in the middle, which is a perfect place to put your signals. There are other ways to put a zero reference in the middle of the range, but they are not always so convenient.

author: otpowell
author_rank: Regular Contributor
timestamp: May 14, 2015, 08:49:41 pm
content: Quote from: IanB on May 14, 2015, 08:46:08 pmOk I get it, thank you! That was the best explanation! So all I need is a 30v power supply, stake it and add a voltage divider and I'll have my +15 and -15. Perfect l!

author: AG6QR
author_rank: Frequent Contributor
timestamp: May 14, 2015, 09:21:48 pm
content: Quote from: otpowell on May 14, 2015, 08:49:41 pmIf you use a voltage divider to split 30V in half, generating +15 and -15, just remember that the voltage divider can't sink or source any current. If you try to put current into or out of a voltage divider, the voltage will change. The voltage divider equation just comes from Ohm's law, but with the assumption that the current in the top and bottom half of the divider is equal, which it won't be if current is entering or leaving.There are "rail splitter" ICs built specifically for this purpose -- they can source or sink current, within their limits. A voltage divider followed by an appropriate buffer (maybe an op amp) will do the trick, and that's more-or-less what's inside a "rail splitter" IC.

author: cpt.charlie
author_rank: Contributor
timestamp: May 14, 2015, 10:20:18 pm
content: Use the output of a linear regulator LM78xx as the circuit "ground"

author: AG6QR
author_rank: Frequent Contributor
timestamp: May 15, 2015, 12:19:59 am
content: Quote from: cpt.charlie on May 14, 2015, 10:20:18 pmThat can source current, but it's probably not so good at sinking it. Often you want the ground of split supply to be able to do either.

author: larsdenmark
author_rank: Regular Contributor
timestamp: May 15, 2015, 08:31:04 am
content: Quote from: AG6QR on May 13, 2015, 11:10:54 pmI love the way you describe how it works internally. Not just elves, but magical elves!Do I understand this correctly that the LM337 actually doesn't know that you want to have a negative voltage? Also the LM317 doesn't know that you want to have a positive voltage (since the definition of positive and negative depends on the rest of the circuit). The defining qualities of the LM317 is that is sources current and the LM337 sinks current.If my understanding is correct you can't use a LM317 to give you 5 V and another LM317 to give you 2V and connect the two outputs with a resistor (without breaking something).

author: paulie
author_rank: Frequent Contributor
timestamp: May 15, 2015, 10:05:24 am
content: As hinted earlier none of these solutions are valid for a practical multi-voltage hobby supply. By far the easiest is to use an old ATX supply with +-12v +-5v and 3.3v. Next best idea would be old laptop bricks or those $2 Ebay wall wart 12v switchers which will provide excellent high current isolated plus or minus voltage.http://www.ebay.com/itm/AC-100-240V-DC-12V-1A-5-5-x-2-1MM-Wall-Charger-Power-Supply-Adapter-US-Plug-New-/181744106398?pt=LH_DefaultDomain_0&hash=item2a50cafb9eps. Adding a cheap adjustable $1 DC converter or LM317 will get you 3.3v-5v logic voltage.

author: tron9000
author_rank: Frequent Contributor
timestamp: May 15, 2015, 12:23:27 pm
content: ICL7660A? -https://www.intersil.com/content/dam/Intersil/documents/icl7/icl7660.pdf

author: paulie
author_rank: Frequent Contributor
timestamp: May 15, 2015, 01:36:51 pm
content: Those are unable to handle 12-15v and the HV version 7662 is too expensive. Anyway both do a miserable job at regulation anywhere more than couple milliamps. They fail totally around 8 or 10 which eliminates most op amp and other analog circuits.It is always surprising how the least expensive and most practical solutions are often rejected offhand.

author: macboy
author_rank: Super Contributor
timestamp: May 15, 2015, 01:44:18 pm
content: Quote from: otpowell on May 14, 2015, 12:32:27 pmIf you are starting with AC, then it is easy. Use a half wave rectifier for each the positive and negative supply. For example,look here.If you are starting with DC, then you can use a "charge pump inverter" or "switched capacitor voltage converter" IC to get a negative supply. This clever device first charges a capacitor across the DC supply, then disconnects it and connect it backwards at the output to charge an output capacitor to give an equivalent negative supply. It repeats this at a relatively high frequency (>>kHz). As you might suspect, there is some noise in the output as a result of this switching, but it is effective. If you start with 15 VDC, you could regulate down to 12 V to help clean up that noise. Many are designed for 5 V or lower, but some with with higher voltages like LT1054 at 15 V. The LT1054 loses only about 1 V at 50 mA.

author: paulie
author_rank: Frequent Contributor
timestamp: May 15, 2015, 01:58:37 pm
content: That first (one diode and one cap) as I suggested on page one is definitely the easiest and cheapest way. Unfortunately AC transformers are rare and costly these days. Charge pumps either have way too low current or cost more than other methods. As I said before a PC supply or Ebay $1 wall warts are the only practical solution for multi-voltage hobby supply.Of course buying one of those $150 triple lab supplies is technically "easiest" but sounds like OP is poor.

author: Kalvin
author_rank: Super Contributor
timestamp: May 15, 2015, 03:49:47 pm
content: I have played with few2W and 3W1W and 2W isolated +5V to +/- 15V DC/DC converters from eBay. These are handy when running the device from a +5V USB power supply and the op amps require dual power supply. They cost around 7 Euros ie. $9 each and you can get bipolar isolated +/-15V output (or +/-9V or +/-12V depending of the device model). However, the output voltages are unregulated and there is considerable amount of ripple. They require minimum load current to operate properly and they are good up to 30 - 50mA as the output voltage will droop as the load current increases. You may want to add some LC-filtering, capacitance multiplier, or linear LDO for less ripple and better regulation.There are also single output versions available in $5 range which will convert +5V input voltage to an isolated 5V, 12V or 15V. For example, the isolated 5V is handy if you need a negative -5V for an op amp. Or you can hook the isolated 5V output to the existing +5V power rail giving total of +10V power rail for op amps requiring higher than +5V operating voltage. The 15V version is nice as you can hook a LDO for better output regulation in order to get regulated 10V ... 12V.edit: Fixed errors with power output and money currency.

author: paulie
author_rank: Frequent Contributor
timestamp: May 15, 2015, 04:17:08 pm
content: I would say the Ebay wall warts are better for at least 3 reasons:1. One third total cost for stand alone operation.2. One or two amps instead of 50ma. For example many audio, RF, and wifi circuits like ESP8266 won't work.3. Very low ripple.DC power supplies are a major area of interest for me and a lot of time has been spent looking for good solutions. USB power is very convenient. Do you have a link to those isolated dual output modules? For some reason only very expensive ones are coming up in my search.

author: AG6QR
author_rank: Frequent Contributor
timestamp: May 15, 2015, 04:49:49 pm
content: Quote from: larsdenmark on May 15, 2015, 08:31:04 amThe idea comes from Horowitz & Hill's description of a simplified model for the way an NPN transistor works. I've forgotten whether the word "elf" is used, but the description involved a creature watching two current meters, and quickly turning a rheostat to make the two needles match, such that the current from collector to emitter is a fixed multiple of the current from base to emitter. It's been a long time since I've read that chapter, so please forgive any misquoting of details.QuoteThere is the fact that the 317 tries to maintain the Vout pin at 1.25VaboveVadj, while the 337 tries to keep Vout 1.25VbelowVadj. So there is a sign change involving voltage. But yes, you're right that there's also a sign change involving current, and this is important. The 317 must source a minimum amount of current in order to maintain regulation, and similarly the 337 must sink a minimum amount of current. If you connected two 317s together as you describe, one or both of them would likely fall out of regulation, at least, and maybe even be damaged. Not necessarily, though -- if you used small enough values of resistors in each regulator's network between Vout, Vadj and ground, and if you used a large enough resistor when you connected the two Vout terminals together, you could perhaps keep both regulators sourcing enough current. But you're absolutely right about the big picture point: both regulators would have to source current through their Vout pins to stay in regulation.

author: Kalvin
author_rank: Super Contributor
timestamp: May 15, 2015, 05:17:04 pm
content: Quote from: paulie on May 15, 2015, 04:17:08 pmPaulie,The isolated dc/dc-converters are 1W and 2W (not 2W and 3W as I wrote, sorry about that). I also recalled the price to be USD $7, but it was 7 Euros. Fortunately the price wasn't too much off due to my mistake with the currencies.Single output 5V/5V 1W, USD $2.50:http://www.ebay.com/itm/B0505S-1W-DC-DC-5V-Power-Supply-Module-4-Pin-Isolated-converter-/281463165342Single output 5V/12V 1W, USD $5.00:http://www.ebay.com/itm/DC-DC-1W-isolated-converter-5V-IN-12V-OUT-MORNSUN-B0512S-1W-/140716432162Dual output 5V to +/15V 1W, USD $9.00:http://www.ebay.com/itm/DC-DC-1W-isolated-converter-5V-input-dual-15V-output-MORNSUN-E0515M-1W-/130663289661DC/DC converter 2W isolated 5V IN +/-15V dual out, USD $13.The seller doesn't have the listing for the devices any more.Br,Kalvin

author: paulie
author_rank: Frequent Contributor
timestamp: May 15, 2015, 05:40:47 pm
content: Thanks. I was using bad search terms and couldn't get below $20. Those $9 ones are by far the most compact way to get dual low current op amp voltage from USB and will come in handy for certain applications. A little more shopping I was even able to knock another buck or so off.For general hobby use IMO an old PC supply or couple wall warts works out better. Also cheap 3 output surplus drive supplies but they are getting rare. There are cases where none of these will do (PCB mount, inside cable, etc.) and the tiny low power modules would be perfect. Thanks again for the links.

author: otpowell
author_rank: Regular Contributor
timestamp: May 16, 2015, 01:51:49 am
content: Quote from: AG6QR on May 15, 2015, 04:49:49 pmThanks great explanation. Thank you! I bought Art Of Electronics 3rd edition about two weeks ago and it's a great read. The "transistor man" the book calls it.O.T.Electrical & Electronics EngineeringSent from my iPhone using Tapatalk

author: otpowell
author_rank: Regular Contributor
timestamp: May 16, 2015, 02:00:41 am
content: Quote from: paulie on May 15, 2015, 01:58:37 pmRadio shack has a sale on a 25.2 V center tapped transformer that I'm going to pickup tomorrow for 7 bucks. I'm gonna mess around with the AC circuit and see what I can do. I have an old ATX supply and an old 3 output external hard drive power supply. I guess I really just want to build a circuit Instead of just using a wall wart. I like those isolated DC/DC converters, I might get one just to play around with.Thanks everyone for all the great information.O.T.Electrical & Electronics EngineeringSent from my iPhone using Tapatalk