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Linear Power Supplies and Regulators

By on November 30, 2014
Linear power supplies waveform









At a time when so many people write about SMPS I felt like doing differently…

Once again I will be the underdog’s advocate by writing about some technology which tends to be replaced by better and brighter things. Are linear power supplies and regulators going to disappear? Not so soon and probably never, let’s see why.

Linear Power Supplies have been around for many years. They have certain advantages, as we will see in this article, but one big disadvantage. They are inefficient, in many cases less than 50% efficiency; because of this they tend to be bulky and heavy.

SMPS are more efficient (some have more than 95 % efficiency), consequently small and light. You could not imagine using a linear charger for your mobile phone that would weight half a kilo and have the size of a brick; therefore the tiny thing, smaller than a pack of cigarettes, is much more like what you want. SMPS circuits are more complex than their linear counterpart but, manufactured in larger quantities, they became really cheap.

SMPS, however, have two disadvantages that Linear Power Supplies do not have:

1)    They are not always safe (see my earlier article on “Double Insulated SMPS”).

2)    They tend to generate Electromagnetic Interferences  (EMI)

I experienced the first problem when I was using a 12 VDC SMPS adapter for a microcontroller project. The microcontroller development board includes a programmer that connects to a PC through one of the USB port. In the semi darkness of my office I noticed a small spark when I connected the USB plug to my laptop. Investigating further let me wrote the article mentioned above, warning about the stray voltages in unearthed SMPS! Indeed, while unearthed SMPS are OK for charging your mobile phone they are not 100 % safe. They are banned from hospitals and dairy farms. It is also recommended that you do not use your mobile phone while it is connected to the charger…

I did not want to damage my laptop! So I needed a clean and safe 12 V power supply that I can use for the microcontroller experiments but also for some RF projects I was working on.

The first contender for this application was a small, light and compact SMPS that can deliver up to 3 A; this is much more current than what I needed but this additional current would be a plus. I remembered having tested this PS for stray voltages. The results were not good when the earth wire was not connected but, with the earth connected, my laptop would be safe. So I could live with that. The label indicates that it is CE certified so we can assume that it complies with the European EMC requirements. Electromagnetic compatibility would mean that it cannot generate more than a certain amount of interference. It also means that it is not too sensitive to interferences…

Linear power supplies

So let’s put it on the testing bench!  No too harsh, I loaded it with a 47 Ohm resistor, so it will only deliver a current of 250 mA. As it is connected to the Earth there is no problem of stray voltages.  But look at the output with and oscilloscope:

Linear power supplies repair

This noise, superposed on the DC 12 V, could also be heard in a shortwave receiver nearby (one of my projects); it was loud and all over the frequency ranges, therefore not acceptable for my RF projects…

It provided me with a good excuse for building a small 12 V power supply the old fashion, using a transformer, rectifier, couple of capacitors and a linear regulator.

Linear power supplies and regulators

Here it is, cheap and easy. It delivers a clean, stable and perfectly safe 12 V, almost as good as a battery. And it looks cute, doesn’t it?

The second problem with SMPS manifested itself by a high pitch noise coming from the speaker that was connected to my Laptop. The speaker has its own amplifier which needs to be powered through a USB port. As I needed to spare the laptop USB ports for other purposes I was using an external USB power supply for the speaker. The noise was caused by Electromagnetic Interferences generated by the USB power adapter. EMI are regulated internationally and SMPS must comply. However, sometimes the minimum allowed is still too much for some applications. Again, the easy solution was building a small linear power supply that I can also use as a USB charger and to power the USB oscilloscope.

Linear power supplies repairing

Like the previous one, by using a small transformer, a rectifier, a couple of capacitors and a linear regulator. In this case a 7805.

About linear regulators efficiency

A linear power supply can be built using only a few cheap and easily available components as shown below. The most expensive component would probably be the transformer.

linear power supplies diagram

Linear voltage regulators have been around for so long that there are many different types available. There are the fixed types such as the 78xx series (xx indicates the output voltage) and the adjustable types such as LM317 or LT1083 where we can set the desired output voltage with two external resistors. Now why are they so inefficient?


A series linear regulator can be seen as a variable resistor controlled by a feedback circuitry. It forms a voltage divider with the load. If the load varies, the feedback circuit will act on the “variable resistor” to keep the output voltage constant.

If we disregard the current taken by the feedback circuit (only a few mA) we can say that the input current is the same as the output current. The efficiency is the ratio between the output power and the input power expressed in %. We can easily calculate the efficiency with the relations below:

linear power supplies formula

Note that the efficiency doesn’t depend on the current, but only on the input and output voltages!

For example if you need an output of 5 V out of a 12 V input the efficiency will be 5/12 = 0.417, i.e 41.7 %.  For a 9 V input with a 5 V output the efficiency will be 5/9 = 0.555, i.e. 55.5 %. On another hand, if you need an output of only 3.3 V out of an input of 12 V the efficiency will only be 27.5 %.

The less the difference between VIN and VOUT is, the better the efficiency will be.

You may wonder where the limit is: could we, for example, use a 7805 to build a 5 V power supply out of a 6V input, hence having 5/6 = 83.3 % efficiency? The answer, unfortunately is no! The 7805 needs at least a 7.5 V input to work. On another hand we need to provide for the input voltage fluctuations. In this case a 9 V input would be safe and we would have to settle with only 55.5 % efficiency. For low power applications this might not be an issue.


Linear regulators might still be around for many years, mainly for low power applications, when efficiency and size are not an issue. In the examples quoted above we could have found SMPS solutions, with a better SMPS and proper filtering and shielding to eliminate the EMI problems. But why making it complicated when simple solutions exist? Simplicity is one of the strength of linear power supplies and regulators.





Gerald Musy

Penang Malaysia

For more information about Gerald Musy and his other superb articles please click on the link below:


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  1. marco tapia

    November 30, 2014 at 6:21 pm

    Thank you Mr,Gerald Musy for this insight on linear power supplies and regulators it was very educational for me, love your articles, thanks again.

  2. Robert

    November 30, 2014 at 7:38 pm

    Good job Gerald. Thanks for the article. People have to be careful and remember that if a PC or device is connected to earth ground, the USB port will usually be also.

  3. ricardo ramos

    November 30, 2014 at 8:15 pm

    Well explained and good and interesting source of information.
    Thank you for sharing your technical knowledge.

  4. Yogesh Panchal

    November 30, 2014 at 8:20 pm


    I also fond of simplicity of linear power supply and reliable also. I found SMPS are Unpredictable and after taken care of Overload protection and short circuit protection, when fuse is found open on test; we found some other components also get damaged.I am in computer Repair field and came across this type of failure in MONITOR SMPS AND PC/Laptops SMPS.

    Informative article thanks for sharing.

  5. Osman AL Sinnary

    November 30, 2014 at 9:01 pm

    Thanks Gerald
    Very nice valuable topic ...

    • macdonald makwemba

      November 30, 2014 at 11:00 pm

      Thanks good article.

  6. Henrique Ulbrich

    December 1, 2014 at 12:13 am

    Good considerations, thanks a lot. None the less, let me add a third and remarkable disadvantage SMPSs have: although almost all of its componentes can be easily bought in the regular market, the transformer is a complicated issue, since it is “tailored” to each SMPS project. In this way, this transformer represents an insurmountable barrier to the experimenter, unless this “hero” is supported by considerable tools, both knowledge and technical resources. As this is rare, SMPSs continue to be not attainable to the regular experimenter. Hooray the linear!

    • sylvester

      December 4, 2014 at 9:55 am

      You hit the nail right on the head-Its hard as hell to find a replacement smps transformer without you having to buy the whole power supply-What a gimmick for the market

      • Robert

        December 5, 2014 at 1:53 am

        Transformers are very robust and rarely fail. The culprit is usually some other component.

  7. Vicken Mardiros

    December 1, 2014 at 1:11 am

    thanks Gerald very interest work God bless you & your work

  8. Ala B.

    December 1, 2014 at 2:18 am

    Gerald always has great articles that are are very clearly laid out and enjoyable to read! Keep up the great work!

  9. Raghu

    December 1, 2014 at 4:23 am

    Thank you Mr Gerald for this good article. It is very much informative.

  10. Vasile Petrica

    December 1, 2014 at 5:23 am

    Yes,I like your article!Bravo,bravo!

  11. randy warren

    December 1, 2014 at 5:36 am

    thanks, Gerald, a very nice and informative article, please make more of these to help us to gain more insight into how electronic theory is applicable to helping solve problems with equipment. As I am taking engineering circuits classes at university, it helps to inform me as to how the real world repair and design can take advantage of this knowledge as well as sniff out obvious design flaws by other techs and engineers.

  12. Andre Gopee

    December 1, 2014 at 5:50 am

    Thanks Very much for the info.

  13. Imraz

    December 1, 2014 at 5:54 am

    Nice Article Gerald. Well explained in simple terms. As we say old is gold. SMPS v/s Linear PS... Linear becomes the winner at the end due to stability and durability.

  14. Anthony

    December 1, 2014 at 9:15 am

    You know what,when I use to tested pwr supplies for a company in the old days,I away check to see if there a ripple no more higher than 50mv on the output ports of the pwr supplies and also check for a float back,overload protection, when increasing the current by adjusting the trimmers at that voltages level.This was done on a power regulator station.If any of these two problems show up,the power supply must be reject and send it back to the vendor.
    Another thing ,did you know that the biomedical equipments should have a safety grounding of the equipment when it hook up to a patient?The patient must be ground too!The power cord grounding resistance for a patient care device should be less than 0.5 ohm. Remember that I put some informations about the "Google Play" softwares on medical devices ,the reading of the ECG waveforms show up on the screen of your Smart phone,I wonder if the Smart phone is grounded when touching the camera's len that become a sensor? If that not grounded,your ECG waveforms will show alittle ripple and it look like you got a heart disease!

  15. Gerald

    December 1, 2014 at 2:38 pm

    Thank you to all of you for your positive and encouraging comments.


  16. dicksy

    December 1, 2014 at 3:12 pm

    pretty good article.thanx mr gerald

  17. cvs naidu

    December 1, 2014 at 5:16 pm

    Dear sir thank you , In future I will be using leaner power supply where ever possible, where earth is not avaible. cvsnaidu

    • Robert

      December 2, 2014 at 7:00 am

      I love switch mode power supplies! In the 2 years I've been learning and fixing electronics, I have only seen one bad SMPS transformer. And the reason it went bad is because my great niece poured a glass of soda in the top air vents of the TV while it was on. Other than that time, the culprit has always been some other problem - never the transformer.

  18. Sameer

    December 2, 2014 at 6:28 pm

    Thanks a lot Gerald,great place to learn.Really enjoying the articles.Keep them coming

  19. Humberto

    December 4, 2014 at 2:15 am

    Hi Mr. Gerald Musy, a very good article from you. By the way, I think like you about Linear Power Suplies (PS). Congratulations and have a good day.

  20. shahid ahmad

    December 13, 2014 at 1:22 am

    thank you sir,

  21. Taring K Arioka

    December 29, 2014 at 4:21 pm

    Very goood articles. Please send more and thank you for sharing.


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