Voltage regulators are an essential part of many projects that require a stable input voltage. Their job is to take an unregulated input voltage and output a regulated voltage, with the only catch being that the input voltage must be higher than the output voltage. If you’ve got a project in the works that needs a specific voltage, here are several options you may consider:
Fixed Voltage – LM78XX
The LM78XX series of linear voltage regulator chips are extremely popular, and for good reason. They’re cheap, easy to use, require few other components, and have built-in circuit protection against drawing too much current. There are different models for outputting different voltages, and the last two numbers in the model number denote their voltage output. For example, the LM7805 outputs 5 volts, the LM7810 outputs 10 volts, and the LM7824 outputs 24 volts.
Fixed Voltage – Zener Diode
You’re halfway through your project, and you just realized you’re fresh out of linear regulator ICs. What can you do? If you’ve got the right voltage zener diode and a power transistor, you can make your own fixed voltage regulator using the circuit diagram above. The output voltage will be 0.6 volts below the diode’s zener voltage, due to the base-emitter voltage drop across the transistor.
Variable Voltage – LM317
When you need to be able to adjust the voltage output of a voltage regulator, the LM317 is right for you. It is very similar to the LM78XX series, except that it has an adjustment pin to change the voltage output. By adding a potentiometer to your circuit, you can use it for purposes like controlling fan speeds or variable voltage power supplies.
A Note on Heat Sinks
The larger the voltage drop across the voltage regulator, the more heat will be dissipated through the component. In order to keep it from burning up, make sure to use a heat sink!
7 thoughts on “Why You Should Be Using a Linear Voltage Regulator”
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Linear regulators are good. They don’t generate any electrical noise, so they’re good for analog circuits, and a good choice as long as your circuit doesn’t draw a lot of current. Beware, though, that they regulate voltage by dissipating the excess voltage as heat. So keep the input voltage close to the desired output voltage. This is more important, as the current (I) drawn by your circuit becomes higher, because P=I*(Vin-Vout) is the amount of heat you will have to get rid of.
And if you use a heat sink, remember that just attaching a lump of metal to the regulator won’t get rid of the heat. To work, a heatsink needs a constant stream of cool air flowing over it, and that cool air has to get into the housing that encloses your circuit and the hot air has to get out. One way of doing this is to have a row of holes at the bottom front, and another at the top back of your box. The heat generated by your electronics will create a flow of cooling ait in the front and the hot air will exit out the back.
Just slapping a heat sink on does not necessarily solve the problem of power dissipation. One needs to choose a heat sink with the proper thermal resistance for the ambient temperature for which the device will be used. In addition, silicone grease and possibly a thermally conductive insulator, such as mica or solid silicone, may be needed to lower the thermal resistance of the contact between the device and the heat sink.
The calculations are fairly simple using an analog to Ohm’s Law. Treat the temperature difference between the maximum junction temperature (typically 150 degrees C) and the ambient temperature as voltage, the power dissipation as current, and determine the required thermal resistance (in degrees C per watt) by dividing the two. The result will be equal to the required thermal resistance which is the sum of the junction-to-case resistance and the case-to-air resistance (both are specified in the data sheets for the devices). We can’t do anything with the junction-to-case resistance, but we can lower the case-to-air resistance by using a heat sink.
The formula: ThetaHS =(Tj – Ta)/Pd – ThetaJC, where ThetaHS is the thermal resistance of the heat sink in degrees C per watt and includes the thermal resistance of the grease and insulator, Tj is the maximum junction temperature in degrees C, Ta is the ambient or environment temperature in degrees C, Pd is the power dissipation in watts, and ThetaJC is the junction-to-case thermal resistance in degrees C per watt.
Hope the above is of some use.
Dave
That’s great information, thanks! I can tell you this though… as a kid I used to read things like this. Then I would go to the local Radio Shack with my tiny little bit of paper route money. I’d look on the racks and not find thermal resistance numbers listed on their generic heat sinks just like I wouldn’t find values listed on inductors or ranges on variable capacitors. I thought I had to have exactly what the project articles I read about called for or exactly what the formulas I read calculated!
Then I would read magazine articles where people talked about building things from parts they scavanged from junk electronics. That would have been great on my old paper route budget! But how did they even know the exact values of the parts they were working with? I thought those people must be absolute geniuses to figure that out or maybe that had million dollar lab equipment or something!
This held me back from progressing in electronics for many years.
Don’t ge me wrong.. it’s great to learn everything you can. Become the best engineer you can. But… sometimes it’s fine to just slap on a salvaged heatsink and call it a day.. or maybe even some piece of metal that wasn’t even meant to be a heatsink in the first place. Look at another linear power supply maybe. If it is putting out the same or more power than you need and you use a heat sink as beefy or more so than it you are unlikely to go wrong. Or.. don’t. Just pick a heat sink and go big with it. Worst case.. you smoke a 95 cent regualtor chip!
Another tip…. Desitin. That cream stuff that parents put on their babies when they change their diapers. Maybe you are a parent yourself and so already have Desitin on hand. Or.. maybe you are a kid with a younger sibling so there’s a bottle in the house. Anyway.. it comes in big bottles, it’s cheap, and it’s made out of the same stuff as thermal grease. It’s probably not as pure.. I wouldn’t use it in a professional environment but for a hobbyist on a budget it makes a decent heatsink compound. I used it quite a bit while my daughter was still in diapers.
Leif, I get where you’re coming from concerning the availability of specs. In my younger days, (over 40 years ago ;-) I used to do the same thing: grab any heat sink I could find and use it. I don’t recall ever having a problem doing that. After grad school in EE and working as a design engineer, I became a bit more of a perfectionist when it came to design, even if it was for a hobby project. Thus my desire to ensure I was using the right parts with the appropriate characteristics.
Concerning the specs: after the internet matured, I was often able to find dimensions on heat sinks of similar size and material to what I had and figured that the thermal resistances would be close. Then, of course, I would over-engineer anyway.
I really like your suggestion of using Desitin. It has to be considerably cheaper than silicone grease.
I like to have a metal lid on projects that use a regulator. Then I just attach the regualtor to the lid since the lid has access to outside air. Of course it doesn’t have fins so that is only good when you only have a little bit of heat to get rid of. I would imagine you could attach a heatsink to the lid directly opposite the regulator. I think I have even seen comercially built devices that did that. If you try it make sure to apply heatsink compond (or Desitin as I described elsewhere) to both junctions, between the regulator/lid and lid/heatsink.
Also.. remember.. with most of thost 3-terminal regualtors the tab is part of the circuit. Make sure nothing else can touch the lid that would cause a problem! The same can be said of a metal heatsink that is fully inside the enclosure.. make sure it doesn’t make electrical contact with any other parts of the circuit.
I believe you can get around that problem by using a mica insulator and a vinyl screw/nut. But.. if you are going to go out and buy those you might as well just buy the whole heatsink too right?
I like to just keep a bunch of LM317s and various fixed value resistors around. I just use fixed resistors instead of a variable one so I still have a fixed voltage supply. This way I never find myself with a drawer full of 5 volt regulators when i need a 9 volt one or some other unfortunate combination of numbers. I just make the voltage I need by selecting the resistors. If I don’t have the right resistors… I make something that is close enough by placing various resistors that I do have in serial or parallel combinations. Then I go buy a big bag of the value I would have preferred to have off of Ebay so that I don’t have to do that again next time.
Bonus… I’ve read that LM317s have less ripple than LM78x anyway!
Something to add… if your supply voltage is a battery, you may want to consider a Low Drop Out (LDO) regulator. This means the difference between the supply voltage and the output voltage can be much lower than for example the 2V drop out rating of the 78xx series. That translates to longer running time for your project on battery. I like the KA278R0x series, which can have Input-Output differentials as low as 0.5V.