#548 LM317 with Transistor for more Current

Episode 548
I use a PNP darlington in combination with three terminal regulator to increase the current capability
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[htomerif]

This was a very well done, succinct video. The thumbnail and title made it clear exactly what we were going to get and it was explained in the right amount of detail for people who would actually be inclined to use this circuit.
I don't personally make use of linear regulators much, but I subscribed because I'm sure there's something on your channel I will make use of.

[valantonescu]

Just found this relatively old video. Thank you for excellent explanations relating this approach.

[RoyAntaw]

Nice I love these practical and electronic circuit overview videos which are useful in anyone's lab.

[davidharms]

Great explanation on this particular circuit! I also appreciate you mentioning the power ratings of the resistors in this particular circuit, a lot of other video’s will give you a resistor value, but never mention the power ratings. And well, power ratings obviously matter in circuit design. Understand the current path in the circuit helps too, and you did a great job explaining that as well. Thanks! 👍🏻

[craxd]

They have the TIP73 rated at 15 amperes but, somehow, I doubt it could take that for long. They similarly rate the 2N3055, but most manufacturers run them at 5 amperes, and this transistor is a TO-3, with more heat dissipation over its case.

[thomaslewis]

This is a cool circuit as power NPNs are easier to find than PNPs. I have a bunch of TIP-42s (PNP 3or4 amp TO-220s) and 2N3055s (NPN 16amp TO-3s) that should do nicely (will post if it works).
These regulators, and their cousins, have been around forever and avoid some of the noise you can get with switch mode supplies. I did find a low power variant (LM317L ?) that will regulate 100ma and comes in a TO-92 package (like signal transistors). We are talking about the one that comes in a TO-220 (or power surface mount case) and can source 1.5A by itself. (So don't order the wrong one.)
As I recall there is an internal reference and differential amplifier that tries to keep the output 1.25v above the adj. terminal, so you can use that idea to analyze your feedback network.

On second thought a TIP-42 might be overkill, the 2N2905 has a 0.6A collector current max.

[paulcohen]

Current sharing resistors are usually in series with the Emitter.
In series with the Collector they just minimize the voltage (and power dissipation of the power transistor).

[td]

Thank you for your effort to share! FYI, even though you explained everything very well, for some reason I was still wanting to see your board bodge. Eye candy, I guess...

[bandittwothree]

A fascinating part that can be paralleled without the ballast resistors is the LT3080. Instead of using an internal voltage reference to drive the error amplifier, it uses a constant current source.

[JustMe-dxxl]

Trap for young players ( & not so young) is a little thing called power dissipation.
From the Texas Instruments datasheet the lm350 will actually handle up to 4.5A of current (* CONDITIONS APPLY .... gotta love the asterisk) so in theory it should have been able to handle your 4A under the right circumstances with minimal voltage drop. We all regularly hear about & see the obvious 3A rating in the specs & think great I only need 2A so more than enough, get excited, switch brain off & carry on. That's where the problem starts as the actual power dissipation is not made so obvious in the datasheets. I do not understand why the power dissipation was not made very obvious & specifically listed in the main tables. I browsed many datasheets many times before the penny dropped & the dissipation issue become obvious. Although very versatile in terms of voltage & current range the TO-220 package can only dissipate 25W & the TO-3 package 30W Max. (20W & 25W respectively to allow a safe margin) prior to thermal shutdown. Although from your experience the thermal shutdown does not function as intended to protect the device anyway. I have never tried to cook a LM317 / 350 / 338 to check its thermal shutdown functionality but would be a interesting feature to test.

From 2013 Texas Instruments LM350 datasheet
Current Limit
VIN − VOUT ≤ 10V Min.3.0A Typ. 4.5 A


VIN − VOUT = 30V Min.0.3A Typ. 1 A

So yes, the dissipation limits are listed in the tables but it is not obvious as we tend to switch the brain off & stop reading after the first line & 1A limit on the second line for the large voltage drop gets ignored. The circuit specifications you mention are realistic but I see many circuits on You Tube claiming ridiculous voltages & current ratings while using pass transistors. I think 60V @ 100A is about the highest I have come across. Even when using pass transistors the dissipation limit of the voltage reg still needs to be given careful consideration when driving the transistor base for large voltage / current ranges. This is where the NPN / PNP combination has the advantage over the single NPN pass transistor.

[xristost]

A little correction: the base-emitter voltage drop of the darlington transistor 2n6287 is 1.3-1.5V, so the current through the 5 ohm resistor would be 1.3/5 = 0.26A, not 1/20 A.
Adding the base current of 0.5A (at 5A output) will result in 0.76A which is not insignificant.
The total minimum voltage drop of this schematic will be about 4.3-4.5V and the input voltage should be at least that much higher than the desired output voltage.
I wonder if the current protection of the LM317 will work at all?

[resitusta]

Aga elektronikte harikasın.Teşekkürler.

[Broken_Yugo]

RE the LM350 blowing. At this point I don't trust any advertised protections in a linear reg. I've lost a couple things worth enough to annoy me because some 7805 shorted and tried it's best at taking a 5V rail to 12.

[bayareapianist]

A couple corrections: 1) since you use a Darlington transistor the voltage drop acros BE is approximately 2x0.7V. 2) when you put two transistors in parallels, you never said why you needed to limit currents. Infact the reason is that thosw two transistors are not usually identical. They usually have different Vbe and current gain (beta). In order to balance the load on those transistors, you need to have a resistor on emitter to cancel the vbe differences which is a fraction of Vbe. Otherwise only one of them would always carry the load. It is exactly the same idea as putting two diodes in series to increase their max voltage or put them in parallels to increase their max current. Infact without using the registers, due to decreasing Vbe with the heat and temperature, the situation will get worse for the conducting transistor and it gets hotter and hotter.

[mouseminer]

Sorry, I don't get one thing, How the voltage can remain constant.Transistor is connected here is like a jumper between input and output. Lets say input voltage is 20 volts and you set the output voltage at 5 volts and then start drawing enough current to turn the base emitter junction. Now the transistor is a jumper between input and output. How can voltage can stay at 5 volts. Please explain. Thanks

[osmanfb]

George C. Sziklai hungarian-american not japanese :-) complementary darlington pair.

[clems]

Will you please please explain the operation of the balancing resistors on the pass transistors. The 10-watt resistor how does that balance the current

[winlose]

Thank you for the video! can I please about what limits the current at the base of the pnp power transistor here?

[frontier]

I've built something after looking at heaps of designs. I really just wanted to share the load between a small transistor BD140 pnp and the LM317. Its only 2A max type of power supply - so since BD140 can do 3A max and 0.5A at the base - maybe that's ok. I"m trying to figure out how much resistance from the input to the base of pass transistor will share. I used 3.3kohm to start with. I used 6.5 ohm 2 watt resistor to turn the transistor on. However I see this ...

[toddanonymous]

Your great videos just keep coming. Great channel. You might want to add a crowbar at the output. A friend of mine found out the hard way that his xcvr didn't like the unregulated voltage,