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.
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! 👍🏻
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...
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).
@@topquark22 Yes, the emitter of a PNP Sziklai is the collector of the NPN, but when paralleling output transistors in Sziklai configuration resistors are in the emitter of the second transistors. It is also used in paralleled output Sziklai power amplifiers.
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,
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
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?
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.
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.
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.
I mistook the 2N2905 for a 2N2955 ( which seemed way too big ). The 2N2905 is more of a signal transistor with a 600mA max collector current. A TIP-42 is probably too big.
If I use an npn transistor connecting the base to the regulator output, collector to input supply and take final output from emitter and ground then will it be good voltage regulation at output on load current of 3 amperes ?
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 ...
the resistor is not really a sharing current. it sets the turn on point of the transistor. the transistor will need 0.7V to turn on. so the voltage across the resistor is 0.7V so if you want the LM317 to 'share' 100mA. then 100ma across 0.7V is what the resistor needs to be. or 7 ohms.
Remember LM317 needs some power to function. So the resistor on unregulated side cannot be more then a few ohms. Also the voltage divider resistors should not be more than a few hundred ohms. Otherwise it won't work as a regulator. I learned these in a hard way too!
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.
@@krzysztofsoja5301 That might be a correct spelling (transliteration) but people will find it easier under "Sziklai". Also to the original comment poster, at 9:21 he says "current hog". Those two words are everything you said in your "correction". The explanation is "to avoid current hogging". This is a 10 minute video, which is how long it should be. He also didn't explain the intricacies of why its difficult to find high power PNP BJTs. We didn't need a half hour explanation of P type silicon majority carrier mobility increasing silicon resistance and requiring more silicon per watt than an equivalent NPN BJT, increasing the cost of PNP BJTs. We didn't need an explanation of economies of scale and the low production of PNP transistors leading to a higher cost.
I am just learning about these. I build an adjustable regulator using a LM317 today in fact, and it works! My question concerns the output voltage with the addition of the bypass transistor. In my scenario, I am using RC LiPo batteries at 12 to 18VDC, and driving an arduino at 5VDC. Does the bypass transistor cause the voltage to rise when it turns on? My current draw is very small, on the order of tens of milliamps. Do i even need to worry about a bypass transistor?
@@IMSAIGuy Yes, on the data sheet schematic! I should have assumed that it was assumed to be there, lol! I ordered two of the PNP power transistors from Jameco just to build this circuit to see how it behaves. Thanks for answering a question on a three year old video, I appreciate it :)
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.
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
if the LM317 is not drawing current, there is not enough voltage drop on the resistor to turn on the first transistor, and therefore no current in the big transistor.
But the resistor in series with the PNP base acts as a load and will draw the current to turn on the transistor, isn’t it? I know it works (I tested it), but I just don’t get why the voltage still gets regulated, even though there is another path with least resistance 🤯
This is old school analog, the transistor only starts to turn on. It never goes into saturation and turns all the way on. The name transistor is from transfer-resistor, which was how they were used (pre-digital). Current drawn out of the base of the PNP (because there is a voltage drop (at least 0.6v) across the 22ohm resistor, because there is current flowing thru the 317) starts to turn on the PNP. The NPN will be off if the PNP is off, because there will be no current thru the 500ohm resistor the base and emitter are at the same voltage. When the PNP starts to turn on, a current of less than 2ma thru the 500ohm resistor will raise the base 0.6v above the emitter and it will start to turn on. The voltage out of the LM317 is controlled by the voltage divider formed by the 120ohm and the variable 1.5k resistor. When the NPN starts to turn on (or if the load is reduced) the output voltage will start to rise and the 317 will start to draw less current. That means less current across the 22ohm resistor, and the PNP will start to shut down, which will start to shut down the NPN. Now even a big slow power transistor can respond to changes within micro- seconds, so a partly turned on equilibrium is quickly reached. By the way, a transistor in the active region (partly turned on) can generate a lot of heat, so you may need a heat sink to not burn up your transistor. To turn on a PNP, you must draw current out of the base in the direction of the arrow (the V-base is about 0.6v less than the emitter). To turn on an NPN, you must push current into the base in the direction of the arrow (the Vbase is about 0.6v greater than the V-emitter. A small current into or out of the base (in the direction of the arrow) controls a much larger current from collector to emitter (NPN), or from emitter to collector (PNP). The voltage across the emitter-collector terminals has very little effect on the current thru them. Using discreet components can be a lot more challenging than plug and play IC's, but you may need to adapt the not-quite -what-you-need IC to your situation. When a few components in a clever circuit make it right, that is a good feeling. Don't give up, it gets easier every time you learn a new circuit.
Hi, thank you for this idea. For some reason I was unable to get more than 170mA from my LM317. With a pnp BC557 in sziclai configuration with 2N3055 (npn) I got more than 2AMP, it is very funny. I guess the 22R resistor can be changed to controle tha pass transitor.
Was the heat sink large enough? Was the voltage within the proper range? Was the LM317 good and not a faulty or fake one? It should easily supply the 170mA current if all was OK.
@@paulcohen1555 may be the heat sink ! I tried also other lm317 from the same box, I fried 2 of them. Anyway I switched to lt3080 which is a better regulator, no complain now !
Whether anyone tried this circuit and could get variable voltage at 5 amp ? I tried this circuit with complimentary pair of transistors tip 2955 and 2N3055 with all recommended capacitors / resistors . The output current I got was hardly above 1 amp . Any clue .....?
check the voltage across the 22 ohms resistor and make sure it is large enough to turn on your tip2955. It might be too low for that transistor and you need something greater than 22ohms.
The only problem with using collector resistors is they have to be able to work in very high current situations and are usually very small values. And such high wattage, small value, resistor can be hard to find...
Hey Dude, Many people forget that we say just as much with our hands & other gestures as we do with our mouth. I find the hands actually adds PERSONALITY & a 'REAL HUMAN' content to the video. Maybe it is a generational thing Later Dude
