Data sheet question. - Page 1
Forum section: electronics / beginners

author: sureshot
author_rank: Regular Contributor
timestamp: July 29, 2018, 12:37:27 pm
content: I have been looking through data sheets to glean some circuits, psu circuits to be more precise. I'm unsure of two resistors value and why there resistance is this high. Normally these circuits have these resistors in a few tens of ohms, but in this schematic they seem very high. The two resistors I'm referring to are the base rsistor for the 2N2905 at 5K and the emitter collector resistor of 500 ohms for the TIP73. I've seem maybe up to 100 ohms to maybe 150 ohms, but never this high as in this schematic. The image is from a genuine data sheet, would anyone know why there so high in value ? Thanks for reading, any help appreciated.

author: sureshot
author_rank: Regular Contributor
timestamp: July 29, 2018, 01:31:51 pm
content: With the above posts question, its as this image below. I know the transistors are different, but there still being used for same purpose. The rsistors in the image below are a lot less resistance than in the image in the post above. And I've no idea why.

author: lapm
author_rank: Frequent Contributor
timestamp: July 29, 2018, 01:38:50 pm
content: Thats current drive. LM317 handels voltage and transistors handle current. 5K resistor is there to limit first transistors base current since its not power transistor and 500 ohm is there to ensure second transistor starts to conduct when enough current flows.Very handy if you need more current drive capasity then lm317 has by itself.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 29, 2018, 01:39:06 pm
content: This schematic may be similar to what you are referring to. In this circuit the added pass transistor is a low gain power transistor so the base resistor is a low value because more current is needed to control the low gain pass transistor. In your circuit there are 2 stages used where the first is a high gain lower power transistor that will work properly with a higher value base resistor. This transistor drives the lower gain higher power pass transistor and the 500 ohm resistor supplies voltage to the first transistor.

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2018, 05:50:58 pm
content: Those resistor values are not critical in any way. The series base resistor protects the 2N2905 from excessive base-emitter current at the start of current limiting. The base-emitter shunt resistor on the TIP73 sinks leakage and removes base charge for better response.

author: sureshot
author_rank: Regular Contributor
timestamp: July 29, 2018, 07:45:32 pm
content: Thank you for getting back to me. I have built the pnp single transistor version, and multiples of the same circuit with ballast resistors. And it works very well, I'm trying to find similar circuits that use the npn transistor. Most of what I've found are npn transistors on the final output, with a base resistor in some of them or a few schematics with none at all. I know these are poorly regulated now, and probably not worth bothering with. In the circuit in my first post, I just didn't understand why the resistances where so high. Thank you for the explanations on this. I've also been looking for circuits on the LM723 with no more than two transistors, a driver then the final output power transistor. It's a bit more tricky finding npn variants of the linear regulator boost current circuits.

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2018, 07:50:57 pm
content: Below is the reference from the National Semiconductor Voltage Regulator Handbook that I use for these boosted current regulator circuits.

author: sureshot
author_rank: Regular Contributor
timestamp: July 30, 2018, 01:39:30 am
content: Thank you for posting that data sheet, it will come in handy. I think the only sensible way to use an npn power transistor is to drive it with a more modest pnp transistor. All those circuits with 2N3055's on the output of circuits are poor on regulation from what's been said and what I've read across the web.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2018, 01:49:40 am
content: Quote from: sureshot on July 30, 2018, 01:39:30 amTheir regulation is not *that* bad especially at higher output voltages and similar regulators using zener diodes without an error amplifier were common before integrated regulators become available. A load regulation of say +/-0.25 volts is acceptable in many applications.Still, there is little reason to use that sort of design today.

author: sureshot
author_rank: Regular Contributor
timestamp: July 30, 2018, 09:59:56 am
content: I just find the linear regulator circuits interesting. Variants of them that look viable. I've even looked at descreate components to build a regulator from scratch.

author: sureshot
author_rank: Regular Contributor
timestamp: July 30, 2018, 10:30:37 am
content: So for a first attempt at pnp to npn output from a lnear regulator, i can do the configeration below with say a 50 ohm resistor on the input to the regulator ? Then add the emitter and base and collector of the image below, keeping the resistors as per that schematic ? Think ive got it right. Thanks again for the help.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2018, 03:11:01 pm
content: Figure 7.4 shows how to replace the input PNP power transistor shown in figure 7.3 with an NPN power transistor and PNP driver transistor.I am not sure if they meant the 50 ohm resistor in Figure 7.4 to replace R3 or be placed in parallel but the value is not critical; anything between 10 and 50 ohms will work fine.

author: sureshot
author_rank: Regular Contributor
timestamp: July 30, 2018, 05:29:50 pm
content: Yes that's what i was wandering, do i need a base collector resistor. As there is already the 50 ohm base emitter resistor. I think i do, but only want the regulator doing a small amount of the current supply. If the additional base collector resistor isn't needed i would probably soon know as it will be not running properly. I have a hard time with the maths, i find it easier to open the circuit and take some current measurements. Also measure across resistors to see what voltage is dropped across them. From them numbers i can do the maths. Is just theory straight off the bat i struggle with, like equation maths, transistor or multiple transistor circuit equations. Thanks again for your help.

author: sureshot
author_rank: Regular Contributor
timestamp: July 30, 2018, 05:39:18 pm
content: The reason i think i need that additional input resistor is the 22 ohm resistor showing in the schematic below. Going by that its got the R3 resistor, although its 5K, but that diagram leads me believe i need that input resistor. But then the figure 1 schematic just has the R3 resistor. Bit conflicting.I think I'm going for the figure 1 schematic with 50 ohm and 75 ohm resistors first. If it's a total flop I will move on to the schematic in the first image of this post. I know I don't need such a punchy transistor for the driver, but I've a few TIP2955, and TIP3055's also a fare few TIP36C's and TIP35's.

author: sureshot
author_rank: Regular Contributor
timestamp: July 31, 2018, 11:08:21 am
content: Just wanted to say thanks again for all the help from everyone.