Why less than 1 ohm resistance?

Question

While examining some circuits, I saw that some use less than 1 Ω of resistance. I searched the internet but couldn't find a satisfactory answer. What could be the role of a resistor of less than 1 Ω in a circuit?

What are the uses of resistors less than 1 Ω and why do we use them?

An example circuit using a resistance less than 1 ohm:

24 volt to 12 volt 20 ampere DC-DC converter

what would be special about the value of 1Ω that would make resistors below and above it be different? (hint: nothing's special. Our – very human – choice for units is purely arbitrary.) — Marcus Müller, Dec 05 '21 at 13:51
That's a completely different question then your title, and this is a strange random circuit (with a lot of things that go under "do not draw schematics like that!", so chances are, it's not a *good* circuit, either) that we don't know. What does the cricuit do as a whole? You#re in a better situation than us to explain that. It's *your* circuit. — Marcus Müller, Dec 05 '21 at 13:59
That is such an awkwardly drawn schematic that even after staring at it for 1 min, I don't understand it. — tobalt, Dec 05 '21 at 14:00
@Jundullah please understand that it's also not ours, and it's pretty bad, so we can only guess, and I don't see why it would pay for you or for us to waste our times on that. — Marcus Müller, Dec 05 '21 at 14:16
https://www.youtube.com/watch?v=UKuTith6lD0&t=175s @MarcusMüller — Jundullah, Dec 05 '21 at 14:21
aha, so it's a schematic from youtube, but at least the youtube author says what it is. So, please **edit** your question to include that. Also, be advised that really, this schematic is drawn so badly that it's pretty clear that whoever made it has no real education in electrical engineering; so, they're most likely just badly copying copies of copies of schematics without understand. Maybe really not the source of information you want to base your learning on! — Marcus Müller, Dec 05 '21 at 14:24
@Jundullah thanks! But as you **yourself** note, the circuit is not working. So, why ask what a component does in it, if it's not working as claimed? There's no indication it was put there through sensible design, so we can't tell you what the design intent was. — Marcus Müller, Dec 05 '21 at 14:27
@Jundullah but... there's nothing to tell you. They are just resistors. End of story. — Marcus Müller, Dec 05 '21 at 14:32
Resistors of **any** value are used either to limit current or voltage. A 1 Ω will drop 1 V when 1 A is run through it. If that's too much then we use a smaller resistor. There is nothing magic about 1 Ω ! You might as well ask what use is a resistor < 1 kΩ. In general, low value resistors will be used in high-current circuits as the required voltage drop will be generated with the chosen R value. — Transistor, Dec 05 '21 at 15:10
In this case the resistors are used to balance the currents so that the transistors have more or less equal currents. .47 ohms is a tradeoff between wasting power and having uneven currents. — stretch, Dec 09 '21 at 21:30
Interesting that less than 1F capacitor in the same schematics did not raise the same question... — Maple, Dec 10 '21 at 16:29
You should be aware that this circuit produces a lot of heat. At full load 240 W heat in all 2N3055. You would neat a very big heat sink to keep the transistors below their limit temperature. — Uwe, Dec 10 '21 at 18:37

Tony Stewart EE75 · Answer 1 · 2021-12-05T19:46:56.743

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Short answer: Vbe/Rs=0.5V/0.5ohm= 1A current sensor for some action.

One might use a sub-ohm resistor for current sensing as a Vbe detected active current limiter which for the NPN load 0.1mA = (Vin-Vt)/Rg , Vbe might be ~ 500 mV so the I_sense resistor = 500 mV/(Imax*Rsense) or 0.5 Ohm for 1A.

It is a Rule of thumb for this NPN Vbe current limit sense design to be 600 mV drop full scale into high Z MOSFET gate. But for better current limiters, keeping Pd max below 50% of the rated power is desirable so Rs=50mV max/Imax is a common value.

The Schematic has some design errors.

It's a booby trap design with everything wrong, so it you try it , you need to learn a lot about electronics and phishing from bad examples to attract you to a custom design service. Be more aware where you get web schematics from and learn the basics found anywhere, including my profile.

edited Dec 05 '21 at 19:46

answered Dec 05 '21 at 14:00

Tony Stewart EE75

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OP has added a youtube link in a comment to a, quote "Mosfet Overcurrent Protection Circuit - Limit Adjustment Possibility", and it's... not good. The whole video is really not good. It's just a thinly veiled advert for a PCB service, and an attempt to generate a few views with that, quite possibly just to make a few bucks out of the ads that Youtube then adds. – Marcus Müller Dec 05 '21 at 14:26
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Yes I was being generous that the circuit has "some" errors" It has many errors! – Tony Stewart EE75 Dec 05 '21 at 14:33
@TonyStewartEE75 Is it possible for you to explain some of the errors in this circuit? – Jonathan_the_seagull Dec 05 '21 at 17:37
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It is drawn completely wrong. Vbe should be across R1, Out +/- polarity is reverse. Gnd symbol is upside down – Tony Stewart EE75 Dec 05 '21 at 17:46
@Jonathan_the_seagull also note that the person recording the video says that he recorded the video to get JLCPCB's sponsorship. In other words, that video wasn't made to explain an engineering feat, but to advertise the PCB service – which is OK, but it reduces the expectations for technical excellence quite drastically. – Marcus Müller Dec 05 '21 at 19:33
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@Jonathan_the_seagull you'll probably enjoy reading [this](https://electronics.stackexchange.com/questions/28251/rules-and-guidelines-for-drawing-good-schematics/28255#28255) answer, which explains a few good rules for making schematics that people universally can read well. Point "3." goes into flow and direction. Generally, technical documentation is read left-to-right (even in cultures where regular text flow is different), which means a signal enters on the left and flows to the right. Input left, output right. High voltages are on top ("high"), lower voltage lower;GND always points down. – Marcus Müller Dec 05 '21 at 19:35
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NPN transistors have the emitter always point down, feedback over transistors is drawn in a way that makes it easy to spot. This whole schematic goes through lengths to obfuscate! – Marcus Müller Dec 05 '21 at 19:40
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This hides a few *important* facts about this circuit: Input supply is *directly* connected to output; all the "fusing" happens on GND, it seems. This needn't be a big deal – but you can be very sure that if I see a fusing circuit I'll want to know against what kinds of shorts it protects. And "short circuit to GND" is pretty common, and this doesn't protect against that at all. In most conventions, the rail you want to interrupt is the VCC rail, not the ground (you'll more often see a fuse in series with VCC instead of the current return path for that reason). – Marcus Müller Dec 05 '21 at 19:42

score 4 · Answer 2 · answered Dec 10 '21 at 15:06

In the circuit OP presented, those resistors are called "emitter degeneration resistors". Besides the reasons mentioned by others, they serve to help balance the differences in beta (Hfe) between transistors when they are used in parallel like shown, to increase current carry capability. They help the transistors to share current more equally than what might otherwise occur, where the transistor with the highest beta would carry more of the current.

Their exact value is not critical. A fraction to several ohms is typical, depending on the current involved.

score 3 · Accepted Answer · answered Dec 10 '21 at 14:52

There's nothing special about the value 1 Ω, just like there's nothing special about the value 1 m or 1 s; just like you can have lengths of less than one meter and times of less than one second, you can have resistances of less than one ohm.

Such low resistances are commonly seen in things like current sense resistors (where you want to minimally affect the current you're sensing) and, as in the example you show, current-sharing resistors, where you need a little bit of resistance to help balance several parallel paths, but not too much that you waste a bunch of power heating resistors. Resistances less than one ohm can also show up as parasitic elements, such as the ESR of a capacitor, the Rds,on of a MOSFET, the DCR of an inductor, or just the resistance of a wire. Of course any of these can also be more than one ohm--there's nothing special about the value 1 Ω.

Why less than 1 ohm resistance?

3 Answers3