# Short-circuit current limiting circuit, requesting comments and critique

#### ebeowulf17

Joined Aug 12, 2014
3,274
Short version:
I've come up with a current limiting system with low dropout voltage, intended primarily for output short-circuit protection. I think it hits a sweet spot in terms of balancing accuracy/repeatability vs cost and complexity. Current flowing through D1, D2, and R-Bias sets a reasonably stable voltage difference between supply voltage and the base of Q3. Current through Q3 can increase until Q3's Vbe plus the voltage drop across R-Sense add up to that supply to base voltage difference. So, the *approximate* current limit can be found with this formula:

Current limit = (Vf(D1 + D2) – Vbe(Q3)) / R-Sense

I'm interested in hearing comments and criticisms:

Long version:
I've recently become interested in simple, compact, cheap solutions for current limiting circuits. These don't necessarily have to be perfectly accurate, not what you'd call current sources, just something to protect the outputs of a board from short circuits. I found an intriguing circuit that just uses a couple extra resistors and is surprisingly effective, but it's pretty finicky about pass transistor Beta, supply voltage variations, etc. and it's not very straightforward to calculate values for. This circuit is what got me curious about other circuit options:

I came up with a system using two common diodes to create a moderately stable reference voltage for the transistor's base (instead of the voltage divider in the circuit above.) This circuit delivers a much more stable current limit when facing varying supply voltage or Beta values. It's also much easier (at least for me) to calculate the values for, although it's still not the most straightforward. I like that I could incorporate it into different designs with less fear that changing transistor component choice or having minor inaccuracies in supply voltage regulation would dramatically change the current limit:

I also looked at a few other known circuits for comparison. There's a circuit shared by @crutschow which is more stable and easier to calculate, basically better in every way, except that it's significantly more complex, so if you had a board with 20 outputs to protect and absolute precision wasn't that important, you might want something cheaper and simpler. Here's that circuit:

The other known circuit I looked at was the LM317 configured as a current limit, which provides great accuracy and stability, but at the cost of high dropout voltage:

Finally, as I was looking at the various influences that would push the current limit on my own design around, I thought perhaps a more stable reference would help, so I tried a version with a TL431, instead of two signal diodes, providing the reference voltage. This did yield more stable results, but at the cost of much higher dropout voltage (although still not as bad as the LM317.)

Some key parameters I looked at when comparing these circuits were:
• dropout voltage
• stability of current limit with varying input voltage
• stability of current limit with varying transistor Beta
• stability of current limit with varying transistor Vbe
Here are the results, based on each circuit being configured for a 5V supply voltage and 60mA current limit. First I looked at dropout voltage. As you can see, the LM317 and TL431 circuits have much larger dropouts than the other 3, making them unsuitable for the applications I'm currently looking at:

Next is the effect of varying input voltage. The graphs below represent 5V +/-0.5V supply voltage. You can see that the two circuits with ICs deliver very consistent current limits, @crutschow's circuit shows very little variation in current limit, my circuit (EBS) shows a few mA of variation, and the Doughty circuit that originally got me curious about all this swings up or down nearly 15mA based on these 0.5V supply variations:

Next let's look at that effect of transistor Beta on current limit set point. I ran these sims with the default 200 Beta setting, as well as Beta of 100 and 300. The LM317 circuit doesn't require an external transistor, and the TL341 and Crutschow circuits appear to be immune to Beta variations. My circuit shows a little variation, maybe a few mA, while the Doughty circuit shows a little over 10mA of total variation as a function of Beta.

Finally, let's look at the effect of transistor Vbe on current limit set point. I ran these sims with varying IS parameters, corresponding to Vbe variations from around 0.65 to around 0.9V (rough estimates on those numbers.) Since all of these circuits except the LM317 rely heavily on Vbe, most of the graphs below show large variations. The Tl431 showed the least variation, Crutschow's circuit and mine were quite similar on this, and Doughty's was the worst, albeit not by terribly much.

Basically, the result of all these sims seems to be that the Doughty circuit works well enough if you have a well regulated supply voltage and you have a pretty good idea what Vbe and Beta numbers to expect. If not, Crutschow's circuit provides the best performance, and I'd like to think maybe mine finds the sweet spot in terms of reasonable performance with a simpler circuit. The other two, while interesting for comparison, don't meet my needs in terms of dropout voltage. If there are other things I should be looking at, like if my circuit is especially vulnerable to temperature effects or something else that I've overlooked, I'd love to know. (@OBW0549, you've given me some good tutoring on other current limiting circuits, a modified current-mirror circuit in particular, so I'd love to hear your thoughts on this arrangement.)

#### Delta prime

Joined Nov 15, 2019
230
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I've come up with a current limiting system with low dropout voltage, intended primarily for output short-circuit protection
Yes, It comes down to the bottom line cost
 .your application so on and so forth. The diodes are a natural progression in terms of stability to a voltage divider for the base.and then one would likely omit the two diodes for a single zener diode then 2 PNP transistors,
Then the complementary pair NPN and PNP to offset the temperature coefficient, then guess what? throw a Fet in there 2 around off everything .

The diodes are forward biased, establishing about a 1.4V drop. Diodes are used because their voltage will not vary significantly with a varying IN voltage, providing a stable reference. The transistor's BE junction needs 0.7V to turn on, leaving the remaining 0.7V across R1. We can choose the emitter resistor R1 to limit the OUT current to 100 mA using R=V/I=0.7/0.1=7ΩR=V/I=0.7/0.1=7Ω.
The value of R2 is much larger: it has to only conduct enough for the transistor to have enough base current drive and let some current through the diodes.
In this circuit, if OUT is short circuited to ground, still only 100 mA will flow (for the 7 ohm value of R1). In that case, the transistor will have some voltage across it, and will be dissipating heat. A power transistor with a heat sink may be needed. For instance, suppose the VCE voltage is 10V when there is a short circuit. In that case, the transistor is dissipating 1W.
You can use a single Zener diode instead of the two silicon diodes. R1 has to be adjusted then to keep the same current, in light of the Zener voltage.

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#### MisterBill2

Joined Jan 23, 2018
5,742
Certainly it appears that the proposed circuit will work as intended, and provide a limit to short circuited current. The trade-off is with efficiency and cost. And there is the consideration that the forward voltage on the added diodes varies with temperature. So the question becomes the benefit versus the added cost of components and complexity.

#### ebeowulf17

Joined Aug 12, 2014
3,274
And can you please watch your tone when regarding Mr.Crutschow.he does nothing but help and does it in such a way that he does not belittle the thread starters.
I didn't think I said anything negative about him, but if I did, I'm sorry. I agree with you that he is an extraordinarily helpful member, and the circuits he's shared in various threads are very well thought out.

My description of his circuit in this case said that, in terms of performance, his circuit was better in every way than the others I looked into. I hardly think that's an overly negative tone. All I said beyond that is that I was interested in seeing what level of performance could be had from a simpler circuit, specifically for situations where the limit didn't need to be quite so precise and where lower parts count might be a priority

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#### ebeowulf17

Joined Aug 12, 2014
3,274
Certainly it appears that the proposed circuit will work as intended, and provide a limit to short circuited current. The trade-off is with efficiency and cost. And there is the consideration that the forward voltage on the added diodes varies with temperature. So the question becomes the benefit versus the added cost of components and complexity.
Thanks, I forgot to mention efficiency. In my world, often focusing on espresso machines which pull 20-40A, a few mA is pretty negligible, so I tend to forget about that side of things. But yes, my circuit wastes quite a bit of power compared to the LM317 or Crutschow's circuit, so that's a definite downside to my design.

And yes, I need to take a closer look at temperature dependence in my circuit. Thanks for the reminder on that one too.

#### MisterBill2

Joined Jan 23, 2018
5,742
One more thought is the option to use an IC driver with built in current limiting. I am sure that such devices exist, given the value that they would provide. So that may be a better solution yet. ST semiconductors company has a huge portfolio of devices, as well as a whole lot of things sold to single customers, so they might already offer exactly what is needed. Just another thought about an alternative approach.

#### OBW0549

Joined Mar 2, 2015
3,300
(@OBW0549, you've given me some good tutoring on other current limiting circuits, a modified current-mirror circuit in particular, so I'd love to hear your thoughts on this arrangement.)
Your circuit seems fine to me; I see nothing wrong with it, provided the dropout voltage and temperature dependence are both acceptable in your application.

In my world, often focusing on espresso machines which pull 20-40A, a few mA is pretty negligible, so I tend to forget about that side of things.
20 to 40 amps? Or did you mean milliamps?

#### ebeowulf17

Joined Aug 12, 2014
3,274
Your circuit seems fine to me; I see nothing wrong with it, provided the dropout voltage and temperature dependence are both acceptable in your application.

20 to 40 amps? Or did you mean milliamps?
Cool, thanks for the double-checks!

As for current draw, I meant the whole machine, not these little current limiting circuits. Yes, the machines I spend most of my time on are 240V, 20-40A systems. As such, subtle differences in efficiency of the low voltage control systems don't feel like a high priority in our world!

#### atferrari

Joined Jan 6, 2004
3,916
Cool, thanks for the double-checks!

As for current draw, I meant the whole machine, not these little current limiting circuits. Yes, the machines I spend most of my time on are 240V, 20-40A systems. As such, subtle differences in efficiency of the low voltage control systems don't feel like a high priority in our world!
@ebeowulf17
Could you mention a model or show just a picture of those machines?

#### ebeowulf17

Joined Aug 12, 2014
3,274
@ebeowulf17
Could you mention a model or show just a picture of those machines?
I prefer to keep a tiny semblance of mystery to who I am and where I work as far as my online presence and public profiles, but I'll send you a private message.

#### BobaMosfet

Joined Jul 1, 2009
1,057
Your R-Sense resistor is essentially doing all the work- as it is in series with your load. Not sure the other components are really doing anything useful. You could use 2 resistors and 2 BJTs to actively control the load. A 3rd resistor allows you to control the output current in a very linear way over a very wide range.

The 10K Ohm resistor is the one you adjust to control output current on bottom-right BJT.

#### OBW0549

Joined Mar 2, 2015
3,300
@BobaMosfet: Take a look at your PNP transistor; the way it's connected, it'll never turn on since Vbe will always be zero.

#### ebeowulf17

Joined Aug 12, 2014
3,274
Your R-Sense resistor is essentially doing all the work- as it is in series with your load.
There is *some* voltage drop across the resistor, but I wouldn't say it's doing most of the work. If you look at the first graph in my original post, you'll see that with a varying load, as current increases from just a few mA up to 60mA, the voltage drop goes from negligible up to about 0.5V. Beyond 60mA, the transistor starts limiting, current is maintained at 60mA, and voltage drop across transistor increases rapidly accordingly.

In fact, all of the circuits I was comparing have a similarly sharp cutoff when the current limit is reached, albeit with different initial dropout voltages. I'm not sure why you think the rest of the circuit has no effect. The resistor is doing a very small portion of the current limiting.

As for your proposed circuit, I'm intrigued. It looks vaguely familiar, but it's not one I remember. I'll have to refresh my memory and learn how it works.

#### MisterBill2

Joined Jan 23, 2018
5,742
@BobaMosfet: Take a look at your PNP transistor; the way it's connected, it'll never turn on since Vbe will always be zero.
Indeed that is correct. The current sense resistor is not included. Probably a drawing goof. And so the LEDs will have lots of current, limited only by the drop in the NPN device. A fairly hot and rather short lifetime, I would guess. So there does not appear to be any mechanism to develop base bias of any polarity on that PNP transistor of the circuit in post #11.

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#### BobaMosfet

Joined Jul 1, 2009
1,057
@OBW0549 @ebeowulf17 @MisterBill2
I beg your pardon-- The transistor is a PNP... I mentally was thinking it was an NPN, hence my comment. I stand corrected ;P

#### ebeowulf17

Joined Aug 12, 2014
3,274
@OBW0549 @ebeowulf17 @MisterBill2
I beg your pardon-- The transistor is a PNP... I mentally was thinking it was an NPN, hence my comment. I stand corrected ;P
Hah! Yes, it would be a very different circuit with an NPN there. I can see why you had your doubts!

#### ebeowulf17

Joined Aug 12, 2014
3,274
Ahhh, internet searches are a funny thing - I was searching for examples of @BobaMosfet's circuit online to see what the standard arrangement is, and in the process I discovered that the circuit I just came up with was already online elsewhere! I can't say that I'm surprised that this idea was already out there, but it still caught me off guard to just happen upon it when I was searching for something else. Here's the circuit that I think @BobaMosfet meant to share (found via a Google image search here: https://www.wikiwand.com/en/Current_limiting):

And here's somebody else's version of my "original" circuit that I just found here:
http://www.ecircuitcenter.com/Circuits_Audio_Amp/BJT Current_Source/BJT_Current_Source.htm

P.S. I tried running the top circuit above through my various sims (varying Vbe, Beta, supply voltage, etc.) and to me it looked like a toss-up, slightly better in one test, worse in other tests. Definitely a viable solution, but nothing extraordinary.

#### MisterBill2

Joined Jan 23, 2018
5,742
Ahhh, internet searches are a funny thing - I was searching for examples of @BobaMosfet's circuit online to see what the standard arrangement is, and in the process I discovered that the circuit I just came up with was already online elsewhere! I can't say that I'm surprised that this idea was already out there, but it still caught me off guard to just happen upon it when I was searching for something else. Here's the circuit that I think @BobaMosfet meant to share (found via a Google image search here: https://www.wikiwand.com/en/Current_limiting):
View attachment 201128

And here's somebody else's version of my "original" circuit that I just found here:
http://www.ecircuitcenter.com/Circuits_Audio_Amp/BJT Current_Source/BJT_Current_Source.htm
View attachment 201129

P.S. I tried running the top circuit above through my various sims (varying Vbe, Beta, supply voltage, etc.) and to me it looked like a toss-up, slightly better in one test, worse in other tests. Definitely a viable solution, but nothing extraordinary.
The difference being that in post #11 R_Sense is missing.

#### OBW0549

Joined Mar 2, 2015
3,300
If you want an even lower dropout voltage than the above circuits provide you can get it, albeit at some extra complexity and cost. The following 5V, 50mA limiter has a dropout voltage of only 50 mV and is also stable with temperature since it doesn't rely on a BJT's Vbe to set the current.

Voltage divider R1/R2 determines the setpoint for the voltage across current sense resistor R5. Op amp U1 provides the control function, and MUST be a RRIO op amp. C1 and R4 discourage instability and oscillations.

U1 and Q1 are a lot costlier than a pair of jellybean BJTs; but if you absolutely must have the lowest possible dropout voltage the above circuit is one way to get it.

#### atferrari

Joined Jan 6, 2004
3,916
If you want an even lower dropout voltage than the above circuits provide you can get it, albeit at some extra complexity and cost. The following 5V, 50mA limiter has a dropout voltage of only 50 mV and is also stable with temperature since it doesn't rely on a BJT's Vbe to set the current.

View attachment 201138

Voltage divider R1/R2 determines the setpoint for the voltage across current sense resistor R5. Op amp U1 provides the control function, and MUST be a RRIO op amp. C1 and R4 discourage instability and oscillations.

U1 and Q1 are a lot costlier than a pair of jellybean BJTs; but if you absolutely must have the lowest possible dropout voltage the above circuit is one way to get it.
After staring at the above, I concluded that the next time I need to design a current limiting circuit the first I would consider is an adjustable (or not) constant current source with enough allowance for a certain variation of the input voltage.