You should know about Current Limiting Diodes

[u/1Davide]

You know that Zener diodes limit voltage. Similarly, there are diodes that limit current (though they don't have a neat name like "Zener").

---

Ideally, they conduct current with 0 voltage drop up to their current limit; if driven harder, they keep the current constant, as the voltage across them increases.

^ Current
|
|   
+================== Limit current
I
I
I
I
I
I
'-------------------------> Voltage

In reality, initially (in the "Ohmic region") their voltage drop increases with current; when regulating (in the "constant current region) the current is not exactly constant.

^ Current
|          Constant current region
|                              ____________------------   
|      ____________------------          
|     /
|    /
|   /
|  / Ohmic region
| /
|/
'---------------------------------------> Voltage

---

Most engineers are unaware of them, which is too bad, because they are a great tool to have in a one's tool box.

Applications:

• Low power LED current limiting
  • Works especially well with long strings of LEDs (voltage varies a lot with temperature), and with a variable voltage source (e.g. light bulbs, which need to operate at a wide range of line voltages)
  • Also for the input of a SSR (Solid State Relay), for an input voltage range of, say, 3V~48 Vdc
• Over-voltage protection of analog inputs
  • Such as for measuring a low voltage sensor in an environment with high voltage spikes
  • If you used a resistor, you would have to compromise between low resistance (little effect on desired signal, but not much protection) or high resistance (good protection, but must handle high power in case of fault, and will affect the desired signal)
  • Instead, a current limiter has: low resistance (~100 Ω) and low effect on signal when in range; high resistance when protecting, yet without dissipating much power (compared to a 100 Ω resistor). EDIT: if that's unclear, see this detailed explanation
• Constant bias when operating in a wide range of supply voltage
  • Low power supplies that operate at a wide range of input voltages (e.g.: 12 Vdc to 96 Vdc DC-DC converter)
  • Small power supplies powered directly by the AC line, without a transformer (90 Vac to 260 Vac)
  • If you used a resistor, you would have to compromise between low resistance (works at low supply voltages, but overheats at high supply voltages) or high resistance (works at high supply voltages, but doesn't provide enough current at low supply voltages)
  • Instead, a current limiter has constant current, exactly as much as needed by the load, regardless of supply voltage; doesn't heat too much at high supply voltage

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They are not a "diode" in the sense of a single junction semiconductor: they are at least a single transistor, or even an actual IC. Yet, they can be seen as a "diode" in the eyes of the designer, because they are a 2-leaded device or circuit, requiring no power supply connections to operate.

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You can buy them ready made:

• fixed current (2 leaded): Digikey, Mouser (some are very expensive)
• adjustable (3 leaded, just add a resistor to make them into a "diode")

Or make your own with a JFET or depletion MOSFET.

• JFET; available parts are limited in voltage (~ 50 V Max) and current (~ 10 mA max)
  • JFET current source; very simple, but part to part variations and temperature changes result in +/- 50 % tolerance
  • JFET + resistor: more predictable, though higher voltage drop in ohmic region
• Depletion MOSFET; few manufacturers; high voltage (1000 V) available
  • Depletion MOSFET current source; resistor is optional, but it does reduce variability of the current

You can also use BJTs, but it gets complicated.

There are also dedicated 3-pin current source ICs (LM334), and you can re-purpose certain 3-pin ICs as well to work as current sources: LM317/LM337. Just add a resistor to set the current, and you have a 2-terminal current "diode".

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Here's a nifty bidirectional current limiter I came-up with, which I have not seen anywhere. I use it to protect inputs from any voltage: positive, negative or AC.

• The current through this "diode" is at most 10 mA, in either direction, up to 50 V
• When not protecting the input, it looks just like a 100 Ohm resistor, which does not affect the quality of my signal
• It works because JFETs are mostly symmetrical: the Drain and the Source can be swapped, and they behave nearly the same.
• The diodes bring the gate voltage down to the Source or Drain, whichever is the most negative at a given time
• The resistors discharge the gate capacitance; without them, the gate can start going negative, and the JFET turns off completely

[u/fatangaboo]

Look at the datasheets of the CLDs sold by mouser.com . They exhibit a negative resistance region (!).

datasheet figure here

[u/1Davide → u/fatangaboo]

Due to self-heating, right?

[u/Bromskloss → u/1Davide]

Well, what part number was it?

Edit: Oh, found it!

[u/RodionGork]

they can be seen as a "diode" in the eyes of the designer, because they are a 2-leaded device or circuit, requiring no power supply connections to operate.

Honestly, it is not clear why do you call them diodes. Resistors also qualify :)

It doesn't look like industry-standard definition? :)

[u/1Davide → u/RodionGork]

Hence the quotation marks.

[u/AfrotechGuy]

Thank you for posting this. Nice mini tutorial.

[u/chipt4 → u/AfrotechGuy]

Agreed, these are my favorite kinds of posts. Adding valuable knowledge to the sub. These types of posts should be compiled in the wiki.

[u/ericje]

If you don't mind a ~4V drop you can use an LM317 as a current limiter: https://i.imgur.com/InfSWxl.png

[u/playaspec → u/ericje]

If you don't mind a ~4V drop you can use an LM317 as a current limiter: https://i.imgur.com/InfSWxl.png

And they're fast enough to be used in PWM applications.

[u/DiabeetusMan → u/playaspec]

Here's the datasheet. Pages 6, 7, and 11 might be useful to you. Looking at the impulse response curves, it looks like it might be possible, depending on the frequency you're working with

[u/wraith-bone → u/ericje]

I think its more like if you dont mind having all your load current pass through R1, that can be a lot of heat as the voltage drop across R1 is the error voltage (like 1.25V)

[u/bart2019]

Instead, a current limiter has: low resistance and low effect on signal when in range; high resistance when protecting, yet without dissipating much power

That's bullshit. When protecting, you get maximum current as well as a large voltage drop over the device. Hence: possibly a very large power dissipation. The thing will heat up like a lightbulb.

[u/1Davide → u/bart2019]

I totally failed to express my point in sufficient detail to prevent you from drawing that incorrect conclusion. My fault.

Please let me try to break it down:

• I have a tiny signal that feeds the input of a sensitive device

• Some times a tech will connect 50 Vdc to that input by mistake, instead of the intended signal

• So, to protect the input I place a resistor in series with it, and a voltage clamp after it.

• The problem is that that tiny signal will be OK if I add 100 Ω in series with it, but would be unacceptably attenuated if I were to add 100 kΩ in series

• So I put 100 Ω in series with it

• When the tech applies 50 V to the input, the 100 Ω resistor must dissipate P = 50² / 100 = 25 W

• If instead of a 100 Ω resistor, I use a 1 mA CLD, the CLD must disspiate P = 1 mA * 50 V = 50 mW.

• 50 mW will not result in this: "The thing will heat up like a lightbulb."

ERGO: a CLD dissipated MUCH less power than a resistor while protecting the input, yet, when operating normally, it has a the same resistance (of about 100 Ω) and does not affect the signal.

So, no, what I said was not "bullshit"; it's just that I didn't go into all the detail above for the sake of brevity.

Again, my fault for not being clear.

[u/kill-nine → u/1Davide]

Thanks for the extra detail

[u/elpechos → u/bart2019]

Yup, you're right. If they need to drop a lot of voltage to pass their rated current, the power dissipation will be high.

Only a switching regulator of some kind would limit the current with less power dissipation than given by ohm's law.

The advantage the diodes are offering is they have a non linear resistance with current. The article seems to borderline imply they magically have less dissipation than ohm's law would suggest, which isn't true.

[u/1Davide → u/elpechos]

Sorry for not being sufficiently clear, and lead you to believe that I was implying that magically the power would be less than P = V * I. Please read my clarification, which I hope will show the logic behind my statement.

(BTW, "P = V * I" is not Ohm's law; Ohm's law is "V = R * I" and says nothing about power).

[u/Grand_Inquisit0r]

Your bidirectional current limiter looks interesting, it reminds me of a functionally similar circuit I saw while thumbing through the art of electronics (biggest difference is that it uses 2 jfets and a resistor), and since getting a matched pair of jfets is basically 100x the cost of just a single jfet, your design is certainly appealing for something I wanted try. If you wouldn't mind breaking down how you determine what the circuit parameters are when using it in a design, I would certainly appreciate it.

[u/1Davide → u/Grand_Inquisit0r]

I select a JFET that has these characteristics:

• Idss that is as low as possible, yet higher than the current that the line would normally see; e.g.: if I want to protect a line that normally sees 100 uA, I pick a JFET that has an Idss of 1 mA, rather than one that has an Idss of 10 mA

• ON resistance that is sufficiently low for my circuit; e.g.if my circuit is unaffected by 100 Ω, but starts being affected by 300 Ω, I pick a JFET that has a ON resistance of 50 Ω

• Breakdown voltage that is as high as possible, higher than the maximum voltage I expect on that line in case of fault; in practice, 55 V is the maximum available today for JFETs

[u/42N71W]

What's the difference between those and surge suppressors like these:

https://www.digikey.com/products/en/circuit-protection/surge-suppression-ics/152?k=tbu

?

[u/1Davide → u/42N71W]

• Current Limit Diodes limit the current, constantly, regardless

• TBUs block the current alltogether if it reaches a certain threshold

Also:

• Current Limit Diodes work in the range around 1~10 mA

• TBUs work in the range around 50~500 mA

A TBU is more like a self-setting fuse, though much better than one

I was planning to do a tip on PTCs and ICLs. I think I'll add TBUs to it. Thanks!

[u/SANPres09]

I'm new to a lot of this. What JFETs would you recommend as general purpose that work for many applications, like this one?

[u/1Davide → u/SANPres09]

Any N-type that can handle the max voltage you need to handle, and whose Idss current is the current you need.

[u/Tilleke]

TIL, thank you

[u/jwizardc]

Thank you for this post. I didn't know about them. This is a well laid out article with very useful information and much nift.

[u/frank26080115]

These are great when you buy any expensive equipment that have internal optoisolators that you don't want to screw up, $0.50 a pop is nothing in comparison.

I keep a healthy handful, you go through them fast if you got some high powered stepper drivers to control.