jfet as current limiter

As I understand it, that's one of the quirks of JFETs - a fairly wide range of process variability (Idss in particular) within any given JFET part number.

The only solution I know if, amounts to part screening... buy a whole bunch of JFETs, test them, and bin them. The tighter you need your current tolerance to be, the more bins you'd need (with each bin using its own specific resistor value, to achieve the desired current limit).

Or, you can buy "current source diodes", which are essentially pre-binned JFETs with gate and source shorted together internally.

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If you can afford a little voltage drop, use an LM317 in current limit mode.

How much voltage will the gizmo be dropping if the load short-circuits? You might be looking at a two-terminal BJT solution, e.g.

R2 = 0.6V/Imax IN OUT

0--*------------ --------*----R2R2---*---0 | \ A | | | Q1 ------ | | | | ---- Q2 | | | / V | *---R1R1-------*------- -----------*

Q1beta_min*(Vdrop_max-0.6) R1 ~= -------------------------- Imax

To reduce the fault current, you can add a bit of foldback by connecting Q2's base to a voltage divider between IN and the emitter of Q1. A bit of figuring is required to get the right current limit and short circuit current.

Don't try folding back by more than about 2/3, i.e. 30 mA I_trip, 10 mA I_SC, or you may have problems starting up. (Depending on the load, this may be more or less of an issue, but it's perfectly possible to fold back to zero current, at which point you have a latch and not a current limiter.

A little bit of foldback is helpful in sharpening up the knee, which otherwise tends to be pretty squishy due to the lowish loop gain and to the current-dependence of Q2's V_BE.

Cheers

Phil Hobbs

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You'd get less spread using a logic level MOSFET with the source resistor fed to a bipolar which in turn shunts the gate voltage - but then you'd need a collector supply/gate bias.

The Supertex depletion mosfets are more repeatable than a typical jfet, but the datasheet limits still aren't good enough to sneak between 23 and 30 mA without adjustment.

One of these would work:

The top one is cute, and glows in the dark, but self-heating of the pass transistor/Vbe change will be an issue. Maybe 2% per degree C or so.

The little fet is a Supertex LND150 or 250, which will current limit at about 1.5 mA.

In the lower one, a bandgappy thing could be used to sense the current, with maybe a mosfet pass transistor, but the minimum voltage drop would go up.

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below is a sim of an inline current limit circuit. I used a loop supply in the range of 6 .. 24 V as a test voltage in the loop with variable load currents to show how it stays with in 4..22mA or so, using different loop supply voltages. I think I can do one with a low voltage op-amp inline that supplies itself via the loop voltage that will give you even better response across difference loop supplies. More on that later.. Jamie

Version 4 SHEET 1 880 680 WIRE 160 32 48 32 WIRE 256 32 160 32 WIRE 48 48 48 32 WIRE 160 48 160 32 WIRE 48 144 48 128 WIRE 160 144 160 128 WIRE 160 144 112 144 WIRE 256 176 256 32 WIRE 112 192 112 144 WIRE 192 224 160 224 WIRE 112 288 112 256 WIRE 256 288 256 272 WIRE 256 288 112 288 WIRE 256 320 256 288 WIRE 256 320 224 320 FLAG 48 144 0 FLAG 224 384 0 SYMBOL voltage 48 32 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL load 208 320 R0 WINDOW 0 43 41 Left 2 WINDOW 3 107 130 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 48 84 Left 2 SYMATTR InstName I1 SYMATTR Value PULSE(0.100 0.004 0 .2 .2 .5 1) SYMATTR SpiceLine load SYMBOL npn 192 176 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL res 144 32 R0 SYMATTR InstName R1 SYMATTR Value 15k SYMBOL References\\LT1634-2.5 112 224 R0 WINDOW 3 -154 3 Left 2 WINDOW 0 -50 33 Left 2 SYMATTR InstName R2 SYMBOL res 144 128 R0 SYMATTR InstName R3 SYMATTR Value 18k TEXT -40 32 Left 2 !.tran 1

r e . d

The benefit of the LM317L is that it can withstand 40 volts, has foldback based on power and temperature, it has a very high dynamic impedance when in current limit and low parts count.

Cct below limits at 23mA |----------|

--------| LM317L |------/\/\/\---+--- |----------| 51R | | | +--------------------+

Il 10/10/2012 04:03, Jamie ha scritto:

Great! The actual circuit should be closer to this:

Version 4 SHEET 1 880 680 WIRE 160 -128 -304 -128 WIRE 160 -96 160 -128 WIRE 160 32 160 -16 WIRE 256 32 160 32 WIRE 160 48 160 32 WIRE -304 144 -304 -128 WIRE 160 144 160 128 WIRE 160 144 112 144 WIRE 256 176 256 32 WIRE 112 192 112 144 WIRE 192 224 160 224 WIRE -304 272 -304 224 WIRE 112 288 112 256 WIRE 256 288 256 272 WIRE 256 288 112 288 WIRE 256 320 256 288 WIRE 256 320 224 320 WIRE 224 336 224 320 WIRE 224 448 224 416 FLAG 224 448 0 FLAG -304 272 0 SYMBOL npn 192 176 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL res 144 32 R0 SYMATTR InstName R1 SYMATTR Value 15k SYMBOL References\\LT1634-2.5 112 224 R0 WINDOW 3 -154 3 Left 2 WINDOW 0 -50 33 Left 2 SYMATTR InstName R2 SYMBOL res 144 128 R0 SYMATTR InstName R3 SYMATTR Value 18k SYMBOL res 208 320 R0 SYMATTR InstName R4 SYMATTR Value 200 SYMATTR SpiceLine "" SYMBOL voltage -304 128 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 24V SYMATTR Value2 "" SYMATTR SpiceLine "" SYMBOL current 160 -96 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 28 Left 2 SYMATTR InstName I1 SYMATTR Value 10m SYMATTR Value2 "" SYMATTR SpiceLine load TEXT 320 -80 Left 2 !.dc I1 0 50m

and the simulation shows the curve I want to see! I'm sorry for my ignorance, but why it not depends on the physical characteristic of the bjt?

Thanks again! Marco

Sure, but it drops 3 volts. The OP wanted the limiter to go away when the current was below the limit. You can almost do that with a circuit breaker, but not at 30 mA.

Cheers,

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

The other approach (used by the OP) was JFets and they drop more than 3 volts if you want any sort of accurate current control. Can yo use a lm1117 as a current source or do they oscillate. That would lower the drop out voltage to 2.5v

The OP wanted the limiter to go away when

The two-transistor model drops 0.6V at low current, rising to ~1.3V at the knee.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Not sure as to what you mean by that last statement? The operation of that circuit does depend on a bjt characteristics. The Vbe and Vce(sat) combined will give much less over loss than what you would get with a jFet or linear regulators

Vce(Sat) should be ~170mV and Vbe being around 0.7v to a sum of 0.870v loss in this type of circuit.

R1 supplies minimum current in series with R3 to achieve ~ 20mA at around 6 volts in the loop supply. WHen loop supply is up to 24 volts, the 2.5V ref will clamp that and attempt to give a steady reference to the base via R3.

bjt's work in the same manner as fets do with the exception of how they are biased of course. With a fet, it's all about the Vgs and the V(Th) with the Vgd that turns it into a constant current device, but the problem his is, you normally have to get up in the Vg(th) region for it to start working, that could be higher than the losses you wish to sacrifice.

With bjt's, it's always current biased instead of voltage and the voltage here starts ~0.7v and is repeatable within reason between batches.

Jamie

Il 11/10/2012 02:21, Jamie ha scritto:

Ok, I've already got it. But...

...here the good news! I'm afraid changing the bjt (batches, not models) I run again into the same issue. The advantage, as you pointed out, is we rely on a simple junction threshold.

I'm building the circuit to test it in the real world :)

Thanks again for the explanations.

Marco

On a sunny day (Thu, 11 Oct 2012 07:43:17 +1000) it happened "David Eather" wrote in :

There is an other simple way, providing you have some stabilized vlatge around: V unregulated | --- V stable \ / -> LED | --- 100 kR1 | | c ----------------- b NPN e | /// Iled = ( (Vstable - .7) / R1) * beta

If you take cheap BC547-B or something like that from one batch it matches close. If you have no stabilized supply use the unregulated one, it still works as limiter. Dropout is less than a volt, Vmax 40V or more. Watch dissipation in transistor.

Beta controlled current? Gack! That's about the worst current source/sink I've ever seen proposed here in SED.

close.

limiter.

On a sunny day (Thu, 11 Oct 2012 11:40:32 -0400) it happened " snipped-for-privacy@att.bizzzzzzzzzzzz" wrote in :

around:

It works very well, I have used it. And this circuit also interfaces to a micro / CMOS output, where the micro is on a stabilized supply. Beta is a physical entity, and you should actually always try to use / take those those into account in your circuits.

Sending transistor into saturation as an idiot and then putting some resistor in series with the LED in case of the micro, simply costs more parts, and gains very little.

close.

limiter.

This is fun, an LM317 as a self-resetting fuse:

I used this to protect some cheap sip dc/dc converters that otherwise suicide if you short their outputs.

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Picosecond-resolution Digital Delay and Pulse generators 
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On Thursday, October 11, 2012 2:31:03 AM UTC-7, Jan Panteltje wrote: [ a current limiter with sharp knee in 23-30 mA range]

It's OK for a quick hack, but requires beta-sorting of transistors; the given target range (23 to 30 mA) is a bit narrow for standard beta ranges of parts. I'd not like the lifetime/aging of this, either: audio amps with this kind of trimmed-gain have turned black on me.

Better by far, if you have negative power supply (-12V is good) would be V unregulated | --- \ / -> LED --- | c GND--------- b NPN e | Rlimit | (-12V)

Iled = (12V - 0.7V)/Rlimit about 1% temperature variation in 0-50C range

or the nearly-equivalent current mirror solution. Both will dissipate Vsupply times Ilimit in quiescent state, though, and require three terminals