I'm playing with a n-channel jfet (2n4392) as current limiter (resistor on source, with gate connected on the other end).
Unfortunately, the limit value depends on the resistor value but on the jfet geometry too. Several transistors bought at the same time, will have a cutoff voltage quite different. Too much for my application.
I need to limit the current under 30 mA, with no effect under 22-23 mA.
Is there a smart way to set the limit value independently of the particular jfet?
The circuit must be a bipole: I cannot rely on other voltages or ground connections.
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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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:
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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
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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.
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 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.
...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 :)
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.
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.
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
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