250mA voltage clamp

It's a voltage clamp. When the input voltage is above 24V, the differential will be Vi - 24V. When the input voltage is below (24V + Vdropout), the differential will be Vdropout. I want Vdropout to be less than 2V, preferably less than 1V. (Though there is no requirement that Vdropout be precise or constant.) I'm sorry if that confused you.

I could simply use a LM317 as a preregulator. According to the datasheet its dropout voltage (at room temperature or higher, at less than 500mA) is around 1.7V. But I don't know whether to trust it to remain stable around and below dropout, which is why I asked about that.

And in answer to Jim's question: it takes about 300mA draw for the internal impedance of the supply to drop the voltage sufficiently. I'd rather avoid a shunt regulator if I can, so as to avoid unnecessary heating. The max-load condition is unusual and is expected (though not guaranteed) to last only a short time; 99% of the time the load is expected to be only 30mA or so.

What's wrong with a 24 volt zener? Pick one that will handle the dissipation when the supply is current limited by its internal impedance. What is the current from the supply when it has a load heavy enough to drop its output voltage to 24 volts?

Jim

The interesting question is how he plans to limit the I/O differential to 2V with 35V in and 24V out- looks like 12V to me, and at 1/4 amp makes for 3W. Sounds like someone is confused, again.

In article , Walter Harley wrote: [...]

If the supply is isolated, you can put the pass element in the (-) leg. This lets you use a low cost MOSFET.

The supply voltage to the op-amp can be regulated by zener. If you really want, this zener could also serve as the reference.

You didn't rule out putting a large capacitor across the output terminals. If you do this, you can make the common source MOSFET stage have its high frequency gain determined mostly by the gm and the output capacitor. BTW: A source resistor can make the MOSFET's gm easier to predict.

By limiting the supply voltage on the op-amp, you reduce how far it has to swing as the supply rises past 24V. The op-amp should be a fairly fast one. The output capacitor should be the important pole in the system.

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kensmith@rahul.net   forging knowledge

OK, a DC-DC converter, buck/boost type between your source and load would solve all your problems at 80% efficiency. And be much easier to implement than some op-amp and pass element scheme.

Jim

A voltage clamp is just a simple emitter follower with a zener from base to GND. The TIP41 with a 24V zener will work well. The trick is to keep the power transistor base drive low impedance and somewhat regulate the zener current against your 18V to 35V input swing. You can use something minimal like this: View in a fixed-width font such as Courier.

. . . V+ . | . .-------------+ . | | . | | . [270] | . | | . | | . +------. | . | | | . | [47] | . >| | |/ . 2N2907A |----+----| TIP41 . /| | |>

. | | | . | | | . | | + '-----> OUT . [1K/1W] Z . | A 24V [1N4749] . | | - . | | . '------+ . | . | . --- . /// .

Surely replacing the highly unregulated power supply would be easier still? I fail to see the point in spending time building a fix for a simple sub-assembly that has a problem, when another PSU would solve it.

Thanks!

That makes sense; regulating the current to the Zener makes it possible to handle the big voltage swing without dissipating too much heat there.

But, this circuit achieves that regulation by shunting the excess current to ground; it's not a lot of current but I got to wondering whether I could do it with a series element instead. First idea was to use a JFET current limiter, but those drop a lot of voltage, which would have meant too-high dropout voltage.

But doing it with a BJT current source seems to work, although it does have one more part:

TIP31 -----o---o---- -------- | | \\ ^ V .-. --- - | | | | | |72 | V '-' | - | | | |< | o-| | | |\\.-----' .-. | 2N2907 | | | 22k| | - '-' ^ 24V Zener | | -----o---o---------------

The current source needs to provide the max base current of the pass transistor, plus enough to keep the Zener in regulation. When the output load is low, all the current passes through the Zener; but that's still only (Iout(max)/beta(min) * 24V) or about 1/4W assuming min beta of 25.

I haven't built it yet, just thought about it.

I still wonder whether an LM317 is stable around and below dropout?

Yes, it just saturates to its dropout voltage (which varies with current, and junction temp, as NSC's datasheet shows in plots.

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 Thanks,
    - Win

It is not necessary to shunt the current to ground- it can be shunted to the output. Since you say the load is 30mA most of the time , this works well. If your loading goes to some minuscule amount, the PNP will just saturate and the output may go to 24.75V which is not the end of the world, and the zener receives max current of (35-24.75)/330=30mA for a

750mW dissipation at no load. At nominal loading of 30mA and maximum Vin, the zener is biased at 0.75/68=11mA (minus NPN base current) and the PNP collector current is 30mA-11mA=19mA direct to the output leaving 11mA for the NPN.

View in a fixed-width font such as Courier.

. . V+ . | . .-------------+ . | | . | | . [330/1W] | . | | . | | . +------. | . | | | . | [68] | . >| | |/ . 2N2907A |----+----| TIP31A . /| | |>

. | | | . | | | . '------|------+-----> OUT . | . | . | . Z + . A 24V [1N4749] . | - . | . | . | . | . --- . /// . .

That circuit is more of a standard, and has lower output impedance at dropout voltage by about a factor 5. That sounds good initially but in the case of moderate capacitor filter/load on the output, combined with a sizable unregulated ripple, the saturated input results in much more ripple current loading of the NPN. It is better in a DC application but may be problematic in an AC application.

You may be thinking of a conditional stability type of thing - and this is not an issue with the LM317 design.

I'm not sure I see why that's an improvement? The current through the 22K will vary between about 0.7mA and 1.5mA depending on the input voltage, but all that current is doing is biasing the two diodes in order to program the current source, and the current source itself is just biasing the Zener, so the effect on the output voltage should be miniscule (and the clamp voltage itself is noncritical, it just needs to hold the output low enough to protect some downstream components). Plus, these current-limiting diodes expensive - $1.50 or more even in quantity, compared to a penny or two for the resistor. Is there something I'm missing about this?

How about this to improve it. Change the 22K to a 1N5305...1N5313

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kensmith@rahul.net   forging knowledge

Perhaps, perhaps not. It depends on how tight you need the line regulation to be. If the diodes are 1N914s, the voltage drop on each will vary by, lets say, 40mV each. This means that the 70 Ohm (IIRC) will have at least 40mV of variation on it.

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kensmith@rahul.net   forging knowledge

It doesn't need to be at all tight - this is just a preregulator. All it has to do is make sure that the output voltage doesn't rise above the point where it would damage downstream circuitry, while not dropping excessive voltage (or wasting too much current) otherwise. So, simpler is better, in this case.

But regardless, a 40mV change in voltage on that 70 ohm resistor is less than 5% change in the current source output; and the only thing that directly affects is the amount of current through the zener. The current through the zener will be changed by much more than 5% as the load, and thus the base current of the pass transistor, changes - in fact I expect at least

10:1 change in zener current. So, precisely regulating the current source doesn't really buy anything in terms of the regulation of the clamp voltage, I think.

Cheers, -walter

To confirm, I hooked up an LM317 to provide 21V, with a load switched between 75R and 1k by way of a MOSFET controlled by a pulse generator. Then, with a scope I watched the output as the load switched on and off, while varying the input voltage from 18V to 35V.

As Win suggests, the LM317 stays quiet and stable when its input voltage is too low to regulate. There was no hint of ringing or noise around the transitions. When the input voltage is too low, the LM317 looks like a 2.5V drop, in series with an ohm or so.

I was a little surprised that the drop was 2.5V, even with only 1k load (in parallel with the ~4k divider network). From the datasheet I expected it to be lower. I was using a JRC part rather than National, but the JRC datasheet still suggests dropout < 2V.

That's interesting.

--
 Thanks,
    - Win

Walter Harley wrote: >

With that much dropout it makes no sense to let it drop out. You can keep the LM317 in regulation by regulating to filtered Vin - 3V up to a clamped upper limit as required. We are talking about a simple level shift and diode clamp- and not a lot of excess current. The extra 0.5V drop is more than paid for by the high performance of the regulator, and you have all the protection the LM317 offers. Don't forget to add all the usual diode protection around the IC. View in a fixed-width font such as Courier.

. . +--------+ . Vin>--+-----+--------------|IN OUT|-----+----+-->

. | | | | | | . | | | ADJ | | | . | | +--------+ [220] | . [4.7K] [10K] | | | . | | | | |+ . | | .--|>|--+----------+ === . | | | | | | . | | | | 1N4748 | | . | | | |< ---/ | . +----------------+-----| Q2 // \\ | . |+ | | |\\ --- | . === | |/ | | | . 100U +--------| Q1 | | | . | | |> | | | . | | | | | | . | | [620] | | | . | | | | | | . | '-|>|-|>|--+ | | | . | | | | | . +----------------+-------+----------+----+-->

. | . ---

That's a nice circuit. Am I right that the function of the diode is to protect Q2's base-emitter junction from reverse breakdown? Or does it do something else that I'm missing?

I like the idea of having the LM317's protection. But I'm still leaning toward the discrete approaches, because I don't like having this much input-output differential in the non-clamped case; it means that the piece of equipment this is powering won't handle brownouts as well. Probably a non-issue, but I hate for my stuff to be the weak link. I'll probably go with one of the discrete circuits instead.

Thanks! -walter

Yes- the NPN current source keeps the base of the PNP at ~Vin-5 while the zener clamps at 22V, so the PNP BE junction starts to reverse bias at Vin=27V.

Nope.

I suppose you could adapt the same approach to an LDO, pre-regulating say 0.2V-0.5V down up until clamp level,- there are a few that handle

40V IIRC, or use a more common and cheaper one and install a zener over-voltage bypass. If you clamp at 24V and Vin,max=35 then the LDO only needs to hold off 11V worst case. The trick is to find one in something easy to heatsink like a TO-220 , and the 250mA current requirement is not an issue.