There's also a slight gain function from the HV ripple developed across the zener modulating Vgs of the FET. I'm not going to speculate about a bunch of what-if's as to what kind of environment the circuit has to tolerate, ju st too open ended. The original post said something about just 10mA load or so and a 48V supply, so that means he can easily power limit his source to something workable and eliminate all this crazy conjecture about spikes, b lown junctions, unlimited rise times, and megalithic energy storage reservo irs all over the place ready to unleash destruction.
Yeah, that's the serious one. 78Lxx doesn't even specify a max dropout, just a typ.
I was thinking
The thermal shutdown of the 78L12 is defeated by the low in-out voltage. So if the load is shorted, the depletion fet gets all the heat and could unsolder itself. The "peak output current" spec of a
78L is 2.2 amps!
Shorting the load could blow the fet gate. Well, probably not.
The fet might do the RF oscillation thing.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
A very clear demonstration of your complete misunderstanding of circuit ope ration. As the 3-terminal unloads the FET, the source-gate junction reverse biases turning the FET off. Since the 78LXX has approximately 3mA load ind ependent internal bias drain through its GND pin, FET leakage to the point of overvoltaging the regulator is out of the question.
There are any number of ways to make the Supertex circuit bullet-proof. This, or anything obviously equivalent to it, takes care of the "That's OK, but it probably doesn't guarantee the 78L12 dropout voltage" problem.
Lots of parts. And why use a depletion fet now that you have the gate drive parts? Enhancement fets are cheaper.
This is simple:
V+-------+-----+ | | | | | | R | | | | | | d +----g nfet, either kind | s | | Z | | +-------78L12----out | | | | C gnd gnd | gnd
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
ish. There are any number of ways to make the Supertex circuit bullet-proof . This, or anything obviously equivalent to it, takes care of the "That's O K, but it probably doesn't guarantee the 78L12 dropout voltage" problem.
Looks like the main advantage to the original SuperTex circuit was that the gate was tied to a tightly regulated node which in turn improves the rippl e reduction at the 78LXX input, making for a exponentially quieter circuit. How about using the FET to make a nominal current source feeding a zener s hunt pre-regulator for the 78LXX, that's pretty simple. I don't think ultra
-low power is a big consideration for a line operated circuit.
Instead of waving your hands in the air, why don't you tell us what is wrong with the circuit?
Na-a-a-aw! Really! I didn't know that >!-)
Quit whining and tell us what is wrong with the circuit.
Are allusions all you have to offer... besides condescending Pecksniffian crap? ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85140 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
operation. As the 3-terminal unloads the FET, the source-gate junction reverse biases turning the FET off. Since the 78LXX has approximately 3mA load independent internal bias drain through its GND pin, FET leakage to the point of overvoltaging the regulator is out of the question.
really?
For someone that is always in trouble presenting their case, does not seem to be in the right corner for making such assessments.
I understand it very clearly, holes in it all over the place.
Get real, take it like a man, drag queen or what ever you are.
I listed my concerns about an hour before you posted this.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
If the FET is dropping at least several times the max in-out voltage spec of the 78Lxx, that would be sort of hard to avoid, regardless. I really like Tyco's idea of laminated polyfuse + transzorb combinations--the heat from the transzorb makes the polyfuse switch faster. Kind of hard to do with packaged parts, of course.
Oscillation is a possibility if the 78Lxx's input cap is small. Of course 78xx devices can oscillate with too-small caps all by themselves.
I do like the general idea of using a well-behaved jellybean with a bandaid wrapped around it, though.
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 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
zener modulating Vgs of the FET. I'm not going to speculate about a bunch of what-if's as to what kind of environment the circuit has to tolerate, just too open ended. The original post said something about just 10mA load or so and a
48V supply, so that means he can easily power limit his source to something workable and eliminate all this crazy conjecture about spikes, blown junctions, unlimited rise times, and megalithic energy storage reservoirs all over the place ready to unleash destruction.
My V+ will be up to 48 volts supplied by the customer, current limit unknown, and I'll have 30,000 uF of capacitance on that. That could make a decent bang/smoke event.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
You bloviated. You gave no specifics. Your Pecksniffian echo listed a bunch of BS.
How about some facts?
I'm not holding my breath. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85140 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
zener modulating Vgs of the FET. I'm not going to speculate about a bunch of what-if's as to what kind of environment the circuit has to tolerate, just too open ended. The original post said something about just 10mA load or so and a
48V supply, so that means he can easily power limit his source to something workable and eliminate all this crazy conjecture about spikes, blown junctions, unlimited rise times, and megalithic energy storage reservoirs all over the place ready to unleash destruction.
Your cluelessness is phenomenal >:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85140 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I don't think there's an appropriate low impedance path.
Certainly no protection against a Hobbs' fart >:-}
You're making stuff up. Trying to emulate Larkin?
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85140 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
So, Jim, when did you become such a Charles Dickens fan?
Great writer, Dickens, no problem there, but suddenly every second post you're referring to "Martin Chuzzlewit", which isn't one of his most popular works these days. Did you read it recently?
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 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
Typical. Where's the guarantee? You're usually the one policing this sort of stuff in other people's posts.
Sorry? Dumping transient current backwards through a 78Lxx is probably the most famous way of blowing one up, other than input overvoltage. Hence all the inverse-parallel diodes in app note circuits for the past
30-odd years. The charge from turning on a 'HV' supply--most of the parts listed are 500V or 1 kV, remember--can be pretty significant.
C2 is the only thing there that can intercept those spikes. If it goes, life can get bad pretty fast. A zener would be very comforting.
The source-drain diode of the FET. Better than good enough.
I invite you to hold the output terminals, with rectified 230V line voltage on the drain, while we test the FET's resistance to lightning surges and so on.
What happens if you reverse the input supply, even transiently? Big current spike backwards through the body diode, 78Lxx blows up. As I said, that's the second most common way of blowing up 3-terminal reg.
Your ego is getting in the way of your judgement again, Jim.
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 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
Too much BS to dispute after several glasses of wine. Would any Freshman in EE like to take down Hobbs a few notches?
I'm considering publishing the "Stheno Diaries" >:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85140 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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