It doesn't. Look at the datasheet of the 1N4001 and especially the graph forward current versus forward voltage. And then look at what the temperature does to the forward voltage.
-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
I've depended on Hfe a few times, when the transistors were tightly binned. The few times I've done it, it was reliable.
Nothing but Ib sets the zener current. And 4.3 volt zeners are very soft. Add a b-e resistor and it gets more reasonable.
-- 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
The forward drop on 1N4001s can be all over the place.
I doubt that anybody fabs wafers that are intended to be 4001s. Most people will fab something that has a couple hundred volt breakdown and sell it as 4001 through 4004 maybe. Or just buy their wafers in a back alley in Hong Kong.
The higher voltage parts, 4005-7, tend to be a PIN structure and have more forward drop.
What can you expect for 1.2 cents?
-- 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
Good job, dagmargoodboat! I don't have the patience for ASCII art. ...Jim Thompson
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| James E.Thompson | mens |
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I love to cook with wine. Sometimes I even put it in the food.
1.2 cents? You buying them one at a time at Starbucks?
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" snipped-for-privacy@interlog.com Info for manufacturers:
LOL. Good one.
Every reel comes with a box of See's candies.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot 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
/ / / /
/ / /
/ |/ /' / /
/ / /| / | | | ' | / )I liked Matt's circuit, but thought it needed illustration...
Fred Bloggs is the ASCII King.
-- Cheers, James Arthur
Since there's no ac-ripple to speak of as the input is also regulated, a si mple current limiting resistor in series with ADJ should be enough. Since o n the modern parts Iadj doesn't budge by 0.2uA typically across all combina tions of input voltage and loading varied from min to max. Please view in a fixed-width font such as Courier.
. . . MAIN RAIL >--+-------------------+---------> Vmain . | | . | | . | - . | ^ 6.25V . | 317 - ref . | ----- | . | | | | . +--|I O|----------|-----+---> Vmain-5 V . | | A | | | . | ----- | | . | | | | . | | | | . === '----[1K]-----+ === C . | | | . | [R] | . | | | . | | | . ----+-------------------+-----+-------- . .
The 317 doesn't enough internal resistance to protect itself against excessive current from Adj to Out with Out shorted in this application, so the 1K in series with Adj is required. Please view in a fixed-width font such as Courier.
The 6.25V reference is best done with a 431 type precision shunt reference IC:
. . . MAIN RAIL >--+-------------------+---------> Vmain . | | . | | . | - . | ^ 6.25V . | 317 - ref . | ----- | . | | | | . +--|I O|----------|-----+---> Vmain-5 V . | | A | | | . | ----- | | . | | | | . | | | | . === '----[1K]-----+ === C . |0.1u | | . | [R] | . | | | . | | | . ----+-------------------+-----+-------- . .
current from Adj to Out with Out shorted in this application, so the 1K in series with Adj is required.
As I opined after your first ASCII ;-) ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 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.
current from Adj to Out with Out shorted in this application, so the 1K in series with Adj is required.
I tried breaking the National LM1117 in such circumstances. Couldn't make it fail, with the adj pin pushed all over the place.
The 1117 is just a lower dropout version of LM317; nice part.
-- 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
Key word: stated.
Quick version: I know the voltage drop from the main rail to the tracking rail is going to vary. The original poster didn't say how much variance was acceptable.
Long version:
I actually did look at one data sheet (Diodes Inc) before I posted the first time, because I had one memory of the nominal forward voltage being 1.0 V and another of the nominal forward voltage being 0.6 V. That data sheet quotes a maximum forward voltage of 1.0 V at 1.0 A forward current, 25 C ambient, single phase, half wave, 60 Hz, resistive or inductive load. (Yes, this application does not meet those conditions.) So I went ahead with 5 of them in series.
That data sheet also has a graph that shows forward voltages of about
0.7 to 0.9 V over currents from 0.01 to 1.0 A at a _junction_ temperature of 25 C, but this graph also has the test conditions of "pulse width 300 us, 2% duty cycle". I'm sure there is a reason for this test condition, but I don't know what the reason is, so it seems a little weird to me. The application that the original poster asked about is probably much closer to continuous than pulsed.I just looked at some other data sheets. Vishay's has a similar If vs Vf chart, with about the same curve, except their test condition is a
1% duty cycle. Fairchild has a similar chart and curve and uses the 2% duty cycle. ON has kind of a different chart; it doesn't have the pulsed test conditions, and has curves for three different Tc (case?). For 25 C, Vf varies from about 0.79 to 0.91 V over If 0.1 to 1 A.The Diodes Inc, Vishay, and Fairchild data sheets don't have any temperature vs forward voltage data. The ON one sort of does, in the three curves on the same graph mentioned above. At a constant If of
0.5 A, Vf varies from about 0.72 to 0.86 V as Tc (case?) varies from 150 C to 25 C.So yeah, Vf will vary over If and temperature. The original post did state a current of "up to" 0.5 A, which implies that it could be less, so If will probably be changing. The original post did not state a temperature requirement; I realize the diodes will heat up some from the power dissipated in them, so temperature will probably be changing.
This means that in this circuit, Vf will change, so the total voltage drop over D1-D5 will change, roughly in the range of 3.5 to 4.5 V. There will be a small additional voltage drop from the fuse (Bussmann says an AGC-1/2 is 0.59 V typical at rated current), so the total voltage drop from the main rail to the tracking rail will vary over the range of roughly 4.1 to 5.1 V.
The original post did not state a tolerance on the 'fixed 5 V drop' requirement. The claim could be made that "4.1 to 5.1 V doesn't meet the 'fixed 5 V drop' requirement", but that would mean that an alternative solution that provides a 4.9999 to 5.0001 V voltage drop also doesn't meet the 'fixed 5 V drop' requirement.
The fanciest DMM I can find in a little bit of Googling is an 8.5- digit one (Agilent 3458A), and its resolution on the 10 V range is
100 nV. So, I guess if there is a solution that provides better than 4.9999999 to 5.0000001 V voltage drop, it would be hard to prove that it didn't meet the requirement.Data sheets referred to:
Diodes Inc
Standard disclaimers apply; I don't get money or other consideration from any companies mentioned.
Matt Roberds
Right, is definitely the way to go, especially if there's substantial variation on the main supply rail.
This circuit looks hopeful, avoids the short circuit through the Adj, uses cheap readily available parts, and accuracy is power supply adequate. Adj currents cancel:
Please view in a fixed-width font such as Courier.
. . . MAIN RAIL >--+-------------------+---------> Vmain . | | . | | . | | Metal Oxide . | [1.0K] 1% . | 317 | 1W . | ----- | . | | | | . +--|I O|------------------------+---> Vmain-5 V . | | A | + | | . | ----- | | . | | 1.25 | | . | | | | . | | - | | . === '-------------+ === . |0.1u | |0.1u . | 317 ----- | . | | I | | . | --|A | | . | | | O | | . | | ----- | . | | | | . | | | | | . | | [200] | 6.25mA | . | | | | | . | | | v | . ----+-------------+-----+-------------+-------- . . .
Many thanks for all the replies.. Tempting though the PIC/Motor based solution is I think the LM1117 with 6.25V ref should do nicely.
Don't forget that you will dissipate 2.5 watts at 500mA.
on the main supply rail.
cheap readily available parts, and accuracy is power supply adequate. Adj currents cancel:
Cute! ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 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.
Looks like designing for production is not your forte ?:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 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.
variation on the main supply rail.
cheap readily available parts, and accuracy is power supply adequate. Adj currents cancel:
Actually more than cute! Damned clever! Into my bag of tricks >:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 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.
Four units is a production quantity, right?
I don't do hardware design for a living; I do software for a living. Recently it seems I have been doing various kinds of embedded software. When I do anything with hardware it's usually for fun. Sometimes it's for work, but then it often consists of providing goofy ideas to the real official hardware guy. Mostly this provides comic relief to the real official hardware guy. Once a year or so, it might help him get unstuck - not by using one of my goofy ideas, but by helping him come up with a goofy idea of his own that actually works. :)
This all furthers my goal of getting the board in my hands sooner, so I can write the software, and then get on to the real fun of arguing with the hardware guy about whether it's his crummy board or my crummy code that is causing it to screw up. Sometimes we have a shootout in the lab at noon with a compiler and an oscilloscope. If neither of us turns out to be a good shot, the sheriff^Wboss comes by and talks to both of us. Usually the boss decides that adding more crummy code is cheaper than revising the crummy board. This gives me more code to write, frees up the hardware guy to start working on the next board, and frees up the boss to slide the bar ends to the left on the Gantt charts, so everybody is happy? Right?
Matt Roberds
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