Zero Volts from a LM 317 - Page 1
Forum section: electronics / beginners

author: nick.sek
author_rank: Regular Contributor
timestamp: May 13, 2012, 02:34:11 am
content: Hey everybody, I just wanted to show you a simple approach to get 0 volts from a LM 317 voltage regulator. Now of course it has a not so good feature, which is it constantly eats 1.4 volts (in theory). It obtains zero volts by using diodes. If you guys have any other approaches on this let me know. So here the link below:

author: Rerouter
author_rank: Super Contributor
timestamp: May 13, 2012, 02:56:43 am
content: kinda large problem with your design buddy, your sense line is before the diodes, and diodes tend to vary quite a bit with current, far better approach is use 2 diodes on the external negative, and have only the set pin pulled down by the true negative, this way you dont loose regualtion with the rest of the circuit about 1.6V less than the input,edit: actually i'm partly wrong, my method would be similarly wrong to yours,edit2: if you dont mind a little error introduced from input voltage fluctuations, use a resistor and a capacitor just to smooth it out, then have a second resistor bias the set pin high, and you would only need to calculate it for your 0V point, as you are setting it manually instead of digitally,

author: amspire
author_rank: Super Contributor
timestamp: May 13, 2012, 03:27:44 am
content: There are much better designs to do this using chips like the TL431 or opamps to cancel out the LM317 offset, but you still need a negative supply. In the suggested approach, you do not need a negative supply, but the regulation would be worse then a simple emitter follower supply.It is very limited in power handling, it's short circuit protection is poor in that it is only design for infrequent short term failure shorts, rather then constant overloads that any power supply supply is expected to cope with.To me, the common feature of almost every LM317 based power supplies is that they are pretty bad power supplies.Richard.

author: MikeK
author_rank: Super Contributor
timestamp: May 13, 2012, 04:55:00 am
content: Would you be able to do it without a negative supply, by using an inverting op-amp to offset it?

author: free_electron
author_rank: Super Contributor
timestamp: May 13, 2012, 05:09:46 am
content: @mikek : and how do you think the opamp would pull its output negative ? it can't go beyond its rails ...As amspire said LM317 stink as 'lab supply' . An LM317 is a really good regulator to make a FIXED voltage that cannot be obtained using of the shelf regulators like 78xx series. its twin, the lm337 can replace the79xx series. It also saves you from having to carry all the 78xx and 79xx in stock. al you need is a big bucket of LM317 and LM337 and a pile of trimpots ... Perfect. be my guest.There is another thread here on the forum form someone that also makes an LM317 based supply. I posted along reply there with all the common pitfals desinging robust power supplies , what diodes to use , where and when to put caps , how to do slow start and slow start recovery , reverse polarity protection and preventing backfeeding the supply.But using the darn thing to make a lab supply is sheer madness !You can make a really good lab supply with a couple of opamps and a darlington transistor. It will regulate down to zero , and have an adjustable current limit as well.In my book : it ain't a lab supply until you have an adjustable current limiter.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 13, 2012, 06:45:33 am
content: I have seen that diode "solution" before. I think it is a common design in the ham community. That doesn't make it in any way a good idea or good design. If you want to do the electronics community a favor pull that video from YouTube. BTW, I am not a big fan of videos when simply showing the schematics in an image would have been done.The only reasonable way to get an LM317 down to zero is to pull the adjust pin below zero. And that isn't without problems. The negative adjust voltage directly affects the output voltage, so it needs to be stable.There is no silver bullet when building a lab power supply from a single IC from the '70th. Even the cheap Chinese "lab" power supply makers haven't found a way. And the Chinese are masters in leaving out "unimportant" components and stretching component specs.

author: Zero999
author_rank: Super Contributor
timestamp: May 13, 2012, 10:12:27 am
content: I like the op-amp method. It may not be the simplest but it needs minimal adjustment and is pretty reliable. Of course, you'll get better results from a low offset op-amp and tight tolerance resistors but I just used the old 741 because I've got loads of them lying around.http://www.silicontronics.com/index.php?action=ezportal;sa=page;p=19You do need a negative supply which is easily generated with a voltage doubler, if you're using a transformer.

author: nick.sek
author_rank: Regular Contributor
timestamp: May 13, 2012, 05:36:00 pm
content: I'm excited to proto-type this circuit, thanks for showing me this op-amp ) volts power supply!

author: free_electron
author_rank: Super Contributor
timestamp: May 13, 2012, 05:41:04 pm
content: but, you don't NEED and opamp if you got a negative supply !just tie the bottom of the resistor divider to the negative voltage and you are done !. the lm317 doesn't give hoot. All it's after is having 1.25 volts between ot and adjust..Code:_______(+20)-----------|IN OUT|---------+---------(out)| | || ADJ | 240 Ohm------- || |-------------+|potmeter 5k||(GND)-----------------------------)---------(GND)|(-1.2 volt)-----------------------no need to go mucking about with opamps.here you go--------------|<|-----------------| _______ |---0---|>|----0---0-------0----|IN OUT|----0-----0-----0----0----0----(out))||( | | _|_ | | | | _|_ |)||( ---|<|- | |___| |__ADJ__| 240 | |___| |)||( | | --- | | --- --- ------0---|>|-|-- | 1000u --------0 /_\* | 100u /_\| | | | | | |0---|<|-0------0-0---0-------------------|-----|-----0---0-----0-----(0)| | _|_ 0-----0 |_|_ | |___| | _|_ ||___| --|>|---- --- 100u 5kpot |___|10u |100u--- | | | --- || | | | | |------0--|<|--------0----------------1k-0-----0-|<|-|<|-all diodes 1n4001 except * : 1n4148fully protected lm317 supply with slow start and adjustable down to 0 volts...

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2012, 01:32:13 am
content: Quote from: free_electron on May 13, 2012, 05:41:04 pmDo you really think I didn't know that?QuoteDiodes are fine if you keep it in a temperature stabilised oven, otherwise the output voltage will drift by as much as 200mV.The diodes also won't be matched to the LM317's 1.2V to 1.3V (1.25V typical) reference so the minimum voltage setting won't be 0V. Going from the typical vales on the datasheets, it'll probably be around 50mV but could be anywhere between -150mV and 200mV, worse when temperature drift is factored in.The circuit you posted should also use 120R and 2k5 for the potential divider as the LM317 has a worst case minimum load current of 10mA.An alternative solution might be a TL431 and adjustment pot which isn't much more complicated but it'll be a hell of a lot more stable than diodes.You don't need an op-amp but it's a good way to make a stable -1.25V reference which is almost perfectly matched to the LM317.

author: amspire
author_rank: Super Contributor
timestamp: May 14, 2012, 01:56:38 am
content: Quote from: Hero999 on May 14, 2012, 01:32:13 amAs you vary the the output load from 1mA to 1A, a pair of 1N4004 diode will have a forward voltage change of 0.6V in total. That is just too much variation to be useful in a lab power supply.What is the use of adding diodes to get 0V out, if the output can vary by 0.6V? Given the choice of decent regulation down to 1.2V or horrible regulation down to 0V, I would go for 1.2V minimum any day.Richard.

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2012, 01:59:18 am
content: The circuit in the datasheet shows a 1.2 V zener. Surely that's got to be better than ordinary diodes unless you are really penny pinching?

author: amspire
author_rank: Super Contributor
timestamp: May 14, 2012, 07:38:08 am
content: Quote from: IanB on May 14, 2012, 01:59:18 amYou can get two terminal 1.2V reference ICs, but not 1.2V zeners. The trouble with reference ICs is they cannot take the full power supply current and that was the whole point of this thread.I think everyone understands that if you have a negative supply, there are lots of ways to get a LM317 to go to zero volts. Not so easy if you do not have a negative supply though.Richard.

author: caroper
author_rank: Regular Contributor
timestamp: May 14, 2012, 08:37:46 am
content: Just out of interest.I was experimenting the other day with getting the 317 down to Zero volts and actually managed to drive the output negative by a couple of volts. I immediately trimmed the offset back up but it raised a question I had not previously considered.I was initially surprised that it went negative at all but will it damage the 317 if it is driven negative for extended periods?I was running the test with no load and a bypass Diode, but I would imagine there is a possibility of the 317 attempting to sink rather than source if it is driven negative.

author: codeboy2k
author_rank: Super Contributor
timestamp: May 14, 2012, 10:29:06 am
content: @free_electronAre you reading Dave's blog on a dialup terminal ? VT100? ADM-3A ?I respect your knowledge, it's clear you know what you are talking about, all your posts are a wealth of great information. Truly.... but...text schematics from usenet ??We ought to take a collection to buy you a free copy of Eagle capturelol:) just poking fun at your ascii art..

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 14, 2012, 11:06:35 am
content: Quote from: free_electron on May 13, 2012, 05:41:04 pmsorry master but with respect... ascii drawing is dead dinasour. i know the feeling of hard to change or in love with what we already used to, but face it, its dead. download ltspice or tina spice you can draw schematics in no time (capture screen, that Prt Scrn button on keyboard and paste/save in paint if you dont have proper graphic tool)... and easier to read toolove ya!

author: free_electron
author_rank: Super Contributor
timestamp: May 14, 2012, 02:45:26 pm
content: Quote from: amspire on May 14, 2012, 01:56:38 amIf you look at my schematic you will see that the output current does not go throught the diodes... The two diodes are only used to make a -1.2 volts reference. So there will be no variation.As for temperature drift : silicon diodes drift about -2 mV per degree c. So if the supply gets 50 degrees above ambient the supply will drift 200mv. Keep the diodes away from the heatsink..Not a problem and not a reason to go mucking about with zeners ( that also drift ) and opamps. If you really want to make it stable use a tl431 based reference.But this whole excercise is crazy. Adding 5 dollar in parts to get a 0.5 dollar regulator, that wasn't designed to go to 0 , to 0 ...Just spend those 5 dollar on an dual opamp some resistor and a transistor and make a real power supply with adjustable current limit.As for ascii art : i like ascii art. Its easy to make , you dont need to muck about with saving as gif ( cant use jpeg because the compression messes it up sometimes ) or png , dont need to upload it to some shady imegahoster. You can just slap it in.

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2012, 03:59:05 pm
content: ASCII art survives a run though with Exchange server and outlook, while an attached image can be mangled, resized to zero or even substituted with a same sized item from the cache.............

author: codeboy2k
author_rank: Super Contributor
timestamp: May 14, 2012, 05:08:06 pm
content: Quote from: free_electron on May 14, 2012, 02:45:26 pmThis is the most important lesson to take away from this exercise ==> it's never good, nor necessary, to over-engineer anything.free_electron is absolutely correct here. When we have learned to make the right decisions, that are appropriate for the situation, onlythen are we rightfully ENGINEERS.Quote from: free_electron on May 14, 2012, 02:45:26 pmAscii art doesn't actually bother me, I was poking fun at you. For me, I grew up on BBS's and USENET news, so I accept it. It is sometimes a littlemore work to understand it, and I am sure its more work for you to create it, but it's just not necessary anymore given the tools that areavailable today. (most of them free)Quote from: SeanB on May 14, 2012, 03:59:05 pmI've never worked in an MS Exchange environment, so I can't comment on that . But I have also seen other mail servers mangle ASCII art just as easily as you sayexchange preserves it. So it probably goes both ways. I think if you're reading and posting on a web forum using an intermediary service, not browsing directly, then all bets are off anyways. And the forum seems to preserve most images that are uploaded, and can be uploaded directly, no need to host them on another server. The forum accepts png images just fine, see attached.Cheers!

author: free_electron
author_rank: Super Contributor
timestamp: May 14, 2012, 07:49:46 pm
content: Quote from: SeanB on May 14, 2012, 03:59:05 pmor worse : be actually a fake image that causes a buffer overlow and downloads nastyware onto your computer ... Adobe just got nailed ... download a 'specially crafted tiff file' to install rootkits ... they are now going to fix it. and we all know about 'poisoned' gif files and other formats that cause trouble.I have yet to see the first ascii art that can infect a computer ...Quotei didn't know this forum hosted its own images ... i am very active on a few others where you need to use an external hoster like imageshack (imagine projectile vomit smiley here ) or some other 'shady' free image hosters...anyway : i still draw faster schematics in ascii art than in a drawing programjust fire up notepad and crank away.

author: george graves
author_rank: Super Contributor
timestamp: May 15, 2012, 12:28:22 am
content: LOL at the ascii art, It's 2012 - just use a jpg.

author: baljemmett
author_rank: Unknown
timestamp: May 15, 2012, 01:12:54 am
content: Quote from: george graves on May 15, 2012, 12:28:22 amUgh, no; JPEG is not designed for that sort of image. Use a PNG. Or just use whatever the hell you want to use when you're giving someone advice for free

author: Unknown
author_rank: Unknown
timestamp: May 15, 2012, 01:29:53 am
content: Simple device for achieving 0V output from the LM317 or other adjustable 3 terminal regulators.

author: free_electron
author_rank: Super Contributor
timestamp: May 15, 2012, 04:32:01 am
content: @ uncle vernon : bwahaaaaa. That's a good one. You are perfectly right. And you can put it in series or parallel withe the output , there is no drop ( pun intended) and the polarity doesn't matter either.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 15, 2012, 06:05:47 am
content: Quote from: baljemmett on May 15, 2012, 01:12:54 amNitpicking: It is, but nobody uses it.The JPEG standard also contains a lossless compression JPEG version which would work well for things like schematics. But similar to JPEG 2000, it never became popular and you would have to be very lucky to find an implementation.

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2012, 08:50:41 pm
content: Quote from: free_electron on May 14, 2012, 02:45:26 pmThe disadvantage with that solution, compared with the LM317 is there's no over temperature or safe operating area protection so overheating and thermal runaway are more likely when a plain transistor is used at the output. It's not hard to design an LM317 PSU that can work down to 0V and has adjustable current limiting.Quote from: free_electron on May 14, 2012, 02:45:26 pmA 200mV drift maybe nothing when the output voltage is set to 10V but at lower voltage settings, the tolerance approaches infinity, even 50mV at 1V is unacceptable for many applications.

author: resistcircuitresist
author_rank: Contributor
timestamp: May 25, 2012, 10:39:19 pm
content: One of the best ASCII circuit diagrams i've ever seen, props.

author: ELEKTRICK GASCHIEF
author_rank: Newbie
timestamp: May 03, 2013, 08:40:45 pm
content: I was looking into the question of operating the lm317t down to zero volts and fell upon this site, hence the reason for digging up an old thread. My idea was to use an ICL 7660 to generate the necessary -ve supply to enable operation down to zero volts. I seen the comments suggesting the use of 2 diodes might be unsteady. What about using an lm337l to get the -1.25 reference.....

author: codeboy2k
author_rank: Super Contributor
timestamp: May 03, 2013, 11:58:26 pm
content: these could all work. The cheapest solution is the LM337 or TL431. The ICL7660 is a little costly and has its own switching ripple, that you already have from the voltage doubler output ahead of it, so why use a regulator that adds more ripple?Just use a linear regulator like the LM337 or a shunt regulator like the TL431. The -1.25V reference output from this solution will be cleaner.

author: digsys
author_rank: Unknown
timestamp: May 04, 2013, 12:49:22 am
content: If you're going to use parts, why not an old tried and tested -It only needs to supply 5-10mA (adj current), easy enough to work out cap values.Good points - not a lot of loss (depends on cap/Zener), works on any AC voltage (No IC restrictions)Edit: Just noticed it is a bit similar to free's cct (couldn't read his ascii art :-) ) #2: oops, fixed incorrect circuit

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 04, 2013, 02:33:39 pm
content: Quote from: nick.sek on May 13, 2012, 02:34:11 amI like it!Another simple thing would be to take 1.5 volt battery for the negative input. No?JPJP

author: codeboy2k
author_rank: Super Contributor
timestamp: May 05, 2013, 02:32:14 am
content: yes, you could use a 1.5V battery off the ground rail, but why would you? Even though the negative rail doesn't need much current to supply the Adj reference point, it will eventually wear down the battery and then you need to replace it.Sometimes replacing batteries is more trouble than it's worth, when it's simple to use just 2 diodes, 3 capacitors and a zener to provide a negative rail.You can stop there, or add a shunt regulator or linear regulator to clean it up even more.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 05, 2013, 01:25:19 pm
content: Quote from: codeboy2k on May 05, 2013, 02:32:14 amI am not far from uncle Vernon wisdom ;-)How do you set all theese? 6 components ^ 6 gives 46656 possibilities.JP

author: Alana
author_rank: Frequent Contributor
timestamp: June 13, 2013, 05:24:55 pm
content: Here is my version. I had some "fun" with it - negative voltage generator needed bigger caps and i used single electrolytic as C5. Ended up with loud bang and some magic smoke coming out of it. Modified version seems to work fine.

author: Paul Price
author_rank: Super Contributor
timestamp: June 14, 2013, 12:37:02 am
content: The Paul Price Solution:Here is my circuitusing all off the shelf parts, and it has good temperature stability and low ripple. Not everyone has a LM337 or a box full of switched capacitor charge pump IC's, but almost everyone has a bipolar type 555 sitting around andthis circuit will work with or without a center-tapped transformer to build a Zeri Voltage Out LM317 power supply.I salvaged the 1500uf/6.3V and the LED from a dead computer motherboard.The voltage regulated 555 high frequency oscillation allows small capacitors andthe RED LED provides a stable reference for the LM317to allow it to drop down to zero volts.It must be a RED LED to give the right regulation bias voltage.R1 and the Zener diode Z1 can be replaced with a 78L05 Regulator.And its a .jpg

author: J4e8a16n
author_rank: Regular Contributor
timestamp: June 14, 2013, 10:55:44 am
content: Quote from: Alana on June 13, 2013, 05:24:55 pmWould you show the shematic?

author: Alana
author_rank: Frequent Contributor
timestamp: June 14, 2013, 03:29:43 pm
content: First attachemnt, over that with photo of the device.

author: c4757p
author_rank: Super Contributor
timestamp: June 14, 2013, 04:16:42 pm
content: Paul, unless I'm missing something, your solution is a bit problematic the way it's designed. First, it'll only oscillate at all if the input voltage is under about 10V, because you're powering the 555 from 5V but charging the timing capacitor from the input voltage. It'll never drop below the lower threshold like that. The 2k2 resistor should go to the 5V rail. Second, with those component values it's going to try to oscillate at a couple MHz! Only a few 555 variants can even hit 1 MHz (the NE555 that most people will have lying around are an order of magnitude less), and the 1N4001s that you're using in the charge pump certainly can't.

author: Paul Price
author_rank: Super Contributor
timestamp: June 14, 2013, 05:34:55 pm
content: Thanks C4757p for pointing out the error in my schematic drawing, I misplaced one end of the 2.2k resistor when copying my hand drawn circuit to my schematic drawing program.The resistor should connect to pin 8-4 of the 555 the 5V, not to the raw DC from the bridgeand the 470 ohm resistor should be 1k. The posted schematic has been changed to reflect this.The 1N4001 rectifiers will work fine(correction The 1N4007 work fine at 12V 555 supply, not 5V). (You could also substitute these diodes with 1N4184 or schottky diodes, if you have them lying around..the idea here is to use the most common parts off the shelf.).Any 555 has no trouble oscillating with the component values shown.The idea is to get the 555 to oscillate as fast as possible to minimize the possibility of the the oscillation noise to appear in the output of the power supply.With the component values shown, the 555 oscillates at approx200KHzwith no trouble starting or running at this frequency.I have tried several brands and the CMOS I7555 and they all work fine with these values.Correction, only at 12V, but they are not well suited for the instantaneous currents of the voltage doubler, use a bipolar 555 instead.I used 1K AC mains rectifiers, 1N4007 or equiv. and they work fine in this circuit.Substituting 1N4184 diodes made no difference in operation.Correction, they did make difference at 5V, they didn't work in this low voltage circuit.It would be a good idea to further bypass the electrolytic cap across the LED reference with a .1 low ESR cap.The idea here is to show that a simple solution using off the shelf parts will work and work well.

author: c4757p
author_rank: Super Contributor
timestamp: June 14, 2013, 05:47:23 pm
content: Just use 1N4148 or 1N914. Who doesn't have those? You're wasting quite a bit of power with such a slow diode. I think the 5V Zener (or even linear regulator) is more likely to be absent than a little switching diode.Quote from: Paul Price on June 14, 2013, 05:34:55 pmIs it really the right idea to give a beginner to go ahead and massively exceed the rating of something by an order of magnitude because "it works for me"? It's clearly not working correctly if it's running at 200kHz - the component values give 5 MHz by the usual calculation. You've got a 1.5 mF capacitor on the output and a shunt regulator for crying out loud, you're not going to have a noise problem even if you take it down to 50 kHz. Throw a little RC filter on that if you must.

author: Paul Price
author_rank: Super Contributor
timestamp: June 14, 2013, 05:56:17 pm
content: This circuit just doesn't work for me, but for anyone who builds it. This design has been extensively tested.I am not rectifying a 200KHz 3 amp signal here.The 1N4007 diodes do not waste any power. If you scope the waveform they are performing just fine and their duty cycle is less than 10%, not that this point matters.The whole 555 bias circuit uses only a few mA of current.If you bothered to breadboard and scope the output of this power supply you will see no trace of any 200KHz spike.I agree with you, though that placing a .1uf low ESR cap would lower the 500KHz spike due to the higher ESR of the big cap, but this change would not change the operation of the power supply.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: June 14, 2013, 05:59:39 pm
content: Alternatively you could also perhaps squeeze a few turns (theres usually a slight gap to thread through ) of fine guage wire on the transformer (remembering of course to keep any xtra turns well away from the mains lugs/winding),you will only need 4 or 5 turns on a medium sized tranny to get your -2V ouput for the adjust pin .

author: c4757p
author_rank: Super Contributor
timestamp: June 14, 2013, 06:09:13 pm
content: I do agree, power consumption really isn't a concern. I stand by both of my points, though. Yes, you've tested it, but you're exceeding ratings forno good reason. There's no reason to prefer a 1N4007 over a more suitable and equally common 1N4148, and there's no reason to configure the 555 to oscillate at 5 MHz if it won't. If you're going to exceed ratings and stated capabilities, you should have a reason for it and explain it.Quote from: Kevin.D on June 14, 2013, 05:59:39 pmThat would work. I suppose I'd hesitate a bit to send people off sticking wire into mains transformers, though; you'd be surprised at the aptitude for the average person to find ways to make that go wrong.

author: Paul Price
author_rank: Super Contributor
timestamp: June 14, 2013, 07:13:11 pm
content: C4757pThank you for you comments. You are absolutely correct. The 1n4007's are not suitable in this circuit at all and the circuit would not work with the the IN4007 type diodes unless the zener voltage was changed to 12V. In my first circuit, I used a 12V zener for the LM555 and there was no problem.If you have a 78L12 or a 7812 and you don't have a 12V 1A zener, then a 78L12 would even work better and the 220 ohm and 12V zener could be elminated.In the 5V circuit the 1N4184 diodes work fine.I just breadboarded this circuit again, and I apologize for not checking my notes for accuracy. The revised circuit is now posted.It works fine at 5V zener and with 1N4184 diodes and a bipolar NE555.The original 12V circuit works fine with CMOS ICM7555 and NE/LM555 timers and with the 1N4007 diodes. I have just breadboarded the 12V version again and tried several CMOS and bipolar 555 and they all work well, but the bipolar 555 is recommended if you have it.I only had two spare Signetics 555's on my shelf and they both worked perfectly. but the circuit does not work as shown with two ICM7555 CMOS timer versions of the 555 at 5V.Please note that the 470 ohm resistor was changed to a 1K resistor and 100pf was actually a 1000pf in the final working circuit I built a few years ago.Thanks to all again for letting me know where my sloppy notes and my assumed memory falls short of the original circuit in my memory.

author: Paul Price
author_rank: Super Contributor
timestamp: June 14, 2013, 08:21:39 pm
content: Here is the 5V version of the same 555 Inverter to -2V bias circuit that works with 1N4184 diodes,but not with the 1N4007's inefficient diodes, but the 1N4007's can be used in a pinch with the 12V version.In either version the 1N4184 diodes are the diodes of choice.

author: Paul Price
author_rank: Super Contributor
timestamp: June 14, 2013, 08:35:25 pm
content: According to the spec sheet from Nat Semi, the LM555 is capable of 3MHz max as a one-shot.With the my circuit it is cruising along at 200KHz due to the loading of the voltage doubler circuit.

author: c4757p
author_rank: Super Contributor
timestamp: June 14, 2013, 08:45:37 pm
content: LMC555, not LM555. LM555 only does 100 kHz (reliably - 200 kHz won't surprise me). LMC555 costs $1 each and is probably one of the most uncommon variants.

author: Paul Price
author_rank: Super Contributor
timestamp: June 14, 2013, 09:41:19 pm
content: C4757p: Thanks again for your insight and comment, however it has been my experience from having worked with this chip for many a circuit, that the ordinary bipolar 555 has no trouble oscillating up to about 500Kz reliably. By reliably, I mean it works every time with every type of bipolar version of this remarkable timer chip.I recently built a current mode SMPS that started up a Vin.2.9V, switched at 500KHz and once started the circuit with the boost bootsrap voltage would work down to a battery voltage of .9V. The circuit was able to supply up to 35mA need to switch N-Chan power MOSFETs for motor control at their lowest RdsOn and also power a 5V MCU, and all with a 3.7V li-ion battery. Because it would not start at battery voltages < about 3V, the circuit has an intrinsic low-voltage lockout on attempted startup to prevent over discharge of the battery. Of course I could have used off-the-shelf chips by TI or National or Maxim or Linear Tech, but the 555 circuit is cheaper and there was no chance of wasting time with inventory or availability issues.The 555 is a very useful, common and even in this modern era, still an intriguing and interesting device that has been produced in several technology variants over the years.I downloaded the LMC555 spec sheet that gave a 3MHz limit, but after carefully looking other 555 spec sheet from Harris and National, I could not find a max. freq. of astable operation spec.The parts that worked best at 5V and 12V were the ordinary NE/SE 555 bipolar devices> I have seen or used the LMC555 but I had about 6 of the the I7555 CMOS devices(all from the same mfg. log) and they worked the same, fine at 12V.

author: Paul Price
author_rank: Super Contributor
timestamp: June 15, 2013, 12:35:34 am
content: Why is there both a 12V version and a 5V version?If someone was using a low-voltage and maybe low current transformer and it was rated at 12V or so, then my 12V version might not have sufficient voltage to regulate at high loads if the transformer was not hefty enough to deliver 1.5A RMS approx., but a 5V version would not be a problem with power transformers from 8VAC up to 24VAC RMS.So, the 5V circuit is more universal but requires more precise components, such as 1N4184's dodes instead of 1N4007's. and an ordinary 555 bipolar timer.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: June 15, 2013, 06:52:55 pm
content: Quote from: Paul Price on June 14, 2013, 07:13:11 pmIs the Adj 220 Oms the good sizeDoes the 1k resistor goes to pin 2 o 4?The 2.2 k resistor does not connect with pin 8.Can I use a 12 volts batterie instead of an isolation transformer?Thanks.JP

author: J4e8a16n
author_rank: Regular Contributor
timestamp: June 15, 2013, 10:59:04 pm
content: Did you mean a 12 volts 78L12?

author: Paul Price
author_rank: Super Contributor
timestamp: June 16, 2013, 03:42:53 am
content: J4a8a16n:If you enlarge the .jpg image you can easily see that where there is junction mark where two wires cross, then there is a connection, otherwise the wire just is passing over.The 78L12 or 7812 or both LM7812 or LM7812 positive 12V regulators. There is only a 12V type of LM78L12 or LM7812 regulators.The 1K resistor goes to pin 2. Pin 4 and Pin 8 connect to the 12V regulator and 1uF capacitor.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: June 16, 2013, 11:31:49 am
content: Pin 4 and Pin 8 connect to the 12V regulator and 1uF capacitor .... and the 2.2 k?Now I have only 2.65 volts as minimum voltage.

author: Paul Price
author_rank: Super Contributor
timestamp: June 16, 2013, 12:27:25 pm
content: I have redrawn the original schematic, it was a bit unclear. See the attached. The purple lines show all the parts connected together at the 555C power pin. The 5V version I have also posted as a schematic is also almost exactly the same schematic but with a 5V zener and may be the best circuit if you have a <15V power transformer.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: June 16, 2013, 04:16:22 pm
content: It goe even a little less than 0 volts :-)It looses only 1.12 volt from the input ...Thanks to you.

author: suppersready
author_rank: Newbie
timestamp: January 20, 2014, 04:36:04 pm
content: Look at my ideahttp://mytecblogg.blogspot.it/