Need analog chips to regulate +/-70vDC to +/-60vDC - Page 1
Forum section: electronics / projects

author: enut11
author_rank: Super Contributor
timestamp: February 17, 2022, 06:40:32 pm
content: I want to regulate a raw +/-70vDC supply to +/-60vDC at less than 100mA. It is a low noise metrology project so aiming to use analog chips.I found the TL783 125v will do the positive side but have yet to find a suitable negative chip.Any suggestions from the analog gurus?enut11

author: strawberry
author_rank: Super Contributor
timestamp: February 17, 2022, 06:48:35 pm
content: LM317 ,LM337

author: ledtester
author_rank: Super Contributor
timestamp: February 17, 2022, 06:53:12 pm
content: I'm not an analog guru, but for the jellybean LM317/LM337 regulators what matters is the Vin-Vout difference (not the magnitude of Vin itself), so I'm curious if the jellybean LM317/LM337 regulators used in a "floating regulator" configuration could work for you, e.g.:https://electronics.stackexchange.com/questions/188734/floating-lm317-used-at-high-voltage-protected-by-two-zener-diodes

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 17, 2022, 07:28:46 pm
content: A capacitance multiplier with transistors and a TL431 as base regulator would be easy.

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2022, 07:39:19 pm
content: Quote from: ledtester on February 17, 2022, 06:53:12 pmFirst, the circuits you link to are cr*p. Typical clueless web-waffle.Second: 70 V input to an LM317? If the output is connected to capacitor (and it is!) Vin-Vout is 70 V. Boom!Question is: how well does the +/- 60 V need to be regulated?Perhaps a simple transistor/zener voltage dropper is good enough. Easily made for both positive and negative supply.

author: ledtester
author_rank: Super Contributor
timestamp: February 17, 2022, 10:25:43 pm
content: Here's a design from the TI LM317L-N datasheet. Perhaps the zener will protect the regulator if the power supply has sufficient output impedance.https://www.ti.com/lit/ds/symlink/lm317l-n.pdf

author: langwadt
author_rank: Super Contributor
timestamp: February 17, 2022, 10:38:37 pm
content: Quote from: Benta on February 17, 2022, 07:39:19 pmso TI is clueless since they show the same schematic in the dataseet?Quote from: Benta on February 17, 2022, 07:39:19 pmthat's what the zener is for, if you short the output bad things happen but that is true for many simple regulators

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2022, 10:47:25 pm
content: Quote from: langwadt on February 17, 2022, 10:38:37 pmI'm not debile. The purpose of the Zener is clear to me. And it's still a cr*p design. Is TI God, or what? Most manufacturers have those kind of suggestions, and they're usually from very old data sheets.That does not prevent you from putting on your engineering hat instead of worshipping TI.

author: magic
author_rank: Super Contributor
timestamp: February 17, 2022, 10:54:22 pm
content: It's clearly a National drawing, though.Quote from: Benta on February 17, 2022, 07:39:19 pmThe fundamental question.

author: langwadt
author_rank: Super Contributor
timestamp: February 17, 2022, 10:54:58 pm
content: Quote from: Benta on February 17, 2022, 10:47:25 pmDid you get up on the wrong side of the bed today?

author: bdunham7
author_rank: Super Contributor
timestamp: February 17, 2022, 11:35:48 pm
content: Quote from: Benta on February 17, 2022, 07:39:19 pmI've seen similar circuits used extensively in older equipment that has many different voltages in the power supply, such as the old Fluke calibrators (my most recent memory). They use combinations of fixed and adjustable regulators with zeners both to float them higher than their specified output and to do other interesting things. They might in fact be crap, but they don't go boom--even with 70 volts into an LMxx regulator. The only consistent issue I've seen is that some of the zeners are dissipating a bit more power than would be ideal for long life, but in this arrangement I just don't see an issue.

author: enut11
author_rank: Super Contributor
timestamp: February 19, 2022, 07:26:08 pm
content: Some heated discussion so far!Thanks for all the suggestions. A bit more context might help. I have been working on an AC Voltage Calibrator which is essentially a very stable AC voltage/frequency transfer unit. So, ppm stability is desirable.https://www.eevblog.com/forum/metrology/an-experimental-ac-voltage-calibrator/1/I currently use LM317/337 chips to regulate an audio amp for an output up to 20vRMS. In Reply#91 of the calibrator thread, I mentioned a new power amp kit that would allow over 40vRMS signal to be generated from a stable signal source.Regulating the power amp rails is just another way to minimise amplitude errors/drift during the transfer process. The amp load will be very a high impedance so less than 50mA will be needed from the +/-60vDC supply rails.enut11

author: magic
author_rank: Super Contributor
timestamp: February 19, 2022, 07:44:22 pm
content: Well, if Benta doesn't like bypassing LM3x7 with zeners...Maybe you could make a 5V zener shunt reference on top of the regulated rail, feed it from the unregulated input and cascode an LM3x7 with some HV transistor.Gotchas: high zener current on output short circuit, high power dissipation in the cascode on output short circuit.Upsides: the regulators could even be LM3x7L and ripple rejection is improved by the ratio of zener pullup resistor to zener dynamic resistance, or something of that sort.

author: Someone
author_rank: Super Contributor
timestamp: February 19, 2022, 11:16:43 pm
content: Quote from: enut11 on February 19, 2022, 07:26:08 pmWill there be some precision references already in that design somewhere? Build your own feedback loops against a well stabilised reference rather than trying to get super stability from a commodity LM317/LM337 (or other old fashioned device).

author: enut11
author_rank: Super Contributor
timestamp: February 20, 2022, 01:36:46 am
content: Quote from: Someone on February 19, 2022, 11:16:43 pmThe project is essentially finished. There are no precision references as part of the design. This latest mod is just extending the capability of the low-volts range of the original design.https://www.eevblog.com/forum/metrology/an-experimental-ac-voltage-calibrator/msg4008427/#newFrom Reply #82 on

author: Someone
author_rank: Super Contributor
timestamp: February 20, 2022, 01:55:27 am
content: Quote from: enut11 on February 20, 2022, 01:36:46 am3 terminal regulator = opamp + reference + pass element + feedbackIf 2 or more of those parts arent meeting your requirements, then you may as well build it out from scratch. Trying to cobble together a bunch of zeners and transistors around a 3 terminal regulator made sense when opamps and references were the expensive bit, but not so much anymore.

author: nctnico
author_rank: Super Contributor
timestamp: February 20, 2022, 02:17:37 am
content: Quote from: enut11 on February 19, 2022, 07:26:08 pmA decent amplifier will have good power supply rejection for low frequencies. An RC filter in the supply lines to keep the HF noise on the supply lines down is likely more effective to get rid of HF noise on the supply lines compared to using regulators.Did you look at using a high voltage power opamp (TI has nice ones!). You could opt to parallel a few for higher output current and lower noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2022, 03:05:27 am
content: Just FYI (and... I guess explaining a bit more than other more kneejerk reactions? Idunno, just skimming over this thread at the moment). Anyway, the idea of having the zener bypass the regulator at high voltage drop is fine,as long as it's rated for the surge current.Which is a pretty big ask for most linear supplies, with the amount of capacitance available. For sure, more than a mere 1W zener will be needed to clear a fuse. A TVS will do better, but it may need even a SMDJ size part.Note that the zener may well save the regulator, but I highly doubt it will save itself, instead failing shorted (or maybe blowing open completely, but with a real fuse in circuit, that probably won't happen). So, if you know that this has happened, and you're going in to replace both components at the same time, because they'll probably fail together -- yeah, that.Or to put it another way, whereas a regular fuse is normally shorted and blows open under excessive current; a zener can be normally open, and blows short under excessive voltage. An antifuse!As for survival, TVS capacity can be calculated, or at least estimated, given ratings of the capacitor, TVS, and maybe a simulation to prove it out. A shorted output will deliver some kind of current surge, so, it's right in the wheelhouse of a TVS (surge rated!) but you need to check that it has the capacity. Shorting out a cap can deliver hundreds of amperes, it's not to be underestimated!Tim

author: bdunham7
author_rank: Super Contributor
timestamp: February 20, 2022, 03:24:36 am
content: Quote from: T3sl4co1l on February 20, 2022, 03:05:27 amThe only reason the zener would ever need to conduct during either startup or a short on the output (barring a malfunction of some other element of the circuit) is that the regulator might not be fast enough to react. In that case, the zener will conduct very briefly until the regulator can catch up. There shouldn't be any scenario where the zener has to do the heavy lifting. If something goes wrong with the regulator or the resistor network, then it would be OK for the zener to fry before the fuse pops. That's just how the cookie crumbles--if things go wrong with the circuit itself, damage may occur.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2022, 06:28:09 am
content: Yup, or including a relatively high capacitance load -- can happen when a lot of bypassing/filtering is used -- regulator goes into current limit and the input charges up to ~nominal voltage while the output lags behind. In which case the zener has to be rated to handle at least that surge energy due to charging the load, while perhaps not also the energy of discharging the main (input) cap. (Or sinking mains fault current once the cap's been discharged; not to leave that out, either.) And for short durations (some µs), yeah, the regulator won't respond instantly either, but that should be fairly small potatoes for a linear supply sort of thing, and it won't take much zener to assist that; response time doesn't mean anything though if we're talking about current limiting under startup/short conditions.Tim

author: strawberry
author_rank: Super Contributor
timestamp: February 20, 2022, 09:05:52 am
content: more power

author: magic
author_rank: Super Contributor
timestamp: February 20, 2022, 09:21:08 am
content: This will blow up on output short circuit without some resistor in series with the base.Try it in SPICE or IRL if you have the balls

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2022, 11:58:06 am
content: Quote from: strawberry on February 20, 2022, 09:05:52 amThe zener should be connected to a transistor, to limit the input voltage to the LM317.Here's an example. Note that the circuit contains an error. It should use the HV version of the LM317, which is rated to 60V, or the zener diode should have a lower voltage rating.

author: magic
author_rank: Super Contributor
timestamp: February 20, 2022, 12:15:39 pm
content: That's the cascode I talked about, and I would consider a 5V zener to also improve PSRR under normal operation. The gotchas apply, particularly with a low voltage zener. Tr2 doesn't seem strictly necessary, but some sort of minimum load on the 317 is.Quote from: Someone on February 20, 2022, 01:55:27 amYou missed one more element: having fun tuning feedback networks around an off the shelf IC opamp rubbish for capacitive load stability

author: mawyatt
author_rank: Super Contributor
timestamp: February 20, 2022, 03:04:56 pm
content: Why go to all the trouble using these lower voltage regulators and having to protect themUnless the OP has some tight regulation requirements a simple series pass transistor with shunt zener is hard to beat for simplicity. A couple zeners, a NPN and PNP pass devices, and a couple resistors (shunt caps for better noise/ripple) and you have a +-60V regulatorOf course one can employ a TL431 and a couple more transistors & resistors for a better performing regulator, but may not be necessary.Best

author: bdunham7
author_rank: Super Contributor
timestamp: February 20, 2022, 03:29:21 pm
content: Quote from: T3sl4co1l on February 20, 2022, 06:28:09 amTrue for larger power supplies, but I hadn't thought of the non-thermal current limiting because I was only referring to the circuit posted above from the TI datasheet that used a 1W zener. You'll notice that the fault current there is limited to below 0.9A under any conditions. Beyond about double that I think it would be better to use a different design, probably one that doesn't depend on a fuse to keep it from going up in flames.

author: bdunham7
author_rank: Super Contributor
timestamp: February 20, 2022, 03:35:25 pm
content: Quote from: mawyatt on February 20, 2022, 03:04:56 pmBecause it is cheap and simple and works really well forlow-power applications.

author: mawyatt
author_rank: Super Contributor
timestamp: February 20, 2022, 03:41:12 pm
content: Quote from: bdunham7 on February 20, 2022, 03:35:25 pmAnd how is this simpler, cheaper than a simple shunt zener with series pass transistor? Granted maybe a little lower overall power consumption.Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2022, 04:11:04 pm
content: Quote from: bdunham7 on February 20, 2022, 03:29:21 pmOh right, there's a series resistor too! Which makes the fuse kind of dubious, but it says 1/8A and 100V 100Ω is 1A short circuit, it'll open eventually.Anyway, the cascode one is the way to go, though it's a bit tight for OP's low overhead application. A CCS pullup instead of the resistor might be worthwhile.Tim

author: TimFox
author_rank: Super Contributor
timestamp: February 20, 2022, 04:37:55 pm
content: Quote from: mawyatt on February 20, 2022, 03:41:12 pmNote that higher voltage Zeners, say 62 V, have a relatively-large temperature co-efficient and will drift due to self-heating.For regulating screen voltage on pentodes, I try to use a series string of lower-voltage diodes.

author: mawyatt
author_rank: Super Contributor
timestamp: February 20, 2022, 05:23:41 pm
content: Quote from: TimFox on February 20, 2022, 04:37:55 pmYes aware of the TC issue, we've even used a simple shunt type ~6V zener with additional shunt transistor (~ zero combo TC) that has worked well for HV applications. Certainly not in the highly stable regulator class but quite sufficient for our needs.Best,

author: TimFox
author_rank: Super Contributor
timestamp: February 20, 2022, 07:42:16 pm
content: The sweet spot for normal Zeners, where the tempco switches polarity, is somewhere around 5 to 6 V, IIRC.A typical temperature-compensated "reference diode", where the tempcos of the Zener and forward junctions cancel at a given current, is a series combination of a 5.6 V Zener and a "normal" diode to give nominal 6.2 V,e.g.the 1N821 to 1N825 series.

author: mawyatt
author_rank: Super Contributor
timestamp: February 20, 2022, 08:37:04 pm
content: Around ~6V a typical "zener" diode has ~+2mv/C TC. In the concept shown above this +2mv/C TC compensates the transistor -2mv/C TC, yielding a net overall control TC of ~0, and since the series pass element (transistor, darlington or NMOS) is "inside" the feedback loop it's TC gets divided by the Open Loop Gain. If the feedback resistors R1 and R2 have a similar TC, then their TC won't matter much either. If more precision is required then the transistor Vbe should be in included with the zener voltage in the equation (Vz) shown, so would be:Vo ~ (Vz +Vbe)(1 +R1/R2)Best,

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2022, 09:12:59 pm
content: Why not use the TL431, rather than a plain zener?To work at higher voltages than 36V, it can be cascoded or a zener connected in series with it. See the thread I made about it a few years ago.https://www.eevblog.com/forum/projects/tl431-common-base-amplifier-stability/msg599580/#msg599580Add another pass transistor, with the TL431+cascode in the feedback loop and you have a high voltage linear regulator.

author: bdunham7
author_rank: Super Contributor
timestamp: February 20, 2022, 09:21:15 pm
content: Quote from: mawyatt on February 20, 2022, 03:41:12 pmI'm not claiming it is necessarily simpler, just on the same order. But they are probably a bit simpler to design with parts that are laying around--so easy a cave man could do it. And the performanceof the simplest implementationis probably better as well.

author: magic
author_rank: Super Contributor
timestamp: February 20, 2022, 09:24:04 pm
content: Quote from: Zero999 on February 20, 2022, 09:12:59 pmFTFY

author: mawyatt
author_rank: Super Contributor
timestamp: February 20, 2022, 09:47:44 pm
content: Quote from: Zero999 on February 20, 2022, 09:12:59 pmYep, see post #24."Of course one can employ a TL431 and a couple more transistors & resistors for a better performing regulator, but may not be necessary."Best

author: TimFox
author_rank: Super Contributor
timestamp: February 20, 2022, 11:00:54 pm
content: In a similar circuit, I have used a TL431 with a highish-voltage Zener in series with the + terminal, but a Shottky diode between the + terminal of the TL431 and the control pin, and a resistor across the TL431, are needed to avoid smoke (preventing the control from becoming more positive than the + terminal, and ensuring current through the Zener to limit the voltage across the TL431).

author: nctnico
author_rank: Super Contributor
timestamp: February 21, 2022, 01:35:42 am
content: Quote from: mawyatt on February 20, 2022, 03:04:56 pmSimplicity yes, but I have a thing for current limiting and thermal protection. It is nice when circuits can survive a short at the output.

author: bdunham7
author_rank: Super Contributor
timestamp: February 21, 2022, 03:23:08 am
content: Quote from: nctnico on February 21, 2022, 01:35:42 amYes, there's no reason not to include all that in such a basic design, someone can thrift it out later if they just don't want it.Here's a very straightforward 'cave-man' solution for the OP to look at. There might need to be a second 11K3 resistor across the negative output if it is operated with no/low load currents and you can add capacitors for filtering depending on the inputs, but I think this is a minimal solution that will provide 'pretty good' results and is pretty well protected. Shouldn't even need heatsinks and the BOM is $9 in single quantities at Mouser. In stock too!Edit: So much for cave men. Iwouldmight have to lower the hold current of the PTC or raise the voltage of the zener--or both--to fully protect it. But those parts are in stock too.

author: enut11
author_rank: Super Contributor
timestamp: February 21, 2022, 05:24:51 am
content: Thanks @bdunham7.The 70v raw inputs for the regulators will be filtered with 680uF caps.The load is an audio amplifier kit and the rail caps are 100uF.enut11

author: bdunham7
author_rank: Super Contributor
timestamp: February 21, 2022, 05:29:43 am
content: Quote from: enut11 on February 21, 2022, 05:24:51 amAre those raw as in direct from a transformer and rectifier? If so, do those inputs feed anything else or just this circuit? If 1) yes and 2) no, then are those capacitor specs set in stone?

author: enut11
author_rank: Super Contributor
timestamp: February 21, 2022, 07:23:24 am
content: @bdunham7The raw 70v is direct from rectifier/filter and the capacitance can be varied.The audio amp input caps are more difficult to change due to space on the PCBThe regulators will only feed the audio power amp.enut11

author: magic
author_rank: Super Contributor
timestamp: February 21, 2022, 09:02:41 am
content: The zeners could be 5W or TVS as they need to trip the fuse/PTC. Note that when the load draws constant 0.18A, the regulators may overheat and go open circuit and it will fall on the zeners to conduct that current.It may be a good idea to have diodes clamping the rails to ground so that neither can reverse polarity if something shorts them together.Quote from: enut11 on February 21, 2022, 05:24:51 amAny known PSRR requirements?

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2022, 09:22:37 am
content: Quote from: enut11 on February 21, 2022, 07:23:24 amJust one question: why are you using a regulated power supply, for an audio amplifier? It's not normally necessary, since the power supply rejection of a well designed amplifier makes voltage regulation unnecessary.If the design you're using is for 60V, but you have 70V, on the filter capacitors, then it's better to modify the circuit for the higher voltage, than to add an unnecessary voltage regulator. It's probably as simple as increasing the voltage ratings of a few components and using larger heat-sinks.EDIT:I've just read the original post again and I'm confused.Quote from: enut11 on February 17, 2022, 06:40:32 pmNow you say it's an audio amplifier.Please post a schematic or data sheet for what you're powering.

author: bdunham7
author_rank: Super Contributor
timestamp: February 21, 2022, 02:22:48 pm
content: Quote from: magic on February 21, 2022, 09:02:41 amI intended to have the zener never to have to conduct significant current under any external fault. I think that works as drawn as long as the 70V input is exact or very close. Note that with the 68R resistor, if the current exceeds about 120mA the voltage drop reduces the input voltage and the regulator drops out and thus will dissipate minimal power. Also my 'no heatsink' statement reflected a room temperature ambient. If any of that changes, then as I said in the edit,mutatis mutandis-- alterations might be needed. The key to this 'floating high' arrangement is that the current and thermal overload functions have to be taken away from the LM3x7.

author: bdunham7
author_rank: Super Contributor
timestamp: February 21, 2022, 02:26:28 pm
content: Quote from: enut11 on February 21, 2022, 07:23:24 amWhat I meant was does the 70V unregulated power feed anything else besides the regulators? And what are the characteristics of the transformer, etc? Is that 70VDC no-load, estimated, exact within +/- x.0 volts, ??

author: magic
author_rank: Super Contributor
timestamp: February 21, 2022, 03:54:46 pm
content: Quote from: bdunham7 on February 21, 2022, 02:22:48 pmGuess you're right about the dropoutAnd I suppose 68Ω is also supposed to limit short circuit current to 1A so that the regulator has no trouble passing it.I'm starting to wonder if the zener is even of any use at all. It looks like internal current limiting is the only condition which endangers the regulator.Rail clamping diodes may still be a good idea.

author: bdunham7
author_rank: Super Contributor
timestamp: February 21, 2022, 05:12:39 pm
content: Quote from: magic on February 21, 2022, 03:54:46 pmThe zeners only conduct when the regulator reaction time is not fast enough to react to a sudden load increase. Or if the regulator shuts down, but that would be a catastrophic event. Protecting against that would require a pretty big device, although it certainly could be done that way in an alternative design.There are many things you can add to improve the characteristics of a circuit like this, but I'm not sure whether they are needed or not without more exact information so we know what to worry about. Diodes are cheap, I suppose. My main concern would be getting the overcurrent protection right, which might require a higher power rating on the input resistor or a different PTC or both. That and the zener should be a good margin above the max input voltage minus 60 volts.

author: enut11
author_rank: Super Contributor
timestamp: February 21, 2022, 07:27:39 pm
content: Quote from: bdunham7 on February 21, 2022, 02:26:28 pmThe transformer is a 50VA toriodal and feeds only the amp via the regulators. 50vAC to 70vDC unloaded. Variability depends on the 240vAC mains fluctuations.Transformer load is less than 50mA as it supplies voltage (not power) to a DMM.enut11

author: enut11
author_rank: Super Contributor
timestamp: February 21, 2022, 07:32:48 pm
content: Quote from: Zero999 on February 21, 2022, 09:22:37 amThere is no schematic supplied by the vendor.Please see reply #11 for context

author: bdunham7
author_rank: Super Contributor
timestamp: February 21, 2022, 08:13:52 pm
content: Quote from: enut11 on February 21, 2022, 07:27:39 pmOK, so then you need to know the actual quiescent and maximum current that the amplifier draws (I'd test that directly if at all possible), as well as the maximum line voltage fluctuation you want to plan for--5% ? 10% ? B/t/w, does your audio amplifier require at least some load for a non-distorted output?If you are drawing directly from a small, dedicated transformer then you have to account for more variation than if you were drawing from a larger regulated supply. Some people have suggested using a much larger zener or TVS that won't blow up in case the regulator does shut down and although what I've drawn doesn't do that, it might not be a bad plan. Those large 5W zeners are not expensive for a one-off device. Also, your transformer isn't as grossly oversized as you might think--the peak voltage will drop quickly under any significant load and the RMS current of the transformer is multiples of the DC current, how many multiples depends on how close to the peak voltage your DC level is. At 50mA I think you'll be fine, at 300mA I'm pretty sure you'd start to have issues.

author: magic
author_rank: Super Contributor
timestamp: February 21, 2022, 08:35:44 pm
content: If found this AliExpress kit without problemI can't find any seller showing the schematic publicly. But what I can see is two trannies marked 1815 on the silkscreen. If these are 2SC1815 and it surely does look like they are, and if they are the input pair, and if they are not cascoded, I have bad news for you.Observe that they only rate this amp for ±56V supplies for some reason

author: enut11
author_rank: Super Contributor
timestamp: February 21, 2022, 09:07:09 pm
content: The 56v rating is for use as a power amp. I am using the module as a voltage amp.https://www.ebay.com.au/itm/124926624992

author: enut11
author_rank: Super Contributor
timestamp: February 21, 2022, 09:14:09 pm
content: Quote from: bdunham7 on February 21, 2022, 08:13:52 pmMin mains voltage is 235v, max is 245vAudio amp quiescent current is 25mANo minimum amp load required

author: magic
author_rank: Super Contributor
timestamp: February 21, 2022, 09:19:50 pm
content: Quote from: enut11 on February 21, 2022, 09:07:09 pmWouldn't matter if the input stage craps out. But it looks like I was wrong and they are the current mirror.https://www.diyaudio.com/community/threads/ljm-audio.180625/page-28#post-4844361

author: bdunham7
author_rank: Super Contributor
timestamp: February 21, 2022, 09:47:58 pm
content: Quote from: enut11 on February 21, 2022, 09:14:09 pmIn that case, it will work pretty much as drawn except you have to carefully select the PTC so that its maximum trip current isn't more than the 68R input resistor can take. If you want even lower ripple and output impedance, you would put a 10uF 100V electrolytic across each of the 11K3 voltage divider resistors.Alternatively, if you want to make it a bit more robust, capable of more start/surge current and perhaps adjustable to a slightly lower output voltage you would change the input resistor to 33R and the zener to two 12V 5W units in series and select a PTC with a trip current under 400mA in all conditions that it is likely to see. For a per-rail voltage adjustment feature, put a 1M trim pot in series with a 100K resistor across the 11K3 resistors, and change the value of the 11K3 resistors to more like 12k 1% to make sure you can get over 60V.

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2022, 09:58:46 pm
content: The circuit I posted previously, can be used with the LM337, for negative voltages, if the polarities are swapped.https://www.eevblog.com/forum/projects/need-analog-chips-to-regulate-70vdc-to-60vdc/msg4020262/#msg4020262

author: Wolfgang
author_rank: Super Contributor
timestamp: February 21, 2022, 10:10:06 pm
content: You could try a negative floating LM723 configuration, if you want long term stability and low noise. See National datasheets.

author: enut11
author_rank: Super Contributor
timestamp: February 22, 2022, 06:50:50 am
content: Thanks @Wolfgang. Low noise is good in this application so well worth investigating a 723 solution.

author: enut11
author_rank: Super Contributor
timestamp: February 22, 2022, 06:54:14 am
content: Quote from: bdunham7 on February 21, 2022, 09:47:58 pmThanks @bdunham7A variable output control for the regulators is a very good idea as it allows final voltage trimming to suit both the input and output conditions.