1MHz diode laser power supply - Page 1
Forum section: electronics / beginners

author: bschwand
author_rank: Contributor
timestamp: February 07, 2025, 08:30:27 am
content: Hi,I am looking into building this modulable laser diode power supply (schematic attached also)https://www.repairfaq.org/sam/laserdps.htm#dpsldd317but of course some of the parts are not available for me.Looking at a few datasheets, would AD8022 be a good replacement for the LT1215 ?https://www.analog.com/en/products/lt1215.htmlhttps://www.analog.com/en/products/ad8022.htmlI also want to replace the potentiometer with a digital pot, as in my application I'd like to control it with a microcontroller. But the digital pot i can find is not 1K but 5k.I see it is basically used as a voltage divider to determine a setpoint for minimum current in the diode but I am not sure how a different value would affect the circuit overall.How important is it in this circuit for the pot to be 1k ? what would be the effect if I used 5K instead of 1k ?Cheers and thanks for all help

author: Gyro
author_rank: Super Contributor
timestamp: February 07, 2025, 10:37:58 am
content: The LM317 datasheet doesn't have sensible figures for operation at anything other than DC, but looking at the typical transient response and output impedance curves, I would be surprised if it produces a clean waveform at 1MHz. I could be wrong of course - I don't run components outside their datasheet specifications.

author: tooki
author_rank: Super Contributor
timestamp: February 07, 2025, 11:17:00 am
content: Quote from: bschwand on February 07, 2025, 08:30:27 amWhy not just use the LT1215 specified? It’s readily available, so just order one. The AD8022 would probably work, but its much higher bandwidth could make it more prone to oscillation.The issue with the 5k digipot might be the impedance of the subsequent op-amp stage. With the 1k pot, the highest impedance is 1k, which is low compared to the following stage’s 3.3k input impedance. 5k, on the other hand, is higher, so you might have problems.Again, 1k digipots are readily available, so just buy one.

author: bschwand
author_rank: Contributor
timestamp: February 07, 2025, 01:50:32 pm
content: Quote from: tooki on February 07, 2025, 11:17:00 amhmm well I said I can't get the parts listed and ask how those I might be able to get would suit, and you tell me to just buy the parts listed...Thanks for explaining the issue with the 5k pot.For the bandwidth, can you explain how this works or point me in the right direction ? I searched for opamps with 50V/us slew rate and the AD8022 has the appropriate slew rate but has a 120MHz bandwidth. However the LT1215 lists the gain bandwidth _product_ as 23MHz so I'd say it is not directly comparable.If I look at the large signal bandwidth, the 8022 has 4MHz for 4Vp-p and the LT1215 2MHz 2.5Vp-pWhat about an opAmp with same slew rate but lower bandwidth ? would that be more stable ?

author: benj38
author_rank: Frequent Contributor
timestamp: February 07, 2025, 04:12:45 pm
content: Quote from: bschwand on February 07, 2025, 01:50:32 pmI couldn't help noticing that according to your profiles both you and @tooki are from Switzerland (in fact, less than 100 Miles from each other). I guess, that if @tooki can easily get these parts so can you. Maybe you should ask him for a recommendation of where to buy them.

author: bschwand
author_rank: Contributor
timestamp: February 07, 2025, 04:17:59 pm
content: Quote from: benj38 on February 07, 2025, 04:12:45 pmgeography is not the sole criteria here.Is this a place to learn or just buy stuff from lists ?

author: benj38
author_rank: Frequent Contributor
timestamp: February 07, 2025, 05:26:00 pm
content: Quote from: bschwand on February 07, 2025, 04:17:59 pmI fully understand you. However, note that this is not a place to only learn, it is a place toget help. Hence, members sometimes suggest solutions that are not simply answers to questions asked. It is all done in a genuine attempt to help.If you just want answers to theoretical questions about circuit design then please make it clear. It was not made clear in your original post, so we also responded to your comment that you can not get these parts, trying to help you get them. I am sure that you can imagine a scenario where a person would be happy to find out that there are other ways to get parts that he did not know about.Now that it is clear that all you want is to learn about the circuit and the effects of your suggested changes, and not anything else, perhaps some other members will try to help you with that.

author: tooki
author_rank: Super Contributor
timestamp: February 07, 2025, 11:42:50 pm
content: Quote from: bschwand on February 07, 2025, 01:50:32 pmWell you said they are “not available to you”. But theyareavailable to you: we live in the same country, so if they’re available to me, they’re also available to you. “I can’t/won’t buy them” is not the same thing.Anyhow, your best chances of success are using the parts specified in the design. The more you start replacing, the more risk it might not work.Quote from: bschwand on February 07, 2025, 01:50:32 pmI’m not an expert on this, but it’s just a rule of thumb that higher bandwidth amps are more susceptible to oscillation. But it also depends on the internal design of the amp (not just the schematic of its insides, but the physical implementation). Again, I’m not saying it definitely won’t work, just that there is a bit more risk with it. (But nowhere near the risk of, say, an amp with multi-GHz bandwidth.)

author: bschwand
author_rank: Contributor
timestamp: February 09, 2025, 09:43:55 pm
content: I really do not want to be curt or rude but, seriously ?Quote from: tooki on February 07, 2025, 11:42:50 pm<rant>tooki, do you know where I am currently ? Do you know what constraints I am living under ? do you have any idea if I am actually living right now in the same region or location ? am I travelling in africa or somewhere else ? do you know my financial means or ability to order or pay or anything else ? Do you know anything about how or why I could or could not procure certain components ?NO, you don't. I was very clear,"some of the parts are not available for me"I thought that set the stage pretty clearly and unequivocally as "I am looking for alternatives"The bottom line is this: I ask if some components would be a good fit as alternative for certain ones that ARE NOT AVAILABLE TO ME. If they were available TO ME, I would get them.This seems to be a common practice, to find and select alternative components.So why is everyone telling me to just buy them and insisting that yes in fact those components are available to me even when I say they are not ? No, they are not and there are zillions of possible reasons for that which you can't possibly know.</rant>with that hopefully clearly settled, thank you very much for the comments on the component selection and the effects, I guess I have more studying to do. If there is any other advice on component selection I will gladly accept it.

author: bschwand
author_rank: Contributor
timestamp: February 09, 2025, 09:48:51 pm
content: Quote from: Gyro on February 07, 2025, 10:37:58 amThanks, I understand that this circuit is kind of a hack taking advantage of the LM317 characteristics, so probably not something to use for a product but instead a good cheap way to achieve the goals for a hobbyist. That is fine with me. I would actually be interested to measure how bad or good this circuit is compared to some supposedly well designed existing products.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 10, 2025, 08:40:45 pm
content: To answer the original question, no I don't think the AD8022 is a viable replacement, mainly for these reasons:1. The input common mode range does not extend down to GND. The first stage opamp is going to have its inputs biased around 0.85V (not sure about the second stage). The input common mode voltage range of the AD8022 is limited to ~0.75V above -Vs, which leaves very little margin to work with.2. The output range doesn't extend down to GND. The datasheet says it can only get within 1.4V of -Vs (that's while sinking current, doesn't say what to expect with a high impedance load). In the worst case the second stage would saturate with its Vout at 1.4V, meaning the LM317 output would never go below 2.65V. If the Vf of your LED isn't higher than that, then the LED will never turn off.That aside, I don't think the circuit shown in the schematic would work even with the LT1215. As shown, the LM317 and U1/B are connected to create positive feedback... and even if that's fixed then the circuit will end up regulating load voltage instead of load current (not good when the load is a diode....).

author: bschwand
author_rank: Contributor
timestamp: February 11, 2025, 12:19:19 am
content: Quote from: mtwieg on February 10, 2025, 08:40:45 pmThanks, makes perfect sense about the 8022.Why would the circuit regulate load voltage and not load current ? The output resistor R11 would be fixing the current, wouldn't it ?

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 11, 2025, 03:06:13 pm
content: Quote from: bschwand on February 11, 2025, 12:19:19 amNo, R11 doesn't really do anything in this schematic (as drawn, anyways).Actually it does regulate output current, just in an unconventional way (not by measuring output current and comparing it to a reference with an error amplifier). The feeback is positive, but not strong enough to make the circuit unstable (at low frequencies, anyways). More like feedforward than feedback, I guess.Ultimately, it should output anode current Ianode = (1.25V - Vpot)/R11, where Vpot is the voltage from the tap of VR1. The written description on the website doesn't actually do any quantitative analysis, just has lots of confusing prose. Seems to be talking about a completely different circuit sometimes. For example:QuoteThere's no zener diode or 100R resistor involved with the second opamp stage. There is no C3 or C4 on the schematic at all. No idea what is meant by the "input socket"...

author: bschwand
author_rank: Contributor
timestamp: February 11, 2025, 05:08:41 pm
content: Quote from: mtwieg on February 11, 2025, 03:06:13 pmThe C3 and C4 are on the PCB artwork actually, but not the schematic. I guess because they are decoupling capacitors for the OpAmpThe 100 ohm input resistor is R9 and the input Zener diode is D1, they are both on the schematic, and I agree it is a bit convoluted but at that point of the description he is talking about filtering the input modulation so I think it makes sense tht R9/D1 (and D1's capacitance) effectively would be an RC filter on the input.Quoteyes, I think that is what he describes asQuoteIt is rather unconventional, which makes it interesting to understand

author: tooki
author_rank: Super Contributor
timestamp: February 11, 2025, 06:47:05 pm
content: Quote from: bschwand on February 09, 2025, 09:43:55 pmThis is just an FYI: perhaps it's a language barrier issue, but at least in English (my native language), "not available to me" isnotthe same thing as "I am unable to buy because of ".Availabilityis whether the market makes the thing available for purchase, it says nothing about your ability to purchase. 1kg bars of gold are available for purchase at my bank; that says nothing about my inability to afford one!So given that it's not uncommon on the forums for people to say "X isn't available" when in fact it is, but they just didn't know where/how to get it, and your choice of wording (which "places the blame" on the sellers, not the buyer), it's absolutely reasonable for people to try and guide you towards sources for the supposedly-unavailable part. It really is not rare for people to ask for alternatives for something because they simply failed to find a source for the original, and they're often quite happy to find out that the original is actually available.So don't get your knickers in such a twist. Accept that your choice of wording doesn't actually mean what you think it does, and that you can tell people "oh, sorry, I actually just meant that there are other factors preventing me from ordering from mainstream distributors" graciously.

author: bschwand
author_rank: Contributor
timestamp: February 11, 2025, 08:09:27 pm
content: Quote"not available for me"the operative part of this sentence isFOR MEI never said the part is not available. You conflated that.I wrote"not available for me"Who is getting his knickers in a twist here ?maybe stop trying to backtrack and justify your own misunderstanding and assumptions and maybe question your own mastery of the english language since it is your native language; instead of sneakily implying that I clearly do not understand and can't write english, unlike you who are clearly a native english speaker and therefore can correct what I wrote so incorrectly to purvey the exact meaning of my thoughts, thank you so much.The reason for my rant is because you (and other) repeatedly insisted that I just buy the parts even though I repeatedly said I am not getting those parts and asked for technical advice, but you keep insisting and making up stuff about my situation that you know absolutely nothing about.QuoteThat is exactly what I did. But it was not enough, you have to keep bringing it up, yet again.PS: and if you really were just so helpfully showing a poor soul unable to order parts online, why didn't you actually give a useful answer: maybe a link to suppliers and price for the part, instead of repeating ad nauseam that the part is indeed available and just do it ? what is the point of that kind of "help" ? there is nothing, no actionable information.I am not interested in that kind of debate, thank you

author: LaserSteve
author_rank: Super Contributor
timestamp: February 11, 2025, 08:21:47 pm
content: Knowing a bit more about the particular designer of that circuit then most, take it cautiously. Test it on a simulated LD first.This was the same gentleman who advocated testing collimated and raw RED LDs by exposing them to 100% back reflections from a mirror before mounting them in arrays. (For most LDs this is often lethal, blows the output end mirror off from intracavity energy density issues.)That circuit is a work in progress at best. I built it many, many, years ago and it had stability problems to say the least.Sam was sent a test board after I flagged the design for issues. It seems some brands of LM317 are more suited than others.Steve

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 11, 2025, 10:03:32 pm
content: Yeah the suitability of the LM317 for controlling fast pulses is dubious at best, would not be surprised at all if different versions from different manufacturers behaved very differently. Also there's the issue that the LM317 has a minimum load current of 10mA in order to regulate properly. So your LED current would likely never go below 10mA.Also don't blind anyone, practice your safety squint, etc

author: bschwand
author_rank: Contributor
timestamp: February 11, 2025, 10:12:57 pm
content: Quote from: mtwieg on February 11, 2025, 10:03:32 pmI was hoping to use that (or some variation) with a higher power LD (in the 3.5A range) with an LM338 instead of the LM317Obviously just experiments and play for nowYeah, I know, don't look in the laser with remaining eye...seriously, I will set up stuff so I am not even in the same room when it fires, and observe with a webcam from behind a nice concrete wall

author: tooki
author_rank: Super Contributor
timestamp: February 12, 2025, 12:21:11 am
content: Quote from: bschwand on February 11, 2025, 08:09:27 pmI didn’t misunderstand or make up anything: you simply didn’t write what you actually meant. (The key being that “not available” — including in “not available to me” — puts the root causeawayfrom you. It means the supplying party is preventing you from getting it.)Quote from: bschwand on February 11, 2025, 08:09:27 pmMy apologies. I thought it was quite clear that I was saying that.Jokes aside, your English is quite good overall. But you are definitely lacking some understanding of nuance and connotation, at least of the phrase in question.Quote from: bschwand on February 11, 2025, 08:09:27 pm…because your wording suggested that you had failed to find a supplier capable of selling them to you, not thatyouare unwilling/incapable of buying from those suppliers who could sell them to you. So being reminded that the parts can in fact be sold to you could be useful information to someone who had simply not known where to buy them. Or perhaps a nudge to try a bit harder to find the parts.But moreover, as I tried explaining to you in the last reply (and others explained to you as well), it is not uncommon for people to respond with an answer that isn’t quite what’s been asked, because what people think they need is not always what they actually need. We see this a lot, and it’s not an insult or attempt at disrespect or anything. It’s an attempt to get you from point A to B the best way. The more detail you provide, the better.Quote from: bschwand on February 11, 2025, 08:09:27 pmNobody made up anything. We can only go by what information you shared, like your country flag. You are, in actuality, getting annoyed at us fornotbeing clairvoyant.Quote from: bschwand on February 11, 2025, 08:09:27 pmWell no, you quite immediately started getting annoyed and snarky. I’ve only tried to explainwhyyou got the response you did. And I did, at the same time, also try to answer your questions about the substitutes to the best of my ability.Quote from: bschwand on February 11, 2025, 08:09:27 pmYou can ask for that. But it’s also not our job to do your homework for you. Being informed that the parts are, in fact, available for purchaseisactionable information, because it means that it is not a waste of time for you to try and find it. It’s important to learn how to find things on your own. When someone has demonstrated that they made an effort (e.g. by stating what they did and where they ran into trouble) people are happy to help out.Note also that I actually said “readily available”, meaning that finding them is not difficult at all. It’s not like I found them on some obscure dealer of rare parts, I found them at the mainstream distributors.(Also, twice is not “ad nauseam”.)But here’s a useful free tip: search for parts on octopart.com. It searches all the major distributors (and some smaller ones). (Reichelt is the only one that is missing that I wish were there.) Search there, change the currency as needed. Saves TONS of time compared to searching each distributor individually.The other tip I’ll give for Switzerland is that the following distributors all ship with customs pre-cleared, meaning no nasty surprise bills from the post/UPS/FedEx/DHL: DigiKey, Mouser, Farnell, Reichelt, Conrad, Distrelec, RS*. The first two have free shipping on orders above ~CHF 54, Farnell for orders above 75 euros. The first three also ship very quickly and have very reliable delivery dates.*Doesn’t sell to consumers, so you must order through a company.

author: bschwand
author_rank: Contributor
timestamp: February 12, 2025, 03:23:54 pm
content: Quote from: tooki on February 12, 2025, 12:21:11 amok, thanks, I am glad EEVBlog has such highly skilled erudite linguistsalthough look what an interesting number one definition Merriam-Webster gives me:available/ə-vā′lə-bəl/adjectivePresent and ready for use; at hand; accessible."kept a fire extinguisher available at all times."components not present, not ready, not at hand, not accessible. I guess I am using the word correctly and even eliminated some degree of confusion by precising "FOR ME"The root cause is away from me, wether the supplier is able or preventing me from getting it or not, since other factors independent from both me and the supplier are at work. I certainly would agree that in the vernacular language, "availability" is mainly predicated on the supplier's capacity to provide the items, and less on extraneous forces, which is why I qualified it with "for me" in an obviously vain attempt to reduce misunderstanding.Yes, other posts not from you in this thread (and even from other places) added insult to injury by also insisting multiple times that I just buy the stuff; it certainly rubbed the wrong way and lead to my rant. When you are trying to make do and learn, and all you hear from everyone everywhere in this consumerist-no-ownership-everything-is-disposable-service-subscription-based world we live in, is to just buy the stuff... it gets really old.Well, I'd love to continue discussing linguistics nuances but obviously it's not the point of this forum.Thanks for the tips on suppliers.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 13, 2025, 11:49:37 am
content: This circuit was intended to be a "high side" driver so LDs could be stacked in arrays by laser hobbyists building their own laser projectors without having to insulate the outer cases of the LD cans. At the time it was created, red diodes were only 250 mw and were harvested from CD players.If you don't need high side drive, you can do much better with more conventional voltage controlled current sources.Watch for startup transients at power on.It's based on an obscure LT app note and 317 was not the output part.Nobody else ever got that to work .Steve

author: tooki
author_rank: Super Contributor
timestamp: February 13, 2025, 05:04:17 pm
content: Quote from: bschwand on February 12, 2025, 03:23:54 pmI think you really misunderstand people’s intent. Far be it from us to dissuade someone from building something, the forum absolutely encourages repair and DIY.Certainly, though, there’s a HUGE difference between “buy this component instead of that one” and “don’t fix that appliance, just buy a new one”. Unless you secretly have a semiconductor fab at home, then every IC you use will be purchased. (Assuming you’re not recycling salvaged parts, which brings its own can of worms…)However, there is also the issue of practicality: if you say you want to build a circuit designed by someone else, then your best chance of success is to build it as provided. The more changes you make — especially when we don’t know why particular parts were originally selected — the greater the risk it won’t work. So when one comes with a comparatively trivial roadblock of “I can’t get the parts" (when you almost certainly actually could because they aren’t exotic, restricted, or obsolete), then it really begs the question whether it isn’t more sensible to just fix that issue, rather than go to the potentially huge effort of selecting alternative parts. (Even when one knows exactly why a given component was selected, finding substitutes can be quite daunting; doing so when we don’t know the original selection criteria, and in a circuit which is, let’s say,creatively misusinga common part well outside its specified specs, then it’s even harder.But this isn’t specific to you; this is how we’d approach anyone’s problem.It’s different if someone asks “I want to learn how to select a substitute part for this circuit because I want to learn.”

author: bschwand
author_rank: Contributor
timestamp: February 14, 2025, 03:27:38 pm
content: Quote from: LaserSteve on February 13, 2025, 11:49:37 amThanks, very useful feedback, it's good to know the background behind the circuit.I am looking into something more along these lines now (attached). That comes from the LT1492 datasheet.

author: bschwand
author_rank: Contributor
timestamp: February 14, 2025, 07:06:12 pm
content: I found a paper on a circuit that would probably be overkill but it explains well how it works, what to pay attention to and how to modify it.https://arxiv.org/pdf/0805.0015

author: LaserSteve
author_rank: Super Contributor
timestamp: February 14, 2025, 08:18:14 pm
content: That one works. There are followup papers on it by other authors. You may need to scale up the current.Please consider a Lasorb (www.lasorb.com) to protect your diodes. I use them,know the company founder personally, and they work.Steve

author: bschwand
author_rank: Contributor
timestamp: February 14, 2025, 10:05:30 pm
content: About that circuit, my understanding is that the modulation input part only adds or substracts a small current to the current set through Vset.In my case, I want to define precisely the low state (as in a current just below lasing threshold), and the maximum current.In the original Libbrecht and Hall paper, they mention modulating the laser (for big current modulation) by altering the set point.So, I'd use 2 DACs like an AD5025 to define two different Vset, then use a voltage adder to present that to the current regulation AD8671 on its positive input. Then use a P-mosfet to connect/disconnect the output from the "max" Vset to the adder, and modulate that way my laser diode.Any comments on that scheme ?

author: bschwand
author_rank: Contributor
timestamp: February 15, 2025, 12:22:39 am
content: Quote from: LaserSteve on February 14, 2025, 08:18:14 pmThe LASORB component is interesting, would it work across LD modules that have strings of LD in series ?

author: LaserSteve
author_rank: Super Contributor
timestamp: February 15, 2025, 07:17:26 am
content: One per diode.Its a patented combination of parts made on a wafer.There should be an appnote or two on the web site.Steve

author: bschwand
author_rank: Contributor
timestamp: February 15, 2025, 10:41:54 am
content: Quote from: LaserSteve on February 15, 2025, 07:17:26 amAh too bad. It wouldn't work for modules that have multiple LD chip in a package like those nichia laser projector modules. But good for those individual ones. good to know.

author: bschwand
author_rank: Contributor
timestamp: February 18, 2025, 06:43:00 pm
content: Any input about this scheme, does it sound reasonable, what should I pay attention to ?Quote from: bschwand on February 14, 2025, 10:05:30 pm

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 21, 2025, 12:34:24 pm
content: Quote from: bschwand on February 18, 2025, 06:43:00 pmSounds reasonable if you just want to transition between two configurable setpoints. I would suggest using an analog switch instead of a P-fet, but it's hard to imagine exactly what your proposed circuit looks like.

author: bschwand
author_rank: Contributor
timestamp: February 21, 2025, 12:57:29 pm
content: Quote from: mtwieg on February 21, 2025, 12:34:24 pmwell my idea was to use a p-fet as a switch, to basically let it pass the voltage from one of the DACs or not into the adder op-amp, so the adder opamp would vary accordingly between two setpoint.Yes, I want to transition between two configurable setpoints, but do it fast and repetitively, at up to 5MHz.I did have the idea of using an analog switch instead of an adder and a p-fet, but are those able to be switched rapidly ? In my application I would select one or the other input at up to around 5MHz.I am also not sure how an analog switch goes from one to the other input, is there some down time or else ?I guess that is quantified by the capacitance and the ton/toff times only ? I guess I would want very low on resistance too. (I was looking at something like the ADG849https://www.analog.com/en/products/adg849.htmlor the SN74LVC1G3157https://www.ti.com/product/SN74LVC1G3157)

author: LaserSteve
author_rank: Super Contributor
timestamp: February 21, 2025, 04:14:05 pm
content: W's page has been "coming and going" with regards to access, but take a look here.I've helped him with his Ion Laser Designs, he's good..https://hololaser.kwaoo.me/Aka W's Laser Project Page in Google Search, If you can't load them, head for the web archive.His driver designs are impeccable.Steve

author: bschwand
author_rank: Contributor
timestamp: February 21, 2025, 05:31:24 pm
content: effectively the page is downHis requirements seem quite different from mine, but there is a lot of very interesting and useful information there, thanks for the link !Quote from: LaserSteve on February 21, 2025, 04:14:05 pm