Bad or Good Idea; 13.8V Linear DC PS for HAM Radios using 1 of these regulators? - Page 1
Forum section: electronics / projects

author: Nipkinz
author_rank: Contributor
timestamp: August 24, 2024, 07:26:23 pm
content: Hi gang,I'm considering putting together a design schematic for a linear DC 13.8V power supply used for Ham Radios. Many linear DC supplies for Ham Radios make use of the famous LM/uA-723 voltage regulator chip. After reading through a few posts here related to the 723, I'm considering using a different regulator. For my Ham Radio requirements, typical load current will range between various radios between 9A-23A, therefore I'm planning for a PS capable of providing up to 40A 100% duty-cycle max. However, typical max current load will likely never exceed 25A at 100% duty cycle.Strongly desired regulator specs would include:- Adjustable output voltage (either via built-in pin(s) or external resistors)- LDO- Low output voltage noise (perhaps <80µVrms)- High ripple rejection (perhaps >50dB (100 Hz)- Input voltage range of 18v-35v (wide range because I've never worked with a LDO regulator and not certain how much drop is associated and/or specified for stable and safe operation)- Current limit protection (short and reverse protection)- Over-voltage protection adjustment to protect the radio(s).This is a big ask, and is my first forum post, so I'll apologize now for saying or asking stupid things, *grin*.So far, in my search, I've come across two possible regulator candidates. What are your thoughts and recommendations?1. TPS7A4701-EP:https://www.ti.com/lit/ds/symlink/tps7a4701-ep.pdf?ts=17244899320332. UC2834M:https://www.ti.com/lit/ds/symlink/uc2834m.pdf?ts=1724513506097&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUC2834MI appreciate any input you provide, have a great day!

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 24, 2024, 09:07:59 pm
content: Building a 30 amp Linear PS is not hard but probably quite expensive. I would look at the Astron or Pyramid PS and find a schematic.But the most expensive part will be your Transformer.If you just need a PS for regular use I would look at Hamfests. Even a broken one would be probably easily fixed, as long as the transformer is OK. You will probably find that an entire used PS will be cheaper than buying the transformer alone.I use a Yaesu FP 1030A PS for testing radios. Since I test broken radios I need something with good safety features and an ammeter. I bought it broken at a Hamfest for $20 and fixed it in about an hour.https://www.arrl.org/hamfests-and-conventions-calendar

author: Nipkinz
author_rank: Contributor
timestamp: August 24, 2024, 09:31:35 pm
content: Quote from: Wallace Gasiewicz on August 24, 2024, 09:07:59 pmHi Wallace, thanks for responding, I really appreciate that. To clarify, I'm not seeking advice on buying a power supply, I asked whichregulatorwould be well-suited for building my own power supply and included two potential candidates for consideration. Thanks again for responding.Cheers

author: bingo600
author_rank: Super Contributor
timestamp: August 25, 2024, 12:55:10 pm
content: Quote from: Nipkinz on August 24, 2024, 09:31:35 pmWhat an attitude for a 2'poster

author: Nipkinz
author_rank: Contributor
timestamp: August 25, 2024, 04:24:02 pm
content: Quote from: bingo600 on August 25, 2024, 12:55:10 pmWhat? I replied with a clarification to the responder who missed the context of my post. I think you're seeking drama where none exists. Interestingly, now there are two irrelevant responses to my post, thanks for your time.

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 25, 2024, 04:34:22 pm
content: I'm not sure with your design requirement. 35 V input, 13.8 V output, 40 A capacity. That's potentially 850 W heat to dissipate.That's... a lotForget TPS7A4701 (unless you want to parallel 300 if them), you need external pass element. Even then you may still need to parallel lots of them.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 25, 2024, 05:28:07 pm
content: with all your needed specs ans so on, better buy an already made one, pfc correction ovp ocp thermal etc ...i dont think even with answering your 2 choices, you'd catch on how it's difficult to get theses specs, and reliably supply your ham radio and protect it against failuresHam radios have a tendency to kick more current (s) for a brief moment when you emit .. and some smps may hate this ..and some rigs could catch the switching frequencies of the DIY smps you want to build ...i have a friend who has rig Ham stuff and we had to test many smps before getting to real reliable brands, and have peace, low to zero harmonics etc ...

author: madires
author_rank: Super Contributor
timestamp: August 25, 2024, 05:29:55 pm
content: For 40A you'll need several pass transistors in parallel (each one with an emitter resistor) and a driver transistor (or two). It's not going to be an LDO. Also, as already explained by ArdWar, the heat dissipation for an input voltage up to 35V will be insane.

author: Nipkinz
author_rank: Contributor
timestamp: August 25, 2024, 05:47:09 pm
content: Quote from: ArdWar on August 25, 2024, 04:34:22 pmHi ArdWar, thanks for responding mate. I don't have a design requirement of 35 V input to the regulator, I noted a range of regulator input voltage specs so that I can then choose from a broad list of potential devices. For example, some regulators max input V caps out at 20V, which, likely wouldn't be quite enough; perhaps 2'ish V short for example unless a LDO regulator could operate between 16V-18V for example, but I've never designed with LDO, so it's uncertain at this time. I can't imagine pushing 35V into a regulator with only needing 13.8V for the ham radios, and wouldn't design a PS with that high of an input. Would you mind helping me understand why I would need 300 TPS7A4701's? When I read through the data sheet, a single TPS7A4701 will push enough current to drive pass transistors just fine. Perhaps I'm not understanding something.

author: Nipkinz
author_rank: Contributor
timestamp: August 25, 2024, 06:04:51 pm
content: Quote from: coromonadalix on August 25, 2024, 05:28:07 pmHi coromonadalix, thanks for responding mate. My list of specs are noted as desired (not absolutely required). I've designed with the uA723 previously, but I'd like other regulator options and leave the uA723's behind me. The uA723 is a great regulator but I'm hoping there are modern / better options these days perhaps? You mentioned SMPS a few times in your response, but as stated I'm designing for linear, not SMPS. You're totally right about the peace of mind with a DIY Power Supply for the expensive Ham Radios out there today. I just saw that ICOM is releasing a brand new IC-7760 for nearly $6,000. Designing for ovp crowbar is the easy part, the LDO with enough input V spec and low noise might be the more challenging aspect of the project.To clarify, I would like to use the least amount of regulator input voltage required for stable operation of this linear 40A 13.8 supply. (keep in mind that nominal operation of this supply would be 25A full duty cycle worst case scenario). And you're right, cooling and proper PCB design are both paramount to success. Let me know if you have any suggestions for regulators you feel might come close to the specs noted above. Have a great day, cheers!

author: iMo
author_rank: Super Contributor
timestamp: August 25, 2024, 06:31:59 pm
content: Quote from: Nipkinz on August 25, 2024, 06:04:51 pmThe 40A 13.8V is a good design goal. What will be your biggest challenge is to satisfy the "least amount of input voltage.." requirement..Your RX current could be 2-3A easily, and your TX current up to 25A with 100W (it depends on the operating mode). Your transformer's output voltage will jump around based on those loads and the construction of the trafo.The minimal voltage could be, say, 13.8V+5V+2V+3V = 23.8V (output, pass transistors, diode bridge, cap filter ripple) at full load, it will jump up to perhaps 30V or more at the low load based on the transformer construction (the core material, geometry, etc).So the power loss at the electronics itself (30V-13.8V)*2A = min 32W at RX, and (23.8V-13.8V)*25A = aprox 230W at TX (or aprox 380W at 40A). Not counting the loss at the transformer.That would be a good result, imho.

author: Nipkinz
author_rank: Contributor
timestamp: August 25, 2024, 07:02:17 pm
content: Quote from: iMo on August 25, 2024, 06:31:59 pmHi iMo, thanks you for helping me keep my sanity; I was struggling with loss-assumptions because I haven't even put this design into schematic format yet. I was waiting to do that if/when I found a great linear regulator that I could get passionate about. A mistake made (there was more than one, *grin*) on my previous uA723 regulator-based linear DC supply was that I went too far and unnecessarily overboard on the bridge rectifier and filter caps. That PS had too much V drop/loss before getting to the regulator, and also experienced slow-start design deficiencies.The UC2834M is starting to look like it's currently top of the short-list. Thanks again mate, cheers!

author: iMo
author_rank: Super Contributor
timestamp: August 25, 2024, 07:07:18 pm
content: I updated the calculation above - the ripple at the capacitor (after the diode bridge) would be much higher than 1V I had there previously, so I put there 3V, what is still a pretty optimistic value, imho.The design of such a power supply is not about some mysterious chips but rather the entire concept. In our ham club we have there a perhaps 40-50y old linear psu (all others are switchers), people from time to time use it with modern 100W output all mode all band rigs, large heavy box with perhaps 6 pass transistors mounted on a large heatsink.The voltage regulator itself (not counting the diode bridge, pass transistors and crowbar with a thyristor) could be almost anything (will work with say 4-5 transistors and a zener, as well as with any modern chip), what is quite tricky to design is all that stuff around it - the heavy metal.

author: Nipkinz
author_rank: Contributor
timestamp: August 25, 2024, 07:44:02 pm
content: Quote from: iMo on August 25, 2024, 07:07:18 pmAye, roger that. My last PS needed 24.2V feeding the uA723 at full load, and jumped to nearly 36V no-load. That was an expensive Transformer and the PS was built for much higher power than the 40A design I'm considering building now. Ripple will be an issue that I still need to solve.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 25, 2024, 08:00:50 pm
content: How about a switching regulator with a small LDO in parallel so that the switcher doesn't actually switch at low current?

author: Nipkinz
author_rank: Contributor
timestamp: August 25, 2024, 08:07:25 pm
content: Quote from: NiHaoMike on August 25, 2024, 08:00:50 pmHi NiHaoMike, interesting and I have never even considered that idea. I'll check into this; very cool, thank you!

author: floobydust
author_rank: Super Contributor
timestamp: August 25, 2024, 08:17:59 pm
content: The pass-transistor array is typically a triple-emitter follower with associated high voltage drop, say 3V. This is overcome with the extra DC bus voltage, but also makes more heat.Instead I would consider using an aux transformer/winding to power the regulator circuit.Had to laugh, just repaired a 20A 13.8V PSU that uses TL431 for voltage regulation.I think it's not about the regulator IC - a vanilla voltage reference, voltage-error amp, current-sense op-amp. Nothing that discrete IC's can't easily accomplish and are far easier to source.I see car batteries used for transient high currents as a way to lower the PSU requirements and cost, as well.

author: Nipkinz
author_rank: Contributor
timestamp: August 25, 2024, 08:29:09 pm
content: Quote from: floobydust on August 25, 2024, 08:17:59 pmHi floobydust, thanks for this recommendation. I really like the idea of an aux transformer for the regulator. Years ago I consulted with a PS design guru who suggested the same thing. I kick myself often for not taking that advice.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 25, 2024, 08:33:40 pm
content: Flooby beat me to it.....We use old Military linear PS at the Radio Shop. They are so big that one person cannot lift one. More than 100+ Amps.Lots of Pass Transistors. So I am familiar with these things. I have also repaired lots of Astrons etc.....Here is what I think: The first step is deciding what sort of Transformer you are going to use.The idea is not to burn off power unnecessarily. The Headroom should not be huge.Power for the regulatory circuit in current Linear Supplies these days is supplied by another secondary on the transformer. Essentially another transformer. Lots of the Lab Grade PS use this type of circuitry.Here is an example of this type of circuit: file:///tmp/mozilla_mint0/yaesu_fp-1030a_sm.pdfThe older Astron PS used a slightly different approach using the 723:https://www.ameradio.com/doc/Astron%20RS-35A%2C%20RS-35M%20schematic.pdfNotice the voltage for the regulator is not coming from the same transformer taps as the main power output.But the idea of regulating the main power with big pass transistors remains the same. You really do not want to down regulate 35 volts to 13.8 Volts, unless you are cold and need a space heater. The voltage in the "housekeeping" supply is not affected by the main power output, but separate and does not change with load on the output.

author: Nipkinz
author_rank: Contributor
timestamp: August 25, 2024, 08:57:30 pm
content: Quote from: Wallace Gasiewicz on August 25, 2024, 08:33:40 pmHi Wallace, with your experience repairing Astrons, you might have seen the following schematic, but perhaps not yet. Astron has made a revision to their big boys in 2020. Let me know what you think, although my schematic will differ of course because I'll be using a different regulator, but concepts remain with regard to pass transistors, etc. Latest Astron VS70M Schematic PDF:https://drive.proton.me/urls/NPX1TDMHJ8#kVxw7qDIko3TCheck out what they're doing with the 7.2 rails; I like it.

author: iMo
author_rank: Super Contributor
timestamp: August 25, 2024, 09:13:10 pm
content: Also Harrison type of linear PSU architecture (3 floating secondaries) may fit your goal..Like this one - couple of transistors, btw..

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 25, 2024, 09:46:09 pm
content: A salvaged AC motor, a salvaged car alternator, belt and pulleys, a car battery, and voila!Hope people realize this is a tongue in cheek answer.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 25, 2024, 10:47:46 pm
content: Astron has always put out a good product. Their new schematic is just fine. Similar to older units in the transformer topology. I kind of like the Yaesu design as it lends itself to different transformers rather than one just made for Astron. All you need is a big transformer for the 13.8 volt output and any little transformer will do for the "housekeeping"With this topology, you can probably more easily source transformers.As to the drop in voltage needed in the main power output, as I recall the huge 100 plus Amp PS we use start with only about a 16.5 volt after rectification that is brought down to 13.8 volt output. Not Much waste. But very large transformers.Also, just a thought, since MosFETS are so cheap and can handle lots of power, why not use mosFETS instead of bipolar pass TO3 transistors. Easier to mount on a heat sink also. Probably require much less drive so probably a whole stage could be eliminated. Good TO3 power transistors are not cheap.

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 25, 2024, 11:11:25 pm
content: Quote from: Nipkinz on August 25, 2024, 05:47:09 pmI guess you can use it that way. Although that will be a bit weird choice since once you use discrete pass element all of their performance/protection bell and whistles are thrown out of the window.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2024, 04:07:51 am
content: For the full voltage range and current as stated, you need to dissipate up to 848W.That's a big heatsink, and at least several large transistors, probably a dozen modest sized ones.The specs are absolutely achievable by SMPS alone -- no need for pre/post reg.For simplicity, I might opt for a hybrid design, iron transformer + rect into buck regulator; LM25119 and similar parts would be a reasonable choice. Power factor is a concern, especially at these power levels (13.8V 40A is 552W; at 90% overall efficiency, 0.5 PF and 120V input, that's 10.2A RMS already!), and I mean just in practical terms. Using a proper offline SMPS is highly attractive.As it happens, I have a stock design (mains to 24V 30A, LLC resonant) I could apply to such a purpose, or buy a unit ready made. It tends to be a bit noisy in the audible spectrum (would not meet the ripple requirement, unless maybe the requirement is low frequency only) so the buck stage would still be necessary, rather than designing it for 13.8V exactly in the first place.I do wonder just how sensitive the load is, if it really needs to be 13.8(00..) V and low ripple, or if anything from 12 to 15V would do, and what the effect actually is. I.e. are you asking for precision out of an abundance of caution, or is there a real and present need for it? Compare class A/B to C/D (maybe E) amplifiers for example; SSB vs. FM/PM use, perhaps. Linear amps generally should have better PSRR, give or take biasing anyway, and it may be less effort to fix the latter than to regulate and filter the former.Even with an SMPS + postreg architecture, the required voltage drop is much smaller than for raw rectifier output. Even if you have a tap changer or variac to compensate for mains fluctuation, the peak-to-peak ripple is still enough to need a hundred watts or so of dissipation with the rectifier. Very low overhead can be used with an SMPS, but mind that loop gain will be lower, and compensation more difficult, as the transistor's operated near or in  saturation.Tim

author: Andy Chee
author_rank: Super Contributor
timestamp: August 26, 2024, 05:44:52 am
content: The preference for linear PSUs over SMPS for ham radio applications, stems from the cleaner output voltage which have fewer harmonic emissions (radiated and/or conducted) that could potentially increase the RF noise floor, making weak signal reception more difficult.IME, living in an urban electrical environment, a linear PSU offers no noise floor advantage. The OP however might be living in a rural area in a relatively electrically quiet zone.PSRR is not the aim of the game in this context. It is radiated harmonic emissions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2024, 05:49:18 am
content: Quote from: Andy Chee on August 26, 2024, 05:44:52 amSurely you mean input conducted emissions--?I mean output too, but output is easily filtered.Either way, filtering is the solution. Even a linear supply's rectifiers will emit harmonics, line-frequency buzzing. (Usually not much at most radio frequencies, but depends on type.)Tim

author: Andy Chee
author_rank: Super Contributor
timestamp: August 26, 2024, 06:28:31 am
content: Input, output, conducted or radiated. It all spells disaster for the RF noise floor, particularly for anyone working 1.8MHz, 3.5MHz, and 7MHz bands.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: August 26, 2024, 06:53:55 am
content: Quote from: Wallace Gasiewicz on August 25, 2024, 10:47:46 pmHigh power MOSFETs achieve their amazing spec by having a very low “on” resistance. A tiny surface mount MOSFET can have a rating of 100A and 60V… but it won’t dissipate more than a couple of Watts. They aren’t suitable for use as linear regulators. I use DMT6002 MOSFETs in my power supply protection boards, controlled by an LTC4368.The hard part of building your own high current linear PSU is (a) sourcing a transformer and (b) dealing with the potential heat generated when supplying upwards of 25A for several minutes. I have a military surplus Racal linear supply that is rated at 35A continuous and it’s a big heavy lump with heatsinks on both sides of a long case and a large fan that draws air through the case, cooling both the transformer and heatsinks.Bottom line is that I no longer use the Racal supply, it is nice but unnecessary and takes up a lot of space… I use an SEC 1235 switching supply with a home made external over Voltage protection board (using the LTC + MOSFET). A linear supply is a nice idea in theory.SJ

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 26, 2024, 07:55:26 am
content: If you put a good SMPS into the neighbour room or even into the cellar and use multiple stages of filtering then no residual RF should be detectable in the air or on the power lines. Do not connect it´s power input to the same distributors as your radio equipment. Use one power filter near the PS that bleeds of the disturbances back to their source (the PS housing) and another filter near your devices that blocks residual RF before getting into your equipment. Use current transformers together with a spectrum analyser to check the filters.There should really be no reason to use a linear power supply in the 1kW range - except if a room heater is what you want in reality. And if you use a linear PS without filtering, then you may have created only little noise. If you use a decent SMPS with effective filtering then you also get rid of all noise coming from mains or elsewhere.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2024, 08:42:32 am
content: Quote from: Phil1977 on August 26, 2024, 07:55:26 amThe key is loop area:Putting some distance between it, is just pushing the problem away, and you really can't get a whole lot of distance inside a single building; and you're not doing much of anything if you're working at long wavelengths where it's still all near-field anyway.Take the PSU, mount it on a metal plate. You've got the wires, in and out, coming off it. Bend those around to the same side. To the mains, attach a line filter -- one of those metal-canned chassis mount parts will do fine here. Maybe a two-stage part if you really need extra. To the DC side, probably just bolt the negative to the plate for grounding anyway, but this isn't required; if not grounding, then simply filter both as if you have "+13.8V" and "-13.8V" (or half-and-half, rather) relative to chassis ground. Power receptacle to one side, DC terminals to the other -- sides of a common edge of the plate, probably.DC side filtering can probably use generous capacitors (depends on your requirements if isolated), like ~uF, and then fairly small chokes are reasonable, ~uH, which is convenient at this current level (if winding your own, use at least 12AWG, preferably 10, at this current). A CLC(LC) structure should do well enough, and cheap (or readily salvaged) high-loss powder cores (e.g. mix #26) are not only acceptable but desirable here.Keep capacitor leads short.Here's a six-channel example:Well not exactly an example, it's a LISN, but that just means the EUT side terminals have a capacitor to RF connector, instead of to GND. Values are ~1uH and 1-10nF so as you can see it doesn't take much to filter the crunchiest stuff, and, just scale up accordingly for lower frequencies. Impedance isn't so important for DC filtering, so, 100nF and 1uH, down to a couple MHz; 2.2uF and 4.7uH, below a MHz; etc..So, just imagine the copper-clad panel is a steel or aluminum sheet, and one terminal block is replaced by the PSU.Mains can't afford as much normal-mode filtering, hence the filter module suggestion; it's mostly CM filtering inside anyway. At the highest frequencies (10s MHz), NM is still fine, as you only need a few nF for that -- not much ground-leakage current draw there. So you can make single channel filters over GND like this (but use Y1 type capacitors!). At lower frequencies, normal mode is ruled out, and just use a big enough CMC instead, and/or multiple stages.Give or take what local requirements are (GFCI/RCD circuit or breaker, how much you care to build to commercial/regulatory standards, if you have a sufficiently permanent ground bonding connection to the panel, etc.), maybe you don't mind a little extra ground leakage anyway, and more and somewhat larger Y-caps are acceptable (few to 10nF each, say?). Or you run the whole thing from an isolation transformer, so that the secondary winding can be floated (who cares about ground leakage) and the panel still wired to earth properly. Transformer brings extra filtering too.Just slap that together, it almost takes as long to do as to write this out. Then put a cover over it, say a folded mesh/perf sheet, and you're good to go, doesn't even take a heroic amount of filtering to make something nearly undetectable.SMPS aren't scary, and they aren't magic, they just need to be deal with properly. Proper dealing can be nonobvious, maybe even counterintuitive, but it's an intuition which can be updated. I've put SMPS on several radio projects before, granted mostly as converters not isolators -- isolation is really the hard part, as isolation (at frequency) is inversely proportional to capacitance across the barrier -- but all the same, a little filtering and shielding, applied judiciously, is really all it takes.Tim

author: iMo
author_rank: Super Contributor
timestamp: August 26, 2024, 08:47:58 am
content: Quote from: Andy Chee on August 26, 2024, 05:44:52 amMy QTH is in a dense urban area and using EFHW antenna. Using the SEC1223 SMPS, rigs (no SDR ones) and PSU aprox 3-4m off the antenna feedpoint itself. I cannot see any practical difference in RX when I power my older Icoms off a battery or off the SEC1223 on 80/40m. I spent some effort with SEC's denoising, however..PS: with modern radios and their great SDR capabilities you may see the SMPS noise in their waterfalls, sure. When living in rural quiet area and striving to see a perfectly clean background in the waterfall the linear PSU (or batteries) is the only way to go, imho..

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 26, 2024, 08:58:38 am
content: Quote from: T3sl4co1l on August 26, 2024, 08:42:32 amYes, loop area creates magnetic fields and they like to couple into other loops.And that is the only reason that some distance can sometimes help, because there are three ways to spread disturbances without a wire:- The electric field (E), non-propagating: This can easily be blocked by conductive shielding- The magnetic field (M), non-propagating: This is not so easy to shield, even ferromagnetic cases or mu-metal are not perfect. But it´s intensity falls exponentially with distance.- The electromagnetic field (EM), propagating: Distance helps not so much any more (else we would have no radio-communication), but emission is extremely weak if the emitting structure is less than 10% of the EM-wavelength in its physical dimension. So it´s usually no problem for SW.Quote from: T3sl4co1l on August 26, 2024, 08:42:32 amLot´s of useful tips and a nice setup in the picture!

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 26, 2024, 09:08:50 am
content: Quote from: iMo on August 26, 2024, 08:47:58 amI would bet that you can get the waterfall background as clear with a SMPS as with batteries if you spend some money on good filters and shielding. But if you just want to run a receiver, then it´s perfectly reasonable to use a linear PS with only a few watts of dissipation.

author: iMo
author_rank: Super Contributor
timestamp: August 26, 2024, 09:18:32 am
content: Yep, that would be an interesting solution - the RX off a linear PSU, while TX you switch to the SMPS.The SMPS will be switched off during the RX of course.. The switchover is not that fast (say 100ms would be enough) such the SMPS's runup would not be able to follow the RX->TX transition..The only issue will be with the CW QSK mode (morse)..

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 26, 2024, 09:24:58 am
content: I have a home brew tiered solar battery supplySolar Panels > 41V Bus > 13.8V bus. There are 3 of 300 Watt DC pwm converters.The 13.8 V is reserved only for sdr receivers, computers, network, auxiliaries. Load is typically 4 to 6 Amp.The 41V bus feeds the transmitter DC pwm Converter 36:200V DC drain supply. This converter is enabled by CAT only during tx.Right now (5.00 am) the two bus converters are running from 120V supply. The noise on 20m is mostly at -110dBm peaking to about -103dBmThere is noise contributed from the two converters but is barely observable on the spectrum compared to when they are switched off and batteries take load.Here is a recent station block diagram with the DC system shown on left side.https://app.box.com/s/tvub9v22ftpdwsmieq8prvp9m5kwnkzcMy recommendation to Nipkinz would be to get the transmitters off 13.8V, onto a higher voltage supply if possible.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 26, 2024, 12:35:46 pm
content: We are getting off the OP's topic with SMPS. He is trying for a linear supply.I have had many SMPS. The ones that come with HAM gear are usually good and pretty quiet but expensive.I have purchased other SMPS (28 V) that were used for cell towers (I think) that were very noisy and put birdies into the ether. These birdies probably did not affect the initial usefulness of the SMPS but made them troublesome for HAM use. Unwanted emissions in SMPS can come from the input and output power leads and this can be dealt with successfully. Some SMPS have spurious emissions intrinsic to the circutry and radiate emissions from that circuitry. These are much harder to deal with since the whole box seems to radiate.Newer SMPS use some sort of freq variation where the internal freq are changing so that no noticeable birdies are generated. I am quite ignorant of how they do this but it works. It is another level of engineering that I am not familiar with as of yet. This is separate from the Pulse Width.Anyway there are very nice SMPS that do not cause problems but there are also lots of SMPS that cause problems.I have a nice TenTec SMPS that is very quiet.Anyway I think we are stuck with SMPS and probably need to learn more about them since they are taking over.Edit: I am sticking with my original suggestion of buying a used or broken Linear Supply and fixing it. It would be a lot easier and cheaper than obtaining the correct transformer

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2024, 02:17:04 pm
content: I would go with the UC3834, which is indeed very much like an improved 723, similar to the discontinued MC1466L.Advantages of the UC3834:- The UC3834 is intended to be used with external pass transistors.- The UC3834 includes facilities for adjustable current limiting, including foldback current limiting.- Like the 723, the UC3834 uses operational transconductance amplifiers providing high performance if you know how to use them.I do not see any noise specifications for the UC3834, however if noise is too high, then it may be reduced with an external reference.

author: floobydust
author_rank: Super Contributor
timestamp: August 26, 2024, 06:42:48 pm
content: No stock for a few months though, and I would be going PDIP (obsolete) so it's easy to replace.

author: Nipkinz
author_rank: Contributor
timestamp: August 26, 2024, 09:10:44 pm
content: Quote from: David Hess on August 26, 2024, 02:17:04 pmHi David, I think you are spot-on regarding the UC3834. Thanks!

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 27, 2024, 04:46:29 am
content: Quote from: iMo on August 26, 2024, 09:18:32 amThe switchover is easily done by just paralleling the switching and linear supplies with the linear set to a slightly higher voltage. The switcher won't switch at all when the linear supply is able to provide enough current.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 27, 2024, 05:50:38 am
content: There are also MIC5157 / MIC1518 parts, linear regulator controllers.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: August 27, 2024, 12:20:06 pm
content: Quote from: NiHaoMike on August 27, 2024, 04:46:29 amWhile off the original topic, the way to avoid switching noise completely is already done by turning off the switching inverter on receive, and turning it on while transmitting, the now discontinued MFJ 4416C “Voltage booster” (12V to 13.8V) operates that way. The change over time is a little slow for CW at more than 20 wpm, but perfectly adequate for voice modes.I used to be a fan of linear power supplies for ham radio use, but the SEC line of switching supplies create no noticeable noise at my site and I have 2 of them.SJ

author: iMo
author_rank: Super Contributor
timestamp: August 27, 2024, 02:09:16 pm
content: Quote from: Solder_Junkie on August 27, 2024, 12:20:06 pmWhile looking at its schematics - it simply passes the input 12V (minus 2x Schottky diodes in parallel) through the switcher's output transformer windings in the RX mode, and runs the switcher in the TX mode such you get 13.8V, it seems.The rigs usually do not provide 100W output at 12V or less, so that difference helps a bit..