120 VDC supply? - Page 1
Forum section: electronics / general

author: james_s
author_rank: Super Contributor
timestamp: December 30, 2022, 07:46:22 pm
content: I'm wanting to build a supply to convert 120 VAC to 120 VDC to eliminate the annoying flicker of some retro style LED holiday lights, I've already verified that the bulbs are happy running on smooth DC by using my Heathkit IP-32 but a tube based linear bench supply is not a practical solution. They do not contain any regulation though so if I give them 170VDC by just rectifying and filtering the line they are WAY too bright and not likely to last long at all. Input 120VAC, max output current around 500mA, I don't need isolation, I don't even really need regulation, low cost and reasonable efficiency are important. My first thought was to use a 120:35V transformer to buck the line voltage down followed by a rectifier and capacitor. I've also found a source of some surplus transformers with ~85V secondaries that ought to be about right. I considered using back to back power transformers, 120V:9V feeding a 120V:12V backwards. I was chatting with a friend about it and he suggested a SMPS which first struck me as overkill but might make an interesting project, perhaps just a buck converter to drop 170VDC to 120VDC. There are even some off-line integrated buck ICs out there that look interesting but these are all aimed at creating low voltage output, an alternative to the old capacitor dropper. I considered a triac dimmer circuit too but those tend to produce a huge amount of hash that swamps my antique radios.Thoughts?

author: TimFox
author_rank: Super Contributor
timestamp: December 30, 2022, 08:10:25 pm
content: A thought: have you considered using a choke-input filter, which for load current above the "critical value" givesVDC= 0.9 xVRMS?Since you don't need isolation, a FW bridge on the line input before the L-C network.

author: bdunham7
author_rank: Super Contributor
timestamp: December 30, 2022, 08:19:30 pm
content: Quote from: james_s on December 30, 2022, 07:46:22 pmTL783 HV linear regulator with a ballast resistor to drop some voltage and lower the heat dissipation. It will operate directly from your 170VDC supply, so just a bridge, a capacitor and some resitors are needed. You can even set it up as a CC supply. I used this setup to convert an old live-chassis AC/DC tube radio to an isolated design with regulated DC supply for everything including the filaments.

author: james_s
author_rank: Super Contributor
timestamp: December 30, 2022, 08:34:18 pm
content: Quote from: bdunham7 on December 30, 2022, 08:19:30 pmEfficiency is my second most important criteria after cost so I decided early on that a linear solution is off the table. Seems a bit silly to use efficient LED bulbs and then throw away ~25% of the power in a linear regulator,, with a load of ~60W that's getting pretty significant. Still less power than incandescent but I should be able to do better than this. CC is not needed or wanted, the lamps are in parallel and the load will vary depending on how many are connected. The TL783HV is an interesting part though that could have other uses,

author: james_s
author_rank: Super Contributor
timestamp: December 30, 2022, 08:37:54 pm
content: Quote from: TimFox on December 30, 2022, 08:10:25 pmI hadn't, clearly there is a gap in my knowledge here, can you point me to an example of this?

author: bdunham7
author_rank: Super Contributor
timestamp: December 30, 2022, 09:12:00 pm
content: Quote from: james_s on December 30, 2022, 08:34:18 pmThen perhaps adapt a PFC front end from an SMPS and modify it to put out the the voltage you want directly. This would give you good efficiency and power factor. Something the size of a laptop brick would give you what you need.

author: TimFox
author_rank: Super Contributor
timestamp: December 30, 2022, 09:19:04 pm
content: Quote from: james_s on December 30, 2022, 08:37:54 pmChoke-input filters are covered in all the tube-era textbooks, such as Terman's "Radio Engineers' Handbook" in its various editions.Here's a '50s QST article:https://www.rfcafe.com/references/qst/power-supply-filters-qst-december-1952.htmFor detailed calculations, Duncan Lab's PSUD2 freeware does choke-input filters, but it assumes a transformer before the bridge.You could insert a 1:1 transformer with low effective resistance into its circuit description.The factor of 0.9 comes from the mean absolute value of a sine wave with a given rms value.The interesting detail is the "critical inductance" or current required: the QST reference handwaves this.

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2022, 01:26:20 am
content: Quote from: bdunham7 on December 30, 2022, 09:12:00 pmI thought of that, but aren't those typically (always?) boost converters? In this case I need an output voltage lower than the input.Is there any particular challenge in using a buck converter for this application? Most of them list a much lower maximum output voltage but I'm not sure why the output voltage couldn't be anything up to nearly the input voltage. They also list a maximum output wattage of typically <10W which I don't completely understand either, I thought that the limiting factor is the current that the switching device can handle, so the wattage ought to scale with the output voltage. I've built plenty of buck converters but I've never tried to get more than about 12V out of one.I also considered trying to modify an existing SMPS, something like the laptop brick you mention although something physically larger would be easier to work with. Rewind the transformer secondary and upgrade the output capacitors to handle the higher voltage. In the past I've had limited success disassembling ferrite transformers without destroying them though.

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2022, 01:30:26 am
content: Quote from: TimFox on December 30, 2022, 09:19:04 pmThat's an interesting idea that I hadn't encountered before, I'll read up on that. I think by the time I have a transformer I'm probably better off just getting a surplus transformer that I can either buck the voltage or one that puts out the voltage I want rather than adding a choke in addition to the transformer but it's still an interesting concept.It also occurred to me that I still have a few cut-C cores from a project several years ago so I could wind a custom transformer from scratch but the cores I have are overkill and from what I recall, smaller cores need correspondingly more turns of finer wire which makes them more tedious to wind. Past attempts to disassemble random E-I iron transformers have not been successful.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 31, 2022, 12:58:15 pm
content: Have you considered a capacitive voltage dropper? Note that it's a little different than the usual capacitive power supply, there's also a capacitor in parallel with the input of the bridge so the output voltage would vary less with load.

author: Faringdon
author_rank: Super Contributor
timestamp: December 31, 2022, 01:18:25 pm
content: Use a low side Buck converter.FET in the low side.Freewheel diode so the current can flywheel in the leds.Just put it straight after the mains rectifier.You dont want PFC i take it?If you do want PFC, use a PFC'd flyback.....theyre dead simple....simple set and check feedback loop.You can hack a L6564 to do the sine current regulationBut you dont want PFC, so low side buck is for you.Give me a shout if you want some sims in LTspice?Use a SiC diode or a nice UF diode if you do continuous mode..However, you could just do it in DCM if you want...to give you less reverse recovery problem.BTW, HV7708B does a nice Constant off time control for you if you want.(EDIT..its not HV7708B, it will come to me soon, and ill get back...its like HV7708B, or something..END OF EDIT)...anyway, you could just use UCC2843 and rig it for constant off time if you wanted....saves using slope compensation...but then again, slope comp off the osc is easy anyway.For led current regulation....there are many ways...or more-or-less open loop, its up to you.BTW, HV7802 can do hi side current monitoring, if you want accurate led current, its up to you.Or here is another led driver chiphttps://www.mouser.co.uk/datasheet/2/268/20005344A-608240.pdfhttps://www.mouser.co.uk/datasheet/2/268/hv9803b-1021767.pdfhttps://www.mouser.co.uk/datasheet/2/268/20005580A-1590346.pdf

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2022, 07:37:53 pm
content: The first two look like constant current drivers, good for bare LEDs in series but I'm using LED lamps with internal resistors in parallel so I need a constant voltage source. The HV9911 might do the job, I'll have to further study the datasheet.

author: Faringdon
author_rank: Super Contributor
timestamp: January 01, 2023, 12:33:44 pm
content: So you could use low side buck with opto feedback to give constant v output.Give me a shout on here if you want a sim of it. ? (LTspice)