Power regulation options for 5V and 18V circuit - Page 1
Forum section: electronics / beginners

author: dsegel
author_rank: Contributor
timestamp: January 09, 2018, 06:18:31 pm
content: I'm building a circuit that needs both 5V and 18V available. Current draw will be on the order of 200mA max for the 5V line, and 325mA for the 18V. I was planning on using a 20-24VDC input and running it through an LM7818 and LM7805, but I wanted to ask first because that doesn't seem like a very efficient solution and I'm worried about the 7805 overheating since it would be operating at the top of its range and burning off a lot of power as heat.So are there better options? Should I go for a lower input voltage and use a boost converter for the 18V? Or vice-versa? Are there any common solutions to this problem?

author: davy peleman
author_rank: Regular Contributor
timestamp: January 09, 2018, 06:51:06 pm
content: LM317 has a rating up to 1.5 amps and 35 volts. i use them al the time for timer ic's and mosfet drivers outputting up to 10 amp pulses . never had any heating issues even without heatsinks they stay cool. you will need 2 resistors to set the voltage tough, but these regulators are solid and also have shortcircuit protection. otherwise you could use a tiny switch mode power supply a buck or boost converter. there are small units that are cheap. but i'd stay with the lm317, that's the cheapest solution. the 18 volt rail won't waste much power when the input voltage is 20- 24 volts, since it only has to bring the voltage down by a little. And in my experience bringing 24 volts down to 5 volt at 200 ma will not cause heating. Steve ward uses lm7805 and sorts for his tesla coils logic and drivers and they do the job just fine, so they're rugged i suppose. also linear regulation causes the least noise whereas smps and sorts brings noise that you should compensate for with bigger electrolytics and sometimes even shielding (also more expensive).Also even these power supplies cause heat. you could also use zener diodes but they will surely heat the most i suppose. Also if you state the application people can help you better. is it for logic, for inductive (motor) or resitive loads (lights)so what's the project and how big is the housing,budget...? these are also important factors for deciding. i'd say lm317 all the way for up to 800ma.Hope this helped.

author: dsegel
author_rank: Contributor
timestamp: January 09, 2018, 07:32:54 pm
content: Thanks, I'll look into the LM317.The 5V is for logic-level controlling L293Ds and an Arduino Nano, the 18V is for some small electromagnet coils that are powered by the L293Ds for polarity purposes. This will all fit into a box about 2x2x6 inches, and I need components that are not much bigger than a few TO-220s and associated caps and resistors. External power (the 20-24V) will be coming from a wall wart.

author: soubitos
author_rank: Frequent Contributor
timestamp: January 09, 2018, 07:48:06 pm
content: Since this is an arduino design, you can and probably should get the 5V from the arduino power supply side. Trying to drop from 20-24V to 5V with say 7805 even at 200-300mA will get hot sooner than later.If i had a 5V power supply for the arduino part of my design, i would increase its A output say by 1A which is easier and use a small booster circuit for the 18V rail which @325mA draw is not at all impossible even from as law as 5V supply... it might not be as efficient at first glance but it is much better than trying to introduce a 20-24V supply in the mix which is a whole other power supply design on its own....

author: dsegel
author_rank: Contributor
timestamp: January 10, 2018, 12:13:02 am
content: That's essentially what I'm wondering about - should I drop the 18V to 5V to power the arduino, or just bring in 5V and boost it to 18V for the coils. I guess I'll look at boost circuits next.

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2018, 09:47:09 am
content: The 18V circuit will be fine with an LM7818 and modest sized heatsink 20oC/W.The LM7805 could be connected to the output of the LM7818 but the heatsink will need to be larger, to account for the extra power dissipation.Another option is to connect a series resistor before the LM7805 to drop the extra voltage. The LM7505 needs 8V to regulate properly and the minimum input voltage is 20V, so that's a voltage drop of 12V, at 200mA R= V/I = 12/0.2 = 60R, so use the next lower E12 value of 56R. The resistor will dissipate 56*0.22= 2.24W, so use a 3W resistor.As mentioned above, you might want to consider a switching regulator, especially for the 5V output, as with a linear regulator, the voltage drop all gets burned as heat.

author: soubitos
author_rank: Frequent Contributor
timestamp: January 10, 2018, 10:54:49 am
content: Quote from: Hero999 on January 10, 2018, 09:47:09 amOP says he wants to get away with it with a few small components, 78xx regulators plus heatsinks dont account for that spec LOL.... since its an arduino thing, he probably has a 5V rail already, no need to create another one and add a 20-24V rail on top of that... 325mA @ 18V is not hard to create with modest cost and small size booster circuit and the choices there are endless... at least that would be my approach to solve this... 1 beefier 5V power supply which can be off the self for cheap or can make one also cheap and a booster and you are done... having a 5V rail for the arduino side, then a 20-24V power supply from which to create 5+18V is a total waste of resources and adds both in complexity and cost

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2018, 11:36:22 am
content: Quote from: soubitos on January 10, 2018, 10:54:49 amYou seem to have misread the first post. He has 20V to 24V on the input, which needs to be converted to 5V and 18V. I agree, using a 5V supply and a boost converter would be more efficient, but if he only has 20V to 24V available, it's not possible. Building a boost converter, is also much more complex, than an LM78xx regulator, although using a cheap pre-built module would solve that problem.I've just realised I missed that using the LM7818 with 20V in would be marginal, because it ideally needs 3V of headroom, although at only 325mA, you'll probably get away with it, especially if it isn't critical.Using buck converters to generate 18V and 5V is another option, but again it's more complex than a linear design, although pre-assembled modules would overcome that.Quote from: dsegel on January 09, 2018, 06:18:31 pm

author: soubitos
author_rank: Frequent Contributor
timestamp: January 10, 2018, 11:46:37 am
content: I stand corrected.... i missed the part where the 5V rail was for the arduino, i thought it was there to power other part of the build.... I agree with your thinking yet, it is dirt cheap and easy to find a 5V power supply, it can be an old 1A capable mobile phone charger ffs..... and like you said, an off the shelf booster module would work just fine for the 18V rail required.... if OP has the possibility to get both (5V PSU and Booster module) he would be fine

author: ogden
author_rank: Super Contributor
timestamp: January 10, 2018, 12:11:44 pm
content: It is no brainer to convert digital 5V rail from power 18V rail, not opposite. So use 18V buck and 5V linear 7805 reg powered from 18V. It's doubtful that Nano will draw more than 50mA, so you can get away with linear reg for 5V.Boost converters are are inefficient compared to buck because of higher currents, thus higher losses in switch and inductor. Also for 18V@0.33A (6W!) boost you need high power 5V rail, around 2A. Such a current needs much bigger capacitors on 5V rail than in buck+linear topology.

author: soubitos
author_rank: Frequent Contributor
timestamp: January 10, 2018, 01:42:08 pm
content: So you suggest having a 20-24V power supply stepped down to 18V via a linear regulator and then a 5V linear regulator... OP asks for 18V/325mA + 5V/200mA that is a total of 6.85W available power plus the power converted to heat from the regulators....My approach is a common 5v-2A power supply delivering the 5V to begin with and a booster module to create an 18V railGood quality 5v-2A wall converter like thishttps://tinyurl.com/y88ysqt3cost 5.44$ and a booster module like thishttps://tinyurl.com/yd9zceu7for a total of 6$ and an extremely small footprint too which as i get it is also required. No heat issues to solve, no heatsinks to take space etc....The parts required to build the booster with MT3608 are as little as 1 ic 1 diode 1 inductor a couple resistors and capacitors... total cost for parts alone can be as little as 0,20$ !!!

author: dsegel
author_rank: Contributor
timestamp: January 10, 2018, 01:59:52 pm
content: I said I needed 20Vin because I was only thinking of reducing it, not boosting it. I could do 5V in and boost it for the 18V line, so I'm going to look at that now. This is all a learning experience for me; as long as it works in the end there's no right or wrong answer.Thanks for all your help and advice.

author: soubitos
author_rank: Frequent Contributor
timestamp: January 10, 2018, 02:24:34 pm
content: actually... for learning's shake... try both approaches and let us know which worked better for you!

author: davy peleman
author_rank: Regular Contributor
timestamp: January 10, 2018, 03:43:45 pm
content: the lm317 is in a to22 package and will be the cheapest option. with that current draw they're fine without cooling. i used one in a variable power supply for a slayer exciter circuit that could draw up to 1 amp and it stayed cool without heatsink. after half an hour it was lukewarm. so try it they're simple, cheap, small and rugged.

author: dsegel
author_rank: Contributor
timestamp: January 10, 2018, 03:53:47 pm
content: Is there any advantage to using an LM317 over fixed regulators like an L7818 or 7805?

author: davy peleman
author_rank: Regular Contributor
timestamp: January 10, 2018, 04:12:29 pm
content: lm317 has a 1.5 amp rating and can regulate up to 35 volts. don't know about the fixed ones. in my country the lm317 is the only one available so never looked into the fixed ones datasheet. if they are rated the same i'd say use the lm7805 and 7818. if steve ward can use them in all his tesla drivers i'd say they're rugged. he uses a heatsink however to cool the one for the mosfet drivers, but that's when driving big IGBT's rated at hunderds of amps. just try them for you purpose they'll be fine, trust me.

author: ogden
author_rank: Super Contributor
timestamp: January 10, 2018, 04:40:36 pm
content: Quote from: davy peleman on January 10, 2018, 04:12:29 pm7805 in TO220 also goes up-to 1.5A. You shall consider lm317 only if you need to optimize your stock or nonstandard/trimmable voltages are needed.This circuit is one why you could possibly want to use LM317, thou lot of new LDO's offer even better performance/precision:

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2018, 06:51:07 pm
content: Quote from: ogden on January 10, 2018, 04:40:36 pmJust one question: why use the LM317 as well as the TL431 when the LM317 will do on its own?Quote from: dsegel on January 10, 2018, 03:53:47 pmYes, it actually has better characteristics. Compare the data sheets. It also allows you to do things like bypass R2 with a capacitor, to increase the ripple rejection.Quote from: soubitos on January 10, 2018, 01:42:08 pmThere's no right or wrong way. Both ways are valid. 5V to 18V might be more convenient, as it's probably easier to get hold of a 5V 10W PSU, than an 18V PSU which isn't overkill, but it's not necessarily better.Using a higher voltage, before the long piece of cable, then stepping it down at the other end, is better than doing the reverse. For the same thickness and length of cable, transmitting 10W of power will be more efficient at 18V than 5V. Work it out. Think about the current needed, the resistance and power loss, in the cable, at different voltages.Given the choice, assuming equal cost and availability of parts, I'd choose an 18V 1A mains power supply and a 5V buck converter, over a 5V PSU and 18V boost converter. Another thing is regulation: the 5V rail probably needs to be better regulated, than the 18V rail and having only a short PCB trace between the 5V buck and device will result in better voltage regulationYou'll find a similar scheme used in laptops. An 18V to 20V PSU used to charge the batteries and a some step-down converters to provide the various lower internal voltages to power the device.

author: james_s
author_rank: Super Contributor
timestamp: January 10, 2018, 07:29:22 pm
content: No matter what linear regulator you use, they'll produce the same amount of heat.Going from 18V down to 5V at a couple hundred mA I consider a switching regulator a no-brainer. Small buck converters are very easy to make now or buy as complete modules. They are far more efficient and run cooler, the only reason to use a linear regulator is if you need really clean power.

author: davy peleman
author_rank: Regular Contributor
timestamp: January 10, 2018, 07:51:17 pm
content: less noise, cheaper and smaller so use the linear option, trust me on this. if it's going to be powered 24/7 i'd go with the buck/ boost but if it's for occasional use go with lm's . This is the last time i will say this or you guys will think i'm an old wining bat addicted in using lm317. i also use smps, buck and boost converters but not when i occasionally use the application and the power consumption is so low then it's a waste of money imo. i pay 0.50 euro's for an lm317 haven't come across quality buck/ boost converters for that price.

author: ogden
author_rank: Super Contributor
timestamp: January 10, 2018, 08:22:23 pm
content: Quote from: Hero999 on January 10, 2018, 06:51:07 pmAlready mentioned: performance/precision. Reference of LM317 is specified as +/- 4% with +/- 0.7% additional temp drift for 0..125oC range. TL431 is 2%, B grade is 0.5%. In circuit TL431 does not dissipate heat, so circuit is much more thermally stable compared to LM317 alone. FYI circuit from TL431 datasheet.QuoteWell, yes. Agreed. On the other hand 7805 shall be considered as close to obsolete today (more expensive than 1117), so we better compare LM317 to 5V 1117

author: Gyro
author_rank: Super Contributor
timestamp: January 10, 2018, 08:26:27 pm
content: Quote from: davy peleman on January 10, 2018, 03:43:45 pmHmm, your circuit wasn't actually pulling anywhere near an amp then. Voltage dropped across the regulator times load current equals dissipated heat. Even at its minimum dropout voltage (2V typ. at 1A) it's going to be dissipating 2W.That's going to be pretty noticeable on a bare TO220 and with a short time constant. There's no getting around the laws of physics.

author: ogden
author_rank: Super Contributor
timestamp: January 10, 2018, 08:34:20 pm
content: Quote from: davy peleman on January 10, 2018, 07:51:17 pmIf heat is not an issue (0.32A * (24-18)V = 2W peak), then indeed you can't beat price of linear regulator. Not always you can afford to build 2W heater out of power supply.

author: Gyro
author_rank: Super Contributor
timestamp: January 10, 2018, 08:41:47 pm
content: Do factor in a heatsink though. Junction to ambient for a bare TO220 is 50'C/W, that will take you too close to shutdown at normal ambient.

author: mariush
author_rank: Super Contributor
timestamp: January 10, 2018, 09:03:10 pm
content: If you have 20v or higher available and you're not afraid of making a small circuit board for a DC-DC converter, you could try using a chip like TPS54383 :https://www.digikey.com/product-detail/en/texas-instruments/TPS54383PWPR/296-23087-1-ND/1790154It's two switching regulators in one, and works at high enough frequency that you can use surface mount inductor and ceramic capacitors and resistors to have a small board ... and it should be efficient enough that you wouldn't need to worry about heat and heatsinks and moving the heat away.Chip is expensive in small quantities, but then again if you add two linear regulators and heatsinks for both and the footprint those two heatsinks will use on your board ... maybe it's not so expensive.In other notes, you may want to consider designing your project to work from 7.5v to 12v, if you think your product it's going to be powered from a wallwart adapter. 7.5v to 12v adapters are very common and therefore cheap.May want to think if you're gonna make a version to run from 4 AA batteries ... in that case you'll have a voltage range from around 4.8v (probably close enough to 5v that your circuit won't care) and up to around 7v - you could use a LDO with as much as 0.1v voltage drop to get ~5v from 4 AA batteries and use a boost regulator to create your 18v (though 18v @ ~200mA would be quite a load on AA batteries).

author: Audioguru
author_rank: Super Contributor
timestamp: January 10, 2018, 09:39:49 pm
content: An error was posted above, here are the facts from the datasheets:A TO220 package creates the same amount of heat from an LM317 as a 7805. Both will shut down within 1 minute when fed 20-24VDC and produce 5V at 200mA unless they have a medium size heatsink.

author: soubitos
author_rank: Frequent Contributor
timestamp: January 10, 2018, 09:48:50 pm
content: if OP HAS a 20-24V (better be 24V) then by all means, 7818-7805 one feeding the other would be an option, he did mention small footprint requirement though.... and that would not be so with proper heatskinking... I used an 7805CV recently in one of my boards, powered from 7,5V and drawing 0,4-0,5A tops, it reached 100C within few minutes... no heatsink, simply laying on the pcb with the GND soldered to a huge pad on it...if OP wants to build something that requires 18V and 5V like he described then i insist its a hell of a lot easier to find a proper 5V-2A wall psu on the cheap like the one i mentioned, and a small boost module would make sure his non-demanding 18V rail was met again, on the VERY cheap..... you cannot beat this logic i am sure LOL....As for the remark about how laptop power bricks come at high voltage etc, you do understand the smaller laptop power brick starts at around 40W power right? And that most chargers over 15V are used to charge 4S battery packs so a voltage of 16.8+drop on voltage regulator and charger circuit is required... so it is only natural to design with a higher voltage in mind, when you really use most of the power in form of 3,3-5-12V rails (and not only)....

author: soubitos
author_rank: Frequent Contributor
timestamp: January 10, 2018, 09:56:22 pm
content: Quote from: mariush on January 10, 2018, 09:03:10 pmNice chip this one... and if you buy LM78xx from the same source or mouser etc, the cost of two LM78xx with heatsinks etc against the tps + proper inductors etc is very much comparable......

author: james_s
author_rank: Super Contributor
timestamp: January 10, 2018, 10:34:40 pm
content: I've used quite a few Chinese buck regulator modules that were 99c shipped, I think I got some once for only 90c. I haven't priced 78xx or LM317 regulators recently but they can't be much cheaper, and by the time you add the heatsink and capacitors it will cost more.

author: soubitos
author_rank: Frequent Contributor
timestamp: January 10, 2018, 10:46:48 pm
content: Quote from: james_s on January 10, 2018, 10:34:40 pmI got LM7805CV from China recently, worked very well, cost 0,76$/10pcs .... a module cost 99c shipped from China with an LM7805 on it is NOT a cheap deal LOL.. On the other hand, 78xx can set you back ~~0,8-1,5$ a piece ordered from "western"sites etc....

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2018, 10:11:48 am
content: Quote from: ogden on January 10, 2018, 08:22:23 pmOh I see. The LM317 could also be replaced with a Darlington transistor, but then there will be no over-current or thermal protection.Quote from: soubitos on January 10, 2018, 09:48:50 pmYes, I mentioned the availability of parts in my previous post:https://www.eevblog.com/forum/beginners/power-regulation-options-for-5v-and-18v-circuit/msg1396846/#msg1396846To simply things: an 18V PSU and 5V buck maybe optimum, but a 5V 2A PSU will be cheaper, than a potentially huge 18V PSU, so a 5V PSU and 18V boost, is more economical.Quote from: soubitos on January 10, 2018, 10:46:48 pmExcept he was not talking about a 99c module with an LM7805 on it. He was referring to a 99c buck converter module, which will be much more efficient than the LM7805 and won't need a heatsink.

author: mariush
author_rank: Super Contributor
timestamp: January 11, 2018, 01:39:45 pm
content: Boosting 5v to 18v is somewhat difficult to do with high efficiency and with cheap ICs .He wants 18v up to 325mA so let's say we aim for 350mA or 6.3w ... with a cheap boost regulator he's probably not gonna get more than 80% efficiency boosting 5v, so we're looking at around 7.5-8w from 5v, or around 1.5..1.6 A and naturally there's gonna be pulses/bursts higher than that. Do you trust any usb 5v 2A adapter to give you around 1.8a..2A for 24/7 (or while your device runs)?Considering the dubious quality of various 5v 2A usb adapters and the losses on the usb cables between adapters and devices (which means i won't even get 5v), I wouldn't trust that i'd get good 5v to use directly without some regulator, so that's why I'm saying it would make more sense to use something in the range of 7.5v and 12v.Allow for some voltage drop on cables, allow use of some LDO for the more "critical" part which is 5v, and make boosting to 18v more efficient by reducing the voltage differential.

author: soubitos
author_rank: Frequent Contributor
timestamp: January 11, 2018, 01:59:57 pm
content: MT3608 is cheap with small BOM around it and can deliver 18V/400mA from 5V no problems... (I tested mine with a led strip @ 12V/0.5A from 3,5-4,2V from 18650 batteries and it was solid - and its not as efficient with inputs under 5V) efficiency with proper inductor etc would be in the range of 85-90% say 80-85% in real life. And the ORICO 5V supply at least based on what i already have and abuse, is good to go for the VA it claims with what it delivers and of course, you can find several others between 2-3A which are both cheap and good quality so its still a cheap solution and definitely the one i'd go with any time.... This for example is a 2 port 5V/2,4Ahttps://tinyurl.com/yb3nes2mmax 15w total charger 5.27$ delivered!

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2018, 05:47:40 pm
content: Quote from: soubitos on January 11, 2018, 01:59:57 pmToo cheap. It might work and be safe, or it could be deadly but is it worth the risk?I'd go to my local supplier and buy something more pricey but more likely to be safe and comply with the relevant EMC standards.The 18V PSU is more expensive, than the 5V, but I'd probably go with it anyway, for the reasons mentioned in my previous posts.http://cpc.farnell.com/stontronics/t6090dv/psu-raspberry-pi-5v-2-5a-uk-euro/dp/SC14026http://cpc.farnell.com/stontronics/t6369st/ac-dc-power-supply-18v-1a-2-1mm/dp/PW02850

author: soubitos
author_rank: Frequent Contributor
timestamp: January 11, 2018, 05:57:53 pm
content: deadly? what makes you say that? i am using MT3608 quite a lot in my designs lately and have not had any issues with them in terms of safety (overshooting in the output for example) even when they eventually failed from (physical) abuse.... what makes an expensive part safer than a cheap one? i really am curious to know if for example the too cheap MT3608 has safety issues as that means i have to redesign half my designs so far......

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2018, 06:41:53 pm
content: Quote from: soubitos on January 11, 2018, 05:57:53 pmDon't be silly, I wasn't referring to the MT3608, but the crappy USB charger on AliExpress of course, hence the links to mains power supplies.Don't cheap out on safety critical components like that.

author: soubitos
author_rank: Frequent Contributor
timestamp: January 11, 2018, 07:52:17 pm
content: Quote from: Hero999 on January 11, 2018, 06:41:53 pmOh.... silly meOn the other hand.... Orico is a very good brand .. Chinese brand .... but a very good one... I have a 4 port 34W (max outp 2.4A you can draw 2,4A from any two channels but must keep it under 1A for the other two) I am so confident for the brand and products I would gladly tear apart the charger i got to show you and possibly others but i can't replace it with anything else and it will take a month or two before i get another ....Mine is different than this one but trust me, its really goodAnd a teardown... shows orico quality

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2018, 10:08:41 pm
content: There are some Chinese brands that are quite good, although you do have to be careful of counterfeits. China seems to have a very lax attitude toward counterfeit products, something will get a good reputation and then dozens of other companies try to cash in by making cheaper copies.