Buck or boost? - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: January 11, 2018, 09:19:58 pm
content: Quick project brief. I bought a small solar system that will run indoors in a window. I know it won't produce much, but I am only intending on using it to charge battery powered devices. Phones, eCigs, power banks, etc. I have been charging my USB devices off a repurposed laptop battery (extracted the cell pairs and put them 3S2P config) with a 5V regulator board I got off ebay. The 3S2P get's charged off the solar load via a proper hobby charger. Charging USB devices it lasts for ages.This works, but it has short comings. I believe it's a linear regulator, it gets hot, it doesn't like to put out more than about 1Amp.So it got me to thinking, if I want to charge things via USB with more than 1Amp and use QC2.0 or QC3.0 adapter etc. Or is I want to run anything less than 12V off the 12V Lithium pack I will need a switched step down buck converter.Now I intend to buy old laptop packs for £8 off ebay and extract the cells, they are low current cells I know.Is it better/easier/more efficient to use the cells in 1S6P and use boost converters to step the voltage up, or build them in 3S2P and step it down?

author: paulca
author_rank: Super Contributor
timestamp: January 11, 2018, 09:38:02 pm
content: Oh, I forgot one thing.Cheap and fairly safe.I have found that switch mode regulators buck or boost are not the cheapest and nor are inductors.Alternatively I can buy Chinese modules for less than the cost of the IC alone, but I would call them "under supervision" devices and am not wanting to leave such a device running overnight (though I have) or while I'm out of work. and the house is empty.

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2018, 10:11:51 pm
content: Buck is almost always more efficient and lower cost than boost. The one caveat I can think of is a failing buck converter can put the full input voltage onto the output while a failing boost converter will typically result in the output dropping down to the input.

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 08:00:27 am
content: Quote from: james_s on January 11, 2018, 10:11:51 pmIs there anyway to add protection against that? Something that will, say, cap the voltage and then fail rather than let it exceed tolerance?I suppose a zener will cap it, but if the current exceeds it's abilities it will then fail and the full voltage will still get out the output rail.EDIT: I was doing a bit of googling, crowbar came up, also Zener clamp. However I wondered. If the zener circuit has no resistor to current limit, if the voltage suddenly rises above the zener threshold with no current limiting resistor could a fuse not be used to fail the circuit as current rushes through the zener and blows the fuse?

author: bd139
author_rank: Super Contributor
timestamp: January 12, 2018, 10:08:52 am
content: That's a relatively naive but effective way of doing it. The only problem is that it depends on how much power the zener can sink compared to the fuse. If you think of a Zener's power dissipation as P = I * V, the V is going to be quite high for a zener which means if your solar panel dumps say 2A through a 12V zener, that's going to have to blow 24 watts out of the window until the series fuse blows. There is a possibility the zener will go before the fuse as most fuses are still pretty slow (sometimes seconds). Also the Zener's "knee" is quite soggy i.e. it starts to conduct a little bit, then gets more and then more etc. There's no "snap" moment.Ergo, that's why you usually use a proper crowbar circuit. This contains a zener, or even better a precision reference and an SCR. When the SCR threshold is hit, the SCR latches down at a very low voltage. SCRs can cope with a lot of current and power. Think clamping a 12A supply in a single TO220 package.If you look at the following Art of Electronics reference, which happens to be the free chapter:https://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdfpp690 (section 9.13), it will go into detail. The best implementation is the TL431 which is a programmable voltage reference that costs literally bugger all. Add a fat SCR, a couple of resistors and a fuse and you're good.Also the chapter above contains lots of info on buck/boost converter characteristics and design etc.

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2018, 10:53:42 am
content: Quote from: bd139 on January 12, 2018, 10:08:52 amA crowbar is a good idea, but beware of nuisance triggering, due to small, sharp spikes or noise.One way round this is to use a zener with higher voltage, than the crowbar and add a capacitor to the crowbar circuit, to slow it down. The zener will absorb short transients, without tripping the crowbar, which will only trip when the overvoltage persists for long enough.

author: bd139
author_rank: Super Contributor
timestamp: January 12, 2018, 11:07:13 am
content: Good point. I have actually had that problem before. Cost me a pass transistor as well

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 11:09:16 am
content: So the particulars in this will be:3S2P Lion 12V -> Buck 5V -> USB* -> Device* (*=multiple)For most things anyway. Other things will most likely be 12V wall wort devices and they usually handle the Lion voltage range fine and won't be bucked.I haven't yet figured out how to do QC2.0/3.0 but I'll leave that for later.Cost benefit would depend on what the device is. If it cooks one of my eCig batteries, I won't be too worried, I have half a dozen of them and they are only £30.However my phone is circa £250 to replace. It has QC and accepts 9V, so maybe it would survive 12V.Just a thought, but a zener, transistor and latching relay is a bit expensive.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2018, 05:56:12 pm
content: Well assuming the regulator is reasonably well designed, then failure is rare. I don't recall ever seeing any form of overvoltage protection in an off the shelf cigarette lighter to USB adapter.

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 06:20:56 pm
content: Quote from: james_s on January 12, 2018, 05:56:12 pmI thought about on the way home as I was using just that. It's been in my car for 5 years. I had one that failed, but it failed 0V. Hard to tell if it is just a linear reg or not though.I have the components. I will use the reference application schematic in the datasheet for the buck IC and see what I get on a breadboard (low current) if it works I'll move it to a perfboard and load it up.

author: bd139
author_rank: Super Contributor
timestamp: January 12, 2018, 06:40:37 pm
content: I had a national semiconductor TO3 voltage regulator go in/our short once. Was on 5v rail so a ton of TTL got very hot very quickly and expired. I think about 16V hit it all. It only gave up when the transformer secondary went phut.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2018, 06:56:35 pm
content: Linear regulators can also fail shorted, I've had it happen myself. In one case it fried a bunch of stuff, in another it magically fixed an arcade game board that had been dead. No idea what happened with that one but the 5V was up over 10V and the game came up but looked blurry, fixed the power supply and it has worked ever since.

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 07:21:00 pm
content: Quote from: james_s on January 12, 2018, 06:56:35 pmAnd as I discovered the other night, they are a route to ground in reverse. Added 5V 'instead' of the regulator output, didn't unwire the regulator, disconnected it's power rail.... the smell got to me before I worked out why stuff wasn't working. I love my current limiting bench PSU, thanks to all on here for telling me it was a good idea to have one for bench stuff. It only took 0.5A as a result and survived.

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 07:23:01 pm
content: Anyway, so that makes tomorrow's project either making an ATTiny85 LED perfboard or a buck converter. Hmm... Can I do both in a day.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2018, 07:45:34 pm
content: You can probably do both in an hour if you have all the parts on hand and don't have to correct any mistakes.

author: paulca
author_rank: Super Contributor
timestamp: January 12, 2018, 09:42:10 pm
content: Quote from: james_s on January 12, 2018, 07:45:34 pmI did the basics of the LED board tonight. Beer is now adding up though.Tomorrow morning I have to sort out the solar system. I'll hopefully get to the buck converter on the breadboard (at least) tomorrow night.My perfboard skills suck

author: bd139
author_rank: Super Contributor
timestamp: January 12, 2018, 10:40:46 pm
content: Everyone’s perfboard skills suck. Some people are just better at hiding the horrible bits.

author: wraper
author_rank: Unknown
timestamp: January 12, 2018, 11:15:03 pm
content: Quote from: paulca on January 11, 2018, 09:19:58 pmDon't do that. Savings are questionable at best compared with new good quality batteries and quiet likely you'll get some worn out batteries.

author: paulca
author_rank: Super Contributor
timestamp: January 13, 2018, 12:00:41 am
content: Quote from: wraper on January 12, 2018, 11:15:03 pmI'm not buying second hand laptop batteries. I am testing the cells. The first set claimed to be 2600mAh, I tested them to be 2600mAh. Not exhaustive, but indicatively. Close enough for what I want.They won't run an eCig though. They are standard low current batteries. I'm rating them as 1C charge, opting for 1/2C instead, 1-2C discharge, aiming for 1C+grace and not asking more. Still that's 2.6 Amp Charge, 5.2 fast charge, 5 amps constant discharge, 10 amp burst.

author: viperidae
author_rank: Frequent Contributor
timestamp: January 13, 2018, 07:28:15 am
content: It's already been done for you.Buy a QC 2/3 car charger. It should work fine on 3S

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: January 13, 2018, 10:25:54 am
content: Be aware that some of the XL6009 boost converters sold on Ebay can 'run away' when the input voltage falls below the minimum for regulation, about 3v. The output can rise to as much as 40v, which could do some serious damage to USB equipment.The XL4015 buck modules seem quite reliable. Bear in mind these sense current in the -ve so the battery -ve must not be grounded.I'd go for a 3S or 4S arrangement and buck. Charging can be via similar buck. Bear in mind the need for a balancing/protection board.To protect the output, put a 5.6v zener across it. If the reg goes out of control this will blow and short the output. You are then relying on the protection board to cut the power. Or, add a polyswitch as well in the battery lead just for good measure.

author: paulca
author_rank: Super Contributor
timestamp: January 13, 2018, 11:10:36 am
content: Thanks guys. I now have a number of options in the post.* 12V car QC3.0 charger plug (with voltmeter), which takes care of sorting that fuss out for the phone. And a socket for it to wire an XT60 onto.* A Chinese 3 port USB buck converter for other things. Remains to be seen how stable this is and what it's based on, but it has a transparent acrylic case and I seem to have a fetish or something for those.I have the parts to build a 3A buck converter, but I'll do that just for fun. Hopefully smoke free.Next thing I need to build is a battery protection circuit. Again this is a rabbit hole as it really 'should' monitor on the balance port and kill the main power rails of the battery if a single cell drops below 2.8V. However, it's probably cheaper to just buy one from an RC shop. I'd have to accept the higher RC high current LiPo safety cut off voltage, often as high as 3.3V, more likely 3.0V, but I'll see what turns up.EDIT: The latter turns out to be trickier to buy than it should be. The only ones that will actually monitor individual cell voltages AND cut out are expensive. £25.I remember looking into this before and the best option is a low cell voltage alarm (£3) and pull the buzzer and connect a relay instead. However, it will need to be a latching relay or it will oscillate.

author: paulca
author_rank: Super Contributor
timestamp: January 13, 2018, 01:35:18 pm
content: So I built the buck converter on a breadboard as per the first application schematic in this:http://www.ti.com/lit/ds/symlink/lm2576.pdfI expect my lm2576T-5 buck IC is fake. It gives me 5.04V open circuit, but with a load pulling 800mA the voltage drops to 4.34V and rises back to around 4.45V. The IC also gets pretty toasty, which is probably normal. I don't think it's even worth putting a heatsink on it, not if it sags 0.6V with that small load.0.6V would be suspiciously like a diode drop, but the only diode is the schotky and it's a 2.something V drop.Was bought from a UK seller on ebay, but they were about 1/3rd the price of RS/CPC. Should have known better.

author: paulca
author_rank: Super Contributor
timestamp: January 13, 2018, 04:47:48 pm
content: I checked the price. RS - £1.48 each in a bag of 5. Ebay: £2.99 for 2. So same price. They are actually ON Semi. Doesn't prove they are real though.Should I really be seeing a 0.4-0.6V drop from the fixed output voltage under load?

author: bd139
author_rank: Super Contributor
timestamp: January 13, 2018, 05:03:41 pm
content: At least the ones on RS are likely to actually work properly.

author: paulca
author_rank: Super Contributor
timestamp: January 13, 2018, 05:12:42 pm
content: Quote from: bd139 on January 13, 2018, 05:03:41 pmYes. Hmm. This seems to behave like a small linear regulator. I might have to probe it and disconnect some pins to find out.Also, how "cowboy" is this... I didn't have a spare heatsink handy. Hunted round my bits and pieces boxes for something I could drill a hole in and use as a heatsink, when I found my power resistors. So I bolted the power resistor to the LM2576 as a heatsink. Works great. Innovate.

author: bd139
author_rank: Super Contributor
timestamp: January 13, 2018, 05:33:15 pm
content: Nothing wrong with that! Bulldog clips and copper pipe caps make reasonable “quick” heatsinks as well. I’ve even built a circuit on a bean can lid because I ran out of FR4.

author: paulca
author_rank: Super Contributor
timestamp: January 13, 2018, 05:47:52 pm
content: Quote from: bd139 on January 13, 2018, 05:33:15 pmLOL. I was considering using a beer can, but figured I might slice myself open cutting the thicker lid section out.I ordered 5xLM2576's from RS. If their inductors weren't a scandalous price I'd order a few of those too.

author: paulca
author_rank: Super Contributor
timestamp: January 22, 2018, 08:15:58 pm
content: Quote from: bd139 on January 13, 2018, 05:03:41 pmThat's if RS could send the right components.Got some random 3 pin SMD thing in a bag that said LM2576T-05 G on it. Returned. Just waiting on a second delivery of the same thing.Anyway. I got two things in the post.Car 12V cigarette lighter socket with QC3.0 port 2x normal ports and a volt meter. The phone loves it, goes straight to 9V and pulls 1.5A. "Turbo Charge Connected". The voltmeter is handy as I'm running it off a lithium pack that is not protected.and this little gizmo:https://www.ebay.co.uk/itm/262407885292Which, asides the worrying green caps, seems to be fairly decent. I've only tested it to about 3A and it doesn't squeal, get warm or even sag voltage. It's holding 5.15V dead steady. The phone likes it, pulls 1.6A from it happily. I seems to have 3 ICs, one is about 20 pins, though I can't think what for as it's not QC. Also seems to have a shunt resistor, so maybe it's current limited. It's pretty too. From the ebay warning about devices not pulling 1A, they seem quite proud of it too.EDIT: Actually this is reported as having 2xQC2.0 ports, which is why the phone is pulling high current. I can assume it only supports 5V though.I think I can tick the USB charging box as DONE now.I'm still going to build the buck converter, just for fun.

author: bd139
author_rank: Super Contributor
timestamp: January 22, 2018, 10:43:54 pm
content: That doesn't surprise me with RS. I regularly get sent the wrong bits. Sometimes they're worth a lot more than what I ordered though! Ordered a 59p pack of Vishay MRS25 resistors and got a reel of tantalum capacitors worth a small fortune. Didn't say a thingLooks good. Some of the little switchers from China are pretty good. I tried to blow a couple up and I couldn't.

author: paulca
author_rank: Super Contributor
timestamp: January 26, 2018, 07:30:00 pm
content: One thing I just found about the chinese buck converter. If it is has no load on it, it fizzes the power input badly. I happen to be powering the buck converter and the headphone amp off the same battery and if I remove the load off the buck converter I hear horrible fizzing. Not loud, but very noticeable.I'll keep it loaded or depowered I think.

author: paulca
author_rank: Super Contributor
timestamp: January 27, 2018, 08:27:19 pm
content: Credit were credit is due. RS helped me sort out the regulators. I ended up having to ask them to do a stock check as they assumed I had ordered the part in error having sent back what looked like a correctly shipped part.They phoned me on Friday and said they did indeed have incorrect stock and have refunded my purchase AND sent a replacement arriving on Monday. I love free stuff.

author: paulca
author_rank: Super Contributor
timestamp: January 30, 2018, 10:20:50 pm
content: So I got the "proper" LM2576s.I'm not sure they were the problem. I got a worse voltage drop! 4.03V at 0.3A.I replaced the inductor with a branded panasonic one that cost me £5! I checked all my capacitors with a component tester. Everything checked out.So I did a bit of double, triple checking, googling and the only answer was "breadboard".So I soldered it onto a perf board. I cramed everything as close as possible and used schotky diode leads for most things. Didn't have time to solder a USB socket onto the board so connected it to a USB breakout board with croc clip leads.4.25V at 0.3A. WTF?I was raging. So... I went to probe pin 2 of the regulator to see the waveform was still there and still 60Khz. I missed and probed pin 4 (output). Low and behold, what did I find? 4.96V. Confirmed with my multi-meter. At the end of the croc-clip leads... 4.24VI expect these croc-clip leads are made out of soft Chinese cheese. Most like about 2 strand aluminium. They are only 0.5m long. So they aren't much use.I expect if I spend the time to solder a USB socket onto the perf board it will give me close to 5V and I can load test it.That said, it was a learning experiment, I've learnt a few things. I don't know if it's worth keeping around and wasting a USB socket on. It's not like I will use it for anything. I might keep the board in case I have a project which needs a 1-3A 5V switching supply daughter board.I also expect the orginal LM2576s (and inductor) were fine and my breadboard was playing funny buggers with the switching of the regulator.

author: james_s
author_rank: Super Contributor
timestamp: January 31, 2018, 03:58:52 am
content: I'm surprised the regulator worked at all on a breadboard, switching regulators are fussy about layout, especially grounding and the feedback loop.I had a similar experience with cheap clip leads. Eventually I bought some high current silicone insulated wire from a hobby supplier, it's the stuff used to carry power to the motors in RC models. I got some quality alligator clips and made up a bunch of jumpers, never had any problems with them.

author: paulca
author_rank: Super Contributor
timestamp: January 31, 2018, 07:56:12 am
content: Quote from: james_s on January 31, 2018, 03:58:52 amMy plan, kinda. The silicon RC wire is expensive though. It's like 1.99 a meter for 18AWG. From hobbyking it's about half that, but still. I'd love to find a 50m reel for a tenner!

author: james_s
author_rank: Super Contributor
timestamp: January 31, 2018, 05:03:03 pm
content: That doesn't seem very expensive to me, especially compared to proper name brand clip leads. I used 18AWG wire from Hobbyking, I made a bunch of 1' leads and some 2' leads, only took a few meters of wire to make the whole pile of them.

author: paulca
author_rank: Super Contributor
timestamp: February 05, 2018, 07:31:18 pm
content: Well I have 10 meters of 18AWG tinned copper silcone wire, which was about £6.99 for each colour.I bought some croc-clips and soldered them onto 0.5m lengths.My multi-meter can't measure the resistance of them, it flickers at 000.1 Ohm and the stablises at 000.0 Ohm.Job's a good en.

author: paulca
author_rank: Super Contributor
timestamp: March 09, 2018, 10:02:24 pm
content: Sorry to bump an old thread, but I wanted to give this little module credit where it is due.https://www.ebay.co.uk/itm/262407885292I thought I had not loaded it much, as I'd only been running it at 1.5A peak load. It never faltered, never dropped it's voltage, never got hot.Then I realised that I was measuring the 1.5A on it's input at 12V. So 12V @ 1.5A = 5V @ 3.6AIt just works perfectly. Does what it says on the tin, doesn't complain. I have barely felt a bit of warmth from it and the inductor gets to be "warm to the touch", but that's about it. To get it to pull 1.5A off a 12V input I have it charging my phone at 1.3A, and two ecigarettes at around 800mA each. (I only have one USB current meter). Input current is floating around 1.4A (currently) off my solar setup 12.4V. Output voltage is 5.20V as always.

author: bd139
author_rank: Super Contributor
timestamp: March 09, 2018, 10:31:41 pm
content: Some of those little Chinese buck and boost converter modules are pretty good. I've got one doing 4-5 amps quite happily doing boost from 12 to 24 volts! Think it cost just under £1.80 delivered from aliexpress.