Step up DC/DC Booster low output voltage - Page 1
Forum section: electronics / beginners

author: Gambini
author_rank: Newbie
timestamp: September 08, 2022, 04:43:50 am
content: Hello there!I have an old 6volts motorcycle in which I installed a 12v digital speedometer by using a small 3A Step up DC/DC Booster. The circuit was working flawlessly for some time until I accidentally shortcutted the battery for a split second when installing a new horn.The booster used to output 12v, now it only outputs 6v (input is 6.3 volts).I replaced the booster by a new one, and the problem persists, it all looks like the battery is discharged by the shortcut. But it still outputs 6.3 volts and all the other instruments and lights work fine. Charging the battery doesn´t seem to help (so far).Can the battery be low on charge and still output the proper (fully loaded) voltage?I connected a 9v transformer to the battery wires and it still didn´t help, but if i do unplug the battery and leave the transformer as power supply, the speedometer works.Any ideas on how to troubleshoot the issue? Considering the speedometer is working, the booster has been replaced and the battery still outputs the proper voltage and all the other devices seem to have proper supply?I´m no expert in electronics, but I do have a decent experience in electricity and basic circuits. Googling about it takes me to things like impedance other concepts that are difficult for me to understand, so i was hoping there´s some rule of thumb things I could try out before setting the bike on fire.

author: rooppoorali
author_rank: Regular Contributor
timestamp: September 08, 2022, 09:47:59 am
content: Hi, have you recharged the battery? Have you tried to test your circuit after Charging the battery fully?

author: Gambini
author_rank: Newbie
timestamp: September 08, 2022, 05:15:41 pm
content: I don´t have a 6v charger, but I connected the 9v transformer and left it "charging" for about an hour. The curious thing is that none of the other devices show signs of a discharged battery, I turn them on all together and they look good and battery voltage is 6.3v, which again looks good to me.Is there any other way to know if the battery is not fully charged?

author: fourfathom
author_rank: Super Contributor
timestamp: September 08, 2022, 05:45:25 pm
content: If you are measuring 6.3V at the battery then your problem is not the battery. The battery may or may not be fully charged, but it's far from dead. What voltage does the booster put out if you disconnect the 12V load? Could you be overloading the booster?

author: Gambini
author_rank: Newbie
timestamp: September 08, 2022, 10:36:20 pm
content: So the situation at this moment is: load unplugged from booster. Output from battery: 6.43v, output from booster 6.3v.So, evidently the battery did charge a bit more (before charging it with the transformer, it was 6.3 on the battery and 6.0 on the booster). So, to me, it all points at the battery not providing enough current to make the booster work properly.So:QuoteThere´s a dismissible loss as you can see, but no boost, when before the shortcut there was double the input voltage.QuoteThe booster is well within its specs.Thanks for the help anyway, unless you or somebody else comes with another idea to try, I´ll try to find another way to charge the battery, since the transformer i was using died.

author: Gambini
author_rank: Newbie
timestamp: September 08, 2022, 11:03:03 pm
content: never realized you were two different guys, thank you both!

author: fourfathom
author_rank: Super Contributor
timestamp: September 08, 2022, 11:04:52 pm
content: Quote from: Gambini on September 08, 2022, 10:36:20 pmWhen a battery can't provide enough current to a load, the battery voltage drops. A 6V lead-acid battery will deliver somewhere between 6.0V and 7V (more or less), depending on state of charge and whether it is charging or discharging. In any case, if your battery is putting out 6.3V then the battery is not the problem. The booster is now unloaded, so it shouldn't be drawing any significant current. Are you sure you have it wired correctly? Can you provide a link to the booster specs?

author: Gambini
author_rank: Newbie
timestamp: September 08, 2022, 11:10:53 pm
content: This is the booster, it´s rated for 3 amp and 3 to 24vI will see if the battery voltage drops when i switch the contact on! I didn´t try that-edit- And, yes i believe it is wired correctly, it´s been working fine for weeks (it´s a restoration i just finished). and it has not had any issues until I caused this little shortcut.

author: Gambini
author_rank: Newbie
timestamp: September 08, 2022, 11:25:15 pm
content: So, battery voltage is 6.43 with everything off, with blinkers on, it ranges from 6.18 to 6.24 and with the booster + speedometer only it drops to 6.39. I believe it´s no significant consumption. There isn´t a single thing in the way, it´s just positive from the battery to the switch and then to the booster (which has an isolated negative) and then back to the battery. There´s a diode between the charging coil and the battery, but I doubt it has anything to do, since this has been all tested with the engine turned off.

author: fourfathom
author_rank: Super Contributor
timestamp: September 08, 2022, 11:26:15 pm
content: Please tell me that you have tried to adjust that trimpot!

author: fourfathom
author_rank: Super Contributor
timestamp: September 08, 2022, 11:31:27 pm
content: Quote from: Gambini on September 08, 2022, 11:25:15 pmAre you sure about this? I've got some similar cheap switching boosters and the input and output negatives are connected together. But if you are testing this with no load connections at all, then the isolation (or lack of it) shouldn't matter. -- no, that module does not have an isolated output. As long as you have a negative-ground system this probably doesn't matter.

author: Gambini
author_rank: Newbie
timestamp: September 09, 2022, 12:10:17 am
content: Quote from: fourfathom on September 08, 2022, 11:26:15 pmNope, but because I believe it´s set to double the voltage, and it proved to be so (before this mess at least). Will toy around with it eventually, i guess. Will have to undo the mounting, etc first.Quote from: fourfathom on September 08, 2022, 11:31:27 pmWhat I mean is that the negative output isn´t grounded. It is an isolated wire that goes from the booster to the speedo. So any parasite interference should be dismissed.I´m starting to believe the problem is that this little thing never worked and that the speedo was working merely by luck because the battery was fully loaded and giving 7+something volts (which may be on the edge of the minimun required for the speedo to work properly). The more things I try the less sense it is making. Will find a way to fully charge the battery first, then will come back with the results. Will try with the knob too. Thanks again!

author: Gambini
author_rank: Newbie
timestamp: September 10, 2022, 01:16:21 pm
content: Quote from: fourfathom on September 08, 2022, 11:26:15 pmRain stopped and saturday came so I could take a proper look at things and tinkered with that trimpot as you suggested and... walaaa I GOT IT WORKING!Turns out I assumed the new unit came adjusted to double voltage, since the old one did. Never skip steps!! Thanks for the help!!

author: fourfathom
author_rank: Super Contributor
timestamp: September 10, 2022, 02:13:01 pm
content: Great news!

author: --Oz--
author_rank: Regular Contributor
timestamp: October 03, 2022, 01:46:55 am
content: Quote from: Gambini on September 08, 2022, 11:10:53 pmI would have to disagree a tiny sot boost converter is 3A output, but thats not the OP point.Do you have a link to that module? tia

author: fourfathom
author_rank: Super Contributor
timestamp: October 03, 2022, 04:03:18 am
content: Quote from: --Oz-- on October 03, 2022, 01:46:55 amI believe that the SOT chip on that module is the MT3608L, which the datasheet claims is good for 2.5 Amps. I have used what looks like a similar module (but without the USB connector and I have not tested it near full load):https://www.amazon.com/gp/product/B07PDGKBQN. The Amazon listing says it is a 2A converter.

author: james_s
author_rank: Super Contributor
timestamp: October 03, 2022, 04:11:25 am
content: Quote from: Gambini on September 10, 2022, 01:16:21 pmYou should never assume that. They will come with the pot set randomly, either however the pot itself was manufactured or however it was left by someone that tested the completed module.

author: Marian_elf
author_rank: Contributor
timestamp: October 03, 2022, 06:35:44 am
content: Quote from: Gambini on September 10, 2022, 01:16:21 pmOne of the most important rule in electronics is never to assume anything, always test as much things as possible!It would have saved you this hustle.Quote from: fourfathom on October 03, 2022, 04:03:18 amAmazon listings claim many things, one should always research more and assume less.In this case it is all about thermal management, and for example MT3608L claim an absolute maximum power dissipation of 0,6W, but that is with the case held at a certain temperature ( about 2*C here ), which is impossible in real life, and of course one should always stay as far as possible from the absolute maximum ratings as possible.At 2,5A and with 80mOhms typical RDSon only conduction losses can be as high ar 500mW, with switching losses adding up, it will cook that sot23...I would say that most of what that board can do is 500mA, maybe 1A with active cooling.

author: fourfathom
author_rank: Super Contributor
timestamp: October 03, 2022, 08:31:22 am
content: Quote from: Marian_elf on October 03, 2022, 06:35:44 amI agree, I was only quoting the top-line datasheet number and the Amazon listing. I'm using that chip on a (hobby) design, but it's only running at about 100mA so I haven't bothered to do the thermal math.

author: james_s
author_rank: Super Contributor
timestamp: October 03, 2022, 03:36:44 pm
content: A general rule of thumb I follow for pretty much any of those various Chinese modules is cut the published specs in half as far as max load, max input voltage, etc. They are often extremely optimistic and if you run one of those regulator modules at full rated load it will usually pop in short order. That shouldn't be an issue for this particular application though.

author: Marian_elf
author_rank: Contributor
timestamp: October 03, 2022, 03:52:31 pm
content: I did tested one of these step-up modules, at just over 600mA load current ( 6,4V input, 12V output just like in the discussed case here ), and after 10 mins or so all the power devices ( IC, inductor and diode ) have well passed 40*C, with no active ventilation and in a sub 20*C ambient in the lab at the time.Meaning i do not think that even half the claimed current can be safely delivered over prolonged time.Still they are so cheap and easy to use ( and such high frequency ) that in some cases they still prove useful...

author: Gambini
author_rank: Newbie
timestamp: October 03, 2022, 05:39:18 pm
content: Well, I will keep you updated then with how long it lasts, guys!Considering the voltage drop is one quarter of the drop caused by turn signals (2 5w bulbs and the blinker). I believe it is within the pessimistic specs too.

author: james_s
author_rank: Super Contributor
timestamp: October 03, 2022, 06:36:51 pm
content: It'll probably be fine. As long as it's under about 70C I would not expect problems.