I dont know Circuit is correct or wrong - Page 1
Forum section: electronics / beginners

author: SKY KALAI
author_rank: Contributor
timestamp: August 31, 2021, 12:50:20 pm
content: Good Evening Everyone,Hope you all are doing well. I joined this forum today. Im not a electronic engineering student, i dont know how to make schematics myself. just only I tried to understand how electronics work. I would like to make a 16v to 5v power supply. Typically 4S (5A) to 5v. Im using LM78M05 SMD voltage regulator, multilayer ceramic 10uf 25v 0805 Capacitor in both IN and OUT. when I connect it with power supply (16.8v), after 5 seconds, the C3 was burnt out. I just attach schematic for reference. after that I checked , whether its a regulator or capacitor problem. regulator works well. So finally I decided the problem is Capacitor. the threshold voltage is 25v but I dont know why capacitor burnt. I simulated it with EASYEDA too. voltage out is 4.95v. My question is why its burnt?, does any other external factor cause this burn?. or I go to electrolytic capacitor?. or I move to higher farad capacitor? can anyone clear my doubt please.Thanks

author: xrunner
author_rank: Super Contributor
timestamp: August 31, 2021, 01:17:35 pm
content: I think you got a bad capacitor. Test another onebeforeinstalling it, and try again.

author: amyk
author_rank: Super Contributor
timestamp: August 31, 2021, 01:17:58 pm
content: MLCCs are somewhat fragile. Measure after installation to ensure you haven't cracked it and created a partial short.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2021, 02:27:21 pm
content: Yes, multilayer ceramic chip capacitors are fragile. You could try an aluminium, or tantalum capacitor, which should be more reliable. Note that it's important to get the polarity right. Aluminium capacitors, have the negative marked with a band, whilst tantalum is the oppositive: the positive is marked. It's also important to generously de-rate tantalum capacitors, so use 16V minimum, for a 5V and 25V minimum for 16V.

author: SKY KALAI
author_rank: Contributor
timestamp: August 31, 2021, 03:30:40 pm
content: Thanks for replying Mr.xrunner, Mr.amyk, Mr.Zero999WIll it work well, when Im connecting with power, Because I made this for RGB led inside the water, so i make it as water proof. Why am asking this, after finishing of I dont want to teardown again.All are giving same advise, I will change it by using 47uf 50v electrolytic capacitor, tantalum, aluminium and told you.Thanks a lot friends.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2021, 03:57:39 pm
content: Quote from: SKY KALAI on August 31, 2021, 03:30:40 pmHow much current does the LED require?It will need to be properly sealed, with a suitable heat sink. Fortunately, if it's not going to be used in air, water has a higher heat capacity, than air, which will make it easier to dissipate the excess heat.Silly question: why not use a 5V power supply in the first place? Mains/12V car power supplies, with 5V USB outputs are cheap. It makes sense to avoid converting one voltage to another, if possible.

author: SKY KALAI
author_rank: Contributor
timestamp: August 31, 2021, 04:32:11 pm
content: Hi zero999yes you are correct, I can directly use 5v car adapter, but fairly to say, here after I would like to make circuit myself to learn. I believe in practical, thats why. I know this is silly, but once I imagine my mind, it will work, I make one that one, after that, if problem comes I dont know how to rectify that. I prepare for solving problems. (my English was very poor, sorry for that, I will improve myself ASAP). thats why I do it myself.My LEDs needs 1A max. thats why I connect voltage regulators (LM78M05) in parallel.Thanks for your reply Zero999

author: mariush
author_rank: Super Contributor
timestamp: August 31, 2021, 04:40:43 pm
content: That's a problem dude.Linear regulators work by dissipating the difference between input voltage and output voltage as heat.So, if your 5v led strip consumes up to 1A, the 7805 will take 16.8v in and produce 5v on the output, but will dissipate (16.8v-5v) x 1A = 11.8 watts.You would need a HUGE heatsink to dissipate 12 watts of heat. Hopefully the regulator and the strip won't be INSIDE the tank, you don't want to boil your fishes.It would make more sense to power it with a 7.5v adapter, the 7805 regulators need at least 1.5-2v above the output voltage to work, so 7.5v is plenty. With 7.5v, the regulator will only waste around 2.5 watts as heat.Individual 7805 regulators can do up to around 1.5A of current, so you don't need need 7805 regulators in parallel, one is enough.BUT considering 5v phone chargers and wall wart power supplies are so cheap, I'm wondering why you have to resort to this.later edit: there are also linear regulators that can handle 3A or more, and they're fairly cheap.for exampleRegulators with 1085 in name -they're made by various companies so there's prefixes in front of the number unique to each company - (https://www.digikey.com/short/f389w4jw) are good for maximum 3A but usually have a lower maximum voltage (20v, 25v, 27v 30v) - depends on who makes themRegulators with 1084 are good for 5A but a bit more expensive.1117 regulators are super common and cheap, but they're typically good for around 0.8A and max 15v input and some (depends who makes them) are picky about capacitors on output and require minimum ESRs so you can't just throw a ceramic capacitor there and be done with it.Keep in mind that most regulators have a thermal limit ... for example let's say max 15 watts, it depends on the chip. So you can't just put 24v at the input and output 3.3v at 1A because you'll dissipate 21 watts of heat and the chip would turn itself off.Realistically, you need to aim for less than 3-5 watts of dissipated heat. With a reasonable heatsink, in air or in a case with ventilation holes, a regulator will be maybe 70-100c hot, dissipating 3-5 watts of heat. 15 watts is doable with a heatsink and fan like you see on some motherboards that fan on chipsets.

author: SKY KALAI
author_rank: Contributor
timestamp: August 31, 2021, 05:14:15 pm
content: Thanks for taking more time to giving idea and teaching me mariushI understand clearly. But in my case capacitor only got burned. Maybe MLCC are in bad quality. But I didnt use P=VI formula. I would like to change Input voltage 16v to 12v. because as you said it dissipates more heat.First I used heatsink in regulator. I just touch the the regulator but there is heat dissipation. (after capacitor burnt out , I check the regulator, it worked well).second Is individual regulator can able to give 1.5A out? because I read a datasheet of ST LM78M05. thats why I choose to connect parallel in 2 regulators. If it gives 1.5A thats enough for me.I will let you know by taking some tests like output current, heat dissipation with load. (also changing capacitors)Thanks you for all guys.

author: mariush
author_rank: Super Contributor
timestamp: August 31, 2021, 07:37:36 pm
content: There's all kinds of 7805 regulators, made by lots of companies.Some like LM78M05 are rated for a lower maximum current, but it's an exception, not the rule.If you go at a distributor of electronic components and filter to show 7805 "class" regulators, you get lots of models based on same design :https://www.digikey.com/short/89z83zdjYou have 78L05 which ismax. 100mA:https://www.digikey.com/en/products/detail/diotec-semiconductor/DI78L05ZAB/13153664orhttps://www.digikey.com/en/products/detail/fairchild-semiconductor/MC78L05ACDX/12613467You have 78M05 which is max.500 mA:https://www.digikey.com/en/products/detail/texas-instruments/UA78M05IKCS/863847orhttps://www.digikey.com/en/products/detail/onsemi/KA78M05RTM/12603045orhttps://www.digikey.com/en/products/detail/rohm-semiconductor/BA78M05CP-E2/2357100You have 7805 that can do max1A:https://www.digikey.com/en/products/detail/onsemi/MC7805CTG/919333orhttps://www.digikey.com/en/products/detail/onsemi/MC7805BDTG/1481212orhttps://www.digikey.com/en/products/detail/onsemi/SCD7805BTG/5845498You haveL7805 orUA7805 (and other prefixes) that can do1.5A:https://www.digikey.com/en/products/detail/stmicroelectronics/L7805ABD2T-TR/585694orhttps://www.digikey.com/en/products/detail/texas-instruments/UA7805CKCT/3458146orhttps://www.digikey.com/en/products/detail/stmicroelectronics/L7805CP/585963These all have different maximum voltages, different maximum power dissipation (some limit at 15w, some at 30w, some don't specify), the thermal resistance varies with the package ... voltage dropout is usually in the 1.5-2v ballpark for all.There's also differences in packaging like TO220 and TO220FP ... the first is package where the tab is metal, so it can be screwed to a heatsink directly ... the second has plastic back,The plastic back means it's radiates heat less efficiently into a heatsink compared to pure metal tab, but the plastic insulates the chip, so you can screw it to a heatsink with other components that may not have GROUND connected to the tab ... so this way you would not have short circuits between components, and you don't have to use mica insulators or other insulators with the chip in that scenario.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2021, 08:05:05 pm
content: Did you measure the output voltage? How long was it powered for? And were all of the LEDs connected to draw the full load current?The LM7805 won't be damaged if it overheats, because it includes built-in thermal protection, which reduces the output voltage, until the power dissipation is low enough for it not to run too hot.What do you intend to learn by using the LM7805? Solder practise, heatsinking, or perhaps more worthwhile that it's unsuitable for this application. You could learn more by building a buck regulator, using a suitable IC, or even buying one of those 12V to 5V DC:DC converter modules and reverse engineering it, than building a few LM7805 circuits.Another option is to use a self-oscillating buck regulator, made with a couple of transistors. Check out the Black regulator. It will obviously need some modifications to work at a higher current, but 5V 1A shouldn't be a problem.https://www.romanblack.com/smps/smps.htm

author: SKY KALAI
author_rank: Contributor
timestamp: September 01, 2021, 12:02:56 am
content: Thanks mariush,Nice explanation. I again read datasheet of regulator I have. It can able to deliver 500mA. So I tried it with parallel again with (12v) first. I also having MT3608. If this problem happened again ad again for initially I move to MT3608. After rectifying problem, I change it.

author: SKY KALAI
author_rank: Contributor
timestamp: September 01, 2021, 12:08:15 am
content: Yeah zero999, I measured a output voltage and current of regulator. 4.95v and clamp meter shows 0.45A. (without capacitor).I attached a MT3608 schematic in mariush reply. First I tried it with electrolytic capacitor and measure the current and voltage. But initially I don't know how to reverse engineer it. I read some articles, YouTube videos. Thta might be very helpful for me. I'm in just 2 weeks of this field. Maybe it takes atleast 3 months to understand.