78XX voltage regulator problem - Page 1
Forum section: electronics / beginners

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 05:49:18 pm
content: why does every time i use a 7808 voltage regulator always burns????first time i used 15v input=deadsecond time i used 12v=dead third time 10volts=dead...NO shortcircuit ..1 pin is input,2 pin is ground and 3 pin is output for the 7808 and for thr 7908 is 1pin for ground,2 pin for input and 3 pin for outputi got 100% dc voltage inputi use this one l7808cvi bought 5 more from a difrend storecan someone tell me the max input

author: tszaboo
author_rank: Super Contributor
timestamp: January 11, 2014, 06:12:40 pm
content: It is not about the input voltage, it is probably about the dissipation. If you dont use a heatsink, you can only dissipate about 2 watt. That is about 250mA for 12V. Use a heatsink? How much current are you pulling?

author: Rudane
author_rank: Regular Contributor
timestamp: January 11, 2014, 06:19:07 pm
content: Are you using a heatsink?

author: Monkeh
author_rank: Super Contributor
timestamp: January 11, 2014, 06:21:29 pm
content: Quote from: christos on January 11, 2014, 05:49:18 pmSorry, but you're going to have to go read and understand the datasheet. Just like the manual for that Keithley. Reading is not optional.

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 06:38:22 pm
content: Quote from: NANDBlog on January 11, 2014, 06:12:40 pmi use heatsink..5x5 and 1amp fuse..but the regulator always gets burnd..how much input should i use max-min

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 06:43:43 pm
content: Quote from: Monkeh on January 11, 2014, 06:21:29 pmi did read the datasheet ..i had some 7805 and replaced the 08 with the 05 and it worked fine..5.1 volts output..put back the 08 and burnd the 4th today ..i did this before 10mins..its the regulator..something is wrong with that..

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 06:44:46 pm
content: anyway..i ordered some more 7808..hope they dont get burnd

author: sync
author_rank: Frequent Contributor
timestamp: January 11, 2014, 06:46:33 pm
content: Quote from: christos on January 11, 2014, 06:43:43 pmWhat capacitors do you use?And how near they are connected to the 7808?

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 06:48:14 pm
content: Quote from: sync on January 11, 2014, 06:46:33 pm2x2200 uF and like 2cm away from the last cap

author: sync
author_rank: Frequent Contributor
timestamp: January 11, 2014, 06:59:47 pm
content: Maybe it oscillates (to death). Add one 0.1uF ceramic cap at the input and one at the output of the 78xx. As near as possible. 2cm is too long. And don't use big caps at the output.

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 07:04:46 pm
content: Quote from: sync on January 11, 2014, 06:59:47 pmwhy near? at the output i use 10 uf

author: arcom
author_rank: Contributor
timestamp: January 11, 2014, 07:33:46 pm
content: Could also be fake ICs especially if the 7805 works and 7808 blows up. I always found the 78XX regulators very difficult to kill since they are pretty well protected. Their only weakness is when the output voltage is higher than the input voltage in which case a diode from output to input fixes the problem.

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 07:37:06 pm
content: Quote from: arcom on January 11, 2014, 07:33:46 pmFake??i blow up 4 of them today and the 7805 works fine with 10 volts input....ill wait for hte next order to see if they will work fine..

author: Unknown
author_rank: Unknown
timestamp: January 11, 2014, 07:45:51 pm
content: Quote from: christos on January 11, 2014, 07:37:06 pmI think you need to review your test procedures.

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 07:53:03 pm
content: Quote from: Lurch on January 11, 2014, 07:45:51 pmill try first with the new ones that i ordered and if i blow them too ill review it

author: henson
author_rank: Unknown
timestamp: January 11, 2014, 07:57:42 pm
content: Maybe you should post a picture of your set up. Something doesn't seem right. You should have plenty of capacitance to avoid oscillation and you really aren't dropping the voltage to much to kill them by heat. Very suspicious.

author: kripton2035
author_rank: Super Contributor
timestamp: January 11, 2014, 07:58:49 pm
content: what power do you dissipate in the regulator ?what is the voltage input, the output should be 8v and what is the current throught it ?

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 08:07:25 pm
content: Quote from: kripton2035 on January 11, 2014, 07:58:49 pmi use a transformer 10 v and 2 amps output

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 08:10:28 pm
content: Quote from: henson on January 11, 2014, 07:57:42 pmif the same thing happens with the new ones i ordered..ill make a vid how the 08 regulator burns and with the same pcb but with the 05 regulator at 10 volts 2 amp input.. may all four of the 08 regulators was bad..

author: Unknown
author_rank: Unknown
timestamp: January 11, 2014, 08:11:41 pm
content: Quote from: christos on January 11, 2014, 08:07:25 pmYou may want to read the datasheet, as already mentioned.

author: tasos987
author_rank: Newbie
timestamp: January 11, 2014, 08:16:28 pm
content: I'm sorry to say that YOU HAVE TO read the datasheet

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 08:21:28 pm
content: Quote from: tasos987 on January 11, 2014, 08:16:28 pmyea i DID IT 5 TIMES..if you live in greece then you know poso lamogia einai oli tous and sorry about not writing in english..many people in greece are like thiefs...they sell many times items that are dead or the buy things that have some kind of malfunction and sell them to us

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 08:26:57 pm
content: Quote from: christos on January 11, 2014, 08:21:28 pmfor example ,when i bought a soldering station the ct -936..i got it with the knob broken ..

author: Unknown
author_rank: Unknown
timestamp: January 11, 2014, 08:27:58 pm
content: OK, well this seems to be going nowhere.

author: tasos987
author_rank: Newbie
timestamp: January 11, 2014, 08:31:30 pm
content: Quote from: Lurch on January 11, 2014, 08:27:58 pmObviously....+1

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 08:34:38 pm
content: so it ends here..ill post on this one if the new l7808 works

author: Monkeh
author_rank: Super Contributor
timestamp: January 11, 2014, 08:35:24 pm
content: Quote from: christos on January 11, 2014, 08:21:28 pmOkay then, what's the maximum input voltage? What does it say about input and output capacitance?

author: mariush
author_rank: Super Contributor
timestamp: January 11, 2014, 08:35:31 pm
content: Do you convert the voltage from the transformer to DC before it gets to the 7808 ?Transformer output is AC, regulators work with DC voltage. You have to use a bridge rectifier or at least one diode and then some capacitors to get DC voltage from the transformer.Is the output of the transformer 10 volts DC AFTER rectification and after using a capacitor, or it says 10 volts on the transformer label?Regulators in the 7800 family need 2 volts ABOVE the output voltage to work. A 7808 will need AT LEAST 10v to output a stable 8v. This is mentioned IN THE DATASHEET.QuoteA knob can be broken during shipping, you can return a broken product and ask for another one or your money back. You're in European Union, so you order something and have it shipped to you, you have the right by law to return anything for your money back, no questions asked, within 10 working days of your purchase. if the seller doesn't agree to send you another knob or change your product, ask for money back within the 10 days allowed by law.

author: Kappes Buur
author_rank: Regular Contributor
timestamp: January 11, 2014, 08:36:29 pm
content: Quote from: christos on January 11, 2014, 08:10:28 pmDo you have a circuit something like thisC1 and C2 should be as close as possible to the regulator.I just noticed, C1 should have said 0.33uF.

author: Monkeh
author_rank: Super Contributor
timestamp: January 11, 2014, 08:41:37 pm
content: Quote from: Kappes Buur on January 11, 2014, 08:36:29 pmAnd this, kids, is why we say 0u33 or 330n.

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 08:54:27 pm
content: Quote from: mariush on January 11, 2014, 08:35:31 pm9.8 volts dc 100%(got osciliscope) before the regulator

author: christos
author_rank: Regular Contributor
timestamp: January 11, 2014, 08:56:01 pm
content: Quote from: Kappes Buur on January 11, 2014, 08:36:29 pmyes but without the D2..why does the cap need to be close to the regulator??

author: underwurlde
author_rank: Newbie
timestamp: January 11, 2014, 09:02:26 pm
content: ^ Inductance old boy.What is it powering? How much capacitance connected to output?Too much capacitance on output = large in-rush current upon powering up = not great.A

author: Kappes Buur
author_rank: Regular Contributor
timestamp: January 11, 2014, 09:21:28 pm
content: Quote from: christos on January 11, 2014, 08:56:01 pmWith 78xx regulators you should always include D2. This will protect the regulator if the input voltage suddenly collapses and the output voltage is trying to feed back into the regulator.I see arcom also mentioned this on page 1Quote from: christos on January 11, 2014, 08:56:01 pmFor stability reasons. Well, this is not a hard and fast rule but it is common sense.It just won't do to have the regulator on one end of a pcb and those capacitors on the far end. Thusly, as close as possible.

author: sleemanj
author_rank: Super Contributor
timestamp: January 11, 2014, 11:14:53 pm
content: Replace the load with resistors to get you about 100mA, see if it works, increase to 150, 200, 250.... until it craps out.Probably "chinese domestic market" parts which don't meet the specs of "the real deal", or simply outright fake.

author: christos
author_rank: Regular Contributor
timestamp: January 12, 2014, 12:17:31 am
content: as you said..chinese all four of them .anyway..im out of 7808..waiting for the new order

author: arcom
author_rank: Contributor
timestamp: January 13, 2014, 12:37:00 pm
content: You know you can use 7805 to get 8V?Just put a voltage divider at the output and connect the GND terminal to the divider, just like an LM317.You have a table here with suggested resistor values:http://www.rason.org/Projects/regulator/regulator.htmUnless you absolutely need an LM7808, in which case please ignore my post

author: christos
author_rank: Regular Contributor
timestamp: January 13, 2014, 01:20:42 pm
content: Quote from: arcom on January 13, 2014, 12:37:00 pmwhy ignore it..i didnt know thatwell..i bought today a 7808 and it works fine..thx for the help

author: lapm
author_rank: Frequent Contributor
timestamp: January 13, 2014, 01:50:14 pm
content: After quick read im thinking you are drawing too much current throw the regulator, leading to massive power transformed to head and your regulator deep frying itself in result.Douple, triple check the pinout. Read datasheet. Anything about maximum limits...Some regulators metal tab is connected to one of the terminals internally. Very easy to miss and cause short that takes out component. Yes even i have been bited by this.Also consider bossibility that if everything else fails, you might have fake component. Im lucky and havent run into one of those yet, but one of these days. One of these days i will.

author: deth502
author_rank: Frequent Contributor
timestamp: January 13, 2014, 02:51:45 pm
content: Quote from: christos on January 13, 2014, 01:20:42 pmboy, that one is a mystery.i believe i have seen at least 5 posts here recommending to read the data sheet.78xx series datasheet:http://www.fairchildsemi.com/ds/LM/LM7805.pdf

author: hobbes
author_rank: Newbie
timestamp: January 13, 2014, 06:33:18 pm
content: Quote from: deth502 on January 13, 2014, 02:51:45 pmOne might wonder what those "SOA protection" and "Thermal protection" in the said data sheet are there for?

author: lapm
author_rank: Frequent Contributor
timestamp: January 13, 2014, 06:37:15 pm
content: Quote from: hobbes on January 13, 2014, 06:33:18 pmWell protection seems to be working. It disables system to stop overload occurring again..

author: sleemanj
author_rank: Super Contributor
timestamp: January 13, 2014, 11:23:19 pm
content: Quote from: hobbes on January 13, 2014, 06:33:18 pmThe presence of such in 78xx from China is not assured. Neither is current capacity, or power dissipation.

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:38:52 am
content: Quote from: lapm on January 13, 2014, 01:50:14 pmnot that problem..it was the regulators..all 4 of them had some kind of problemthe new one i replaced works fine with no problem

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:41:08 am
content: Quote from: deth502 on January 13, 2014, 02:51:45 pmwhy shoud i not read the datasheet..i did it many times..the regulators was the problem..all four of them died in 1 sec..bought a new one from another e-shop and it works fine

author: lewis
author_rank: Frequent Contributor
timestamp: January 14, 2014, 10:52:39 am
content: Fake parts = false economy

author: Monkeh
author_rank: Super Contributor
timestamp: January 14, 2014, 08:29:29 pm
content: Quote from: christos on January 14, 2014, 10:41:08 amBut you show no signs of having absorbed any of the information in it.

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 09:39:23 pm
content: Quote from: Monkeh on January 14, 2014, 08:29:29 pmi knew that the problem was from the regulators from the Reply #34..i didnt know that there was chinese regulators...but why shoud a 7805 work in the same pcb and not the 7808 at 10 volts dc...i tried difrent value of volts and the same thing ..all burnd out..anyway..thx for the help..i did learn something from the replays about the diode and the most important to know what to buy.. second mistake i made without realizing what im buying and spending money for nothing..the first mistake was when i bought a micro sd 32GB and the second day my phone didnt reconize it cuz it went to 8MB from 32GB

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 09:43:07 pm
content: and one more question..in this forum, do i have any list of firnds? can i add friends? if yes then let me know and when my frind comes to greece ill ask him the short circuit protection with only 2 or 3 hight values resistors..still trying to figure how he did it

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2014, 09:54:17 pm
content: Quote from: christos on January 14, 2014, 09:39:23 pmIf the circuit has a constant resistance, more current will flow with an 8V regulator than a 5V regulator so that might be your problem.

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:01:08 pm
content: Quote from: Hero999 on January 14, 2014, 09:54:17 pmthen how do you explain when i replaced the new ones, the regulator works fine and still works?

author: lapm
author_rank: Frequent Contributor
timestamp: January 14, 2014, 10:23:06 pm
content: Quote from: christos on January 14, 2014, 10:01:08 pmWell hard for us to say exactly what was wrong in your circuit. We have to rely only to information you have provided. I am glad you finally got it working.Your last comment how ever seems to verify that this is either case of fake Chinese parts (Bossibly with alternative pinout) or you had short circuit there causing massive overload and removed it while changing components, maybe without even realizing it being there.

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:29:32 pm
content: Quote from: lapm on January 14, 2014, 10:23:06 pmwhat i did was 1)cut the power of 2)remove the regulator 3)clear the solder that was left 4)place the new one 5) solder it so it shines 6)make a continuity test if there was a problem ..i did that 4 times and the 5th time it worked