7812 vreg Fluctuating - Page 1
Forum section: electronics / projects

author: GoatZero
author_rank: Regular Contributor
timestamp: March 28, 2014, 07:26:07 am
content: Im kind of confused about whats going oni just did a PCB using 3 Voltage regulators,UsingLM337 for a -1.2v to -12v works as intended7912 for a -12 fixed works as intended i can actually see exactly -12v in the multimetter7812 for a 12v fixed, this is the one its making me nervous, its not giving a steady 12v instead its slowly fluctuating from 11.53v and then goes up to 11.85v and then again slowly goes down and repeats the processThis does not happen with the other to regulators especially the 7912, it stays right there at -12vthe positive input of the circuit its pumping only 13.8v DChope anyone can give some feedback

author: Skimask
author_rank: Super Contributor
timestamp: March 28, 2014, 07:36:57 am
content: Look in the data sheet for the term "dropout" voltage.

author: ElektroQuark
author_rank: Unknown
timestamp: March 28, 2014, 07:39:27 am
content: 7812 need a minimun input voltage of 14.5V.

author: GoatZero
author_rank: Regular Contributor
timestamp: March 28, 2014, 07:44:25 am
content: i see the 7912 dropout iots only 1 volt, thats why it can give a steady 12v, so the only solution its to get a transformer that can provide a higher voltage? lets sey a 25v transformer instead of the 18v im using

author: Bored@Work
author_rank: Super Contributor
timestamp: March 28, 2014, 08:40:16 am
content: Quote from: GoatZero on March 28, 2014, 07:44:25 amNo, the magic letters are "LDO". Look for a 12V LDO (low-dropout) regulator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2014, 08:55:58 am
content: Not enough headroom for variation in line voltage and transformer loading. Get a 25V transformer.Tim

author: Monkeh
author_rank: Super Contributor
timestamp: March 28, 2014, 10:36:32 am
content: Quote from: EdoNork on March 28, 2014, 07:39:27 amNo, it actually doesn't.

author: ConKbot
author_rank: Super Contributor
timestamp: March 28, 2014, 01:38:42 pm
content: Quote from: Monkeh on March 28, 2014, 10:36:32 amNo actually, it does.http://www.fairchildsemi.com/ds/LM/LM7812.pdfthe line regulation is only specified for 14.5v up. The dropout is listed as 2vtypical, so if you feed one 14.0 and you have poor line regulation because its not fully out of dropout region, thats too bad, so sad, because its only a typical value and not a max.Since a LDO would be cheaper than a new transformer, I'd suggest going for a 1A LDO instead of a 7812.

author: Monkeh
author_rank: Super Contributor
timestamp: March 28, 2014, 01:53:39 pm
content: Quote from: ConKbot on March 28, 2014, 01:38:42 pmWe'll just ignore all the other specs there and real-world measurements too, then. Temperature and load are clearly irrelevant, also.Banner specs are all you need, I guess.

author: mariush
author_rank: Super Contributor
timestamp: March 28, 2014, 02:43:36 pm
content: Replace that 7812 with a 1117 - 12 ... It only does 0.8-1A but has a voltage drop of about 1-1.2v so it will run fine from a 13.8v input.

author: ConKbot
author_rank: Super Contributor
timestamp: March 29, 2014, 12:23:39 am
content: Quote from: Monkeh on March 28, 2014, 01:53:39 pmId hardly call specs on the 7th page of the datasheet "banner specs"You can call fairchild though and tell them to update their incorrect datasheet though 408-822-2000

author: Monkeh
author_rank: Super Contributor
timestamp: March 29, 2014, 12:26:58 am
content: Quote from: ConKbot on March 29, 2014, 12:23:39 amI didn't say it was incorrect, but your interpretation of it is. The line regulation is only specified under certain conditions, one of which is a fixed junction temperature! And considering how wide the range of that 'specification' is, it's not even a useful specification!You paying for the call?

author: ConKbot
author_rank: Super Contributor
timestamp: March 29, 2014, 12:48:42 am
content: Quote from: Monkeh on March 29, 2014, 12:26:58 amFair enough, but the pass element will be a transistor on the die, so increasing temperature will lower the BE saturation voltage, but increased current will raise the BE saturation voltage. So it could go either way with temperature. Regardless, the TS's circuit is obviously not in regulation with 13.8V, and while some units may work fine with 14 or 14.2, or 14.499 instead of 14.5. Going for the datasheet 'safe' value is going to be a better start then messing around to see if it will work at a certain temperature etc...Either way, rather then being pedantic over datasheet values vs what can be done in real life, I dont think there is any arguing that a replacement with an LDO is in order.

author: Monkeh
author_rank: Super Contributor
timestamp: March 29, 2014, 12:57:37 am
content: Quote from: ConKbot on March 29, 2014, 12:48:42 amWell, the Fairchild datasheet is rather sparse. Other datasheets will fairly clearly indicate decreasing dropout voltage with increasing die temperature, as well as with decreasing output current. So, the 'load regulation' figure at a fixed current and die temperaure is of.. limited utility. Especially as they don't elaborate further on their test methodology.QuoteWell, we don't know if he's got a ripple free supply, or.. Incomplete information makes for a lot of shooting in the dark.QuoteOr with a suitable switching regulator. Certainly it's about time the 7800s died, though. It's not the 70s any more.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2014, 02:47:29 am
content: Why are you people arguing about tenths of a volt? The fundamental engineering error is not that the regulator's dropout is 0.2V too high, it's that the supply is 5V too low!

author: David Hess
author_rank: Super Contributor
timestamp: March 30, 2014, 06:10:57 pm
content: Quote from: Monkeh on March 29, 2014, 12:57:37 amUnconditional stability and easy frequency compensation are so passe.They work as inexpensive easy to bias alternatives to the LM395.