7812 Stability - Page 1
Forum section: electronics / projects

author: itdontgo
author_rank: Regular Contributor
timestamp: July 26, 2017, 05:16:21 pm
content: I feel like I should know this...If I run a 7812 for example below 12v input, will it be stable? Really I want a 12v max power supply but the thing could be run at 30v or 11v. I have a SMPS for a 4v section and don't want to implement another SMPS for this section as it's not critical. It's only a low-ish power audio amp <1/3W.

author: MosherIV
author_rank: Super Contributor
timestamp: July 26, 2017, 05:24:23 pm
content: QuoteA 7812 needs a minimum overhead voltage, usually around 2V. So to be clear, 7812 needs inout of 14V or more.If the input voltage drops below that then the output voltage drops as well.What current will it be running?The 7812 dissipate the difference in V as heat. So if your input is 15V and it is supplying 1A, the 7812 must dissipate 3W of power. Doable with heatsink. It starts to be a problem above 20W.

author: DBecker
author_rank: Frequent Contributor
timestamp: July 26, 2017, 05:49:47 pm
content: To clarify, you want a power rail that is limited to 12V but may be lower if the power input is lower.The 7812 will do this, and it should be stable. But it won't be awesome. It's going to have a negative PSRR (less than 0 db, meaning that it will amplify power supply noise) when it's not regulating.

author: itdontgo
author_rank: Regular Contributor
timestamp: July 26, 2017, 06:14:16 pm
content: That's not what I mean. Obviously 11v in 9v out.Will it be unstable/will the drop out voltage massively increase/increase with current... That's what I don't know.

author: MosherIV
author_rank: Super Contributor
timestamp: July 26, 2017, 06:23:22 pm
content: QuoteSorry if I am teaching you to suck eggs.I do not think the behavior is specified in the data sheets, hence you being unsure.I think if it works, it should be stable. The basic block diagram of how the 78xx works is that the output is monitored by an opamp against some reference.I think the main problem is that the output transistor may not get enough current to work properly and may become unstable as you fear.Time for an experimental circuit ?

author: floobydust
author_rank: Super Contributor
timestamp: July 26, 2017, 06:28:51 pm
content: 7812 has Darlington pass-transistor with current-source drive, so I think undervolting the input will give the usual 1.5-2V drop, as the error-amp just goes hard to one end. Nothing to go unstable, but it probably has a long response time coming out of it. If you go from 11V to 30Vin output overshoot might be a problem.The pass-transistor should have the same base-drive current even if the input is less than Vout.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 26, 2017, 06:57:21 pm
content: Dropout Characteristics plothttps://www.jameco.com/Jameco/Products/ProdDS/902127.pdfTim

author: DBecker
author_rank: Frequent Contributor
timestamp: July 26, 2017, 08:19:55 pm
content: Quote from: floobydust on July 26, 2017, 06:28:51 pmI doubt that the 7812 will have a problem with output overshoot going from 11V to 30V input. It's the same as powering up with a slow rise time, and we don't see 7812s killing downstream components.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2017, 10:12:57 pm
content: Quote from: itdontgo on July 26, 2017, 05:16:21 pmDo you have a schematic of the amplifier?The problem with the LM7812 is the output voltage could be too low, when the input voltage is on the low side. It would be much better if it could be designed to work from the correct voltage in the first place.

author: Blinkenlights
author_rank: Contributor
timestamp: August 02, 2017, 01:12:05 am
content: Will a 7812 be stable when it goes into dropout? In my experience, yes.I tested with national semiconductor, ST, and TI 7812.I remember winding the variac down and seeing the 100Hz dips start to appear in the output - definitely below 11V.The first indication of instability would have been some sort of spurious oscillation - probably at the start of end of the dip - I did not see that.My load current at the time was 500ma.Not all 7812 parts will be the same, I don't think lower currents will cause more problems, and make sure you have (good) bypass capacitors close to the regulator,input and output. I think it will all work for you.

author: CJay
author_rank: Super Contributor
timestamp: August 02, 2017, 06:19:50 am
content: A better question to ask yourself might be 'would my circuit perform at 9V' and then use a 7809 so you still have the headroom and guaranteed specifications of the regulator instead of relying on some characteristic of the device design that may vary by brand or even batch of regulator...

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2017, 10:47:21 am
content: I have never seen the 78xx series of regulators become unstable when operating below dropout. Just make sure to include a bulk decoupling capacitor on the output and input decoupling.

author: CJay
author_rank: Super Contributor
timestamp: August 02, 2017, 11:00:12 am
content: Quote from: David Hess on August 02, 2017, 10:47:21 amI haven't seen them oscillate or produce more noise than usual*, but that's not the same as them being able to regulate an output voltage, I'd be more concerned that the 'regulation' was just input V - Dropout V, I.E no actual regulation other than the various forward voltages of the junctions which could have some unpleasant side effects in the circuit it's powering.* had one that would wipe out local VHF radio and a digital meter on the output displayed a higher voltage than was being input to the regulator

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2017, 11:29:27 am
content: Quote from: CJay on August 02, 2017, 11:00:12 amObviously it will not regulate at all when in dropout and the dropout voltage itself varies with load current and temperature so the output voltage will vary as well.QuoteThat is caused by poor decoupling. Long long ago when I first tried using one, it produced a higher voltage at the input (!) of the regulator. Adding a 0.1uF input decoupling capacitor solved that.

author: CJay
author_rank: Super Contributor
timestamp: August 02, 2017, 11:58:04 am
content: Quote from: David Hess on August 02, 2017, 11:29:27 am*nods* Exactly what I was driving at, I reckon the OP would be best designing for 9V if he's intent on using an 11V minimum input.Quote from: David Hess on August 02, 2017, 11:29:27 amAbsolutely, I saw it on a linear voltage dropper for a truck, they had capacitors but they were cheap arse things, just a fuse, a switch, the 7812 and one or more 2N3055s, crude but effective.I had a batch of 50 or so to repair, various faults but this one had a label 'stops radio working' or something similar.Inside it had a different brand regulator to all the rest, changing the regulator fixed the problem, I stuck some new capacitors on it as well.