Having problems with some audio gear. Few 5536 low noise opamps doing fairly standard audio things. The power supply is +/-8V, from a pair of HW rectifiers onto 470u caps, providing about +/- 13v unregulated. The +8v supply is provided by a 7808 reg, and the -8 volts by a resistor and a zener in series.
When the power is applied, the 7808 sometimes fails to kick in, its output holds at a *negative* voltage of about -1 volt. There's no instability evident, it seems to be some sort of startup transient relating to the fact that the two rails come up together, causing the 3-terminal device to latch low.
I'd appreciate any suggestions on how I can get the 3-terminal reg to come up reliably. TIA
Does your circuit have protection diodes from the output rails to ground? If not, and the negative regulator "comes up" faster than the positive and pulls the positive regulator's output below ground, the regulator can latch up. If the diodes are in place, than capacitance across the supply rails can cause a similar problem. See here:
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(From Douglas Self's book "Small Signal Audio Design")
On Sat, 7 Jul 2012 16:59:10 +0800, "Bruce Varley" wrote:
--- Since you're doing +/- half-wave rectification from the same end of the winding, the positive and negative rails _can't_ come up together, which is probably where the problem lies.
That is, depending on where the mains are when you power up the supply, either the positive or the negative rail will go hot first, and since the latch-up is unpredictable that sounds like it may be the problem.
Version 4 SHEET 1 880 852 WIRE -768 -16 -992 -16 WIRE -432 -16 -624 -16 WIRE -112 -16 -432 -16 WIRE -432 80 -432 -16 WIRE -112 80 -112 -16 WIRE -768 160 -768 -16 WIRE -624 160 -624 -16 WIRE -992 176 -992 -16 WIRE -432 272 -432 144 WIRE -400 272 -432 272 WIRE -288 272 -320 272 WIRE -112 272 -112 144 WIRE -80 272 -112 272 WIRE 32 272 0 272 WIRE -432 368 -432 272 WIRE -288 368 -288 272 WIRE -112 368 -112 272 WIRE 32 368 32 272 WIRE -992 528 -992 256 WIRE -768 528 -768 240 WIRE -768 528 -992 528 WIRE -736 528 -768 528 WIRE -624 528 -624 240 WIRE -624 528 -656 528 WIRE -608 528 -624 528 WIRE -432 528 -432 432 WIRE -432 528 -608 528 WIRE -288 528 -288 432 WIRE -288 528 -432 528 WIRE -112 528 -112 432 WIRE -112 528 -288 528 WIRE 32 528 32 432 WIRE 32 528 -112 528 WIRE -608 608 -608 528 FLAG -608 608 0 SYMBOL diode -96 144 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D2 SYMATTR Value MUR460 SYMBOL diode -448 80 R0 SYMATTR Value MUR460 SYMATTR InstName D1 SYMBOL ind2 -784 144 R0 WINDOW 0 -43 40 Left 2 WINDOW 3 -43 75 Left 2 SYMATTR InstName L1 SYMATTR Value 10 SYMATTR Type ind SYMATTR SpiceLine Rser=,1 SYMBOL voltage -992 160 R0 WINDOW 3 24 104 Invisible 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value SINE(0 13 60) SYMATTR InstName V1 SYMBOL ind2 -608 144 M0 WINDOW 0 -46 46 Left 2 WINDOW 3 -44 73 Left 2 SYMATTR InstName L2 SYMATTR Value 10 SYMATTR Type ind SYMBOL res -640 512 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 1G SYMBOL cap -448 368 R0 SYMATTR InstName C1 SYMATTR Value 470µ SYMBOL cap -128 368 R0 SYMATTR InstName C2 SYMATTR Value 470µ SYMBOL res -304 256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 220 SYMBOL res 16 256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 220 SYMBOL zener 16 368 R0 WINDOW 0 43 32 Left 2 SYMATTR InstName D4 SYMATTR Value BZX84C8V2L SYMBOL zener -272 432 R180 WINDOW 0 -43 32 Left 2 WINDOW 3 -130 -4 Left 2 SYMATTR InstName D3 SYMATTR Value BZX84C8V2L TEXT -752 136 Left 2 !K1 L1 L2 1 TEXT -592 552 Left 2 !.tran .1
--- How much current are you pulling from the +/-V rails?
This is actually a well-known problem with the 78xx / 79xx series of linear regulators when used for dual supplies. The 790x is usually slightly faster, and the 780x cannot start up if it sees a negative voltage on its output.
It would also stop and sit there unable to restart if its output gets driven negative at any later time (e.g. during an overload transient).
The trick here is to never allow the outputs of 78xx / 79xx regulators (especially the positive 78xx) to be driven to opposite polarity.
This can easily be done using a transistor or MOSFET of appropriate polarity (PNP or PMOS for the positive side, NPN or NMOS for the negative side). In your application (8V output level) it's easiest to use MOS, because the gate can be grounded directly and no other parts are needed - just connect the source to your existing regulator and the drain is now the output. Make sure the transistor or MOSFET is rated for the output current.
This keeps the output unloaded, so it won't swing to opposite polarity, until it has risen to the gate threshold voltage, at which time the regulator has already successfully started up and is providing output power charging the output capacitors at its internal current limit. Since at this time the regulator is sourcing maximum current and the voltage is ramping up fast, it's no longer in danger of not starting any more.
If the output is external and connected to user-accessible cabling, and you therefore expect transients there, make sure to connect clamping zeners from outputs to ground and freewheeling diodes across the regulators (output-to-input in reverse polarity). This keeps the power supply happy should the outputs accidentally get zapped by ESD.
Note: If you need a lower output impedance for dynamic loads than the FET provides, you can use an additional capacitor (marked *) at the output. See below for details.
+------||-------o----o-+ +------o-------o---- OUT -
Add a schottky diode, 1N5819 maybe, from the reg output to ground. That will keep the negative supply from pulling the +8 rail very far in the wrong direction.
Yup. 7800s are famous for this behaviour, but iirc 317s and 7900s are okay.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
78Lxx and 79Lxx are _not_ just junior versions of 78xx and 79xx. At least the 78Lxx ones have *horrible* supply rejection, far worse than the 317L. I had a bunch of them once and threw them all out so I wouldn't be tempted to use them.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
The freewheeling diodes protect the regulators against reverse-polarity transient currents by diverting the overcurrent that can happen in either of two possible situations:
the output experiences low-energy overvoltage transient with moderate risetimes (as would be typical with a relay or some other inductor switching), and
the input is discharged very fast (additional heavy load before the regulator) and there is a lot of additional capacitance on the output, so that the output capacitors would try to discharge "through" the regulator back to the input during turn-off.
If there is a transient that is energetic enough to overcharge the input capacitors above the regulators' maximum ratings, it needs to be clamped before it can reach the regulators and capacitors. This requires a dissipating device: a varistor or zener.
Similarly, dissipative clamping is also needed if the transient is very fast (micro- to nanoseconds) and can exceed the regulators' ratings before the (somewhat resistive and inductive) input electrolytics can bring it down to manageable levels. This can happen with ESD, as these discharges have very fast risetimes. Here, ceramic output capacitors dampen the fastest parts of the impulse, and the zeners eat the rest.
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