Re: Need a simple improved voltage regulator

In article <46b0cffc$0$21932$157c6196@xxxxxxxxxxxxxxxxxxxxx>,
"Mogens V." <mogensv@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Too many of my (guitar) rack devices use simple 78/79 regulators.

I've seen some with a transistor supposed to shunt noise to ground, but
seems more like a phase inverter, depending on accurate resistors to
match the emitter resistance. Seemed to also depend on how many, and
size of, capacitors dropped around the board.
Another one used two high speed swithing transistors.

Would be nice merely replacing the 78/79 and lifting the output leg to
hook in a filter.

Anyone have links to simple add-on filters, so I don't have to invent a
commen wheel?

Those are actually pretty good regulators, and the problem may be
somewhere else. I'd suggest you go back and check out the way the power
circuitry is wired. It's very common for that to get done wrong, and
cause all sorts of hum problems.

No current flows from the transformer during most of the power cycle.
Only when the voltage there is greater than the voltage in the filter
caps will any current flow at all, and when it does flow, the capacitor
looks like a short circuit and the current can be very high. The voltage
drops from those high currents, if not isolated, can couple directly
into the output of the regulator -- and even a perfect regulator would
not help because the hum has cleverly bypassed it entirely.
Specifically, the resistance of even a short wire can be sufficient to
cause problems. Also, there can be large magnetic fields due to the
large currents, and those must be isolated too.

The wiring should be like this (in order):

From the transformer to the diodes; from the diodes to the terminals of
the filter capacitor; from the capacitor terminals to the input and
common of the regulator. Do not connect the regulator common to any
other place than the appropriate terminal of the capacitor --
specifically not to any other place on the common lead, say nearer the
diodes or transformer. To minimize the magnetic field problem, use
twisted pair -- from the transformer to the diodes, and from there to
the terminals of the capacitor.

Take the power from the output and common of the regulator to the
circuitry. Make no ground connections anywhere else. The common pin of
the regulator should be the ONLY place the input common and output
common are connected. It should be a "zero-length" lead.

Isaac
.

Relevant Pages

  • Re: Need a simple improved voltage regulator
    ... I've seen some with a transistor supposed to shunt noise to ground, but seems more like a phase inverter, depending on accurate resistors to match the emitter resistance. ... Only when the voltage there is greater than the voltage in the filter caps will any current flow at all, and when it does flow, the capacitor looks like a short circuit and the current can be very high. ... The voltage drops from those high currents, if not isolated, can couple directly into the output of the regulator -- and even a perfect regulator would not help because the hum has cleverly bypassed it entirely. ... From the transformer to the diodes; from the diodes to the terminals of the filter capacitor; from the capacitor terminals to the input and common of the regulator. ...
    (rec.audio.tech)
  • Re: Wanna see what happens when you dont read the data sheet?
    ... poorly controlled and unstable resistance from a high ESR capacitor is a ... Do you have any data to show trends of ESR change over time and ... In the case of the regulator in question it can use a fairly wide ... the main filter capacitor may be essential for initial stability. ...
    (sci.electronics.design)
  • Re: Connecting a Voltage Regulators - TL750L05CLP
    ... It shows how much resistance must be *in series with* the output capacitor to keep the regulator stable versus load current, when using a 10 uF capacitor. ...
    (sci.electronics.design)
  • Re: Basic Voltage Regulator Help
    ... When applying such regulators, it is common to include a .1uF or larger capacitor between input and common, to help stabilize the regulator, since wiring inductance in the raw supply lead can destabilize the regulator, and to also add a .1 uF or more between output and common to improve the high frequency noise rejection of the regulator, since its feedback voltage control has an upper frequency limit of less than 1 MHz. ... I have no idea what current your logic needs, but if the regulator gets warm enough to need a heat sink, you can also transfer some of that heat to a resistor in series with the input side (to waste some of the excess voltage, and also help filter noise spikes from the 12 volt supply with the help of that input capacitor), as long as there is at least 5.3 volts left at the input of the regulator. ...
    (sci.electronics.design)
  • Simple Low Power Constant Current Test Set
    ... The Constant Current Regulator ... volts applied to resistance in the 2N4143 emitter circuit. ... If you were now to connect a transformer winding of 300 ohms to the test ... Compare Power Diodes plots the forward characteristic of three common ...
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