[serengetiplains]

Paul, are your small series regs effective at video frequencies?  I installed the main supply reg in my scaler, that 6A series reg you built for me which is now powering the scaler.  It made a nice difference, so I assume the answer to my question is yes.  There are a host of smaller SOT regs in the circuitry.  I might replace some of them with small series regs.  Given the heat this machine throws (lots of FPGAs), I don't think the smaller shunts would pass enough current.  What do you think?

[Paul Hynes]

The small series reg uses the same topology and error amplifier components as the 6 amp series reg. The output stage is scaled down for low power use. They are able to provide good regulation at video band frequencies. The usual three terminal type regulators installed in audio and video equipment, having limited bandwidth, have stopped regulating at these frequencies.

You would have to re-organise the power supply to provide enough headroom for the series reg dropout voltage, which is around 4 volts with the output device used with this low power reg.

The Z17/Z78 shunt type regulator has a much lower dropout voltage in the region of 1v2. It also has a bandwidth 10 times wider than the series reg as the error amplifier has a gain bandwidth product of 1 GHz. I have had a 3v3 shunt running at 250ma on a 5 volt rail without thermal issues. Of course using shunt regs in the video scaler will increase the working temperature in the chassis. This may not be an issue as it is obviously designed to run hot but you may need to improve ventilation the chassis.

If you can find out the average and peak current draw on the each of the regs you are proposing to change and the actual input and output voltages to each reg I can assess the viability of the shunt/series regs for you.

Regards
Paul

[serengetiplains]

Thank you, Paul.  Before I decide whether to plunge into this project, can you help with a technical matter regarding replacement?  Some of the SOT regs are adjustable and don't have a ground connection immediately near the reg.  If I were to replace those regs, and assuming I power the replacement directly from the main PSU, what is the best advice for connecting ground on the replacement reg to the circuit board?  I can find a ground connection sometimes within an inch, sometimes within 2 inches of device to be replaced.  Is using such connections ok, or would that defeat the purpose of replacing the poor regs by somehow mucking up ground currents?

[Paul Hynes]

Take the reg ground connection to the point where the SOT device voltage setting resistor goes to ground. It may not be the ideal place but it should give you a good upgrade over the SOT device.

Regards
Paul