PSU5 No Noise

This version of the PSU5 suppresses the loud noise at switch off reported on some pinball machines.  The module has exactly the same functionality of the original PSU5 but includes extra circuitry to suppress the shutoff oscillation of the original PSU5.

Most pinball machines have very large electrolytic capacitors on the supply to the original LM323K.  These capacitors are the cause of the noise generated by the original PSU5.  Capacitors exhibit a phenomenon called dielectric absorption, see here for details.  It results in the capacitor voltage rising after the load disappears and it is particularly troublesome in large electrolytic capacitors.

The photo shows a particularly bad example of the PSU5 repeatedly turning on as the capacitor voltage (blue trace) recovers.  The rapid on and off is what causes the noise on the speakers.

I finally found a way to duplicate the issue on my test bench and the screen capture below shows the turn on and off of the PSU5 with a small load.

The yellow trace is the output of the PSU5 and the blue is input to the PSU5 showing the PSU5 repeatedly turning on as the input voltage recovers.  The PSU5 has a small amount of hysteresis between the turn on and turn off voltage preventing it from immediately turning off as the voltage on the input sags.

The screen capture above shows the clean turn off of the new revision of the PSU5, the PSU5_NN as the input voltage decays slowly over time.

To distinguish the modules from each other the new module is manufactured with a yellow solder mask. The only other difference besides the turn off behavior is that the trim pot now increases the output voltage when the pot is turned clockwise.

The PSU5 is a high-efficiency replacement for popular three-terminal LM323K linear regulators and it is pin-to-pin compatible with the common and now obsolete LM323K linear regulators. The mechanical design allows the PSU5 to fit anywhere where an LM323K was used. The bottom of the PCB is ground, just like the case of the LM323K. The maximum continuous output current is 3A and at room temperature the PSU5 does not need a heatsink to maintain this current indefinitely. All the required capacitors are included on the module, no external capacitors are required and additional input capacitors do no harm. The output voltage guaranteed to be within +/-1% as the load varies. The module has a trim potentiometer and the voltage is factory adjusted to within 50mV of 5.03V with a 12V input and a 1A load. The original LM323K had a rather loosely specified output voltage and it could vary by as much as 250mV without load effects and at room temperature. Many designs incorporated some circuitry to raise the output voltage to prevent under voltage resets of microprocessors. The PSU5 LM323K replacement design can be adjusted by up to 500mV in either direction with the trim pot to ensure compatibility with almost all circuits that used the LM323K. The module has thermal shutdown and current limit protection. The absolute maximum input voltage is 20V. The PSU5 can be mounted on existing heatsinks, even ones that use sockets under the heatsink. The hole diameters and pin diameter is exactly the same as the original package dimensions.

Features:

• Clean turn on and turn off.
• Drop-in replacement of the obsolete LM323K and LM78H05K  or equivalent linear voltage regulator.
• Guaranteed 3A continuous output current.
• Identical input voltage range of 7.2V to 20V
• Suitable for use in Pinball machines and video game consoles
• High efficiency switching regulator design reduces power dissipation with superior voltage regulation compared to the LM323K.
• Thermal shutdown and current limit protection
• Board dimensions are identical to the TO3 package of the LM323K.
• Heatsinks are optional.
• All components are mounted on one side of the PCB
• Highest component is the inductor at 5mm above the PCB.
• Sold with pins soldered in place.
• Gold plated pins and PCB to withstand harsh environments over the long term.
• Can drive inductive loads such as solenoids and DC motors.
• 500kHz Switching Frequency
• Made in the USA
• The photographs show an LM323K next to the PSU5 module for a size comparison.