Supply

Logic 5V and miscellaneous 5V power supply

Application switch mode power supply

This power supply provides two independent power outputs, one for logic digital circuits and one for track circuits and miscellaneous loads like illumination devices. The power supply for the track circuits and miscellaneous components is a independent unit to avoid the wrong operation of track circuits. At the other hand the power supply for logic digital components is a independent unit to avoid the faulty operation of logic digital components due to noise.

Two AC/DC switch mode power supply modules are used. They have a power rating of 7A. This is higher than required but avoids reformation of the plastic enclosure due to overheating. It is the same enclosure used for the above 'Dual plus and minus 15V power supply'.

Description components and wiring

The used switch mode power supply is a Mean Well RPS-65-5 5, see image below. The price of each unit was around €17.00 in 2020.

The logic 5V and miscellaneous 5V power supply circuits have each its own mains power switch S1 resp. S2 and fuse F1 resp. F2. These and the neutral are connected to the hot side of the switch mode power supply X1 resp. X2 by a female crimp plug J1 resp. J2, see photo below.

The 5VDC cold side of the switch mode power supply for logic circuits is connected to a violet pole terminal J5 and the one for miscellaneous devices to a brown pole terminal J6. The GND of each switch mode power supply is individually connected to the black pole terminals J7 resp. J8. All these 5VDC and GND wires are connected to the switch mode power supplies with 4 pole female crimp plug J3 resp. J4. The current rating for the DC connectors and wires is 22 A.
The 5VDC for logic circuits is connected via a current limiting resistor R1 to a blue LED D1. The 5VDC for miscellaneous devices is connected via a current limiting resistor R2 to a green LED D2.

Part list with components are stated below.

Immunity

This page is about measures on printed circuit boards (PCBs) to avoid disturbances in logic circuits due to noise on supply lines. and also avoid this with logic IC families with high tolerance to voltage variations.

Decoupling capacitor

In electronics, a decoupling capacitor is a capacitor used to decouple one part of a circuit from another. Noise caused by other circuit elements is shunted through the capacitor, reducing its effect on the rest of the circuit. Rigorous calculations for these capacitors is complicated, but in early days there was as a rule of thumb to place by one 10nF ceramics MULTILAYER capacitor near every IC, and one electrolytic capacitor of 10µF (preferred Tantalum one) between main traces that supply groups of 10-20 IC. The 10nF caps need to be small—preferably 0402 or at most 0603 sized—to avoid the lead inductance from the package nullifying the effect of the capacitor.

The part list with Decoupling capacitors is shown below.

Voltage variations logic families

All ICs used for my model railway have a supply voltage of 5 volts and they must have immunity to voltage variations on the supply lines between 4.5 and 5.5 volts. Logic families with this property are mentioned in the next two sections and are contenders for electronic circuits depending on their other properties. Also stated is how popular a particular logical family is so you can see if they are readily available.

Microchip ATmega

The voltage variation for the Microchip ATmega48P/88P/168P/328P/808/809/1608/1609/4808/4809 is 5V ±0.5V.

74xx families

Logic 74xx families with immune for supply voltage variations between 4.5 and 5.5 volts are:

1. 74ACxx (POPULAR)
   Advanced CMOS Logic (ACL for short). The AC is faster than the HC subfamilie. Not pin compatible with TTL. Improved noise immunity.
2. 74ASxx (RELATIVELY POPULAR)
   Advanced Schottky TTL – Improved design over 74Sxx Schottky types optimised to increase switching speed at the expense of power consumption of about 22mW
3. 74ALSxx (POPULAR)
   Advanced Low Power Schottky TTL – Lower power consumption of about 1mW and higher switching speed of 4ns compared to 74LSxx types
4. 74HCxx (VERY POPULAR)
   High Speed CMOS – CMOS technology and transistors to reduce power consumption of less than 1µA with CMOS compatible inputs
5. 74HCTxx: (POPULAR IN CERTAIN CASES)
   High Speed CMOS – CMOS technology and transistors to reduce power consumption of less than 1uA but has increased propagation delay of about 16nS due to the TTL compatible inputs. This is the reason for the existence of this logical family because TTL or NMOS outputs are incompatible with the inputs of the HC family unless a pull-up resistor from the TTL output to V_CC is used.