Mark 4, 20 Amp Power Supply, upgrade notes (Mark 4.2)

Back in the early 1990's, I was asked by the MDRC committee of the day if I would like to take on the role as Electronic components and project officer, I didn't see any problem with this and agreed. After being elected into the position I soon found out that a little more time would be required than I expected, most of which was spent answering mail. One item that kept popping up was a couple of small problems with a project released prior to my association with the club, being the 1988 Mark 4 Power Supply Project. The following is the upgrade notes to rid the problems without a total rehash...

UPGRADE to MARK 4.2 as written in 1993
To see circuit diagram page down...

For over 20 years now the Moorabbin and District Radio Club has supplied kits to the amateur fraternity. Our current mark 4 has been quite popular since its introduction in 1988. However, recent constructor feedback has necessitated the need for an upgrade.

The regulator circuit of the mark 4 employs a linear design based on the long standing industry standard "723" regulator from Fairchild (tm). Current trends obviously lean towards Switch Mode designs as they are inherently more efficient, physically smaller and cheaper to produce. However many switch mode designs are often noisy, even when displaying good regulation characteristics and it is for this reason that we have maintained a linear design.

The Mark 4 as with earlier MDRC supplies is equipped with over Volts protection. This prevents the unregulated 24 to 28 Volts DC from causing damage to your equipment in the event of a regulator failure. It is with this part of the power supply circuit that constructors are experiencing problems, the symptoms are as follows...

Cut-out on switch on, indicated by the over volts LED.

Cut-out on the application of a load, also indicated as above.

The over volts protection circuit is a simple resistor, zener diode arrangement with transistor switching. In the event of a output of more than 15 volts the zener with begin to conduct, once sufficient Voltage is developed across R16 (about 1 Volt) TR11 will be biased on. since TR10 is connected in a feedback configuration with TR11 both transistors and thus the relay will be ON. This condition will remain until removal of the mains and discharging of capacitors C1-C6.

A problem with this type of circuit is its sensitivity to spikes or noise. Spikes, caused frequently by switch bounce, may develop sufficient volts across R15 to switch TR10 & 11 ON. This spike need only be for micro-seconds or less as the BD139 / 140 transistors can now switch in the hundreds of Megahertz range.

The following mods have been carried out on several units without further problems...

1/ Replace R15 (1k) with a 1k2 if the secondary voltage is equal to (=) or greater than (>) 20 Volts AC.

2/ Replace C12 (originally 0.22uF) with a 2.2uF Tantalum (10 to 25V), be sure to observe polarity.

3/ Add C15 (10uF) and C16 (0.1uF) across the supply output (position not critical).

4/ Place a 470 Ohm, 1/4W resistor (see R10 on schm) across the Emitter and Base of TR10 (BD140), solder under PCB.

NOTE: Capacitors C7 & C8 were reversed in the original 1988 AR article, this error is not on the original schematic or PCB artwork.

Mark 4.2 Circuit Diagram

To print this, change your printer settings to Landscape...

Printed Circuit Overlay for the Mk 4.2

Printed Circuit Board layout for the Mk 4.2

NOTE: PCB measures 6 inches Sq

Economic construction for lower power applications...

The original Mark 4 was designed to handle currents of around 24 - 25 Amps but there's no reason why you can't taylor make a unit to suit your own application. If you only require something like a 6 or 12 Amp power supply, you can reduce the number of filter caps and pass transistors to suit a lighter duty transformer. A 2N3055 transistor has a good series pass regulation capability of 3 Amps, however the current gain will fall off rapidly on exceeding this level, thus reducing regulation capability. So a good rule of thumb is "1 x 2N3055 for every 3 Amps". With that in mind, a 6 Amp unit would require only 2 x 2N3055 pass transistors (TR2, TR3) plus the on PCB driver transistor TR1 and an 18 VAC, 6 Amp mains transformer. If you are using the standard 5600uF filter caps - use 2 for a 6 Amps and at least 3 for 10 - 12 Amp operation. Please note that  these figures apply to mains transformers with secondary voltages of 18 - 20 VAC, with 18 VAC being the optimum.

Some points on Heat Sinking

Depending on your application the power dissipated in each pass element device (TR2 - TR9) can be as much as 30 Watts giving a total of 250 Watts for 8 x 2N3055. In most amateur radio type applications we would be using the unit for SSB equipment so the duty cycle will be much lower, say 20 - 30% and dissipation on average will be more like 10 Watts per device. A reasonable heatsink for 2 x 2N3055's for the 6 Amp situation should have a thermal resistance of around 2 deg C per Watt. For higher powers - mounting 4 x 2N3055 on the one heat sink would require a thermal resistance of around 1 deg C per Watt. Physically smaller heatsinks can be used in conjunction with fan forced cooling.

For information on PCB's please contact me via Email

Regards, Chris VK3CAE

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Email VK3CAE: cja@rmit.edu.au