Calculating Power Supply Output Capacity

[mjrippe]

If you have a mixing board with no power supply, can you calculate the approximate capacity needed by looking at the schematic?  Specifically this is a Yamaha M1532.  It has several voltage rails but let's focus on the +/- 25V supply.  The two legs of the CT secondary have 6A fuses.  Obviously this is AC current and pre rectification and regulation.  Is there a formula or method to determine the available DC current at the output?

 

[JohnRoberts]

In general you can connect a higher current PS than needed. The current limiting in a PS is often to protect the PS as much as the console.

Looking at that schematic, the current limiting is set by two 4.7 ohm resistors in parallel in series with the output with the voltage rise feeding the base of TR3

Cheating from the nominal voltages marked on the schematic they indicate 2.7V/2.35 Ohm  or a little more than 1A nominal current.

The current limiting turns on TR3 to starve the base of TR2. This voltage is fed through a resistor divider, to provide some holdback to the current limiting.

The max current output at full rated voltage would occur with roughly 25.5V at base of TR3 so current flowing down 11k from + pre regulated must match current flowing down the 56k to turn on the current limiting so pre-regulated would rise to above 30V before 25V output is current limited at Approx 2.3A. This is not precise as exact Vbe of TR3 could vary with temperature etc.

After current limiting the 25V will drop and the current flowing down the 56k will be less, reducing the fold-back current limit point. Into a dead short it will fold back and current limit at something much lower like 250 mA. 250mA is still a lot if power in the pass transistor but not as bad as full current with no fold-back..

JR 

 

[mjrippe]

In general you can connect a higher current PS than needed. The current limiting in a PS is often to protect the PS as much as the console.

Yes, a very good point, but larger PSUs are more expensive ;-)  We are looking at using Power One linear supplies to build a replacement.

Looking at that schematic, the current limiting is set by two 4.7 ohm resistors in parallel in series with the output with the voltage rise feeding the base of TR3

Cheating from the nominal voltages marked on the schematic they indicate 2.7V/2.35 Ohm  or a little more than 1A nominal current.

The current limiting turns on TR3 to starve the base of TR2. This voltage is fed through a resistor divider, to provide some holdback to the current limiting.

The max current output at full rated voltage would occur with roughly 25.5V at base of TR3 so current flowing down 11k from + pre regulated must match current flowing down the 56k to turn on the current limiting so pre-regulated would rise to above 30V before 25V output is current limited at Approx 2.3A. This is not precise as exact Vbe of TR3 could vary with temperature etc.

After current limiting the 25V will drop and the current flowing down the 56k will be less, reducing the fold-back current limit point. Into a dead short it will fold back and current limit at something much lower like 250 mA. 250mA is still a lot if power in the pass transistor but not as bad as full current with no fold-back..

JR

Wow, thank you John.  I hope to someday understand power supplies on this level, but until then you are an invaluable resource!