High-Current 24 VDC Power Supply |
Overview
This is simple +24 VDC power supply project capable of supplying around 20 amps. It's based around two Cosel UAW250S-24 +24 VDC switching power supply modules operated in parallel. Each individual power supply module is capable of sourcing around 11 amps at +24 VDC, and two of them in parallel will give an easy 20 amps. A maximum of five units can be connected in parallel.
The Cosel UAW250S-24 switching power supplies themselves are very well constructed and can sometimes be found for fairly low cost on eBay. The modules contain voltage and current balance connection points for maintaining the proper voltage and current balancing when operating in parallel. The modules are also capable of "remote sensing." This allows the power supply to increase its output voltage to compensate for the resistive losses when using long DC power wires on a high current load. This makes the power supply ideal for powering high-current +24 VDC RF power amplifiers.
The power modules do require attention to detail when operating in parallel. If there isn't adequate airflow, fans should be added to blow cool air over the modules. There should also be a single-point common ground on the output. The 24 volt ground should also be isolated from the Earth/frame ground. These steps will help minimize interference from ground loops.
Each Cosel UAW250S-24 module will draw 6 amps on their AC input when run at full load. This means the AC mains should be able to handle at least 15 amps continuous. Each power supply module also has its own 10 amp fuse, EMI filter, output over-current limit, and thermal shut-down protection so there is no need to add these features externally.
When operating the Cosel UAW250S-24 modules in parallel, one unit will be designated the "master" unit while the other unit(s) will be the "slaves." The final output voltage should be tweaked ONLY on the master unit. The slave unit should have its voltage adjust potentiometer rotated all the way clockwise (when the terminal block is facing you).
When using remote sensing in a parallel operation, the sensing wires should ONLY be connected to the master unit. The "Monitor +" & "Remote Sense +" and "Monitor -" & "Remote Sense -" terminals on the slave unit(s) should be shorted.
The power supply modules have AC mains in-rush current protection, but avoid rapidly turning the units on and off.
Output over-current protection is built into the power supply modules. It will come into effect when the output current draw of the module reaches 105% of its rated current. The power supply module will automatically recover when the over-current fault condition is corrected.
Output over-voltage protection is also built into the power supply modules. It will come into effect when the output voltage of the module reaches 115-140% of its rated voltage. The power supply module should be shut down for a minimum of five minutes after an over-voltage fault condition occurs.
Pictures & Construction Notes

Overview of one of the Cosel UAW250S-24 switching power supplies.
The open-frame modules are designed to be attached to a metal surface for further heatsinking.
Additional forced-air cooling should be used if the power supply modules will be run at full load for an extended period of time.

Overview of the edge-mounted input/output voltage terminal connections.
120 VAC mains input is on the right.
+24 VDC outputs are on the left.
The pinout for the connections are (from left-to-right):
Terminal Number Label Description 1 V+ Output + (1 & 2 are connected together) 2 V+ Output + 3 V- Output - (3 & 4 are connected together) 4 V- Output - 5 FG Frame Ground (Earth ground from AC mains) 6 AC AC Mains Input - Live (Black wire - usually) 7 AC AC Mains Input - Neutral (White wire - usually)
The output voltage adjust potentiometer (light blue) is to the left of the terminal block. Output voltage is increased by turning the potentiometer clockwise and is decreased by turning the potentiometer counterclockwise.
The green LED lights when the unit is powered.

Remote sensing and voltage/current balance connections on the slave unit.
I didn't have the proper matching connector for these terminals so I just soldered the wires to the exposed pins.
The "Monitor +" & "Remote Sensing +" (pins 8 & 9) terminals are shorted together, so are the "Monitor -" & "Remote Sensing -" (pins 10 & 11) terminals.
The GREEN wire goes to the Voltage Balance connection on the master unit.
The WHITE wire goes to the Current Balance connection on the master unit.
The BARE wire goes to the common Voltage Balance/Current Balance Ground on the master unit.
These connections are mandatory when operating the units in parallel to maintain proper voltage and current balancing between the master and slave unit(s).

Overview of the CN1 (left), CN2 (center), and CN3 (right) terminals.
The pinout for the CN1 connections are (from left-to-right):
Terminal Number Description 8 Monitor + Short 8 & 9 together on slave unit. 9 Remote Sensing + 10 Remote Sensing - Short 10 & 11 together on slave unit. 11 Monitor -
The pinout for the CN2 connections are (from left-to-right):
Terminal Number Description 12 Voltage Balance (VB) 13 Voltage Balance/Current Balance Ground 14 Current Balance (CB)
The pinout for the CN3 connections are (from left-to-right):
Terminal Number Description 15 Voltage Balance (VB) 16 Voltage Balance/Current Balance Ground 17 Current Balance (CB)
The CN2 and CN3 terminals are the same.

Mounting the slave unit in an old ammo case.
A small little terminal block was attached to the unit for a common point to attach the incoming 120 VAC mains.
Ferrite beads where added over the incoming AC mains wires.
The power supply modules have several threaded M4 screw attachment points.

Behind the front-panel overview.
A standard 120 VAC IEC connector is along the bottom for the AC mains input. A SPST power switch is just above that. A green neon lamp acts as a power indicator.
An isolated BNC jack (middle - to the right of the power switch) is used for the remote sensing connections. An isolated connection is required as the remote sensing should take place only at the load. The small RED wire from the BNC jack center goes to the "Remote Sensing +" (pin 9) terminal on the master unit, and the small BLACK wire goes to the "Remote Sensing -" (pin 10) terminal on the master unit.
Standard banana jacks (mounted 3/4-inch apart) will be used for the +24 VDC outputs.

Wiring overview.
The slave unit is on top, the master unit along the bottom.
The two units are wired in parallel (V+ on the master to V+ on the slave, V- on the master to V- on the slave) with the other V+ and V- connections going to an optional 10,000 µF filter capacitor.
This capacitor adds a little extra filtering in case of a current spike and also acts as the connection point for the single-point ground system.
The AC mains and DC output wires should be as physically separate as possible.

Behind the front-panel overview, wiring complete.
Slave unit is on the left, master unit is on the right.
Two ferrite beads were added over the remote sense wires.
Output wiring should be with heavy-gauge wires (#12 or #14) and properly crimped ring terminals. Add a bit of solder to the ring terminals for extra resiliency.
A 3 kohm, 5 watt resistor was added across one of the +24 VDC output terminals to drain the filter capacitor when the power is off.

Completed power supply overview.
The three sets of bananna jacks are for the +24 VDC outputs.
120 VAC mains comes in via the IEC connector on the bottom-right. The Cosel UAW250S-24 power supply modules will work from 85 VAC to 270 VAC without the need for any reconfiguration.
The isolated BNC jack is for remote sensing at the load. To use the remote sensing feature, run shielded wires or piece of coax from the +/- power connections at the load to the remote sense input BNC jack (center pin is +) on the power supply.
As the load draws more current and the power supply's output voltage begins to sag, the remote sense feature will slighty increase the output voltage from the two power supply modules to compensate for this.
If the remote sense feature is not used, it must be connected back to one of the +24 VDC outputs. This is shown above with a small BNC jumper cable and a BNC-to-banana jack adapter.