Date: August 29, 2002
Rev Date: September 18, 2002
1.1. The Dummy Load Unit is an active power supply load that can be adjusted with a multi-turn control. The DLU is a building block that can be replicated to any level as required. Multiple units can be paralleled for larger current draws. Each unit is electrically isolated so they can be arranged to test +- power supplies.
1.2. A chassis containing 4 individual units is a general bench device that can be used during power supply testing and evaluation. This chassis construction as well as the DLU is described in this engineering note.
2.1. The DLU is a very simple device, connect the supply to the studs on the back of the chassis and control the current level with the pot on the front. The front panel meter provides visual feedback of the load current.
2.2. Each load unit is capable of dissipating a minimum 200Watts of power. Each unit is however limited to 20Amps of current. If the load is 5V then 20Amps of current can be drawn but only 100Watts will be dissipated. If the voltage is 10V then 20Amps can be drawn and 200Watts will be dissipated. At 15V approximately 13Amps can be handled and so on.
2.3. Each unit is self-contained, except for the cooling fans; therefore they can be connected as needed to provide special loading. That is they may be used on +/- supplies or paralleled to provide larger current loading.
2.4. The front panel holds an Ammeter, current control knob, momentary zero button and a 10-20 Amp full-scale range switch for each unit.
2.5. The momentary zero button is useful when testing power supplies that fold-back or when close to the trip point. The current can be lowered slightly and the zero button pressed to restore the supply without turning the current adjustment all the way down. Since the load is self powered some instablities may occur if the supply folds back during testing.
2.6. The range switch is used to set the full scale current to either 10 or 20Amps.
2.7. The chassis back panel contains stud connections for each load unit. Polarity should be observed to ensure proper operation. If the connections are reverse the unit will not draw any current. The positive terminal is on top.
3.1. Current Range: 20mA-10Amps / 20mA-20Amps set by the range switch on the front panel and controlled with the multi-turn pot. Note that the current does not go all the way to zero because there is a small quiescent current needed to operate the IC.
3.2. Voltage Range: ~3 to 40Volts with 500VDC common mode isolation between DLU and chassis ground.
3.3. Power dissipation: 200W per module minimum, 800W per chassis assembly. The DLU transistor is rated at 300W when the case is at 25°C and derates linearly to 0Watts at 200°C (see figure 1). The transistor case may operate 50°C above ambient at 200Watts.
3.4. Figure 1 Transistor Power Dissipation vs. Temperature
3.5. Thermo-protection: the circuit is protected with a thermo switch set at 70°C. At this point the transistor can dissipate 223Watts as seen in figure 1. When the trip point is reached the base drive is removed from the transistor. The switch automatically resets when the temperature goes down 5 to 10 degress from the trip level.
3.6. Cooling: the chassis requires 120VAC to operate the fans that provide forced air-cooling. Air-cooling was chosen over water cooling to make the unit portable, compromising only the physical size and power dissipation levels. Care should be taken to ensure the air flow is not blocked during operation.
4.1. This design is adapted from Toffoli, Tommaso "Self-Powered Dummy load Checks Out multiple-Output Power Supplies" ELECTRONIC DESIGN 'Ideas For Design' April 17, 2000, pg. 118.
4.2. The circuit is a simple series pass regulator using an OpAmp that contains a built-in voltage reference. The desired portion of the reference is compared with the output voltage across the shunt to provide the constant current load. The components connected to the base of the power transistor add stability and thermo protection.
4.3. A small PC board mounts directly to the transistor and holds most of the components.
4.4. Figure 2 Dummy Load Schematic
5.1. For the DLU to operate in a useful manner an efficient method to eliminate the dissipated power must be employed. A special copper heat sink spreads the heat to a larger set of aluminum extrusions allowing heat dissipation to the air. The idea is to keep the temperature rise on the transistor case to a minimum allowing the maximum power dissipation.
Figure 3 Aluminum Extrusions The extrusion is drilled for mounting the copper heat spreader, thermo-switch and chassis mounting.
Figure 4 Copper Heat Spreader The copper spreader holds the pass transistor and connects to two aluminum extrusions. It serves as the base for the regulator assembly.
Figure 5 Regulator Assembly with Extrusions Bottom View The printed circuit board mounts directly to the transistor and shunt.
Figure 6 Regulator Assembly Top View This view shows the thermo-switch mounting and the pass transistor buried in the pocket of the heat sink.
Figure 7 Front Panel Suggested Layout
Figure 8 Back Panel Suggested Layout and cutaway of stud assembly.
Figure 9 Side Panel Suggested Layout
Figure 10 Chassis Assembly bottom cut-away view
|Bill Of Materials||September 3,2002||12:00:00||8 Units|
|Item||Quantity||Reference||Part Description||Stockroom Items||Unit Cost||Total Cost|
|1||1||A1||LM10||OpAmp with voltage reference||LM10CN||$2.66||$21.28|
|8||2||R6,R3||1||wire wound 1/4W||$0.02||$0.32|
|11||2||SW1||Zero||Momentary push button||1160-189000||$2.75||$44.00|
|12||1||S1||SW SPDT_0||Range switch||1160-200000||$7.00||$56.00|
|17||1||6-32 x 1/2"||S.S. Screw||1226-256000||$0.10||$0.80|
|18||1||1/4-28||Brass Hex Nut||$0.10||$0.80|
|19||5||6-32||Brass Hex Nut||$0.02||$0.80|
|18||8||MT1,12||1/4-20 x 3/4"||Brass hexhead bolts||1224-117000||$0.11||$1.76|
|21||2||Cord||Fan cord & plug||1175-060000||$1.15||$4.60|
|22||2||Top/Bottom||Chassis Top & Bottom||1775-281000||$33.68||$134.72|
|26||4||terminal||1/4-10Ga Ring Tongue||1110-428000||$0.17||$1.36|
|27||1||terminal||#6-10Ga Ring Tongue||1110-426500||$0.13||$0.26|
|28||8||#6 washer||Belleville Washer||$0.00|
|29||6||#8 washer||Belleville Washer||$0.00|
|30||6||8-32 x 1/2"||S.S. Screw||1226-161000||$0.01||$0.12|
|31||8||6-32||Nylon Hex Nut||1214-050000||$0.04||$0.64|
|32||8||6-32 x 1/2"||Nylon Screw||1230-100000||$0.08||$1.28|
|33||16||NT1,24||1/4-20 nut||Brass nuts||$0.00|
|34||16||NW1,24||1/4" washer||Nylon Flat Washer||1222-026000||$0.03||$0.96|
|35||1||F4||acplug||Plug and Cord||1170-134000||$3.25||$6.50|
7.1.1. The transistor case must be in very good thermo-contact with the copper heat spreader and the spreader must be in good contact with the aluminum heatsinks. Heatsink compound must be used at both interfaces and only enough to displace the irregularities of the two surfaces. Too much will keep the transistor case from actually touching the copper piece and to little will allow voids of air. Air is an excellent insulator so too much thermo compound is better than to little. The same theory applies for the copper to aluminum junction and the shunt.
7.1.2. Each mounting should be done using cupped washers (Belleville) to keep pressure applied to the joint. The thermo compound will migrate with time and the cupped washers will help maintain a good joint. Thermo-cycling will also cause the junction to loosen.
7.1.3. The copper heat spreader with the circuit board is an assembly. This assembly with the two aluminum extrusions is an assembly and is a single DLU without the front and rear panel components.
7.2.1. Each Dummy Load Unit is loosely mounted inside the chassis. The units merely sit on nylon screws that hold them in place when the top and bottom covers are installed.
7.3.1. A cooling channel is required to keep airflow over the heatsinks. The front half and rear half act independently. This is accomplished with simple G10 spacers.