================================================================================ SCHEMATIC: 100W DRY (AIR-COOLED) DUMMY LOAD TM-DL-100W-DRY Rev A ================================================================================ RESISTOR NETWORK — 50 OHM, 100W Star/Distributed layout for minimum parasitic inductance 4x Non-inductive resistors: 200 ohm, 25W each (Ohmite 270-1-R200E or equiv) Connected in parallel = 50 ohm, 100W SO-239 / N-Type Jack | CENTER PIN | +----------+----------+ | | | | | | +------+---+ +---+---+ +--+----+ +-------+ | R1 | | R2 | | R3 | | R4 | | 200R 25W | |200R | |200R | |200R | | | |25W | |25W | |25W | +----------+ +-------+ +-------+ +-------+ | | | | +----------+----------+-----------+ | GROUND (CHASSIS) = Connector shell = Heatsink base TOP VIEW (component side, looking down): ___________________________________________ | | | R1 R2 R3 R4 | | [===] [===] [===] [===] | | \ / \ / | | \ / \ / | | \ / \ / | | \ / \ / | | * * | | +-------+-------+ | | | | | [SO-239] | |___________________________________________| STAR WIRING (minimizes loop area): All R1-R4 high ends wire-bonded to center conductor stud. All R1-R4 low ends wire-bonded to chassis/ground bus. Lead length <= 10mm each side. Resistors oriented FLAT on heatsink surface. COOLING SYSTEM -------------- Heatsink (required for 100W continuous): Theta_sa <= 0.95 C/W for 25C ambient, 120C max resistor case Recommended: Extruded Al heatsink, 150mm x 100mm x 40mm fins Wakefield 433-40ABP or equivalent Naturally cooled: theta_sa ~ 2.5 C/W (max 50W cont.) Fan-cooled: theta_sa ~ 0.7 C/W (100W continuous OK) Fan mounting: 80mm x 80mm x 25mm, 12VDC brushless Airflow path: through perforated bottom plate, over heatsink fins, out top Fan guard: required (injury hazard, coax debris ingestion) Temperature sensor: NTC thermistor, 10 kohm @ 25C, B=3950 Mounted on heatsink base adjacent to R1 Connected to CYD monitor (see cyd/ directory) or LED cutoff circuit OVERHEAT CUTOFF (optional): LM56 dual setpoint temp switch Warning LED at 80C Relay/solid-state switch opens antenna relay at 100C Requires 12V supply for fan and control circuit +12V ---+--- Fan (80mm) | +--- LM56 VCC (pin 6) LM56 T+ out1 (pin 2) --> Warning LED (80C) LM56 T+ out2 (pin 3) --> NC relay coil --> open at 100C LM56 GND (pin 5) --> chassis GND Thermistor: pin 1 to 10k NTC, other end to GND CONNECTOR --------- For HF (to 30 MHz): UHF (SO-239) acceptable. SWR < 1.1:1 to 30 MHz. For HF/VHF (to 150 MHz): N-type (female panel). SWR < 1.3:1 to 150 MHz. For VHF/UHF: N-type mandatory. SWR < 1.5:1 to 500 MHz. Note: SO-239 is NOT acceptable above 150 MHz. Connector is the primary SWR-limiting factor at VHF. Use N-type for any load intended for 2M use. PERFORMANCE SPECIFICATIONS -------------------------- Impedance: 50 ohm nominal Power: 100W continuous (fan), 50W continuous (no fan) 200W for 30 seconds (SSB peaks), 150W (no fan) Frequency: DC to 500 MHz (N connector) SWR (DC-30 MHz): < 1.05:1 SWR (30-150 MHz): < 1.15:1 SWR (150-500 MHz): < 1.5:1 VSWR deteriorates at UHF due to lead inductance of resistors. For VHF/UHF precision: use multi-chip SMT design (see sch_vna_cal.txt) PARTS LIST ---------- Qty Reference Value/Part Number Description --- --------- -------------------- --------------------------- 4 R1-R4 200R 25W non-inductive Ohmite 270-1-R200E or TE Connectivity HSC25R200J (DO NOT use wirewound inductive) 1 J1 SO-239 or N-type Panel mount, chassis GND ring 1 HS1 150x100x40mm Al Extruded heatsink 1 F1 80mm 12V fan Brushless DC, 2-pin 1 TH1 10k NTC thermistor B=3950, through-hole 4 -- 6-32 x 1/2" SS screws Mounting resistors to heatsink 4 -- 6-32 nuts and washers 1 -- Thermal compound Dow TC-5026 or equiv, all R faces 1 -- Enclosure See openscad/enclosure_100w_dry.scad ================================================================================