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TTI LD300 Service Manual

300w dc electronic load
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LD300
300W DC
Electronic Load
Service Manual
Book Part Number 48581-1360 Issue 1

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Summary of Contents for TTI LD300

  • Page 1 LD300 300W DC Electronic Load Service Manual Book Part Number 48581-1360 Issue 1...

  • Page 2: Table Of Contents

    Table of Contents Specification Safety Installation Connections Dismantling Instructions and Servicing Notes Circuit Description Calibration Parts List Component Layouts & Circuit Diagrams...

  • Page 3: Specification

    Specification Accuracy specifications apply for 18°C – 28ºC. Setting accuracies apply with slew rate control at mid position and at 50W load power; regulation specifies variation at other powers. INPUT Maximum Input Ratings Current: 80 Amps max. at rear panel terminals. 30 Amps max.
  • Page 4 Constant Conductance Mode (CG) Conductance Ranges: <0·01 to 1 A/V (1 mA/V resolution); <0·2 to 40 A/V (0·01 A/V resolution). Setting Accuracy: ± (0·5% + 2 digits + 30 mA). Regulation: < 2% for 90% load power change (V > 2 Volts, using remote sense). Temperature Coefficient: <...
  • Page 5 Oscillator Sync Output Connection: Terminal block on rear panel. Opto-isolated open collector output conducts during Level B phase of internal transient generator. Max Off State Voltage: 30V. Collector Current: 2mA (typical). DROPOUT VOLTAGE The load will cease to conduct if the applied voltage falls below the Dropout Voltage setting; active in all modes except Constant Voltage.
  • Page 6 External Voltage Control Operating Mode: The applied voltage determines the operating level within the selected range. Scaling: 4 Volts full scale. Accuracy: ± 0·3% ± accuracy of selected range. Common mode rejection: Better than –66dB. External TTL Control Operating Mode: The applied signal selects between Level A and Level B settings.

  • Page 7: Safety

    Safety This instrument is Safety Class I according to IEC classification and has been designed to meet the requirements of EN61010−1 (Safety Requirements for Electrical Equipment for Measurement, Control and Laboratory Use). It is an Installation Category II instrument intended for operation from a normal single phase supply.

  • Page 8: Installation

    Installation Mains Operating Voltage The operating voltage of the instrument is shown on the rear panel. Should it be necessary to change the operating voltage from 230V to 115V or vice-versa, proceed as follows: Disconnect the instrument from all voltage sources. Remove the screws which hold the case upper to the chassis and lift off.

  • Page 9: Connections

    Connections Front Panel Connections Load Input The INPUT terminals for the load circuit on the front panel accept 4mm plugs into the end, 4mm diameter wire or plugs into the cross hole, or ¼ inch spade connections (with a maximum blade width of ½...
  • Page 10 Remote Control Input The CONTROL VOLTAGE terminals are used in two operating modes of the instrument (as selected by the front panel LEVEL CONTROL switch): In EXTERNAL VOLTAGE mode an analogue signal applied here sets the level of the load; the scaling is 4 Volts full scale.

  • Page 11: Dismantling Instructions And Servicing Notes

    Dismantling Instructions and Servicing Notes Service Handling Precautions Service work or calibration should only be carried out by skilled engineers. Please note the following points before commencing work. Most of the integrated circuits are CMOS devices and care should be taken when handling to avoid damage by static discharge.
  • Page 12 Power Stages The complete assembly of the two power stages and their heatsinks is attached to the chassis by two screws accessible from underneath. Support the weight of the heatsinks while removing them. It should then be possible to repair the assembly without detaching the power wiring from the rear panel terminals.

  • Page 13: Circuit Description

    Circuit Description In the following descriptions, items in bold are a cross reference to a section with that name. System Overview Although the load can operate in various modes – constant current, voltage, power, resistance or conductance, it is fundamentally a constant current load. The other modes (all except constant voltage) are implemented by measuring the value of the voltage across the load terminals and using an analogue function multiplier (in some modes, as a divider) to calculate the magnitude of the current required to satisfy the defined condition.
  • Page 14 Fault Finding Hints First check that the problem is not a stability issue caused by the characteristics of a particular supply source. Note that instability caused by inductance in the load circuit can trip the over- voltage protection. In other cases instability can trip the excess power detection circuit. Constant Voltage mode is particularly likely to be affected by the characteristics of the supply source.
  • Page 15 Load Mode The ten position Load Mode switch is encoded to a 4-bit binary code by priority encoder IC4. All signals associated with this encoder are active low. The switch pulls one input line low and the encoder produces the binary output corresponding to this input. Should a fault occur causing more than one input to be low, the encoder will ignore all except the highest numbered one.
  • Page 16 Main PCB Two sets of four-pin test points give access to the most important signals on the board: TP2 has FS, IFB, VS and 0V while TP3 has PROD, REMV, DMD and 0V. TP1 has the internal oscillator signal (when it is in use). Voltage Sensing The internal (IC19) and external (IC32) voltage sense amplifiers are very similar.
  • Page 17 Remote Control The external remote control voltage is applied to the unity gain instrumentation amplifier IC38 which removes the common mode component (up to ±100V is allowed). VR31 provides offset adjustment. The analogue control range is 4·00V for nominal range (80 Amps etc.). For External TTL mode (when the user applies a logic signal the remote input), IC20-A compares the input voltage to the logic threshold of 1·5V and generates the corresponding digital signal XOSC which is used to switch between the Level A and Level B settings in the level select...
  • Page 18 When STRT (from the slow start logic) is low, B3 and B0 are both low and IC7 selects 0V. When SR (“select Remote” from the Control PCB) is low, B3 is low and B0 high and IC7 selects REMV. When SB (“select Level B” from the Control PCB) is low, B3 is high and B0 low and IC7 selects LVLB.
  • Page 19 through R68). Adding and subtracting logarithms is equivalent to multiplying and dividing the original values, so the collector current of IC18-D is proportional to the product of product of the two multiplication inputs, divided by the division input. IC24-B converts this current to a voltage across R82 to create the PROD output.
  • Page 20 In voltage mode the system works differently – IC43 acts as an error integrator. Its output changes to adjust the conductivity of the FETs, in turn changing the current drawn through the source impedance of the user's supply, until the terminal voltage matches the required value. Because the FETs invert, the connections to IC43 appear reversed –...
  • Page 21 The negative VXO subtracts from the positive VS at the input of IC23-B. For as long as VS is the larger, D15 conducts and holds the amplifier output at -0·7V. When the magnitude of VS falls below VXO, the output goes positive, and starts to turn on Q5 (normally R267 & R268 set SSH at -0·75V, so the emitter of Q5 is a little below 0V).
  • Page 22 Slow Start Logic The slow start facility causes the load current to ramp up slowly from zero when enabled and to ramp slowly back down to zero when disabled. The speed of the ramps is determined by the slew rate settings. A slow start is triggered when both the input enable signal IEN is asserted and the user's voltage applied to the terminals of the load is above the threshold set by the drop-out voltage control.
  • Page 23 Metering The metering is a completely separate function implemented with an 8-channel A to D converter IC9, a processor IC5 which drives the multiplexed LED displays on the display PCB. The processor detects the operating mode of the load by reading the state of the control signals through IC1 and IC2 and scales the display readings appropriately.

  • Page 24: Calibration

    Calibration In the factory, a test jig is used in place of the front panel to generate accurate control settings. In the field, control settings can only be made to the accuracy of the internal meters. Equipment Required Calibrated 6 digit DVM. A Frequency counter.
  • Page 25 Adjust reference voltage Note: The internal reference voltage is adjusted to match the gain of the external control input. Connect: Remove the short circuit and connect a DC supply to the rear panel remote control voltage input, leaving the negative connection linked to the Load Input negative. Set: Set the voltage at the remote Control Voltage input to 4·000 V (use the DVM if necessary).
  • Page 26 Duty Cycle span (at 5%) Set: Duty Cycle = 5.0%. Scope: Set main timebase to 20µs/cm and delayed timebase to 1µs/cm. Adjust delay so both edges of low pulse are visible on delayed trace. Adjust: VR11 for pulse low width of 5 divisions (between 50% point of the edges). (Note: Watch the front panel reading and adjust Duty Cycle control as required.) Duty Cycle span (at 95%) Set:...
  • Page 27 Set Mid range (other modes) slew rate Set: Mode = 80V. The Slew Rate display should show about 25.57 V/ms. Scope: Set delayed timebase to 500µs/cm. Adjust: VR2 for 5.1 cm (= 2.56ms) Set Lo range (other modes) slew rate Set: Freq Range = Lo, Slew Range = Lo, The Slew Rate display should show about 819 V/s.
  • Page 28 Function Multiplier output offset Adjust: VR35 for zero (±2µV) on DVM at PROD. Voltmeter adjustment (external sense). Connect: Remove the short and connect the High voltage supply to the rear panel Load Input terminals. Connect the DVM to the front panel Load Input terminals. Select: Sense slide switch = external Meters = Volts &...
  • Page 29 Current Meter calibration Select: Meters = Volts & Amps Adjust: VR28 for Current (RH) display to read 40.00 (or to match actual current). Current Monitor Output calibration Connect: DVM to current monitor output. Adjust: VR26 for monitor output = 2.000 V ± 0.4mV. High Current check Select: Meters = Level A &...
  • Page 30 Dropout Clamp operation (this is a functional check, not a calibration) Select: Level Control = Level A. Input Enable = out, then in. Set: Source voltage = 6 V. Check: Unit is conducting 40 Amps. Select: Meters = Slew & Dropout. Increase the Dropout setting to 5.8 Volts Check: Unit is still conducting 40 Amps.
  • Page 31 Connect: DVM to current shunt (if used). Adjust: VR15 for 40.00 Amps actual current. Note that this method requires that the terminal voltage be exactly correct. Otherwise, measure the actual terminal voltage, compute the required current (at 20A/V) and set for that figure.
  • Page 32 10 Ohm mode setting calibration Select: Enable = out, Mode = 10 Ω. Meters = Level A & Level B, check Level A setting is 5.00 Ω. Set: Supply output to 15 Volts (current required: 3 Amps). Select: Enable = in. Meters = Watts & Ohms. Check: on scope that the system is stable.

  • Page 33: Parts List

    Parts List PCB ASSEMBLY - MAIN (44115-1790) Part Number Description Position 22218-0212 SWITCH SLIDE RA PCB MTG DPDT 22467-0130 TERMINAL BLOCK 2W R/A TB1-5 22573-0041 HEADER 2WAY STR SIL STD/GOLD LK4, TP1 22573-0070 HEADER 4WAY STR SIL STD TP2-3 22573-0071 HEADER 5WAY STR SIL STD 22573-0208 HEADER 8 WAY STRAIGHT .156P...
  • Page 34 PCB ASSEMBLY – MAIN – SM STAGE (44115-1791) Part Number Description Position 22041-0020 BEAD FERRITE SM0805 FB1-5 23105-0010 RES SM0805 1R00F W1 R60, R83, R129, R234 23105-0220 RES SM0805 22R0F W1 R35-37, R191, R278, R280 23105-0470 RES SM0805 47R0F W1 R149, R239 23105-1100 RES SM0805 100RF W1...
  • Page 35 PCB ASSEMBLY – MAIN – SM STAGE (44115-1791) continued/… Part Number Description Position 23106-2910 RES SM0805 9K10D W1 25PPM R188, R270 23106-3100 RES SM0805 10K0D W1 25PPM R226-227 23106-3200 RES SM0805 20K0D W1 25PPM R68, R78, R80, R82, R88, R100, R107, R207 23106-3220 RES SM0805 22K0D W1 25PPM...
  • Page 36 PCB ASSEMBLY – MAIN – SM STAGE (44115-1791) continued/… Part Number Description Position 23405-0331 CAP SM0805 330P CER COG C34, 155, 156 23405-0681 CAP SM0805 680P COG 23406-2474 CAP SM0805 470NK 16V CER X7R C89, C91, C129 23409-0470 CAP SM1206 47PJ 500V CER COG C110-113 23410-0332 CAP SM1206 3N3J 50V CER COG...
  • Page 37 PCB ASSEMBLY – MAIN – SM STAGE (44115-1791) continued/… Part Number Description Position 27239-0000 IC SM 74HC00 IC51, IC56 27239-0020 IC SM 74HC02 IC52, IC57 27239-0320 IC SM 74HC32 IC33 27239-1470 IC SM 74HC147 27239-1660 IC SM 74HC166 IC1-2 27239-2380 IC SM 74HC238 IC49 27250-2140...
  • Page 38 PCB ASSEMBLY CONTROL, DISPLAY & PSU (44115-1780) /continued… Part Number Description Position 23347-0540 POT 50K LIN 10 TURN WW VR5-6 23424-0443 CAP 10NZ 1KV CER D10 P5 C8-9 23424-0459 CAP 4N7 250V AC CER Y C6-7 23557-0664 CAP 1000U 35V ELEC RE2 P5 C101 23557-0667 CAP 220U 25V ELEC RE2 P3.5...
  • Page 39 MECHANICAL ITEMS (51153-4201) Part Number Description Position 20010-0254 RIVET SNAP-LOCK 4.1D X 5.5T FRONT FAN FIXING 20010-0255 RIVET SNAP-LOCK 4.1IDx7.5TH REAR FAN FIXING 20030-0263 WASHER M3 ZPST PBV TO PLATE, PLATES, BRACKET , FRONT BASE, FRONT SIDES, MAIN/TUNNEL, HERMISTER/HEATSINK 20030-0266 WASHER M4 ZPST EARTH 20037-0301...
  • Page 40 MECHANICAL ITEMS (51153-4201) continued/… Part Number Description Position 20500-0020 FAN GUARD 92MM 20611-0013 WASHER NYLON 6.5IDx12.7ODx.8TH 10-TURN POTS 20612-0011 WASHER NYLON M3 CONTROL PCB, DISPLAY PCB 20613-0016 SIL-PAD INSU.RECTIFIER 20651-0016 CLIP CABLE RICHCO MWSEA2-2-01 HEATSINKS, MAIN TO FET 20651-0016 CLIP CABLE RICHCO MWSEA2-2-01 FOR HEATSINKS 20661-0247 SPACER HEX STUD M3 x 12 NPBR...
  • Page 41 CASE LOWER 33537-1020 CASE UPPER 37113-0260 BUTTON S/BANK L/GREY F/R 37151-0490 KNOB 12.5MM D-SHAFT L/GREY F/R 37151-0520 KNOB 21MM D(5MMA/F) L/GREY F/R 37151-0540 KNOB 16MM D(5MMA/F) L/GREY F/R 37151-0550 KNOB 16MM RND 1/4IN SHAFT EXTD 48511-0630 INSTRUCTION MANUAL - LD300...

  • Page 42: Component Layouts & Circuit Diagrams

    Component Layouts & Circuit Diagrams...
  • Page 43 Main Board Page 42...
  • Page 44 PSU Board Page 43...
  • Page 45 Display Board Page 44...
  • Page 46 Control Board Page 45...
  • Page 47 Main Board Sheet 1 of 2 Page 46...
  • Page 48 Main Board Sheet 2 of 2 Page 47...
  • Page 49 PSU Board Page 48...
  • Page 50 Display Board Page 49...
  • Page 51 Control Board Page 50...

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