Harmer & Simmons SM24-50 Setting-up Document










The Control Board should be adjusted for: Output Voltage, Current Limiting &


Over-voltage protection.


Check that there is 12 volts on TP1 and 5 volts on TP2, each with respect to TP3.


Absence of the 5volt line means the 78M05 regulator may be faulty. IC1.




Output Voltage: 27.6v +/- 0.2v




With the rectifier powered and No Load connected, adjust R16 for 27.6 volts


R16 is the 4th pre-set, multi-turn pot from the top of the board.


Check the range of output voltage available which is typically 20 to 30 volts.


If the Over-voltage Red LED lights during this process then adjustment of the


“over-voltage” and “over-voltage minimum current” pots R42 & R50 respectively need to be checked. See below.




Current Limit:




Set the output voltage set to 27.6 volts.


Connect a suitable “Load” to the output.


This may be a resistance of 0.5 Ohm, rated at 50 Amps minimum or 1.5Kw.


Connect a Clamp Ammeter to one of the load connecting wires.


Connect a DVM voltmeter to the output terminals.


Switch on the Rectifier and note the current in the load.


Adjust R25 for a limited current of 52Amps. (Top pre-set pot)


Observe the output voltage with the load current at 52Amps which should be around 26 volts. The output voltage is designed to “fold back” to zero under short circuit conditions.


Switch the Rectifier OFF and disconnect the load when safe to do so, the load may be very hot!




Over-Voltage trip:




The over-voltage trip normally requires an output current to be flowing before it will trip and shut down the supply. In order to adjust the over-voltage setting it is necessary to override the “minimum current” requirement by adjusting R50 fully anti-clockwise.




Adjust R50 (bottom pot) fully Anti-Clockwise.


Power the rectifier and adjust the output voltage to 29.0 volts (R16 4th pot down)


The over-voltage LED should be OFF.


Adjust R42 (2nd pot from bottom) until the over-voltage LED lights and the unit shuts down. De-power the unit and turn R16 (4th pot down) anticlockwise a couple of turns so that on power up, the output is below the trip voltage.


Power up the unit and re-check that the unit trips at 29.0 volts.


Re-power and set the output to 27.6 volts.




Proceed to the Over-Voltage minimum Current adjustment.






Over-Voltage Minimum Current:


This adjustment ensures that the rectifier trips out on “over-voltage” when delivering a small load current, but does not trip out with no load current being drawn.




Adjust R50 (bottom pot) fully Clockwise.


Power up unit and ensure input and output LED’s are lit.


With No Load connected, adjust R16 (4th pot down) for an output of 29.0 volts.


The over-voltage LED should NOT be lit.


Connect a small load to the output so that about 1amp max is drawn.


Adjust R50 (bottom pot) until the over-voltage LED lights and the unit shuts down.


De-power unit and remove load.


Re-power unit and ensure over-voltage LED is NOT lit.


Re-connect load and confirm the unit trips – over-voltage LED is lit.


Remove load and re-power unit, reset R16 (4th pot down) to give an output voltage of 27.6 volts.




Setting up the Inverter Frequency.




The main Inverter should operate at 33 Khz however, some have been found to operate around 27 Khz. The efficiency may be impaired at this lower frequency.


To check and set it up requires an oscilloscope with a X10 probe.




Set up the scope for 5uSec/div timebase and AC coupling.




Connect the scope probe to TP15.


(TP15 is located just beneath TR3 on the rear of the control board about one third of the way down from the top)


With the rectifier powered and a load connected of at least 1 Amp observe the drive waveform on the scope. A triangular shape with a square wave shape below should be seen. Measure the periodic time of the drive waveform which should be about 30 uSecs. If it is greater than 30uSecs then adjust R27 (2nd pot down from top) until the period is around 30 – 32uSecs.


The duration of the square drive pulse will depend on the current load and should be about 1uSec for the 1Amp test load and no greater than 12uSec on current limit. R13 adjusts the duration of this pulse but is apparently not critical unless the drive pulse is much more than 12uSec.


Check the rectifier’s performance as set up earlier if the Frequency has been altered.




























Starting with a completely dead rectifier:


Check the Fuse (if fitted) on the small board containing L1 choke and red/black/green/yellow wires from the EMC mains input box on the front panel.


If blown, check for shorts on the red VDR voltage suppression resistors on the L1 choke board and also for short circuit bridge rectifier diodes/thyristors comprising MR1. Also check for short circuit on the 3 470uF black reservoir capacitors on the 385 volt HT line. CAUTION: IF CHARGED THEY HAVE A LETHAL VOLTAGE STORED – DISCHARGE WITH A RESISTOR BEFORE TOUCHING.






Rectifier powered but no output:




The INPUT amber LED may be partially lit.


The HT line may not be fully up due to a faulty bridge rectifier assembly MR1 or more likely because R5 has gone open circuit.


R5 is a 68 ohm 17Watt wire wound resistor in the start-up path for the HT line.


Because it soft starts the HT supply, it takes a lot of current initially until the inverter starts up and triggers the thyristors in the second arm of the MR1 bridge rectifier. R5 can go open circuit by virtue of its life style and old age! Replace with exactly the same component type. However, if it is shattered or badly burnt it is a crime scene and the most likely perpetrator is the inverter transistors TR1 & TR2 on the Drive Board. When they fail they produce a short circuit on the 385Volt HT line. Check for a short circuit on the reservoir capacitor but be careful in this area due to lethal voltages!!!


If the transistors are short circuited then the rectifier may be considered as “scrap for spares” as the cost of BUV93A’s are in excess of £35 each.






If the 385 volt HT line is present and no output is seen then the Inverter may be “shut down”. If the Over-Voltage LED is lit then check the adjustment of R42 first and then R16 for setting the output to 27.6volts if there are signs of life after re-powering the rectifier.






If the current limit LED is on and no output LED then the current limit pot R25 may need adjusting. Switch off and adjust R25 fully clockwise (10 turn pot) re-power the rectifier and if the output is now present, proceed to the adjustments described earlier.






A common fault is failure of the 5 volt regulator on the control board, check for 12 volts on TP1 and 5 volts on TP2. If IC1 78M05 is replaced, replace C1 1000uF 25 volts as a precaution as it has probably started to dry out.


















If the over-voltage and/or minimum current adjustments prove impossible to set,


more detailed checks can be made as follows:




Over-voltage adjustment R42.




With the rectifier de-powered, connect the –ve lead of a DVM to TP3 on the Control Board. This is located on the bottom of the board by the edge connector and is very close to the toroidal transformer.


Locate IC6 and pins 1, 2 & 3. for measurements.


Power on the rectifier and set the output voltage to 27.6 volts.


Measure the voltages on pins 2 & 3 (typically around 2.5 volts)


Adjust R42 ( 2nd pot up from bottom of board) so that the voltage on pin 2 is higher than Pin 3. ( typically 2.60v and 2.51v) this is to ensure the output volts can be set to a high value without tripping. Pin 1 should be Low (0.7V).


Connect a 1Amp Load to the output terminals.


Set the output voltage to 29.0 volts by adjusting R16 (4th pot down).


Adjust R42 so that the over-voltage trip operates and the LED comes on.


If the unit does not trip, no over-volts LED lit, Adjust R50 fully anti-clockwise and re-check as above. The minimum current adjustment needs to be done afterwards.


Reset the output voltage to 27.6 volts.




Minimum current adjustment R50.




Set up the test meter as in the over-voltage adjustments with the –ve lead on TP3.


Power on the rectifier and ensure the output is 27.6 volts.


Measure the voltage on IC6 pin 5 and pin 6.


Adjust R50 (bottom pot) so that IC6 pin 5 is about 0.1 volt less than pin 6.


(typically 2.55v & 2.65v respectively)


Adjust R16 (4th pot down) so that the output voltage is 29.0 volts.


Measure the voltage on TP7 (beneath IC6) should be about 0.7v.


Connect a small load to the output so that about 1 amp max is drawn.


Adjust R50 (bottom pot) until TP7 just goes High (13.5v) and this should have now tripped the voltage and the over-voltage LED should be lit.


Disconnect the load then re-power the rectifier to reset the trip.


Reset R16 (4th pot down) for 27.6 volts.




It may be necessary to repeat the R42 over-voltage adjustment.




The Rectifier should now be set up for 27.6volts current limited to 52 amps and with an over-voltage trip at 29.0 volts on-load.




If these adjustments are not achievable then there may be a fault on the control board.
















RCD Power Tripping.




If the RCD earth leakage trip in the cabin keeps tripping out with the rectifier powered up, but does not trip with it disconnected from the supply, it is possible that there is a leakage fault in the mains input EMC filter unit, which is bolted to the front panel and contains the IEC mains input plug. This can be proved by substitution with another filter unit if available or by temporarily removing the mains earth connection to the rectifier for test purposes only. Suitable safety precautions should be observed.




Under no circumstances must the mains earth connection be left disconnected!




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