Friday, January 10, 2014

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Inverter Overload Protector With Delayed Auto Rest

An overload condition in an inverter may  permanently  damage  the  power transistor array or burn off the transformer. Some of the domestic inverters sold in the market do not feature an overload shutdown facility, while those incorporating this feature come with a price tag.the circuit presented here is an overload detector which shuts down the inverter  in  an  overload  condition. 

Inverter Overload Protector With Delayed Auto Rest Circuit diagram:

Inverter Overload Protector With Delayed Auto Rest -Circuit -Diagram

It  hasthe following desirable features:
  • It shuts down the inverter and also provides  audio-visual  indication  of  the overload condition.
  • after  shutdown,  it  automatically restarts  the  inverter  with  a  delay  of  6 seconds. thus, it saves the user from the inconvenience  caused  due  to  manually resetting the system or running around in darkness to reset the system at night.
  • It  permanently  shuts  down  the inverter  and  continues  to  give  audio warning,  in  case  there  are  more  than three  successive  overloads.  Under  this condition, the system has to be manually reset.(Successive overload condition indicates that the inverter  output  is  short-circuited or a heavy current is being drawn by the connected load.)
Inverter Overload Protector
Inverter Overload Protector With Delayed Auto Rest

The circuit uses an ammeter  (0-30a)  as  a  transducer  to  detect  overload condition.  Such  an  am-meter  is  generally  present in  almost  all  inverters.  this  ammeter  is connected between the negative supply of the battery and the inverter, as shown in Fig. 2. the voltage developed across this ammeter, due to the flow of current, is very small. It is amplified by IC2, which is wired as a differential amplifier having a gain  of 100. IC3 (NE555) is connected as a Schmitt ‘trigger’, whose output goes low when the voltage at its pin 2 exceeds 3.3V. IC4 (again an NE555 timer) is configured as  a  monostable  multivibrator  with  a pulsewidth of 6 seconds. IC5 (CD4017) is a CMOS counter which counts the three overload  conditions,  after  which  the  sys-tem has to be reset manually, by pressing push-to-on switch S1. the  circuit  can  be  powered  from  the inverter battery. In standby condition, it consumes 8-10 ma of current and around 70 mA with relay (RL1), buzzer (PZ1), and LED1 energised.

Please note the following points carefully:
  • Points A and B at the input of IC2 should be connected to the corresponding points (A and B respectively) across the ammeter.
  • Points C and D on the relay terminals  have  to  be  connected  in  series  with the  already  existing  ‘on’/‘off’  switch  leads of inverter as shown in Fig. 1. this means that one of the two leads terminated on the existing  switch  has  to  be  cut  and  the  cut ends have to be connected to the pole and N/O contacts respectively of relay RL1.
  • The  ammeter  should  be  connected in series with the negative terminal of the battery and inverter, as shown in Fig. 2.Move the wiper of preset VR1 to the extreme position which is grounded. Switch ‘on’ the inverter. For a 300W inverter, connect about 250-260W of load. Now adjust VR1 slowly, until the inverter just trips or shuts down.  repeat the step if necessary. Use good-quality preset with dust cover (e.g. multiturn trimpot) for reliable operation.the circuit can be easily and success-fully installed with minimum modifications to the existing inverter. all the components used are cheap and readily avail-able. the whole circuit can be assembled on a general-purpose PCB. The cost of the whole circuit including relay, buzzer, and PCB does not exceed Rs 100.

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