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PV System Common Grounding Problem Solution Using Optocoupler

1. Why isolation is important:
A ground loop may occur when several circuit elements which should be at ground (i.e. 0 Volts),
but are not quite at ground, are connected. Generally, a ground is constructed by connecting a
wire to a central point which is defined as the “official” ground. Unfortunately, all wires have
some resistance which is generally fairly small, but which can produce a significant voltage drop
if enough current is flowing.
In the digital circuit , ground loop noise is a common phenomenon which will appear when
someone connects two separate and relatively noiseless circuits.

2. What is Opto Isolator-Opto Coupler:
Opto-isolators also known as opto couplers are useful for eliminating ground loop noise, but are
also used to connect devices that operate at vastly different voltages. For example, opto-isolators
are used to exchange data with a device that is floated to several thousands volts as is frequently
the case in particle accelerators in the physics lab experiments. Also, medical instruments like
brain imaging machine which connect to humans must be isolated from the wall power by
mandatory isolation circuits. The other areas of Opto couplers’ application are audio/music,
computing, data communications.
There exist other isolation technologies, such as transformer isolation and capacitive isolation,
but these do not isolate as well as opto-isolators, since electrical noise can still get through a
capacitor or a transformer. Light conversion can potentially provide total isolation between two
circuits because there is no capacitive or inductive coupling at optical frequencies.

Optocoupler Circuit diagramSchematic of Opto Isolator

3. How it works:

a) Using Op-to Couplers:

Solving of common grounding using Optocoupler in PV

 

The above circuit is a simple solar Charge Controller, its function is to regulate the power
flowing from a photovoltaic panel in to a rechargeable battery. The MOSFET circuit becomes
ON as long as Gates gets a small positive voltage while as OFF as soon as Gate voltage becomes
zero or negligible. The activation of the MOSFET precisely the Gate voltage in this circuit is
being governed by the rating of Zener diode and the way the input Gate voltage tweaks to zero.
When the battery voltage falls below to a certain level, PV should charge the battery and in case
of satisfactory level of battery voltage, it should not get charged and ideally Battery should be
discharged through the connected load whatever it might be. The way control circuit is designed
it can sense the battery voltage level and in case of low it sends 0 voltage to the LED thus LED
remains off and in the shown ABC path voltage across Zener remains good enough so that Gate
of the MOSFET gets a positive voltage level say 2 to 3V to keep activated the MOSFET. Thus,
MOSFET drain to source will become act as short, support to draw a large amount of drain
source current through the battery. On the contrary, in case of battery voltage level high, control
circuit will send to high voltage say 5V, which will turn on the LED of the isolator and thus
photovoltaic transistor will act as short and there will be no current flow through the battery or in
other words battery will not get charge in this situation. The shown diode D between B and D is
used just to prevent opposite current flow battery to PV.

To determine at what voltage level of Battery, PV should start charging the battery, rating of
Zener diode, the value of Resistance R and the MOSFET VGS,th are crucial for the circuit design. Say for example if we wanted to charge a 12V battery of 2V cell, by float mode wise it has to be maintained at 13.5V(2.25vx6). Reverse Zener diode rating, MOSFET threshold voltage rating, the value of R should be chosen such a way, so that level Vz maintains at a level of voltage which is equal or greater than the VGs,th of N Channel enhancement MOSFET.

The choice of N channel enhancement MOSFET is justified here for number of reasons. Firstly in
normal condition it is OFF(minimum current flow flows drain to source) and a small positive
voltage turns it ON meaning permits large drain to source current and for that does not require a
gate current –all these are suitable for a faster switching operation. Secondly due the low leakage
current MOSFET acts as a reliable current source, in this circuit battery is connected to the
MOSFET load line and for an optimum charging of battery constant current is required.

By voltage wise this circuit got two sections, one is low voltage side which is the detector circuit
operates with a range of usually 3V to 5V, the other portion is high voltage side it could be 12V
or 24V depending of the rating of battery and PV. Opto-isolator here isolates the low and high
voltage part. Battery negative terminal, the control logic circuit and the LED portion of the
isolator are sharing the common ground while as PV, MOSFET, Zener diode and the
Photovoltaic transistor are sharing the separate ground terminal. By doing that the possibility of
un-necessary inner circulating current or any other un-necessary small voltage drop are avoided
otherwise the Gate voltage could have a possibility of manipulation as it turns on with a very small voltage.
However, the above circuit is very simple, and indeed lot areas to be addressed in case of
commercial battery charging circuit operation. For example, how to take care too frequent
charging/ discharging, how to take care a complete discharged battery, how the over energy flow
from PV can be taken care, how end user created polarity error at battery end can be taken care,
the flexibility of the circuit to take care different rating of battery for example 12 v vs 24 voltage
battery.

b) Using Relay :

Solving of common grounding using relay in PV

In the above circuit, a npn transistor is used to control a relay. Precisely, the npn bipolar
transistor acts here as a current-flow control valve. If the detector circuit can sense the voltage
level of battery is at satisfactory level it will send 0V to the base of BJT. With no voltage or
input current applied to the transistor’s base lead, the transistor’s collector-to-emitter channel
will become open(high resistance), hence there will be no current flow through the relay’s coil
thus PV will remain disconnected with battery. However, for the case of low battery voltage,
detector circuit will send 5V signal to the base of BJT .As soon as BJT will get sufficiently large
voltage and input current at it’s base lead, the transistor’s collector-to-emitter channel becomes
short, allowing current to flow through the relay’s coil. Thus, PV will be connected with battery
and battery will get started for Charging.
The diode shown in the relay coil circuit is used to eliminate voltage spikes created by the
relay’s coil. The diode shown between A and D is to avoid the reverse current flow between
Battery and PV. The relay must be chosen according to the proper voltage rating, should be in
accordance with the BJT rating. The switch shown here is SPDT. The SPDT and relay
arrangement here is the key to keep separated the driving circuit from the battery charging
circuit.

Mainur Rahman
 

Technology and Information Manager(Customer CTO), Ericsson. Key Skills: End to end Network Solution analysis System Integration Resource Management Multi skill: RF, Core, OSS, BSS ,IP,CDMA,2G/3G,Wimax Business development.Graduated from Bangladesh University of Engineering and Technology

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