This informative article addresses an approach by which any ready made SMPS may be transformed into a variable current smps circuit utilizing a couple of external jumper links.
In one of the earlier content we found out to create a variable voltage SMPS circuit by utilizing a basic shunt regulators stage, in the present hack also we utilize the same circuit stage for applying a variable current output characteristic.
In the above connected post we mentioned how an opto coupler presented a significant role in offering the essential constant output feature for almost any SMPS.
The work of the opto coupler might be realized with the following brief justification:
The opto coupler offers an inbuilt LED/photo-transistor circuitry, this device is integrated with the SMPS outputs phase such that when the output tends to get over the hazardous threshold, the LED inside the opto lights up forcing the phototransistor to perform.
The photo-transistor consequently is configured across a delicate "shut down" point of the SMPS driver stage wherein the conduction of the photo-transistor causes the input stage to shut down.
The above situation leads to the SMPS output to also immediately turn off, nevertheless the instance this switching initiates, it corrects and returns the output to the safe zone and the LED inside the opto deactivates which once again switches ON the input stage of the SMPS.
This operation keeps on cycling quickly from On to OFF and vice versa making sure a continuing voltage at the output.
Steps to make an Adjustable Current SMPS Circuit
To be able to attain a current control feature inside any SMPS we one more time look for the help of the opto coupler.
We carry out a simple modification making use of a BC547 transistor configuration as demonstrated below:
Adjustable Current SMPS Circuit
Speaking about the above design we get an obvious idea concerning how you can change or make a variable current SMPS driver circuit.
The opto coupler (mentioned by red square) will probably be display by default for all SMPS components, and providing the TL431 is not present then we possibly may have to configure the complete configuration connected with opto coupler LED.
If the TL431 stage has already been a part of the SMPS circuit, subsequently we simply have to think about establishing the BC547 stage which turns into exclusively accountable for the offered current control of the circuit.
The BC547 can be viewed associated with its collector/emitter across the TL431 IC's cathode/anode, and the base of BC547 can be viewed associated with the output (-) of the SMPS via several selectable resistors Ra, Rb, Rc, Rd.
These types of resistors being in between the base and emitter of the BC547 transistor commence operating like current sensors for the circuit.
These are generally properly determined such that by shifting the jumper connection across the appropriate contacts, various current limits are presented in the series.
When the current tends to increase beyond the set threshold as determined by the values of the related resistors, a potential difference is produced across the base/emitter of the BC547 which turns into adequate to switch on the transistor, shorting the TL431 IC between the opto LEd and ground.
The above activity immediately lights up the LED of the opto, delivering a "fault" signal to the input side of the SMPS via the opto's in-built photo transistor.
The condition instantly attempts to implement a shut down across the output side which often prevents the BC547 from carrying out and the circumstance fluctuates from ON to OFF and ON quickly making certain the current by no means surpasses the predetermined threshold.
The resistors Ra...Rd might be determined by utilizing the following formula:
R = 0.7/cut-of current threshold
For instance if guess we would like to connect an LED at the output obtaining a current rating of 1 amp.
We are able to set the value of the corresponding resistor (chosen by the jumper) as:
R = 0.7/1 = 0.7 ohm
Wattage of the resistor may be merely obtained by multiplying the variants, i.e. 0.7 x 1 = 0.7 watts or simply 1 watt.
The determined resistor helps to ensure that the output current to the LED by no means crosses the 1 amp mark, therefore preserving the LED from harm, other values for the remaining resistors might be properly determined for obtaining the wanted variable current option in the SMPS module.
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