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<p class="has-white-color has-text-color has-large-font-size"><strong>Educational Engineering Team</strong></p>
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<h2>Power Source Circuit using a subsistence transformer</h2>



<p>Most electronics that are powered by a <a href="https://eduengteam.com/2022/02/2022-batteries-and-their-types.html" data-type="post" data-id="4219">battery</a> require a voltage source to be set to a certain value. However,there are other events that are more interested in how many electrons are going rather than the voltage,and this is where a constant current source is particularly helpful! These are commonly used in laser and electroplating applications.</p>



<figure class="wp-block-image is-style-default"><img src="https://maker.pro/storage/sLdiPUA/sLdiPUAABjiZf20VDgwbv7ft2hZJyoH1h2Fqtizq.jpeg" alt="Power Source Circuit using a subsistence transformer"/></figure>



<figure class="wp-block-image is-style-default"><img src="https://301o583r8shhildde3s0vcnh-wpengine.netdna-ssl.com/wp-content/uploads/2017/05/constantcurrentsourceschem.jpg" alt=""/></figure>



<h3>How does the power supply circuit work?</h3>



<p><a href="https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/voltage-to-current-signal-conversion/" target="_blank" rel="noreferrer noopener">Voltage causes a current</a>,not the other way around! So to make a device that provides a constant current no matter the load,We need to convert the current flowing through our load into a voltage with a negative feedback loop. There is a very straightforward way to accomplish this by using a small ohm resistor (in our case,a 0.1-ohm resistor). The current through this resistor is proportional to the current (due to V   IR),and we can use that to work out the current through the circuit. The voltage across this resistor is fed into the negative input of an operational amplifier and a known fixed voltage is fed into the positive terminal. The output of the op-amp is connected to the base of a power transistor (ignoring the Darlington pair) which controls how much current can flow through the circuit. The op-amp in this circuit (U1A) is in an open-loop since the negative input and output are connected to one another (through Q3) and the op-amp will try to maintain the 0 and +3 terminals at equal voltage.</p>



<p></p>



<p>The best way to demonstrate how these circuit works is an example.</p>



<p>If the current on the load is less than 1 amp,then the voltage across the 0.1-ohm resistor will be less than 0.1 volts,and this is seen at the negative terminal at U1A. Because the positive terminal is greater than the negative terminal,the op-amp will become more positive,such that Q3 will transmit a greater current. The increase in conduction of Q3 during this month facilitates more conductance of the electrical current to flow. The current flowing through the resistor should not exceed 1 amp,and if this limit is surpassed,then the voltage across the 0.1ohm resistor surpasses 0.1 volts. Unbalanced inputs to the op-amp U1A component cause the output and input to become unbalanced. As this imbalance is exacerbated by an increase in the op-amp U1A input voltage,the lower leg QnF6 places a charge on the capacitor CnF1. As the charged capacitor CnF1 discharges,the current flows through the resistor 601-Kohm,and 000 ohms is observed from the resistor.</p>



<p>The current flowing through the load is measured by a voltage meter that is connected with an amplifier (U1B). The function of the amplifier is to amplify the voltage over the 0.1 Ohm resistor to an optimal level for an inexpensive LCD display. D1 is utilized to prevent energy spikes of electrons that may be brought on by a disturbed transistor Q3. Q3 ought to be enclosed in a TO-3 case,with additional energy dissipation suitable for currents above 100mA,along with requiring both a heatsink and an additional fan,which will increase current above 1A.</p>



<p></p>



<figure class="wp-block-image is-style-default"><img src="https://maker.pro/storage/th9tBYy/th9tBYy3zqbdbwBO9y9sNRGH08kMwpi4bfAvzaS0.jpeg" alt="PCB layout for a constant current source "/><figcaption>PCB layout for a constant current source.</figcaption></figure>



<h3>Materials Required</h3>



<ul><li>| Component | Schematic Reference | Quantity</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/?p=10K+Resistor" target="_blank" rel="noreferrer noopener">10K Resistor</a> | R1,R4 | 2</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/?p=1K+Resistor" target="_blank" rel="noreferrer noopener">1K Resistor</a> | R2,R5 | 2</li><li>| 0.1 Resistor (Power Metal Film) | R3 | 1</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/?p=100nF+Capacitor" target="_blank" rel="noreferrer noopener">100nF Capacitor</a> | C2,C3,C4 | 3</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/?p=470uF+Capacitor" target="_blank" rel="noreferrer noopener">470uF Capacitor</a> | C1 | 1</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/?p=1N4148+Signal+Diode" target="_blank" rel="noreferrer noopener">1N4148 Signal Diode</a> | D1,D2 | 2</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/datasheet/LM358DT-STMicroelectronics/" target="_blank" rel="noreferrer noopener">LM358</a> | U1 | 1</li><li>| 8 DIP Socket | U1 | 1</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/?p=100K+Linear+Potentiometer" target="_blank" rel="noreferrer noopener">100K Linear Potentiometer</a> | RV1 | 1</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/?p=2N3904+Transistor" target="_blank" rel="noreferrer noopener">2N3904 Transistor</a> | Q1,Q2 | 2</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/datasheet/2N3055-NTE-Electronics/" target="_blank" rel="noreferrer noopener">2N3055 </a>| Q3 | 1</li><li>| <a href="https://www.allaboutcircuits.com/electronic-components/?p=Digital+LED+Voltmeter" target="_blank" rel="noreferrer noopener">Digital LED Voltmeter</a> | P4 | 1</li></ul>



<h2>Constructing a Constant Current Source</h2>



<p>In this particular case,an old computer power supply (ATX) was used to provide the circuit with the voltage it requires to operate. The constant current source is currently using an outer layer of plastic in place of the casing so the construction is visible.  This circuit,despite outputting large currents,uses very low voltages (less than 12V) so this circuit is very safe (unlike <a href="https://maker.pro/education/variable-power-supply-unit-with-fixed-outputs" target="_blank" rel="noreferrer noopener">mains-powered projects</a>).</p>



<div class="wp-block-image is-style-default"><figure class="aligncenter"><img src="https://maker.pro/storage/qTxmFpi/qTxmFpines3BUkCTDDrJabtW1hPbyCw6xQOLhqDV.jpeg" alt=""/><figcaption><em>Circuit before wired up </em></figcaption></figure></div>



<div class="wp-block-image is-style-default"><figure class="aligncenter"><img src="https://maker.pro/storage/h9u3M3V/h9u3M3VfjyAFPbUGfff7J8UwQvqrxopcJTVmUgJe.jpeg" alt=""/><figcaption><em>Side view of test constant current source </em></figcaption></figure></div>



<div class="wp-block-image is-style-default"><figure class="aligncenter"><img src="https://maker.pro/storage/IulFTa6/IulFTa6FgaEzPdhXVG8P4t5klv32Kjj8Qy8Zl07j.jpeg" alt=""/><figcaption><em>Wiring to front panel</em></figcaption></figure></div>



<div class="wp-block-image is-style-default"><figure class="aligncenter"><img src="https://maker.pro/storage/4fctWrc/4fctWrc4HjJj6CNoMLc905XGjUDzpBd3gxGOlWGG.jpeg" alt=""/><figcaption><em>TO-3 mounting and fan assisted cooling</em></figcaption></figure></div>



<div class="wp-block-image is-style-default"><figure class="aligncenter"><img src="https://maker.pro/storage/tf3330p/tf3330p6N7RqxWwMQfpxFEvjoApsXsOxjGi2jvn7.jpeg" alt=""/><figcaption><em>Front display,output prongs,and potentiometer </em></figcaption></figure></div>



<p>A power source circuit that produces a constant current can be created by using a subsistence transformer. This type of transformer uses a small amount of input power to create a larger output power. This is beneficial for circuits that require a constant current,as it will ensure that the current remains stable even when the input power fluctuates.</p>

Develop a power source circuit that produces a constant current by using a subsistence transformer.

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Develop a power source circuit that produces a constant current by using a subsistence transformer.

Develop a power source circuit that produces a constant current by using a subsistence transformer.

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