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DC Power Supply with Adjustable Current & Voltage Limits

adamrodgers

Feb 18, 2020
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Feb 18, 2020
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Hi, I am currently working on a project that involves building an adjustable power supply with adjustable current and voltage limits. I'm fairly new to electronics, so I have been relying heavily on the web to make my circuit.

I have attached a schematic of what I have come up with so far, however, I only have a basic understanding of how it actually operates. Would anyone be able to talk me through the full operation of this circuit (i.e. what each component is doing, what different sections of the circuit are doing and how they work etc.), and potentially suggest any improvements/corrections?
upload_2020-2-18_19-44-33.png

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Thanks for your help, it's much appreciated :)

Adam
 

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Alec_t

Jul 7, 2015
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Welcome to EP!
BR1 rectifies the V1 voltage and C1 smoothes the result, to form a supply voltage for Q3 and Q4 which are connected as a Darlington pair.
D1 (an obsolete type) and R4 establish a base voltage for Q2 which is 2.4V below the said supply voltage. D1, R5 and Q2 form a constant-current source. This current provides base current for Q3 and hence turns on Q3 and Q4. Q4's emitter voltage is the output.
A fraction of the output voltage, as established by the potential divider R6/R7/RV2, is tapped off at the wiper of RV2. If this fraction exceeds about 0.7V Q5 turns on, robbing Q3 of some base current and hence, via Q4, reducing the output voltage. Thus output voltage is regulated to a value determined by the setting of RV2.
Load current passing through R2 results in a voltage drop across R2. A fraction of this drop is obtained from potential divider R3/RV1 and if it exceeds about 0.7V then Q1 turns on, robbing Q3 of base current as before. Thus load current is limited to a value determined by the setting of RV1.
 

adamrodgers

Feb 18, 2020
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Feb 18, 2020
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Thanks for the help.

Yesterday I built up this circuit on a breadboard and everything worked fine, however as soon as any form of load was connected the output voltage would drop dramatically, even with the current limit set high. For example, I hooked up a motor and the output would not turn past 9 volts. Is there any way this can be improved?

And one other thing - I would like to add an indicator LED that illuminates when the circuit is current limiting. My idea was to base this off the voltage change in Q1 when it turns on and use some sort of comparator circuit to sense the change. Is this a good way of going about things / how could it be done?
 

Alec_t

Jul 7, 2015
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What is the rating (voltage and current capability) of V1? What is the motor rating?
A sensitive LED in series with Q1 collector should give some indication of current limiting occurring.
 

adamrodgers

Feb 18, 2020
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Feb 18, 2020
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V1 is 14V @ 2A, and the motor is rated at 3-6V, not sure of the current rating.
 

Alec_t

Jul 7, 2015
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Double-check that the value of R2 is indeed 0.47Ω. With that value the current-limiting shouldn't kick in below about 1A.
 
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