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Posts posted by audioguru

  1. The original Greek kit and the first Chinese copy wrongly said to use a 24V/3A transformer that does not work and becomes overloaded. Its voltage is too low for the project to produce 30VDC at 3A and its voltage is too high for the opamps with a low maximum supply voltage rating of only 36V. The original opamps also needed an additional negative supply with a fairly high voltage.

    We corrected the circuit and used opamps with a 44V maximum supply voltage rating and a 28V/4.24A transformer (the peak of 28VAC is 39.6V). With an output current from the project of 3A then the transformer must supply 39.6V x 3A= 188.8VA. Then the current rating of the transformer must be 118.8VA/28VAC= 4.24A.

    The improved circuit also works with a 30AC transformer and you can calculate its current rating needed.


    The current limiting is variable from about 2mA to 3A and has excellent current regulation. If the output is shorted then the current will not cutoff, instead it will be at the current setting.


    Many stores do not provide datasheets that are available online from the manufacturers. Wakefield is a heatsink manufacturer and has datasheets of all their heatsinks at their online site. 

  2. If you use a 24V/4A transformer then its maximum allowed output is 24 x 4= 96VA. Then the project maximum output current must be calculated with the 34V peak voltage (96VA/34VDC= 2.8A).


    You can simply calculate how much heating (volts times Amps) a 2N3055 will survive by its thermal spec's and the heatsink thermal specs on their datasheets.  

  3. If you use fine and coarse controls then one will always be in the wrong position. Use larger knobs instead.

    Higher output current will cause a heat problem in the output and driver transistors. The improved version of this project already uses two output transistors to share the heat produced at 3A. Use a huge heatsink and a fan for 4A output. Change R7 to be 0.35 ohms/10W.         

    A 24VAC transformer is too low for this project to produce 30VDC output at 3A or 4A but the original TL081 opamps will have a voltage too high for them if the transformer voltage is increased. The improved version of this project uses TLE2141 opamps rated at a supply of 44V and a 28VAC or 30VAC transformer.

    A transformer rated at 24V/4A has a limit of 24V x 3A= 72VA. But here the 24V is rectified to produce a peak voltage of 34V. Then if the load on this project is 3A the transformer produces 34V x 3A= 102VA. It must be rated for 102VA/24V= 4.25A. If the output of this project is 4A then the transformer must be rated for 5.7A. 

  4. A zener diode is used with reverse bias voltage. A temperature sensing diode is used with forward voltage that is completely different.

    Look at the datasheet of a 1N4148 diode. Its forward voltage is shown at -40 degrees C, at +25 degrees C and at +65 degrees C. The forward voltage drops as the temperature of it rises.

    Test it at a current of 0.1mA to 10mA at different temperatures. 

  5. The circuit will never have a negative voltage in it.

    P1 will smoothly give 0V to +5V to pin 2 as it is rotated. The input current of pin 2 is so low that a 10k or 20k pot will make no difference.

    R4 limits the T1 base current to  only 0.3mA then tie relay might not be activated if it needs 6mA or more. The current in R4 must be at least 1/20th the current in the relay coil. The BC547 works poorly at currents above 50mA.

  6. You mentioned relays. Then you do not want speed control? Jerky on-off is called "bang-bang control" in cheap kids toys.

    What will happen to the motor when it is running forward at full power then you suddenly change it to run in reverse at full power? The fuse will probably blow to save the motor.

    You need a circuit to delay switching directions.

  7. A 24V transformer has a voltage too high for the TL081 opamps. Use a transformer with a lower voltage like 15V. Replace the opamps that were strained.

    Replace the old fashioned 2N2219 with a modern BD139 transistor on a real heatsink.

    Replace the single 2N3055 output transistor that might be shorted.

    Re-calculate the values for the resistors that set the voltage and current.

  8. ebay means "manufacturer's rejects" and "sweepings off the floor".

    Don't buy anything from ebay, instead buy from a reputable and reliable Western electronics parts distributor.


    Haven't you see a datasheet of an old 2N2222 transistor? Like all transistors its hFE is a range of 1 to 3 times and the actual hFE also varies with the collector current. 

    With a collector current of 1mA the minimum hFE is 50 with no guaranteed maximum hFE.

    With a collector current of 150mA the hFE is guaranteed to be from 100 to 300.


    hFE is DC current gain that is listed and shown on a graph on a datasheet.

    hfe is AC current gain that is usually a little less than hFE at low frequencies and is usually not shown on a datasheet.

  9. I can buy a transformer with any voltage that I want. A 28VAC one or a 30VAC one will work in this modified circuit and the circuit will have a maximum regulated output of 30VDC at 3A.

    If you use a transformer that is 24VAC then the maximum regulated output from this modified circuit will be about 25VDC at 2.5A, and if the voltage and current controls are at maximum then the output will produce lots of unregulated ripple.

  10. Why are you using the TIP122 darlington to short the 3V battery? The collector and emitter currents will be whatever the shorted battery current will be which might be 8A if the cells are new or be a few mA if they are old. The current gain of a TIP122 is a range from 1000 (the minimum) for some of them to maybe 10,000 or more for others. The Arduino output voltage at 3mA will be less than 5V, maybe 4V so the base current of the TIP122 might be 2mA.

    Your current meter has a voltage drop so it measures currents low. Maybe the Arduino pulses the Currents and your meter shows the lower average currents.

    The high current is not going into the Arduino, instead it is going from the battery into the TIP122. 

  11. The ebay kit that is a copy of the original faulty Greek power supply on this site is also sold by Banggood and Amazon. Banggood is selling it at a clearance price since they probably had many complaints. They have modified it a little and are selling it again as a kit with a 28V/2A rating and an LCD meter for it.


    The very nice looking 30V/10A power supply is extremely cheap. Its ad has no detailed spec's. It does not even say if the voltage and current are regulated.

    If something looks to be too good then it probably is not true.

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