audioguru

You did not provide the coil voltage and its current of the relay and did not provide the output current of the sensor when its voltage is only 3V. The coil produces a voltage spike of hundreds of volts when the relay turns off. Usually a reverse-biased diode is connected parallel to the coil to prevent the spike.

The corrected latest version reliably regulates the maximum current to be 3.0A. Then an additional protection circuit is not needed. Buy name-brand parts from an honest electronic parts distributor, not cheap fake junk from ebay and the project will work fine.

I did not look at everything on Picmaster's schematic to see if it has any more problems. Liquibyte made a new thread showing the latest version of the 3A supply here: http://www.electronics-lab.com/forum/index.php?topic=39481.0

Why was this garbage circuit posted as a project here? It is without an explanation of how to connect it and how it works. Maybe its author built the power supply but its current regulation did not work. Instead of asking for help to fix it in the forum then he posted this garbage circuit instead?

I have Picmaster's schematic of 10/10/2010 and it says 5A but it has only two 2N3055 output transistors that will fry. It might have other problems.

The 3A version uses two output transistors. Each emitter resistor has a max voltage of 1.5A x 0.33 ohms= 0.5V across it. When the output voltage is shorted or is low then each output transistor heats with (30V - 0.5V) x 1.5A= 44.25W. The 5A version uses three output transistors. Each emitter resistor has a max voltage of 1.67A x 0.33 ohms= 0.55V across it. When the output voltage is shorted or is low then each output transistor heats with (30V - 0.55V) x 1.67A= 49.2W. But both versions use a single BD139 driver transistor which heats a little more in the 5A version. The peak voltage of a 28V transformer is 28V x 1.414= 39.6V. If the output voltage is low or is shorted with 5A of current then the transformer must deliver 39.6V x 5A= 198VA so a 200VA transformer will be hot but will be fine. If the transformer is 30V then it will be a little overloaded (212VA). The circuit is designed for a max current detection voltage across the shunt resistor of 1.4V so the 3A version uses 0.47 ohms (3A x 0.47 ohms= 1.41V). The 5A version uses 0.27 ohms (5A x 0.27 ohms= 1.35V). It is simple to calculate the heating (power rating) which is 5A squared x 0.27 ohms= 6.75W so use a 10W resistor which will get pretty hot.

I guess nobody replied is because we cannot understand your broken English. 1) "Remove AC source". Turn it off? 2) "Direction". North, south, east or west? 3) "Applied DC solution". DC applied to what? Solution of what problem?

Lacquer thinner will probably dissolve plastic parts in a pot and deposit it as a mess all over the resistive track. My contact cleaner is "safe on plastics, leaves behind zero residue and is fast drying". Why not use it?

Hi Red Baron, I have Picmaster's schematic, revision 2.10 dated 10/10/2010, which is almost the same as mine. It uses TIP3055 output transistors instead of 2N3055 ones. The TIP3055 has an epoxy case, not metal and is rated at 90W max instead of 115W max so one might be shorted C to E due to excessive heat or maybe moisture from humidity got through the epoxy. If the BD139 or the voltage regulation opamp shorted then the output voltage would be less than the unregulated positive supply.

I think there is a certain supply voltage where the opamps in the project suddenly begin working, maybe with severe overshoot. maybe why most audio amplifiers do not have soft start. Instead they use a relay for delayed start so that the circuit can stabilize before it has an output.

The fast TLE2141 opamp needs to have a couple of compensation capacitors (like the long posted project) so that the phase shift from the slow 2N3055 transistor does not cause oscillation. The opamp will not fry if it oscillates because it does not have a high current load.

I think a TL2141 opamp driving the amplified output section will oscillate because the slow 2N3055 output transistor delays the negative feedback. The single output transistor will get fried if the output voltage is low or shorted and the current is high.

I use "contact cleaner" to fix scratchy sounding pots. Sometimes a coupling capacitor leaks DC current into a pot that makes a scratchy sound that is fixed by replacing the capacitor.

The original project has errors and problems: 1) Many of its parts are overloaded so it is not reliable. The transformer, driver transistor and single output transistor overheat. Your 24V/3A transformer is rated at 72VA maximum but the full wave bridge rectifier operates from the peak which is 24V x 1.414= 34V and with an output of 3A then the transformer power is 34V x 3A= 102VA. You can calculate the heating in the driver and output transistors yourself. 2) Without a load the peak voltage from the transformer is about 36V which is reduced to about +34.8V by the bridge rectifier. The opamps use an additional negative -5.6V supply so their total supply is 34.8V + 5.6V= 40.4V but their absolute maximum rating is only 36V. 3) There is no way the project can supply 30VDC at 3A because the circuit losses reduce the output to about 25V at 3A. If you increase the transformer voltage then the opamps will be destroyed even quicker and more heat will destroy the driver and output transistors quicker. Therefore we fixed the circuit and improved the parts. Maybe you should research the original parts and calculate how much they are overloaded. Also show your teacher the losses that prevent 30V at 3A. The original circuit would be reliable if it uses a 20V/4.3A transformer. But its maximum output at 3A would be only about 18VDC or 20VDC.

I removed R15 from the original circuit because it wasted some important voltage headroom. I removed D10 because I saw no purpose for it. Try adding D10 back. Can you try a non-inductive resistor for the current sensing 0.47 ohms?

I suspect that the inductance of the 0.47 ohm resistor and the 10uF output capacitor cause the darlington connected driver and output transistors to ring at about 10MHz when the power is turned off. Maybe the emitter of the clamp transistor should connect to the output ground on the other side of the 0.47 ohm resistor.

I have not made this project. I make many circuits on stripboard instead of on a pcb then half the wiring are the parts and a few jumper wires. In my electronics career I made custom one-of-a-kind circuits (some were very complicated) and the prototype made on stripboard worked perfectly and looked good enough to be sold and installed. I use Microsoft Paint program to copy and paste things and make schematics.

I gave your white schematic as much contrast as is possible which caused its background to be a little grey. I noticed that you changed most of the parts designation numbers so that when most of the other schematics used R11 and R12 to set the gain of the voltage amplifier now you have R18 and R24 doing it. Therefore do you have a new parts list that matches your new schematic?

I downloaded it and tried to make it a positive image instead of a negative black image. Then many of the lines are not shown. There is some garbled text on it:

I answered you and another kid in your class about your crippling of this project on another website. Earlier today I saw this schematic that uses a 20k fixed resistor for R12 instead of a trimmer resistor. The trimmer resistor changes the voltage gain to allow an output as high as 30.0V. With your 20k resistor for R12 then the maximum output voltage is +24.6V.

Your soldering looks bad. It should look smooth and shiny. Did you use plumber's solder? Maybe your soldering iron is too small to solder those big wires. A shorted capacitor does not hold a charge. Its charge quickly discharges through the short. Charge it then use a high input resistance meter to measure its voltage with the charger removed. If the voltage stays for hours then it is fine. But if the voltage drops quickly then it is shorted or is very leaky.

On the other website you wanted to remove its current regulation. Then it will blow up and will be destroyed if its output is shorted or has a current higher than 3A.

When a charger or power supply has no load then its power consumption is very low. If you detect that it has no load and disconnect it then the new detect/disconnect circuit might waste more power that you save.

What powers your bike? We don't know what powers your bike because you did not tell us. My bike is powered by my legs. Maybe your bike is powered from a battery, gasoline or diesel fuel. Maybe a steam engine with coal on fire?

Why not use a proper real heatsink with some thermal grease? A real heatsink has many fins for a high surface area. Then the bridge rectifier can work at closed to its 50A rating.