Dear Audioguru,I worked on this circuit 16 years ago at www.electronicslab.com. Then it was a defective kit from Greece.
1) The TL081 opamp Q2 is destroyed by over-voltage as mentioned in our thread here. The TL081 maximum allowed supply is only 36V but the circuit needs a much higher voltage.
The transformer must be 28V minimum to get 30V at 3A from this project. Without a load the transformet might produce 29.5V which makes a rectified +40.3V and my negative supply is 1.4V so Q2 is powered with 41.7V which is less than its maximum allowed voltage of 44V. TWO output transistors must be used on a large heatsink to share the heat.
I used TLE2141 opamps because their maximum allowed supply is 44V and Q2 works fine with only 1.4V for the negative supply (the TL081 must have at least 4V for its negative supply).
2) I increased the value of C1 to 12000uF.
3) Like mentioned here, I used 5mA zener diodes instead of the ones in the kit that must have a much higher current.
4) I increased the power ratings of a few resistors so they don't burn up when the output of this project is 3A.
The Chinese copy is of the original old defective kit.
Please can you help me?
I have a Smartkit 1138 0-30 volt 3 amp power supply which I purchase as a kit about a decade ago. I am an electronic dummy and my limit is assembling a kit. I assembled this kit and it has worked flawlessly all these years for the bits that I do with it. I doubt that I have ever drawn more than 1.5 amps off it. So all went well until I decided to upgrade it with a digital voltmeter - ammeter. I connected the ammeter incorrectly and that was the end of my power supply. With my limited knowledge I removed all the transistors and found that the 2N2219 and the BC548 were both damaged. So I replaced them with no success. So I purchase 6 new TL081's and fitted them with no success. So I took out the transistors again and found that the 2N2219 was again destroyed. So I replaced that again and removed all the diodes except the rectifier ones and tested them. They all checked out good. The zeners measured 5.12 and 5.2 volts respectfully. So I reassembled and fitted my last 3 TL081's and no result. I have checked my transformer voltage (25.2 ac) and the rectifier and that seems to be OK. I was very careful with the transistors and checked the data sheets for the pinouts so I am sure that they are mounted correctly. The 2N2219 gas a finned heatsink fitted to it.
I do not know what else I can do to fix my power supply.
Would it be possible for you to spoon feed me a bit here and tell what to test and where to test it and what to replace? Please treat me like the idiot that I am and try and get my power supply working again.
I contacted the original supplier of this kit with a view to buying a new one but the tell me that it discontinued so no linger available.
If you have any suggestions as to what components I should upgrade to make this power supply a little more immune to my trying to destroy then please let me know and I will purchase and fit the parts.
Thank you in advance if you can help me and I do hope that I am not wasting your time.
Moderator Edit: New thread created from this.