Hello everyone,
Like in a thread just below this one started by SpeedMaster57, I'm also having a problem with my DIY kit. I'm feeding it with a 24V 2A AC-AC transformer.
After assembling everything, I'm reading 0,13V on the output, LED stays off and turning pots doesn't do anything.
I can measure following values:

- On Fan Output I'm getting 24V as I should
- Across rectifying diodes D1 and D2 (I think?) I'm getting 17.2V
- Across rectifying diodes D3 and D4 (I think?) I'm getting 16.3V
- Across C1 Cap I'm reading 36.1V
- Across C2 Cap I'm reading 23.1V
- Across C3 Cap I'm reading 7.6V
- Across C7 I'm getting no voltage
- Across D11 I'm getting no voltage
- Across D7 I'm getting 1.46V
- Across D8 I'm getting no voltage
- Across D6 I'm getting 3.24V
- Across D5 I'm getting 2.06V (I'm really not sure, but somehow I get the feeling that this diode should be turned other way around, could somebody verify that? I did put it in the circuit as it was labeled on the PCB, but following that diagram, I'm under impression that anode of D6 should be getting on cathode of D5?) I'll put a detailed photo in the thread...

If any other values are necessary, don't hesitate to ask, I'm in my workshop all day today...

 

Hello,

When you are measuring 36 Volts accross C1, the transformer will be about 27 Volts AC.
The 36 Volts is the absolute maximum for the TL081 chips.
As U2 has also a negative voltage applied, the voltage accross that will exceed the the ratings of the chip.

Bertus

 

Hi Bertus!
Yes, actually, transformer is giving about 26.4V of AC out, but I thought it will be fine after the rectification?
Any suggestions?

 

Are you getting -5.1v between pin 4 of u3 and r3

 

Nope...
I'm not getting anything.
I'm not a easy quitter, but I think this is out of my league, especially since I can't find any additional info on what to check...

 

Hello,

Is there some kind of manual delivered with the kit?
I would like to see it if possible.

Bertus

 

This is what I found...

 

There is no manual but check these links out http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html?m=1
http://www.qsl.net/z33t/dc_0-30v_0-3A_eng.html

 

On top of all, I think I just blew up my rectifying diodes... Arghhhhh!!!!

 

I just saw this video where at the end of it guy explains a thing or two...
Can somebody make something out of it?

 

Hello,

The person in the video is making one big mistake.
He is putting DC on the input capacitor.
There is no way the negative voltage can be made this way.

Using AC on the input of the brigde rectifier will give the circuit the possibility to build up the negative voltage, needed to regulate the output voltage to 0 Volts.

Bertus

 

Hello,

The negative voltage is created like in this page of the eBook, except the diodes are the other way around:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/voltage-multipliers/

Bertus

 

Thank you Bertus, I really appreciate your time.
Yes, he actually came to the same conclusion at the end of the video. Now, since there's no negative voltage on that rail, MOSFET is preventing any or at least minimum voltage to appear on the output if I understood that correctly? If so, can we narrow down the list of possible issues with mine and Speedmasters PSU?

 

Hello,

With your supply, the input voltage is to high.
In the blog page that speedmaster posted, there is a zener (D13) in the opamp lines:


He also added a meter in the schematic in that blog page.
In that schematic there is one mistake, the 82 Ohms resistor should be on the AC input and not on the capacitor C1.

Bertus

 

Hello

@speedmaster57 , Please stay in your own thread, to avoid confusion.

Bertus

 

As this is a DIY build, it is most likely that you have both made a component placement error. Double (triple) check that you have not mixed up the zener diodes with the 1n4148's, and also that the ic's and capacitors are correctly fitted. Set the 100k pre-set to about mid-range to start off with.
The resistor colour codes can be difficult to read correctly, use a magnifying glass and in natural daylight re-check that they are correct. Sometimes the colours can easily be misread, reds, browns and orange are often not well defined on some Chinese resistors, if in any doubt, lift one end and measure the value with a meter.
Some makes of TL081's can act strangely when used near their maximum voltage and the 100k trimpot could cause U2 to fail if it is set close to one end and maximum input voltage is applied without having the minus 5 volts present.
I would fit sockets for the i.c's to make replacement easier. If buying replacements, only get them from a good supplier, avoid the ones on e.bay like the plague!
I used dozens of TL071 and TL081's in the production of a timing advance circuit I designed, and only ever had problems with a batch I mistakenly bought from e.bay.
The ones from R.S and Farnell all performed faultlessly

 

Hey Recklessrog!
Actually, I've taken out both Zeners and 1n4148's to check if I've put them correctly. No mistake there. I suspect that overvoltage might've killed either TL or U2 or whatever.
Anyway, since I killed rectifying diodes as well in the meanwhile, what do you guys suggest?
Should I take everything out of the board, buy components from some other supplier and maybe some higher values (ie 1/2W resistors insted of 1/4 ones etc)?
Or, should I just throw everything away and buy a new kit? I've been waiting for it to arrive for 2 months so I would really hate to do that?
Is anyone willing to accompany me through this build if I buy components and start over?
And, yes, since I only have this 24V (26,7V) 2A AC supply and several DC supplies with even higher current capability, I would like to make something that will work with that as well...
I bough Volt/Amp meter, Two 100K multiturn pots (by mistake), one 10K multiturn pot, enclosure... So I would really like to finish this up.

 

If you can temporarily reduce the transformer voltage, or use a lower voltage transformer so that it will not exceed any part limitations, that could be handy. It could be a lower current one because you won't be drawing much current until the thing is working. Also, I see what looks like a bad solder connection to the second IC behind the one variable control. I don't know if that could cause the problems that you are having.

 

Thank you MrBill2
Actually, I do have a about 1-1,5A transformer that on one of his windings can output 16V on a center tap. I already tried it, however when I moved my probes to check the voltage over the diodes, I must have shorted them out through that aluminum heatsink so I couldn't say what's going on with that transformer. So I'll definitely take those diodes out, test them and replace bad ones to continue troubleshooting. And I'll definitely inspect that solder joint although I've really pun a lot of effort to check for continuity and shorts after I did soldering. But, hey, it won't hurt and I do have time to do it.

 

Just an update...
Apparently my rectifying diodes were not blown...
I've took them out, measured around 0,5V drop in forward bias and no Voltage is going across when I put them in reverse bias. I've put them back on and I attached the PCB onto my 16V transformer. Now, I'm getting 0,14V on the output, I'm getting 20,1V out of the LM7824, and I am getting around -3V across ground and 4148 diode...