Hello,

I'm a Newbie to electrical engineering and I bought 2 DIY kits. One on Aliexpress which works without any problems and one from Ebay where I only get 33 mA current from the power supply. I can only drive a small LED with it, if I connect a small DC motor it immediately goes into the limit and the red LED lights up. With the other Board, the DC motor runs without problems and everything can be adjusted as desired.

I have already read a lot here and found the PDF file operation-and-maintenance.pdf with various measuring points. I have measured all the points given there and all values are correct. I have already exchanged the 3 OP Amps TL081 with MC34071 but the problem remains.

I use an external power supply AC 24V 3A for the input of the boards.

If I don't connect anything to the output, I can regulate the voltage quite normally, it also works with the connected LED and 2 connected resistors, I can regulate voltage and current, but current no more than 33 mA. The 33 mA i measured with my multimeter, which I connected to the output alone. I have checked various components, everything is soldered correctly as on the working board. I can't find anything wrong and I don't know where to look.

I still don't understand the circuit diagrams that I found on the net and here, so I have problems finding out the cause.

Do you have an idea which components I could check to solve the problem and make it work like the other one ?


Best Regards

Stefan

 

Post the schematic of the one that doesn't work.

 

Hi,

here is the schematic, i think it is the right one.



And the manual i could found in the net.

 

And here is the PDF with the measuring points that i have checked.

 

One on Aliexpress which works without any problems and one from Ebay where I only get 33 mA current from the power supply.

Since you have a working example, it should be straightforward to use it to troubleshoot the one that isn't working.

What is the voltage across R7 when you're only able to get 33mA?

I'm a Newbie to electrical engineering and I bought 2 DIY kits.

Assembling kits isn't going to make you an electrical engineer.

 

Since you have a working example, it should be straightforward to use it to troubleshoot the one that isn't working.

That's what I was planning to do and it's the reason I bought another one. I've been trying for days but I can't find the problem. I'm a newbie

What is the voltage across R7 when you're only able to get 33mA?

1,65V

Assembling kits isn't going to make you an electrical engineer.

No, but it requires learning new things. The reference to a newbie was more intended to point out that I probably can't implement tips from a professional.

 

I've been trying for days but I can't find the problem.

The voltages on the two supplies should be in the same ballpark with the same operating conditions. If you can't figure out what the problem is, it will give us some clues because we don't have the benefit of having the circuit in front of us to take measurements.

1,65V

What are the voltages on the inputs and output of the current limit comparator?

 

This schematic will be more useful.

 

The voltages on the two supplies should be in the same ballpark with the same operating conditions. If you can't figure out what the problem is, it will give us some clues because we don't have the benefit of having the circuit in front of us to take measurements.

I find differences but I don't know what the reason is. For example, your last asked value (across R7) is 0.028V on the working one if I just connect my multimeter and limit the current to 41mA, and 1.65V on the non-working one. But I don't understand the reason and that's why I'm writing here.

I also don't know how I can create the same conditions for both. The non-working one has the current and voltage potentiometer set to 100% and it's getting about 36-43mA.

With the working one, I have now set the potentiometer so that I get 41 mA displayed on my multimeter. Am I doing this right?

Then I measure the following differences in the OP amps, but as a newbie I don't know why.

Not working one:            Working one:
-----------------------------------------------
U3:
VCC+ = 33.4V             VCC+ = 33.7V
OUT = 22.3V                OUT = 33.6V
IN- = 1.61V                IN- = 0.03V
IN+ = 1.81V                IN+ = 0.46V

U1:
VCC+ = 33.4V                VCC+ = 33.7V
OUT = 12.69V                OUT = 9.99V
IN- = 7.17V                IN- = 5.02V
IN+ = 7.21V                IN+ = 5.01V

U2:
VCC+ = 33.4V                VCC+ = 33.7V
OUT = -0.80V                OUT = 1.18V
IN- = 1.42V                IN- = 0.03V
IN+ = 1.01V                IN+ = 0.03V

The last few days I've checked whether I've soldered the diodes, transistors and capacitors the right way around. I desoldered some of them and checked if they were ok. I couldn't find any error.

I followed the PDF from #4 and all values fit as specified there. Only on page 17 I did it a bit differently, I don't have a 100 ohm 2W resistor here so I left my DC motor connected and then measured. The result was ok as stated there.

My hope was that a professional could narrow down the problem and tell me what to check and how.

What are the voltages on the inputs and output of the current limit comparator?

Please tell me a little more precisely where I have to measure for this.

I think the schematic will be more useful for you, but not for me. I make an effort to learn.

 

Did you do the setup procedure?

 

yes.

 

yes.

Are you sure that R7 in the non working unit is 0.47 Ohms and not 47 Ohms, which is what you get with 1.65V x 33mA, give or take tolerances?

 

Your measurements indicate that there is a problem... one of things that certainly stands out is R7 voltage drop which is way too high. to get 1.65V across 0.47 Ohm, current would need to be over 3A. so either you are using wrong value or this is not soldered properly. post pictures of your PCB front and back

 

There are no 47 ohm resistors in the kit and it would be pretty difficult to confuse the 5W resistor with any other resistor.

 

There are no 47 ohm resistors in the kit and it would be pretty difficult to confuse the 5W resistor with any other resistor.
View attachment 285692

Hi Dennis,

You're right, there aren't, but we never know what mislabeled thing might sneak into one of those kits, do we?

 

Are you sure that R7 in the non working unit is 0.47 Ohms and not 47 Ohms, which is what you get with 1.65V x 33mA, give or take tolerances?

Thank you very much. This was the hint i needed.

A wrong resistor was included with the kit and I didn't realize it. It's really 47 Ohms instead of 0.47 Ohms. During my research on the Internet, I found evidence that some of these kits are delivered with incorrectly printed circuit boards or defective diodes. For example, diodes were printed there upside down. I have checked this multiple times. I also measured various resistors, just not this one.

I replaced the resistor and now everything works as expected. Thanks very much !

 

A wrong resistor was included with the kit and I didn't realize it. It's really 47 Ohms instead of 0.47 Ohms.

Was it mislabeled? Or just the wrong part?

During my research on the Internet, I found evidence that some of these kits are delivered with incorrectly printed circuit boards or defective diodes. For example, diodes were printed there upside down.

That's what you get buying that cheap stuff from China. They know, or care, so little that they can't even copy existing designs. I have a number of Arduino clones that have LM358 instead of LMV358. LM358 are less expensive, but they aren't a replacement for LMV358.

I replaced the resistor and now everything works as expected.

Did you use a power resistor?

 

Was it mislabeled? Or just the wrong part?

It was a wrong part.

That's what you get buying that cheap stuff from China. They know, or care, so little that they can't even copy existing designs. I have a number of Arduino clones that have LM358 instead of LMV358. LM358 are less expensive, but they aren't a replacement for LMV358.

It was clear to me that I couldn't expect much for less than 4 €, the parts are much more expensive individually. I also like to tinker, but in this case I was too stupid to recognize the problem. But I've learned something new and that's good!

Did you use a power resistor?

No, I had another one here for spares from the working kit from Aliexpress and took the 5W 0.47 ohm resistor from it. I would swap this for a 15W in the future or what do you recommend?

The order from Aliexpress was ok, the one from Ebay had the wrong resistor. The layout of the circuit board is also slightly different.

 

No, I had another one here for spares from the working kit from Aliexpress and took the 5W 0.47 ohm resistor from it. I would swap this for a 15W in the future or what do you recommend?

That resistor will dissipate over 4W if you operate at 3A. A 2W resistor would let you operate at up to 1.5A (after derating the resistor by 20%).
\( P = I^2R => I = \sqrt{\frac{2W}{0.47\Omega}}=1.88A \)

The order from Aliexpress was ok, the one from Ebay had the wrong resistor. The layout of the circuit board is also slightly different.

I wouldn't buy anything on eBay that ships from China (sellers are getting wise to this). You can get junk from China on Amazon too.

 

That resistor will dissipate over 4W if you operate at 3A. A 2W resistor would let you operate at up to 1.5A (after derating the resistor by 20%).
P=I2R=>I=√2W0.47Ω=1.88A

I'm not sure if I understand you correctly. But with a 15W 0.47 ohm resistor I could theoretically use 5.64A and thus have a high reserve. Or is that wrong?