Troubleshooting LM7805 with low Vin (less that min. spec)

Thread Starter

fmashockie

Joined Dec 11, 2020
10
Hello All,

First time posting here so I apologize if I don't provide enough information. I am also a bit new to electronics repairs as well. Okay so I am a Lab Mechanic and having issues repairing an old and broken centrifuge at my job. It keeps providing an error that is caused by incorrect voltage output on its LM7805. Upon initial investigation, I measured a Vin = 3.5 VDC and Vout = 0.7 VDC on this regulator. After doing some research online (the instrument manual and the manufacturer's tech support are no help whatsoever. There's no schematic available for it either) about this error and what might cause it, I found someone who mentioned it might be due to a bad upstream capacitor causing the LM7805 to become full of 120 Hz due to high ripple voltage. I attempted to test for AC voltage at the regulator but my multimeter is one of the cheaper variety and when you test for AC on DC circuits it just multiplies the DC voltage being measured by 2. But I decided to replace all the electrolytic caps including the large power supply filter cap (see photos attached) anyway. That didn't solve anything. I noticed there were some burn marks on the PCB by a string of four 18 ohm resistors. 3 of the four looked burnt but were still measuring correctly. To be on the safe side, I replaced those as well with some axial metal film ones just to test it out. That didn't change anything either. Finally, I decided to replace the bridge rectifier and then lastly the LM7805 itself. When I replaced the LM7805 with a new one, the Vin I measured = 5.00 and now Vout = 0.6 VDC.

I have other centrifuges of the same make and model in good working condition so I decided to take some measurements of one of those. On the good centrifuge, I measured a voltage across that pink smd capacitor (in photo below marked with yellow dots) = 7.1 VDC, 1.1 VDC across each of the four 18 ohm resistors, and a Vin = 11 VDC on the LM7805 (marked with red dots). For the one I am trying to repair, the measurements for the capacitor, the voltage across each of the resistors, and Vout of the LM7805 are 15 VDC, 2.5 VDC, and 5 VDC respectively. Based on the info and photos I am providing, would anyone have any suggestions or intelligent guesses as to what might be causing this to occur? I've taken many measurements of components upstream (mostly resistance and continuity tests with power off as this centrifuge has some parts live up to 150 V) and I can't seem to find anything wrong. The only thing I haven't tried is removing each component one by one to test individually, but I work on a lot of other equipment at my job and it would be hard to make it worth my time (You can buy these used for a fairly good price). Any help would be greatly appreciated and I apologize for the long-winded explanation! PXL_20201211_151155026.jpg

PXL_20201211_151245447.jpg

PXL_20201211_151245447_2.jpg
 

dl324

Joined Mar 30, 2015
12,453
To be on the safe side, I replaced those as well with some axial metal film ones just to test it out. That didn't change anything either. Finally, I decided to replace the bridge rectifier and then lastly the LM7805 itself. When I replaced the LM7805 with a new one, the Vin I measured = 5.00 and now Vout = 0.6 VDC.
It is unwise to replace components that aren't known to be bad. You could end up causing more damage removing and replacing; even if you have good soldering skills.

Does your lab have an oscilloscope?

I apologize for the long-winded explanation!
Try using paragraphs to organize your thoughts. It'll make it easier for us to read and see your questions.
 

crutschow

Joined Mar 14, 2008
26,380
Try to find where the voltages on both circuits are the same and then continue on until you find a significant difference.
That would then be the area to concentrate on to find the fault.
 

Thread Starter

fmashockie

Joined Dec 11, 2020
10
It is unwise to replace components that aren't known to be bad. You could end up causing more damage removing and replacing; even if you have good soldering skills.

Does your lab have an oscilloscope?


Try using paragraphs to organize your thoughts. It'll make it easier for us to read and see your questions.
Thanks for the tips Dennis. And unfortunately no in reference to the oscilloscope. Although I work for a lab (biotech) you'd be surprised how little resources I have for fixing their equipment lol.

-Frank
 

dl324

Joined Mar 30, 2015
12,453
And unfortunately no in reference to the oscilloscope. Although I work for a lab (biotech) you'd be surprised how little resources I have for fixing their equipment lol.
The 5V input to the regulator is something to check. It should be at least 7V. The output being at 0.6V may or may not be significant; the regulator won't work unless it has sufficient headroom (input voltage higher than the output).

What were the burned resistors connected to? Are they on the input or output of the 7805?
 

Thread Starter

fmashockie

Joined Dec 11, 2020
10
Try to find where the voltages on both circuits are the same and then continue on until you find a significant difference.
That would then be the area to concentrate on to find the fault.
Good idea. Once I get some time to open up the working one, I’ll get them side by side again and take more thorough measurements.
 

Thread Starter

fmashockie

Joined Dec 11, 2020
10
The UC2844ad is a Switchmode pwm chip, this looks like a mains powered PSU , here is the pinout,

View attachment 224663
Thanks for pulling this up! I’ve taken some measurements around this chip. So far everything checks out according to the data sheet. I found Vref = 5 VDC, Vcc = 14.3 (can be between 10-16 V after start up), Vcomp = 2.3 V, and Vout = 2.3. Based on those measurements, I don’t think the pwm is the issue. But I’m a novice at this at best so I could be wrong.
 

Thread Starter

fmashockie

Joined Dec 11, 2020
10
The 5V input to the regulator is something to check. It should be at least 7V. The output being at 0.6V may or may not be significant; the regulator won't work unless it has sufficient headroom (input voltage higher than the output).

What were the burned resistors connected to? Are they on the input or output of the 7805?
Those burnt resistors go directly to the Vin of the LM7805. There’s a cap before the resistors as well. Whats strange to me is replacing the LM7805 brought Vin up from 3.5 V (on the original) to 5 V after replacement.

Another thing that I don’t understand: There‘s a cap, followed by 4 18 ohm resistors, and another cap right before Vin on the LM7805. On the working unit I measured 7 V across the first cap, 1 V across each resistor, and 11 V at the cap right before Vin.

On the broken unit. I took the same measurements and observed 15 V across the cap farthest from Vin, 2.5 V across each resistor, and 5 V at the cap on Vin of the regulator.

I guess my question is, why is the voltage drop across the resistors happening in different directions for the broken unit and the good unit?
 

dl324

Joined Mar 30, 2015
12,453
On the broken unit. I took the same measurements and observed 15 V across the cap farthest from Vin, 2.5 V across each resistor, and 5 V at the cap on Vin of the regulator.

I guess my question is, why is the voltage drop across the resistors happening in different directions for the broken unit and the good unit?
The broken unit has a higher load current, so the 18 ohm resistors have larger voltage drops. 5V at the input of a 5V regulator means it isn't regulating.
 

Thread Starter

fmashockie

Joined Dec 11, 2020
10
In other words, it seems that at least now the 7805 is not the cause of problems. Is anything getting hot now?
Nothing appears to be getting too hot now. Initially, before changing the resistors, there was a slight smell of something burning. Possibly due to the old ones being burnt. One of them was cracked as well but they were all still measuring correct resistance. I'm not sure what caused them to burn up initially.
 
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