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I am a veterinarian and I need some help. I purchased two very expensive lift/weigh table/scales. After about 5 years of service one of them failed. I determined that the main PC board had failed. If you look at the cover of the PC board the cover is smoked over the voltage regulator. I removed the operating PC board from my other identical scale and noted a similar degree of smoke (see attached picture). The voltage regulator is an L7808 (8 volt) regulator. The fact that the other scale had a similar degree of charring/smoke made me concerned about a design flaw. When I did the math it seemed to point out that the application was exceeding the thermal limits of the regulator. I measured the input voltage at 19.5 volts (on both of my tables and two my brother has as well) from the AC/DC transformer installed in the base of the table. I think they should have placed the regulator on a heat sink or they should have a lower input voltage. Attached are pictures of the board and the housing that it smoked. Here is a reply from the company as well. There is no way this unit has the input voltage they claim. My question is: would the input voltage of 19.5 exceed the design specifications of this voltage regulator in this application (also not has no fan on it and is covered by a tight metal cover). They want $600 for a new board (which I don't mind paying); however I don't want a repeat performance at that price and my other scale tends to indicate I will!

Company response:

Dr. Wittman,

I am sorry you did not receive a call on Monday, I was out of the office unexpectedly on Tuesday the 13th. This is the measurements and readings from a Canis Major coming off our line today,

14.6VDC Measured input with load
190mA Current Measured with load
Pd=(14.6V-8V).190+14.6.008A*=1.38W
Junction Temp=Rthja*Pd+Ta=65°C/W*1.25W+25°C=106°C
The temperature of the voltage regulator is within the required specification on the datasheet. The part however will get hot, like most any linear regulators.

PC board with smoked cover

voltage regulator image

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5 Answers 5

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I don't think there is necessarily anything wrong with the unit. Let me play devil's advocate here.

The "charring" just looks like it might be some dirty air that got mixed around by convection currents. The PCB is not discolored such that it shows in the photos (maybe it does show on the bottom).

If you measured the voltage with no load applied (just measuring the adapter output) you'll get a much higher voltage than with the load applied, from an unregulated adapter, hence it's completely plausible your measurement is correct, but irrelevant. Feel free to clarify this point if you measured it with the full load applied.

The power dissipation is much higher than I would use in a highly reliable industrial design- I would aim for about half of that, but I'm pretty conservative, and I anticipate much higher ambient temperature than probably exist in a comfortably air-conditioned veterinary office (more like hellish industrial situations such as steel mills and non-air-conditioned plastics factories), so it doesn't seem outrageously off to me.

I also don't see any definitive indication that it was the regulator that actually failed.

On the other side of the coin- the calculation they used is suspect. The ambient in the box (assuming it is closed and fairly small) will not be 25°C. It might be 40°C or 45°C because it will get warm from the internal power dissipation of 2.8W. So the actual die temperature, even with their numbers, could be more like 125°C, which is inching up on the absolute maximum even with a comfortable room temperature. I would prefer to see a small heatsink as Nedd suggested or simply use a different adapter that provides a regulated 12V in.

There is one very simple check you can make that might settle the matter, at least in terms of the failure at hand. If you have the 'bad' unit (and assuming it stays non-functional) and attach the adapter and probe the input and output of the regulator with a multimeter set to 20V scale- left pin to tab is input, right pin to tap is output- if the input is >10V and the output is ~8V, then the regulator is just fine and it's something else, for sure. If the output is not about 8V, then the regulator or something else failed (not a definitive result).

Edit: Well, from your comments below, I would say 90% sure the regulator has failed (it still could be something else that's pulling it down, but the (somewhat) high line voltage and dubious cooling is likely the culprit). Even at 25°C the junction will be running close to absolute maximum, and the air inside that enclosure will be warmer, and 5 years is plenty of time for such a latent defect to manifest. If it is the regulator, you can fix it yourself for about $1 in parts and save the $600 (and add small heatsinks to keep it from happening again). I suggest an overmold type that is insulated. And/or simply replace that adapter with a regulated 12-V output one (will save a bit of energy if you leave it powered up continuously). Oh, and maybe I should bill the manufacturer for diagnosing a marginal design.

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    \$\begingroup\$ Thank you, you have educated me! I tested the functioning unit. Wall outlet voltage 123.1, input voltage to board (this time under load - my mistake) is 15.80, output on voltage regulator is 7.93. The bad boat has 17.8 input voltage under load, and 1.74 volts output. I have attached a picture of the board showing is cover enclosure. Very tight space in my opinion with very few holes for airflow. \$\endgroup\$
    – namby
    Commented Jan 15, 2015 at 22:56
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It may be that the power adapter used to power the device is not well regulated (a simple transformer) and your AC wall power is on the high side. The regulator circuit on the board might be designed without consideration of a high-line voltage.

Adding a heat sink onto a power tab part like this is usually easy. There are many designs available that specifically fit that type of component, (see for eg. Digikey.com, or Mouser.com). Just be sure not to short out any other circuitry in the assembly.

Here is a list from Mouser of heat sinks to fit a T0-220 component: http://www.mouser.com/Thermal-Management/Heat-Sinks/_/N-5gg0?P=1yzuf8yZ1yzuf8zZ1yztmtcZ1yztms4Z1yztmr0Z1yztmquZ1yztmqdZ1yzuf99Z1yztmwcZ1yzvkzyZ1yzuf94Z1yzvkzqZ1yzvkz8Z1yztn8hZ1yzvl04Z1yztn7xZ1yzvkzwZ1yztn11Z1yzvl0jZ1yztn5mZ1yztn0aZ1yztn88Z1yzvl05Z1yzvkzuZ1yztmvsZ1yzvl0gZ1yx3qhiZ1yzuf93Z1yzujc0

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  • \$\begingroup\$ The input voltage spec (LM7808) is 35v dc, so the 19.5v is within spec. There is also a thermal overload protection so it should not fail if over heated. Be sure you are using the recommended power adapter and that there are no shorts (metal tabs, open wires, etc.) on any circuitry areas, including the load sensor wires. \$\endgroup\$
    – Nedd
    Commented Jan 15, 2015 at 4:36
  • \$\begingroup\$ Also be sure you do not have any peripheral connections/interfaces (eg. printers, PC, etc.) that might be drawing excess power from the scale. \$\endgroup\$
    – Nedd
    Commented Jan 15, 2015 at 4:43
  • \$\begingroup\$ If adding a heatsink, ensure you use some thermal compound to make sure the heatsink and component are in good thermal contact. \$\endgroup\$
    – abligh
    Commented Jan 15, 2015 at 9:50
  • \$\begingroup\$ thank you so very much Nedd for your time to answer this. I did not measure input voltage under load. voltage on the functioning unit I have is 15.80 (they said it 14.6 in their calculations) which is much lower but still higher than they claim. Output good board is as expected - 7.93 volts. The bad board is 17.8 input voltage with 1.74 output. Line voltage to wall outlet is 123.1. How do we know if this voltage regulator has thermal overload protection? Is that indicated in the numbers on it, and if it does not, should I buy one with protection? I have picture of cover over board above \$\endgroup\$
    – namby
    Commented Jan 15, 2015 at 23:09
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The regulator looks normal to me. The PCB area around excessively warm part will noticeably change color, 5 years is long enough for this to happen if regulator was indeed overheating. The "soot" on the cover could be caused by outgassing; this happens to plastic at any temperature.

It's not clear from your post what kind of help you require. The pictures are very good though.

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I agree with Oleg and Spehro's point about the PCB color. I think the soot on the cover is suspicious, however. Did you get the 19.5V power adapter voltage measurement under load?

In any case, the input voltage does not exceed the spec. But their temperature calculation assumes 25 'C environment, which may be wrong, especially inside the box, and besides that 106'C is a little too close to the upper limit, which is normally 125 'C. Cheap, bad design, in my opinion. The part itself could fail; another possible problem is that a crack can develop in one of the solder joints due to repeat temperature cycles.

Don't buy another board for $600 yet if you have a little bit more time to spend on this. Check the solder joints of the regulator or just resolder them and see what happens.

You could check the voltages on the regulator pins vs the ground plane; the one connected to the adapter should be between 14 and 20 volts, depending on who's right, you or the Canis Major guy, and one of the other pins, the output, should be at 8 V on the working board and possibly 0 on the failed one. If that is the case, try replacing the regulator.

And yes, add a small heatsink.

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  • \$\begingroup\$ Thank you so much, I did not get the 19.5 under load, I retested the working board this am and got 15.8 V and 7.93 out. Bad board 17.8 V in and 1.74 V out. Wall outlet voltage 123.1 V I looked under mag for cracks in solder and don't see any (I know they can be small). I have attached unit under its metal cover (above) to show the tight space it was in. I think I will try my hand at replacing the VR and see if I can manage the heat sink. I want to make sure it does not happen again (saving the money is nice too :). \$\endgroup\$
    – namby
    Commented Jan 15, 2015 at 23:16
  • \$\begingroup\$ Sure, no problem. Replacing the VR will probably take care of it, although there is a small chance there's something else on the board that killed it. You can probably find one on digikey.com or mouser.com or maybe even at Radio Shack. Make sure the heat sink won't touch the other pins or the cover, even though it looks like it's covered with enamel. Same places to look for the heatsink (see Nedd's link), or you can make one yourself out of ~1.5mm or thicker copper, aluminum or brass. Put a thin layer of thermal paste between the VR and the heatsink if you have any lying around your clinic :) \$\endgroup\$ Commented Jan 16, 2015 at 7:50
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The voltage measurements will come out different if different voltmeters (True RMS or averaging) are used. The voltage coming off the wall-wart will not be sinusoidal when loaded with an ordinary rectifier. Anyway, I'd put in some heat sinks and save $ 590,-!

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