SMPS transformer in VFD display module gets very hot - Page 1
Forum section: electronics / repair

author: canadaboy25
author_rank: Regular Contributor
timestamp: January 31, 2025, 05:28:29 am
content: I have a VFD display module for a large engine that was not working. The problem ended up being a broken solder joint on an SMD transformer for the VFD cathode drive. Resoldering the connection repaired the display.The display module has 3 boards. A power supply board, a processor board, and a VFD display board. While I had the unit apart, I noticed the transformer on the power supply board gets very hot. I've attached images of the board and a thermal image of the board while running. Please excuse the poor alignment of the thermal camera, I don't know why it is off by so much. The transformer seems to be around 93°C. I don't know if the resistor beside the transformer gets hot itself, or was heat damaged by the transformer. I reflowed one joint on that resistor as it looked very bad, but did not touch any other joints as I am nervous to mess with that crusty transformer.Does this temperature seem reasonable? The supply is still operating fine. Or is there some type of fault that could cause the transformer to heat and still work fine. It just seems odd to me that it gets that hot. The unit draws 0.6A at 12V from the power supply. No components on the processor board or VFD board get hot. It has even slighly charred the inside of the plastic enclosure. However it is over 25 years old.Let me know your thoughts.

author: Harry_22
author_rank: Frequent Contributor
timestamp: January 31, 2025, 07:37:21 am
content: Hi!Seems this was the normal operation. You can see that the resistor is fried only near the transformer. It took a time. Stay it as it is and not spend a time. Change only both cans near the transformer.

author: cte
author_rank: Regular Contributor
timestamp: January 31, 2025, 09:03:28 pm
content: Is this 47µF/16V cap leaking?

author: inse
author_rank: Super Contributor
timestamp: January 31, 2025, 09:21:15 pm
content: I don’t think you can fry a resistor with 96C from outside, it looks like it is operated above its ratings.Maybe the transformer has developed a winding short which raised losses but still allows it to operate?Unfortunately not curable…

author: benj38
author_rank: Frequent Contributor
timestamp: January 31, 2025, 09:29:52 pm
content: Quote from: cte on January 31, 2025, 09:03:28 pmIt looks more like black silicone to me, but obviously the OP will know.As for the original question, I second @Harry_22. I have seen this kind of discoloration of the PCB more than once.Some circuits are just designed to run that hot, which is obviously not the best thing to do, but there you have it. They usually run like that for many years until some poor component succumbs to the heat and dies.

author: inse
author_rank: Super Contributor
timestamp: January 31, 2025, 09:39:51 pm
content: Well, maybe but >7W to drive a VFD?

author: canadaboy25
author_rank: Regular Contributor
timestamp: January 31, 2025, 10:48:59 pm
content: No, the caps are not leaking. The black stuff at the bottom is indeed silicone. The entire board and all components have a heavy layer of conformal coating, and the crust on the top of the cap is just some of the coating.I kind of suspected the consensus would be "it is what it is". The board is 25 years old and no replacements are available. I suppose the only thing to do is put it back together and deal with it if it dies in the future.It does seem like an excessive amount of power for a little display module and processor. However, it was designed in the 90's.Appreciate the responses, I just wanted a second opinion.