Re: 78L24 regulator failure
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Jim, Given the high input voltage, voltage spikes are certainly a possibility, but I would also investigate how hot the device is running. High device temps accelerate failure rates. If I remember correctly, the protection diode across these devices is recommended when there is a large filter cap on the output. When the input voltage is removed, the device becomes reversed biased. The higher the output voltage and the higher the capacitance, the more likely that there can be a problem. Bill N0CU
On Sun, Apr 7, 2024 at 10:13?AM Jim Shorney < jimNU0C@...> wrote:
One of my TR7s suffered a 24V regulator failure yesterday. It was just sitting there running and failed overnight. Anecdotal evidence suggests that the 78L24 is a high failure rate part and in my limited sample of <10 I have now replaced three over the years in different rigs. But why? Inquiring minds want to know.
Other 78xx and 78Lxx devices are used in the 7 line gear and they don't fail as often. Maybe it is the voltage? One data sheet that I found with specs for the 24V chip lists 40V as the maximum input voltage. Input on the TR7 PS board was measured at about 36V unloaded. Maybe spikes stressing the IC causing eventual failure. Reducing the input voltage with a small series resistor is something that could be investigated. Ten volts of headroom in this circuit seems like overkill (pun intended). I have not tried this.
One thing in the back of my mind was that a protection diode across the regulator is suggested in some 78xx data sheets. I had to go back to an older TI data sheet to find it.
The idea here is that the regulator can be damaged if for some reason the input voltage drops below the output voltage. A back biased diode from output to input supposedly prevents this. I don't know that this is what is happening but diodes are cheap (or sometimes free) so I added one.
If both me and the rig are still around in 20 years I will let you know how it works out.
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73
-Jim
NU0C
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