7805 / 7905 Power supply problem
7805 / 7905 Power supply problem
(OP)
We have eight IF boards in our equipment. Most of the IF part run on +15V.
The last demodulator (controlled by a shift register running at 40 MHz) runs on +5V, and the ADCs run on +/- 5V.
The analog filter on the ADC inputs runs on +/- 15V.
The principle in the power supply is shown in the attached schematic.
Now for the problem part.
Every now and then, when we turn the +/- 15V on, the 18 Ohm resistor (R1, surface mount) burns! (usually jumps to 3-6 kOhm or so)
At normal operation the voltage drop over it is about 2V, ie. 110 mA or so, which is as expected.
At 'burn time', that current has been measured in the 600 mA area! A quick turn off usually saves the resistor. (max. 1 sec!)
We have tried separate + and - 15V supplies, and turning them up individually, in different order and slowly.
Something inside one or both the 7805s or in the load still pulls a large amount of current from time to time.
The questions are what and why?
All regulators are from Motorola in standard TO-220 housing.
There are no protecting diodes. Not across the output and not backwards across the regulator.
The designer of the board has long retired, unfortunately.
The last demodulator (controlled by a shift register running at 40 MHz) runs on +5V, and the ADCs run on +/- 5V.
The analog filter on the ADC inputs runs on +/- 15V.
The principle in the power supply is shown in the attached schematic.
Now for the problem part.
Every now and then, when we turn the +/- 15V on, the 18 Ohm resistor (R1, surface mount) burns! (usually jumps to 3-6 kOhm or so)
At normal operation the voltage drop over it is about 2V, ie. 110 mA or so, which is as expected.
At 'burn time', that current has been measured in the 600 mA area! A quick turn off usually saves the resistor. (max. 1 sec!)
We have tried separate + and - 15V supplies, and turning them up individually, in different order and slowly.
Something inside one or both the 7805s or in the load still pulls a large amount of current from time to time.
The questions are what and why?
All regulators are from Motorola in standard TO-220 housing.
There are no protecting diodes. Not across the output and not backwards across the regulator.
The designer of the board has long retired, unfortunately.
Talk To Other Members
RE: 7805 / 7905 Power supply problem
I would suspect the equipment attached to the 7805's pulling more current before I suspected the 7805's themselves... slow slew rates can do bad things for a lot of logic, including power shoot-through. You have a resistor followed by a cap, which will provide a longer-than-necessary slew rate.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: 7805 / 7905 Power supply problem
RE: 7805 / 7905 Power supply problem
You can always try a zener across the resistor to limit the voltage drop, if that is the problem.
RE: 7805 / 7905 Power supply problem
If the output of a positive voltage regulator gets pulled below ground because a negative supply (the LM7905) starts up first, it can cause the positive regulator to latch up and basically look like a short circuit. I quick look at an LM78MXX regulator (dated 2005) shows no mention of this, so it may be a concern. They typical fix is to put a Schottky diode between the 7805 output (cathode) and ground (anode), so the output cannot get dragged more than 0.3V below ground.
If your purchasing department recently changed vendors for the 7805 or 7905, that can easily cause this type of issue.
In your schematic, the LM7905 capacitors are shown with improper polarity (LM7905 output is -5V, not +5V).
Z
RE: 7805 / 7905 Power supply problem
RE: 7805 / 7905 Power supply problem
Keith Cress
kcress - http://www.flaminsystems.com