VOLTAGE REGULATOR QUESTION

[4krow]

If a voltage regulator is expecting 12V but only receives 6V, what might happen? (The regulator output is set for 6V). A fuse has blown in one of my projects, and I am uncertain as to why. For now, until my customer sends it back, I understand that the filaments are not lit in the tubes. This will be some kind of lesson for me, I'm sure. I will know more when it arrives.

[Paul Birkeland]

This depends a bit on the regulator itself, do you know which chip it is?

[VoltSecond]

A 6V LINEAR regulator getting 6V usually output 1-3V less than the input voltage.

http://www.fairchildsemi.com/ds/LM/LM7806.pdf

At 1 amp, the LM7806 would put out 4V.  As the voltage dropped, the output would eventually shut off. Not likely a smoke generator.  

HOWEVER

Into a filament load (tube, light bulb) these regulators can get stuck in a thermal or current limit. This normally happens when the heatsink is too small to handle the start up or run power. The regulator goes into thermal shut down and the output drops to an odd output voltage (say 1V) and never recovers.  IF B+ is applied, bad things could happen to the tubes.  With 1V on the filament, they won't look like they are lit.

This latch up can sometimes be solved by placing a power resistor from input to output so that at high input voltage to the regulator, the resistor conducts 60 to 80% of the load current.  Rate the power resistor for ~3 times the actual power it will dissipate. (3W of loss  = buy a 10W resistor )

This problem happened to me a couple decades ago. . .

[4krow]

Thanks guys for the info. The regulator is an LD1085. After thinking a bit about it, I also remember that the AC inlet failed me once. I 'adjusted' the power cord, and never had a problem since. After I sold the unit, my customer says that the unit just quit working. HMMMM says I, and now, will wait for it's return. Probably only to find out it is a defective AC inlet. Lesson learned. I didn't like these inlets from the first I used them. Now, I pay the piper ..

[4krow]

OOOOO,   I should add that the customer says that the fuse is fine, leading to the power connection thing again. But while I'm at it, I did notice a misprint in the instructions. I believe that the voltage regulator should be wired for full wave rectification. In the directions, I see that looks to be wired as a voltage doubler....not good. However, for whatever reason, the voltage measured at 6.3 volts, just as it should. Again, I can't wait to get this things in my torture chamber one more time. More to follow...

[4krow]

L'ill update about the voltage regulator problem. When the unit was shipped back, I replaced the fuse anyway, and plugged it in. Lets just say, it performs fine, and after 3 weeks, it is still in my system. I will never know what the deal was.