running lm3915 with AC current

Thread Starter

zazas321

Joined Nov 29, 2015
433
I tryed to run my 10 led vu meter with AC 10V,2A adaptor and I heard cracking sound, the lm3915 got very hot and apparently its not even working even with my 9V battery anymore.. Can anyone explain why this circuit wont work with alternating current? whats bad about it :eek: I mean i know the differences between alternating and direct current in the theoretical way but i cant use it the practical things:(
 

#12

Joined Nov 30, 2010
18,186
That chip is not designed for its power supply voltage to become more negative than its ground pin. Try feeding it AC for a power supply and it's going to die suddenly.:eek:
 

wayneh

Joined Sep 9, 2010
16,164
Well, you just learned one thing about the practical differences between AC and DC: Things meant for DC will be destroyed by AC!
 

Thread Starter

zazas321

Joined Nov 29, 2015
433
Yeah definately thats some good experience. So u guys mean lm3915 cant go any more down than 0V or it will be destroyed?
 

crutschow

Joined Mar 14, 2008
24,125
Yeah definately thats some good experience. So u guys mean lm3915 cant go any more down than 0V or it will be destroyed?
Exactly.
If you apply a reverse voltage to most IC power pins it forward biases the substrate isolation diode and large currents will flow, zapping the IC.
Most semiconductors require a steady DC voltage of one polarity and not AC voltage which is constantly varying and reverses polarity every half cycle.
Only a few devices are designed to directly operate with AC voltages such as SCRs and TRIACs.
 
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