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Old 7th December 2015, 06:44 PM   #1
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Default quick ,dirty voltage lowering with light bulb

light bulb "regulator" question.
i was bored and had no 7815 ,7915 regulators at hand to finish ebay bought subwoofer crossover, as i re used regulators in something more importatnt.so in their place i put two 300ohm resistors to achieve virtual ground,and small 5w bulb in series with 24v ac trafo secondary, then diode bridge , on output i have very silent 20.5volts dc.

question is, is there possibility that this produces some problem in future, i am not aware of now. bulb is barely lit and not hot. is this safe 100% to use this way? nothing is hot and it plays nice .
thank you for your time.
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File Type: jpg 2015-12-07 19.27.15.jpg (440.8 KB, 93 views)
File Type: jpg 2015-12-07 19.27.41.jpg (338.7 KB, 92 views)
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Old 7th December 2015, 06:47 PM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by tonitonitoni View Post
is there possibility that this produces some problem in future, i am not aware of now.
bulb is barely lit and not hot. is this safe 100% to use this way? nothing is hot and it plays nice .
It's an RC filter, somewhat like an LC filter but with less sharp filtering
and some DC loss. This is not regulated, so the voltage can vary,
and could possibly damage the circuit from over-voltage.

Last edited by rayma; 7th December 2015 at 07:00 PM.
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Old 7th December 2015, 07:44 PM   #3
M8e is online now M8e  Sweden
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Well, one could argue that an incandecent lamp works more like an PTC thermistor. Higher temp = higher resistance

So there is some kind of current regulation/limiting happening.

Last edited by M8e; 7th December 2015 at 07:53 PM.
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Old 7th December 2015, 07:55 PM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by M8e View Post
Well, one could argue that an incandecent lamp works more like an PTC thermistor.
Higher temp = higher resistance So there is some kind of current regulation/limiting happening.
Pretty bad circuit.
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Old 7th December 2015, 08:07 PM   #5
M8e is online now M8e  Sweden
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Quote:
Originally Posted by rayma View Post
Pretty bad circuit.
Well, yes.

Might be better to use a bulb that would run closer to spec. Then it can function like a fuse. :P
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Old 7th December 2015, 08:12 PM   #6
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pretty bad i agree but i had no regulators at hand , and hoped this is safer than runing opamps on 31v where in datasheet says its limit 30v, it worked but i had bad feeling it will blow when i need it most. there is probability that this circuit will be connected to small generator ,or inverter from time to time so i wanted to be 100% sure that this will work from 220-240v ,at 235v it was 31v dc at opamp pins without bulb, with bulb 20.5v. now i was concerned that it has any other negative effect that i am not seeing now.
i hope it is safe to work on 240v mains as it is working on 20v on 235v mains. i am still testing it and it is working for almost 5 hours now and even boxed there is no noticable heating, maybe few degrees over ambient.

edit:thank you m8e i forgot to put fuse
the bulb is fully lit when capacitors are charging
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Old 7th December 2015, 08:20 PM   #7
DF96 is online now DF96  England
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A bulb in series with a power supply is a rough approximation to a constant current supply, not a constant voltage supply. In most cases this is the opposite of what you want.
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Old 8th December 2015, 09:11 AM   #8
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here is sim, but with resistor i dont know how to put bulb resistance in ltspice
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Last edited by tonitonitoni; 8th December 2015 at 09:13 AM.
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Old 8th December 2015, 10:52 AM   #9
JMFahey is online now JMFahey  Argentina
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What DF96 said, bulbs are *constant current* devices, even plain resistors are better than them.
Anyway, you were out of regulators on Saturday or Sunday and tried this ... you got lucky .... fine ....................... so far.

But today shops are open so go get the proper regulators, you have no excuse for that.

If you are dreaming about leaving that circuit as-is forever ... forget it.

Now you have an anti-regulator which will do the opposite of what you need, it will *amplify* mains voltage variations.
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Old 8th December 2015, 11:40 AM   #10
DF96 is online now DF96  England
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Quote:
Originally Posted by tonitonitoni
here is sim, but with resistor i dont know how to put bulb resistance in ltspice
Simulating a different circuit from the one you have built will not give much useful information.
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