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"Eh, I just had it laying around" How to know if it's okay?

c131frdave

Oct 4, 2013
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I need a .33uf Capacitor for the attached regulator circuit. For whatever reason, Radio Shack in their infinite wisdom will sell you the regulator, but they don't have any .33uf capacitors...

I have asked other questions on these forums, and often I get "well, if you have an xxxuf to around xxxuf capacitor, use that. How do I know what is acceptable for substitution? I have a .22uf ceramic cap here, but no .33uf electrolytic. Is that good enough? I need 5 steady volts because it is powering an IC that will blow up if the voltage goes over 5.5V (and really doesn't like anything over 5V). Should I just bite the bullet, order the correct part and wait?

Thanks.
 

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BobK

Jan 5, 2010
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If you had to substitute, a larger cap would be better there. The .22u will probably work fine though. If you have 2 of them you can parallel them. All it is doing is providing current quickly if the demand changes.

Bob
 

c131frdave

Oct 4, 2013
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So bigger is better? I have about 20 100uf electrolytics laying around. That's 300X bigger than what the schematic calls out, but that would make plenty of extra current if needed... Is there a rule of thumb or anything?
 

BobK

Jan 5, 2010
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What kind of power supply is feeding it? If it is a battery and it is close to the regulator, you could get by with no cap. A 100uF in parallel with a .22uF would probably be good.

Bob
 

c131frdave

Oct 4, 2013
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Car battery/alternator. So it will fluctuate between 12V and 14V. Battery is long way away, about 10 feet I would guess. I found a 1uf titanium cap I can use. It doesn't have a voltage rating on it.

Thanks for your help. I really appreciate it.
 

BobK

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If you use this with the car running, you need a lot more circuitry to protect from voltage spikes that exist in an automotive system. You may end up blowing out the regulator with this simple circuit.

Bob
 

shrtrnd

Jan 15, 2010
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That's probably a 'tantalum' cap, not titanium. They're usually epoxy dipped solid tantalum and they don't like voltage spikes like you'd find in a car alternator circuit.
 

c131frdave

Oct 4, 2013
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Lol. Yes, you're right. Brain voltage spike, I think.

Well, I'm searching for a circuit to protect my voltage regulator. Does something like this make sense? http://homemadecircuitsandschematics.blogspot.com/2012/05/make-this-voltage-stabilizer-circuit.html

I have power going two directions. First, 12V from the car goes to a 12V dc motor driver, and then on to two brushed motors. The motors could draw up to 2.5 amps a piece, or 5 amps total (the driver has a 5 amp cut off). The second direction is 12V from the car going to my 5V regulator, which will power an ATTiny85 and tilt sensor (very little power required). It's going on my truck and lifts or lowers the back gate.

Any help is appreciated. Thanks.
 

BobK

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That circuit is to regulate the voltage for the entire car. It is not what you want.

What you probably need is a series diode + resistor before the regulator, a zener diode from there to ground, and a large capacitor. But I am not an expert on this. There are others on this forum who know this much better than I do.

Bob
 

davenn

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Sep 5, 2009
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yes, Bob is correct, that circuit doesn't have any protection on its input

in this link, look at post #4 from KrisBlueNZ

https://www.electronicspoint.com/threads/voltage-regulator-or-help-the-noob.268688/

He has a good input protected PSU for running off a car battery
Use the same LM317 regulator and set the adjustment for what ever voltage you want out
You may have to take the trimpot resistor up to a 2k2 instead of 1k

cheers
Dave
 

BobK

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Or simply replace the LM317 part of the circuit with your 7805 circuit.

Bob
 
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