Having issues converting 12v dc power to 5v

Using Arduino General Electronics
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Hello, I need some help working with the L7805cv and a 12v DC power source.

I am trying to build my first project using an ATTiny85 chip. This is for a 12v LED light strip that I currently have running from an Arduino.

I want to build a board with an ATTiny85 to run the program on 5v but power the 12v light strip. I got a L7805CV and trying to wire it up on the breadboard but I'm blowing up capacitors and I cant figure out what I am doing wrong. I have a feeling it is because of my power source. I purchased some (what I thought were) 12 DC power supplies and upon closer inspection, I don't see "DC" on these things anywhere.

I put a barrel jack on my breadboard and wired it up with a 220uf capacitor on the input side to the input to the L7805CV. L7805CV was grounded and then a 100uf capacitor on output side. Plugged in the 12V power supply and after a couple seconds I heard a sizzling sound and before I could unplug the power supply the 220uf capacitor popped like a firecracker!

I'm thinking I am using the wrong power supply. I see so many examples on youtube of using this L7805CV successfully and super easy that has to be it BUT I'm not sure. I'm just trying to power a 12v LED strip that probably pulls less than 1amp of power as I am only using half of it. LED strip I am using is WS2811. What power supply SHOULD I be using? Else, what the heck am I doing wrong?

Thank you in advance,
Chris

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I suspect you are right that you have an AC power supply, either that or you have the polarity the wrong way. If you don't have a multimeter then buy one, even a £20 / $20 one is way, way better than not having one.

And I suggest you use a buck converter, not a 7805, much more efficient.

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4

Are you sure that you observed the polarity of the capacitor when you connected it?

What is the measured DC output voltage? The measured AC voltage?

A L7805 is kind of wasteful. It must drop 7V and it will do so as heat. A good buck converter is much more efficient.

That said, there are times when a linear regulator is called for. Lower noise when using the ADC to measure a low voltage for example. Controlling a LED strip is not one of those times.

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Thanks Perry. Wasnt sure which area to post this. Thanks for the info as well.

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Hi Fungus,

Yes I made sure the Capacitor's polarity was correct. I did not have enough time to put the multimeter on it as it blew up in maybe 3 seconds. I suspect my 12v power adaptor is not DC. I ordered some from Amazon that I made sure were DC and lower (2amp output) need to figure out how to determine if my power supply is AC or DC with a multimeter.. I'll youtube it.. I'm a newbie... And yes I did discover how wasteful the L7805 is and ordered some buck converters as i would like to put this in a 3d printed case. Thank you for the info. I appreciate it.

Chris

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If you don't have a multimeter, connect a regular LED in series with a 0.5-1Kohm resistor, then check if it lights up when connected to the PSU in both directions.
Single direction only - DC PSU.
Both directions - AC PSU.

1 Like
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I have a multimeter and just checked my power source. 12.3V and .174amps. Then set to AC setting and got nothing or wierd numbers. The rating on the back of the power source is 12v 5amp. Is that just too much power for my circuit? Is this too much power for 220uf 25V dialetic capacitor?

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What do you mean by .174 amps? There is no meaningful measurement you can make of a power supply with a meter set to measure amps.

12.3V sounds like it is DC, so I think maybe you either had the capacitor the wrong way, capacitors rated for less than 12V, connected the power supply the wrong way or have faulty capacitors.

Circuits take the power they need, the 5A means that's the maximum the power supply can supply. Does your 10W table lamp explode when you plug it into a socket capable of supplying 3kW?

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After I checked the voltage I switched to amperage and checked thought someone asked. That is the reading that appeared. I thought that was correct (the circuit takes the power they need) but I wasn't sure if that had something to do with it or not. Thanks

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Don't do that!

Some power supplies have fold back current limiting, which means that if you short them out they reduce their output current. By measuring with the meter on A you are shorting out the power supply.

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Ok thank you.. BTW I discovered what the actual issue was with the capacitor. I had to barrel jack wired backwards!!! I intentionally put in a capacitor backwards since the first blew up. Low and behold, it works. Atleast I now know the polarity of the damn barrel jack. I looked everywhere for it and couldn't find it. Lol.. Newbie mistake

Thanks everyone.
Chris

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That is why we use the meter before we hook things up.

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The rating on the back of the power source also specifies AC or DC. :roll_eyes:

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