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7805 can't take it? No, it couldn't! *SOLVED*

I've got a 7805CT regulator running off a 11.1V lipo (yes, I know it's a big voltage difference). The 7805 is powering a PICAXE, two Maxbotix EZ4 sonars, the logic voltage for a L293N, and it's giving high voltage to the '293s enable lines. When only the PICAXE and EZ4s are plugged in I get 3.3V. When the '293 is plugged in I get just over 2V and the EZ4 readings cut out.

This doesn't seem right. The regulator doesn't even get warm. Shouldn't I be able to squeeze around an amp out of this thing? I'm puzzled.

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The suggestion to swap out the L293 got me thinking. Although the L293 is brand-new the voltage regulator was far from it. Just for kicks I grabbed another junked PCB with a 7805 on it from work today. After some "flood-and-yank," a little cleanup, and a nice heatsink (from the junkpile too lol) I plugged it in and it works like a champ. I get 4.9V at the VLS pin on the '293 and the sonars return happy stable numbers. I guess I finally found a side effect of sourcing components from the junkpile... YMMV.

Thanks to all for the help anyway!

Glad to hear you got it figured out.

When in dought.....  Replace everything....

A thought.... Have you tried pushing a different 293N into the circuit?  Could the one you have be cooked.

You've wired something up wrong.

Have you included capacitors on the input and output pins of the regulator and ground? Add at least 100uF to both the input and output. The regulator can become unstable and oscillate without these caps.

Because of the voltage difference I would put a heatsink on the regulator.

I've got the prescribed 0.33uF cap plus a 150uF cap on the regulator's input and the prescribed 104 cap plus a 470uF cap on the output. FWIW when I was testing just the PICAXE and the EZ4s it took a 100uF cap on the output to keep meaningful sonar readings. Going with the 100% true "bigger is better" rule (hehe) I threw a 470uF at it, hoping it would help. Alas...

Also, I have one of those clip-on heat sinks strapped to the 7805 too but it doesn't get warmer than ambient temperatures, even without it. A preparation for the predicted worst is all it is at this point.

I wouldn't be surprised if it was a simple miswiring issue, OB. I'll have to get some visuals for you guys. As I mentioned in my reply to Dan, The Sandman is calling and I feel powerless... ahh....  :snort: huh?... zzzZZzzzzZZZzzz....

Check to see if the 7805 is getting warm, but the heatsink is not. Did you use a heatsink compound to ensure good thermal bonding?

This makes me wonder, what does the 11.1 volt reading do?  Does it stay up or is it being drawn down as well?

Yes, you should be able to get 1 amp easy. (I think it is 1 amp max continuous, but up to 2.5 amps intermittant peak)

What does the circuit look like? --how is it wired?  You should have an output of 5.0 volts plus or minus 0.2 volts

 

 

It stays up; 11.8V to be precise. I will have to wait till tomorrow to get you guys some better info. Tomorrow approaches and my pillow beckons... Love the kimono in your new avatar too, btw \o/

Thanks. It is my favorite one.

Ok, maybe you can give us a sketch of the schematic.  I agree with Oddbot that it sounds like something has to be wrong in the wiring. If the circuit were drawing down the voltage, then the regulator would be burning hot. Perhaps it is something like a connection that looks good, but is not giving a good bond. Ok, rest well. Will check this page tomorrow.