Author Topic: 7805 voltage regulator only giving 3v when L293D connected  (Read 710 times)

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Offline DennisVRTopic starter

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7805 voltage regulator only giving 3v when L293D connected
« on: August 15, 2013, 03:26:20 PM »
This boggles me, which isn't hard since i'm new to electronics, but i would like to understand what's going on.

I have a 7805 voltage regulator connected to a 12v battery(Pin1:12v,Pin2:GND,Pin3:5V). It outputs 5v perfectly but when i feed the 5v to  pin 16 of a L293D(and Pin13 to GND) the voltage drops to around 3v.  Adding a 10uF capacitor(5V side of 7805) seems to solve the issue but i'm not sure i understand why.

Can anyone give me some insight ?

Offline waltr

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Re: 7805 voltage regulator only giving 3v when L293D connected
« Reply #1 on: August 15, 2013, 06:51:29 PM »
Sounds like the regulator is oscillation when connected to the L293.

What caps do you have at the input and output pins of the 7805?
Are they the values recommended in the data sheet?

Offline jwatte

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Re: 7805 voltage regulator only giving 3v when L293D connected
« Reply #2 on: August 15, 2013, 11:06:47 PM »
Regulators are rated for a maximum current load. Different regulators behave differently when this maximum is reached. Some will shut off completely; others will lower the voltage until the current is within spec; yet others will gladly burn themselves out.

Separately, regulators generally need stabilizing capacitors on the input and output pins. If there are not capacitors there, certain circuit conditions will cause the regulator to oscillate (as it can be a positive-gain system.)

The example diagrams in the 7805 data sheet only show a 0.1 uF capacitor on the output and 0.33 uF capacitor on the input, so that's probably a good minimum. If you have bigger caps layout around, and can use them instead, then that'll just be better (as long as the caps are rated for the voltage, temperature, and ripple current they will see during load.)

https://www.sparkfun.com/datasheets/Components/LM7805.pdf

Offline DennisVRTopic starter

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Re: 7805 voltage regulator only giving 3v when L293D connected
« Reply #3 on: August 16, 2013, 02:28:58 AM »
Waltr and Jwatte, Thanks again!

Where's the "Helpful"-button ?

Should i invest into an oscilloscope ? This being just a hobby ofcourse.

Offline jwatte

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Re: 7805 voltage regulator only giving 3v when L293D connected
« Reply #4 on: August 16, 2013, 11:12:02 AM »
If you are going to be doing a fair bit of electronics (for a hobby,) then an oscilloscope is very helpful.
I find a stand-alone to be a lot more useful than a computer-based one, because the computer is usually busy with other things... Here's the one I ended up getting -- very basic, but gets all the job done that I need:
http://www.amazon.com/gp/product/B007T6XNCA/ref=as_li_ss_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B007T6XNCA&linkCode=as2&tag=enchage-20

Another thing you will want if you start integrating a lot of modules (sensors, controllers, etc) is a logic analyzer. Here, you pretty much have to go with a computer-based one, because the stand-alone ones are too expensive and not flexible enough for protocol analysis:
http://www.amazon.com/gp/product/B004G4ZKA6/ref=as_li_ss_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B004G4ZKA6&linkCode=as2&tag=enchage-20
(Note that this Saleae is just barely fast enough for Arduino-based things -- I wish there was something faster that had as good a UI and wasn't significantly higher cost)

Offline DennisVRTopic starter

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Re: 7805 voltage regulator only giving 3v when L293D connected
« Reply #5 on: August 16, 2013, 02:17:15 PM »
Don't shoot me but i just ordered a Uni-T UT81b  :-[  I needed an upgrade for my crappy $20 multimeter anyway.  The oscilliscope features are limited but probably enough for what i'm doing right now.

Offline waltr

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Re: 7805 voltage regulator only giving 3v when L293D connected
« Reply #6 on: August 16, 2013, 07:05:31 PM »
Nice. That should really come in handy.

Offline jwatte

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Re: 7805 voltage regulator only giving 3v when L293D connected
« Reply #7 on: August 16, 2013, 08:44:24 PM »
Don't shoot me but i just ordered a Uni-T UT81b  :-[

I bet it'll be a lot better than no scope at all :-)

 


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