7805 voltage regulator question

Thread Starter

alexx

Joined May 29, 2011
29
hello!

I made a volt regulator using a standard 9V battery, a 0.22 uF mylar cap on the input, and a 0.1 uF ceramic cap on the output. This gives me an output voltage of 4.96V BUT i need it to be 5.05V because the 4.96V is not powering my device (no, the device isn't broken). my question is : can i replace the .22 uF cap with a different cap to increase the voltage a little?

thanks!
 

t_n_k

Joined Mar 6, 2009
5,455
Pardon my incredulity. Surely 90mV doesn't make that much difference. The regulated output is a function of the 7805's internal electronics - not the capacitor values.
 

Thread Starter

alexx

Joined May 29, 2011
29
thanks for the response,

I thought that too, so i ran 5.05V to the device to see if it was broken but it worked :S and then again ran 4.96V to the device (from the regulator) and it didnt work

by changing the 0.22 uF cap to a smaller cap would it make for a smaller voltage drop through the circuit resulting in a higher output voltage?
 

t_n_k

Joined Mar 6, 2009
5,455
Having aroused my curiosity you might divulge some additional info on the nature / type of device you are testing.

Sounds close to broke to me if it's a commercial electronic device.
 

SgtWookie

Joined Jul 17, 2007
22,210
A 7805 sources about 5.5mA via it's GND pin; so if you place about 16.3 Ohms between the 7805's ground pin and GND, you'll increase the output to 5.05v.

However, it's more likely that your load is heavy enough to cause your 9v battery's output voltage to drop when you try to power it.
 

t_n_k

Joined Mar 6, 2009
5,455
by changing the 0.22 uF cap to a smaller cap would it make for a smaller voltage drop through the circuit resulting in a higher output voltage?
No it wouldn't. The capacitor is a shunt component - not a series component. If a mylar cap was in series with the regulator you wouldn't get any output at all, since good capacitors block DC. You seem not to have a clear grasp on the nature of capacitors and their role in this application.

The 7805 will have a typical output variation of 4.8 to 5.2V - due primarily to manufacturing tolerances.
 

t_n_k

Joined Mar 6, 2009
5,455
Agreed, it's the most likely explanation. Depends if the OP measured the regulator output voltage with the load connected or not.
 

Thread Starter

alexx

Joined May 29, 2011
29
oh okay so changing the cap wouldnt help, thanks

so will adding the resistor work? or do i need to add a resistor and increase the voltage supply too
 

SgtWookie

Joined Jul 17, 2007
22,210
If your battery cannot source enough current to keep the regulators' input above ~7v, then the regulator output voltage will drop regardless of how much resistance you place between the ground terminal and GND.

9v batteries have very limited output capacity. When fresh, you can actually get more than an ampere of current from them; but that lasts only a few moments.
 

Thread Starter

alexx

Joined May 29, 2011
29
okay, so a resistor is probably not needed. I will try making the current source greater by changing the 9V to 18 V, and then power the device

thanks!
 

SgtWookie

Joined Jul 17, 2007
22,210
What IS the device?

If you try to use a couple of 9v batteries in series to power the 7805, then (18v-5v) * current = Watts of power wasted in the regulator, and 5v * current = Watts of power used in your device. This means that your circuit will be less than 38.5% efficient.
 
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Thread Starter

alexx

Joined May 29, 2011
29
the device is a viewfinder from an old sony video camera. and the old camera supplys the viewfinder with 5.05 V (measured with out viewfinder attached) and 0.13 A (measured in series with viewfinder)

so i am matching these values using the regulator. my regulator gave me 4.96 V, and with a 47 ohm resistor folowing the positive output of the regulator, gave me 0.11 amps. but this did not power the viewfinder
 

SgtWookie

Joined Jul 17, 2007
22,210
130mA is much too much of a load for a 9v battery to power for very long.

You would be much better off to use four AA NiCD or NiMH batteries without a regulator.
 

wayneh

Joined Sep 9, 2010
16,390
Then you should try wookie's suggestion, and perhaps test other 7805s as they might vary slightly, but enough.

Oops, nevermind, didn't see page 2
 
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Thread Starter

alexx

Joined May 29, 2011
29
Are you able to handle the viewfinder completely separated from the camera?
what do you mean handle? i still have to figure out how to connect the viewfinder, after i power it, to an alternate video camera (a much smaller one, like a pinhole camera but not as expensive). so i guess the answer would be no i dont know how to handle it/operate it seperated from the viewfinder. all i expect is a blue screen on the viewfinder right now, then i will attemp to solve the video part

another interesting note: when i measured the voltage being supplied by the sony camera to the viewfinder, with the viewfinder attached/with load, the multimeter went up to a steady 5.12 V , and current stayed 0.13 A

also: i tried adding a 22 ohm resistor after the ground pin of the 7805 like sgtwookie said, and the output voltage went up to 5.07 V. so cool! BUT a regulated 5.07 V with and 18 V power supply and a current of 0.11 A still didnt power the viewfinder :(
 
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