Maker Pro
Maker Pro

Voltage regulator voltage too high

Chumply

Nov 12, 2019
6
Joined
Nov 12, 2019
Messages
6
I’ve bought 1000+ L7805CP voltage regulators - made by STMicroelectronics. On all the data sheets it say there is a +-10% voltage tolerance yet the voltage I’m getting is +55%. It says it’s input voltage range is 35v - 7v and its output is a nominal 5v (4.5-5.5v) yet I’m getting 7.85vdc. My power source is a 28.8v battery. Is this just a faulty batch or am I overlooking something? Thanks

thanks.
 

bertus

Moderator
Nov 8, 2019
3,305
Joined
Nov 8, 2019
Messages
3,305
Hello,

Do you have the decoupling capacitors mounted on the regulators?
Without them, the regulator may oscillate and show a to high voltage.

Bertus
 

Chumply

Nov 12, 2019
6
Joined
Nov 12, 2019
Messages
6
Hi Bertus,

I am not using a decoupling capacitor. I have used the same voltage regulators before the only difference being before they were 12v output & not 5v. But I never had to add anything else to the circuit. Could you recommend a decoupling capacitor I could use? Thanks
 

Martaine2005

May 12, 2015
4,932
Joined
May 12, 2015
Messages
4,932
Hi Chumply, linear regulators like the 78xx series usually require a load in order to regulate correctly.
The datasheet will have this information. Try a 1K resistor as the load to verify whether this is the case.
Also try without the caps. Caps are usually only required if the input voltage is a small distance away. The output caps are usually small ceramics if required at all.
Read the datasheet. It will have sample circuits showing approximate cap values.

Martin
 

Chumply

Nov 12, 2019
6
Joined
Nov 12, 2019
Messages
6
Why not look up the data sheet?
Try a 100nF cap on the output and more on the input.
Thanks Duke37,
I have read the 57 page data sheet - but not being too confident in these matters I thought it best to ask. Thank you very much for your suggestion - I’ll give it a go.
 

Chumply

Nov 12, 2019
6
Joined
Nov 12, 2019
Messages
6
Hi Chumply, linear regulators like the 78xx series usually require a load in order to regulate correctly.
The datasheet will have this information. Try a 1K resistor as the load to verify whether this is the case.
Also try without the caps. Caps are usually only required if the input voltage is a small distance away. The output caps are usually small ceramics if required at all.
Read the datasheet. It will have sample circuits showing approximate cap values.

Martin
Thanks Martin,
I’ll definitely try this and let you know how I get on.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
If they were really cheap, you might not have what you think you have. I found some cheap lm337's that were not lm337's :-(
 

bertus

Moderator
Nov 8, 2019
3,305
Joined
Nov 8, 2019
Messages
3,305
Hello,

Here is the mentioned picture in the datasheet about the capacitors:

3-pin_low_power_regulators_with_capacitors.png

Bertus
 
Top