Greetings,
I recently picked up a few 5v regulators. (LM7805C) I have x2 18650s in parallel as a battery pack with a LM7805C hooked up. The pack reads 7.8v. But the regulator is reading 6.8v.
I've tried two of them and get the same results. I hooked up the In and Out pins according to the packaging. I noticed the min voltage for the regulator is 7.5v, so will I have to get x3 batteries to get the voltage output down to 5v?
Cheers!
Yes, they need a high input voltage. That is why often LDO (Low Drop Output voltage regulator) are used.
They also need a capacitor at input and at output.
I don't know about the 18650s but normal 3.7V Li-ion cell can be 3 to 4.2V.
You sure it's hooked up right? With insufficient supply voltage, the output voltage should be lower than expected, not higher.
With a 7805, you do need three batteries. But with a modern LDO regulator, two is okay. Note that battery life of 7805 with 3 batteries and LDO with 2 will be the same 
18650 is LiPo.
Don't forget to connect the ground on the regulator.
Peter_n:
Yes, they need a high input voltage. That is why often LDO (Low Drop Output voltage regulator) are used.
They also need a capacitor at input and at output.I don't know about the 18650s but normal 3.7V Li-ion cell can be 3 to 4.2V.
I meant in series, derp moment. Not parallel. Working atm to test on a psu 12v rail to just eliminate battery issues from the topic.
DrAzzy:
You sure it's hooked up right? With insufficient supply voltage, the output voltage should be lower than expected, not higher.With a 7805, you do need three batteries. But with a modern LDO regulator, two is okay. Note that battery life of 7805 with 3 batteries and LDO with 2 will be the same
18650 is LiPo.
Thanks for the tip on the battery life. That's interesting.
Wiring is the same as:
MorganS:
Don't forget to connect the ground on the regulator.
I didn't have the back ground connected to the negative of the batteries. I did, and it didn't change.
Perhaps it will when I hook up an adequate power source. Report back soon.
A regulator should never give too large a voltage at the output.
You have followed the guidelines in the datasheet about input and output
capacitors? They are not optional.
Update,
With a 10uf 35v capacitor and with 12v. I get 11v out.
Switching around the in and out, I get 11.5v.
Caps may not entirely be necessary (I generally use on on the regulator output). The regulator ground pin is actually a voltage reference pin, and if it does not have a good connection to ground the output may read battery voltage minus the voltage drop across the regulator.
zoomkat:
Caps may not entirely be necessary (I generally use on on the regulator output). The regulator ground pin is actually a voltage reference pin, and if it does not have a good connection to ground the output may read battery voltage minus the voltage drop across the regulator.
I soldered a wire from the back of the regulator to the middle pin. And currently there are two 10uf 35v caps on the in and out. I just don't think things like this would give me a 1 volt difference. Seems rather ridiculous.
Seems rather ridiculous.
Electronics generally work the way they are wired. Most likely you have some type of bad wiring. Have you checked your wiring with a multimeter?
zoomkat:
Electronics generally work the way they are wired. Most likely you have some type of bad wiring. Have you checked your wiring with a multimeter?
No I tested the voltage with my tongue.
The large metal on the top and the backside is for a heat sink. You don't have to connect it to something.
Is it a 7805C ? Perhaps it is a 7905C or it is broken. Perhaps you have a weird clone of 7805 with different pin numers. Can you make a close-up photo of it and show it to us ?
The tab on the regulator and the middle pin are part of the same solid leadframe on which the IC die is mounted so there is no reason to connect them together - you may in fact if it is convenient, use one to connect some of the attached components, and the other for the remainder of the attached components.
On positive regulators, this is the ground or reference pin, while on negative regulators, it is the input pin as the die which mounts there is (almost) always the most negative point.
dritchie:
No I tested the voltage with my tongue.
Whether or not that comment was serious, we are going nowhere without a perfectly focused photograph clearly demonstrating the whole setup.
On positive regulators, this is the ground or reference pin, while on negative regulators, it is the input pin as the die which mounts there is (almost) always the most negative point.
Always check the datasheet. 7805 has tab/gnd. 1117 regulators like the Arduino's use do not.
CrossRoads:
Always check the datasheet. 7805 has tab/gnd. 1117 regulators like the Arduinos use do not.
Interesting design. The tab is - as always - the substrate and generally the negative point, but the "reference" connection is a special part of the die, especially isolated (presumably, by a "moat").