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The output voltage of my L7805cv is only 4.76v with a load resistor of 100ohm using 45mA. The input to the regulator is about 11v. I use a 100ohm resistor from the ground terminal to the negative rail to raise the output to 5v. Why isn't the output 5v as designed with the ground terminal connected directly to negative or ground?
L7805CV output only 4.76v
- Thread starter Arjune
- Start date
Assuming the 7805 is good in the first place and wired correctly:
The ground terminal is designed to be at ground. Adding a resistor between GND on the device and ground will indeed raise the output voltage a bit but regulation will be affected. You shouldn't need something like that.
4.76V out is within the 4.75V specified minimum output voltage for the -CV device so maybe it's just a lame one. See pp 14/54 of the datasheet. Try a tighter spec'd 'ACV' part.
Other things to consider:
Do you have the capacitors installed as called out in the datasheet?
Is the input supply well filtered or is your 11Vin an average of a heavy rippled supply? That could cause the output to have some dropout ripple which would show up as a lower voltage on a DMM.
Is it getting hot? A TO220 should run OK without a heatsink for that low current but these have thermal shutdown that will reduce the output voltage to keep the junction temperature within spec (about 17 degC above ambient in this case according to the 'Lil Professor)
Did you get the 7805 from a franchised distributor or some junk house like Amazon or eBay? These are commodity parts and there are a lot of counterfeits out there. You could also have gotten a reject that found its way back into the stream.
That's all I can think of. Properly used these are pretty goof-proof. And yes, a schematic and photo of your hookup would help.
Good luck!
The ground terminal is designed to be at ground. Adding a resistor between GND on the device and ground will indeed raise the output voltage a bit but regulation will be affected. You shouldn't need something like that.
4.76V out is within the 4.75V specified minimum output voltage for the -CV device so maybe it's just a lame one. See pp 14/54 of the datasheet. Try a tighter spec'd 'ACV' part.
Other things to consider:
Do you have the capacitors installed as called out in the datasheet?
Is the input supply well filtered or is your 11Vin an average of a heavy rippled supply? That could cause the output to have some dropout ripple which would show up as a lower voltage on a DMM.
Is it getting hot? A TO220 should run OK without a heatsink for that low current but these have thermal shutdown that will reduce the output voltage to keep the junction temperature within spec (about 17 degC above ambient in this case according to the 'Lil Professor)
Did you get the 7805 from a franchised distributor or some junk house like Amazon or eBay? These are commodity parts and there are a lot of counterfeits out there. You could also have gotten a reject that found its way back into the stream.
That's all I can think of. Properly used these are pretty goof-proof. And yes, a schematic and photo of your hookup would help.
Good luck!
Last edited:
DickCappels
- Joined Aug 21, 2008
- 10,661
They make higher precision versions, but if this is only to power 5 volt logic higher precision will ot be of any benefit.
That is within the spec (4.65V-5.35V) given in the datasheet.
I guess you all are not open to the beliefs of a schizophrenic because my poll was deleted. I don't think they would make a 5 volt regulator to deliver 4.75 volts. Preexisting conditions are changing as facts change. I should have had 5 volts exactly because that is my understanding. I don't think eBay would sell me a bad regulator-yes I bought them on eBay. The input to the regulator is low ripple for a 45milliA load (100ohm is at output)
. It wasn't too hot. Don't be too certain about the reliability of reality when you make judgements.
. It wasn't too hot. Don't be too certain about the reliability of reality when you make judgements.
Output voltage range typically quoted at 4.8V to 5.2V according to the datasheets I have read as already mentioned.
You don't have the required input and output filter capacitors as specified by the datasheets as already mentioned.
Is your voltage measuring device accurate enough? What is your measuring device?
Why do you need exactly 5.00V?
You don't have the required input and output filter capacitors as specified by the datasheets as already mentioned.
Is your voltage measuring device accurate enough? What is your measuring device?
Why do you need exactly 5.00V?
I haven't done the math but that input capacitor feels a bit small for a 100Ω load on the output. If you never have any larger load it might be OK but in my opinion it wouldn't hurt to double it. That 33µF on the output is doing no good, you could leave it or remove it and it wouldn't matter. A small 0.1µF ceramic would be useful there to squelch oscillation whether you have the electrolytic cap there or not.
There are tons of counterfeit parts on eBay. I'm not saying you got one, but it needs to be in your mind when you buy there. Personally I think it's worth the risk for non-critical parts that are fairly easy to repair or test. For something "fancy", I'd stick to a reputable supplier.
5V is the aim, but inevitable manufacturing tolerances mean that there is an acceptable range about that value.
General purpose 74xx TTL is fully specified to run between 4.75V and 5.5V so you shouldn't have any problems with a supply voltage that is a bit on the low side as long as it is within spec.
http://www.ti.com/lit/gpn/sn7400
http://www.ti.com/lit/pdf/szza036
If you really just want to mess with 5V TTL, I'd recommend a cheap switching wall wart like one of these from Mouser. No muss, no fuss.
5V 2A $7.80 qty 1
Good luck!
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