7805 VR output is 6.24v ! !
LittleTyke
Posts: 34
I've just finished building a small 5v power supply for use with my additional circuit board to go with the BS2 super carrier board. I got a cheapo 7805 from Maplins (CH35 'value' brand). Completed the circuit (Instructables 5V breadboard mini PSU), connected a 9v battery and measured the output voltage. 6.24 volts!! Aarrrgggh! I checked the tolerance and it is +/- 4% so 6.24 is completely off the wall. I checked the 7805 with a magnifying glass and it is definitely a 7805.
So I rummaged everywhere and found an old circuit (20 years old at least) and unsoldered its 7805. Whacked it into the new circuit. Bingo! 5 volts on the button.
I am gobsmacked that a 7805, despite being a cheapo brand, can be so faulty. Is this a frequent occurrence? I've only used about four 7805s in my hobby lifetime and this is the first time I've had a problem. Quite reassuring, however, that I had a used replacement handy, otherwise I'd have had to wait until Monday thinking I'd cocked up the circuit in some way.
So I rummaged everywhere and found an old circuit (20 years old at least) and unsoldered its 7805. Whacked it into the new circuit. Bingo! 5 volts on the button.
I am gobsmacked that a 7805, despite being a cheapo brand, can be so faulty. Is this a frequent occurrence? I've only used about four 7805s in my hobby lifetime and this is the first time I've had a problem. Quite reassuring, however, that I had a used replacement handy, otherwise I'd have had to wait until Monday thinking I'd cocked up the circuit in some way.
Comments
-Phil
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com
After I replaced the device with the old one and got the circuit to work as expected (i.e. 5v), I thought, why not stick the new, assumed duff, one temporarly in the old board and see what output voltage it produces. Well, in the old board it produced 7.31 volts!
NB: Before removing the old 7805 I first connected a battery to the circuit it was in and checked that it was working. It was. Voltage in the OLD circuit was 5v.
"What kind of filter cap are you using on the 5V side?
The circuit uses a 10uF and a 100nF on the 5v side. (See http://www.instructables.com/id/SR38OV5F6S8RU65/ and scroll down a bit.)
"In addition to the output filter cap, were you testing with a load of any sort, or was it open circuit? It should regulate at 5 volts, but maybe "cheap" means a little bit leaky. Maybe it would work better with a 5.1 kO resistor (1 mA) minimum load across the output."
There is an LED on the 5v side wired in with a 460 Ohm resistor. Other than that, the output rails were unconnected when I applied the voltmeter. However, note that the *replacement* 7805 (from that old board) delivers 5v on the nail. That is is only component I replaced. Same test conditions exactly. PSU Circuit with known good (but old) 7805: 5v. Circuit with NEW 7805: 6.24v in the one circuit; 7.31v in the other.
Maybe these Maplin jobs are gray-market culls or something.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
· -- Carl, nn5i@arrl.net
I wonder if the Input and Output pins were swapped. This would not be any fault of yours, but some manufacturers of regulators like to swap these pins sometimes.
That said, I just did a test with a 7805 regulator using a 12V supply (measured at 12.8V) ... reversing the Input and the Output produced an even 11V, ... a 1.8V drop.
If you are powering the regulator with a 9V battery, a 1.8V drop would give you something closer to what you were observing... even closer if the 9V was not 100%.
Just food for thought.
BTW) Reversing the Input and Output pins only caused the regulator to get slightly warm... by correcting the wiring the regulator seems to have suffered no ill effects.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe
IC Layout Engineer
Parallax, Inc.
I have irrefutable proof that this is so and a search of the internet will confirm this practice. I have had "brand name" parts which look very similar to the real stuff but there are subtle differences. There was even a 5-pin switching regulator that I checked out for someone else that turned out to be a different kind of regulator from what it was branded. There was nothing wrong with the part if you hooked it up as it's more common cousin but would not (could not) work as the part that it was branded as. Subtle differences in packaging, logos, and part numbering are the only external clues.
The same story goes for ICs too but more often than not the parts are counterfeit and substandard or rejects that are rebranded to suit demand. If a customer/distributor is chasing after cheap regulators then it would seem a shame not to supply them with what they want when all you have to do is have a bunch of surplus/reject chips your happen to have on hand rebranded. This is a big big worry for the "brand name" manufacturer as they eventually cop the abuse only to find it's not theirs. It's criminal but these anonymous counterfeiters get away with it because they live in countries where certain laws may be non-existent or difficult to enforce.
Caveat preemptor
*Peter*