Muerto
Well-known member
I have changed a componet (a 7905C -5V) with a 7905CV -5V, but the thing is now it´s reading -12V...??? - can the game vork with that, or does it has to be -5V (+-1V or 2V??)
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Could this be a Bob Roberts "Chicken Little" issue? I was not happy to read Bob's page about this because I have always depended on the voltages to measure at their expected values even without a load. Now I have to add a load of the right amount (I am no EE, so not trivial for me) or risk that the voltage is really bad and damage PCBs when I plug them in!
http://arcadecontrols.com/BBBB/7905.html
7905 should regulate relatively close even without a load. Bob is wrong.
I would have agreed with this....until I got a couple from Bob. I don't know how they do it....but they don't regulate worth a damn without a load. Put a load on them....and they (seem) to be fine. I don't get it.
Edward
I would definitely check to make sure you don't have a bad ground, Are you measuring the voltages straight from the 7905? Or test points?
The 7905's Bob sells must be garbage. I've never seen a regulator need a load to stay within a couple hundred millivolts.
OK - this has my curiosity peaked.
Does anybody know the brand of these bad 7905's?
Any manufacturer markings on them? ST?
Ed
I still have one of Bob's, and it is marked as follows:
f1 J29
LM7905C
7905 should regulate relatively close even without a load. Bob is wrong.
Looking at datasheets - all of these linear regulators DO have a 5mA minimum load current. If there is no load on the output - the output is unregulated and often undefined. Many DMM's can read a real high reading as they present nearly no load to the power supply. Some analog meters are more of a load and will read closer to normal value.
These regulators need about 5mA to attain proper regulation. For the most part, a simple 10mA LED indicator would have done the trick. If you are testing a power supply with no load then it would be advised to put a small load resistor on the output. A 470ohm resistor *should* do the trick.
Ed
Yet another reason why I trust my "old school" Simpson 260 over any of my DMM's.
It would seem that the 260 would put enough of a load on the regulator to get a proper reading no?