UA78M33CDCY regulator problem....

[mtolesen]

Hey out there,

I got a problem, i need 3,3 volt for a mems microphone test, so got some small SMD version (SOT223) UA78M33CDCY voltage regulators.
They are connected the standard way, 0,1 nF to ground on output, and 0,33 nF on DC input to ground, and i also had a 560 ohm resistor on output to ground to ensure some current draw.

Here is the problem, it regulates fine to 3,3 volt at input supply voltage betveen 5 to 11 volt, but above 12 volt in, it suddenly raise output voltage dramatically, 5, 6,7 volt, and then often burn off, sending input voltage directly to output.......
As i can read from datasheet, these regulators shoul be able to take Vin from 5 to 25 volt........so why are mine crashing above only 12 volt?

 

[JonSnell Electronic]

Have you a large land for a heat sink? No heat sink and the current is very limited.

 

[mtolesen]

Well, i draw 5 mA by means of the 560 ohm resistor on the 3,3 volt regulated output, so the regulator do not even gets warm.
I also tried without the 560 ohm load, and the same happen, when the input voltage exceed 11-12 volt, the output start to go much above 3,3 volt.....and burn off.

 

[JonSnell Electronic]

What was the source of the regulators?
There are a lot of rejects on fleabay.

 

[mtolesen]

You know what, i did buy them on Aliexpress......i just did not think that semiconducters could have such fundamental flaws.....even one bought on Aliexpress.......do you think they are faulty, all of them?

 

[mtolesen]

Now i tested another of mine ua78m33, and what i can see is that it is pretty stable 3,3 to 3,6 at input voltage from 5 to 13 volt dc in, and then at 14 volt input, output jump from 3,6 to 7 volt!........i simply cannot figure out whet i wrong......

 

[nigelwright7557]

I have had troubles with 3v3 regulators.
The low drop out ones only have max 5 volts in.
I also found I needed 47uf on output or I got massive noise on output.

The higher input voltage ones should be fine, I haven't had any problems with them.

Make sure you got the right type.

 

[mtolesen]

Hi nigelwright,
Thanks for the input.
Am i right the these 3,3 volt regulators, according to datasheet, should be able input voltage up to around 20 volt?
I have looked on the AC noise on the output, i did not see a lot.
Again, when i reach 14 volt in.......the output jump from 3,6 volt to 7volt, what de you think is wrong?

 

[jan.didden]

Have you a large land for a heat sink? No heat sink and the current is very limited.

That would drop the output, not raise it. I looked at the data sheet, and the max Vin is 35V. However, IF they are fake, that spec may be a lot lower.

One thing to check is whether it oscillates, 0.1u at the output is really too low. Oscillating output can look like a large DC output when measured with a regular DC voltmeter.

Try a regular 10uF at the output, at least then you know it is stable.

Jan

 

[nigelwright7557]

Hi nigelwright,
Thanks for the input.
Am i right the these 3,3 volt regulators, according to datasheet, should be able input voltage up to around 20 volt?
I have looked on the AC noise on the output, i did not see a lot.
Again, when i reach 14 volt in.......the output jump from 3,6 volt to 7volt, what de you think is wrong?

You need to look up the spec for your regulator.
3v3 regulators vary a lot.
From 5 volt input low drop outs to standard higher input voltage.

I would recommend buying them from a reputable dealer like RS Components or Farnell.
There is so much ebay and Chinese fakery around now.

Your UA78M33CDCY does have up to 25 volts input.
So could be wrong decoupling caps making it unstable.
Fake device ?

 

[mtolesen]

Hi Jan,
I checked the output with my Rigol 1054 scope....no crazy oscilation, it just jump from 3,6 volt to 7 volt when i raise the input dc from 12 volt to 14 volt......idid try to connect a 10 uF on the output.....still the strange jump.

 

[nigelwright7557]

I would be inclined to build up a simple circuit on prototyping board and check it again.
It will rule out other circuit components.
If it works on its own then its the other part of the circuit.
If it doesn't work on its own the probably fake regulators.

 

[jan.didden]

Hi Jan,
I checked the output with my Rigol 1054 scope....no crazy oscilation, it just jump from 3,6 volt to 7 volt when i raise the input dc from 12 volt to 14 volt......idid try to connect a 10 uF on the output.....still the strange jump.

The thing is that this behaviour is to be expected when you exceed the allowed maximum input voltage (or rather the max input-output difference). But the data sheet I looked at put that max at 35V.
I know there are differences between manufacturers in this spec. Do you know the manufacturer of your chip? Then we could verify the data sheet on the max Vin spec.

Then again, you could use it with no more than 10V input and presumably all would be well.

Jan

 

[Mark Tillotson]

Stick a 7808 in front of it perhaps?