Electronics > Beginners

LM7805 unstable

<< < (5/8) > >>

ebastler:
TME is a „proper“ distributor; so the regulators should be genuine. I think the tendency on this forum to conclude „counterfeit!“ is a bit too pronounced...

bd139:
Yes. Even the counterfeit ones tend to work properly as well.

It's that huge dropout voltage I reckon. All the designs are marginal above 20V. Says so on every vendors' datasheets, including Linear who product exceptional stuff.

tsman:

--- Quote from: ebastler on February 18, 2019, 11:39:02 am ---Hmm... How do you know that? Can't recognize anything obvious that seems suspicious  (beyond the low price, of course) in the Amazon description?

--- End quote ---
The low price nearly guarantees that it'll have a counterfeit LM2596 chip. It has been tested by multiple people online so just search for "LM2596 counterfeit". If you put a scope on the output then you'll see the switching frequency is far lower than the 150KHz for a real LM2596.

The modules do work but you shouldn't draw anything approaching the advertised maximum current and you must provide good cooling. Whatever chip they're actually using doesn't have thermal overload protection and will fail with a input to output short.

mariush:
Some datasheets don't say a minimum load is required, but you basically satisfy that condition through the current used to set the output voltage.

In case of an adjustable regulator like LM317, datasheets recommend a 120 ohm or 240 ohm resistor and a higher value resistor to set the output voltage ... using the simplified formula Vout = Reference Voltage (1.25v typ.) x ( 1 + R2/R1)  where  R1 is usually 120 ohm or 240 ohm.

So you get a few mA of load just by setting the output voltage.

Often I see people just connecting a red led with a 1-2k resistor in series on the output, so you basically get 3-5mA of current consumed by the led which also functions as a convenient "power on" indicator.

If you know the average current consumed by your circuit, you could put a resistor before the regulator to drop some voltage ... for example, if your circuit will use maximum 100mA, then use V = I x R ... so if you use a 150 ohm resistor, you have a voltage drop of V = 0.1A x 150 = 15v  and the power dissipated on the resistor is P = IxIxR = 0.1x0.1x150 = 1.5w , so you'll probably have to use a 3w or 5w resistor.

Jwillis:
Sometimes data sheets can be a little unclear .This is the one I use and I have never destroyed a 78xx/79xx with this information.It clearly states the maximum input voltage for each rated regulator on page 3 .https://www.sparkfun.com/datasheets/Components/LM7805.pdf. I find the Texas Instrument Data sheets to be the clearest to understand.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod