September 27, 2015 at 3:56 pm #40367
I am trying to use Ray’s AC/DC design based on LM2596 which is a very popular switching PS in my project, and I need some help, please, here is the site I am using:
I am using 24Ac for power source, I measured the input to LM2596 is around 37 volt DC with no load, which is expected , but when I measured the output volt is about 1.8, it suppose to be 5 volt after 5.6 Zener diode. I have tried at lease 3 different LM2596 and the result were the same! Any hint or clue in this matter would be great.
ThanksSeptember 28, 2015 at 9:20 am #40380
Check if your LM2596 is a 5V fixed output voltage version (LM2596-5.0) — if you got one with adjustable output voltage, then you need two resistors to set the output voltage. The datasheet has more details:
http://www.ti.com/lit/ds/symlink/lm2596.pdfSeptember 28, 2015 at 9:37 am #40382
Thanks for reply, my first post was not totally accurate, because I did use your exact circuitry in OSPi circuit, like the attached file. So LM2596 is adjustable version, and I am using a combination of a Zener diode and 10 ohm resistor just like your design, but output volt on pin2 is 1.8V. so either LM2596 is bad, or some other component. I am ordering some more LM2596 from DIGIKEY to try them out. The first 10 that I bought was from Ebay very cheap, may be they were all bad. I would appreciate any troubleshooting guide.
Attachments:September 30, 2015 at 8:16 am #40399
Could you please explain why you choose a Zerner diode and resistor combination for adjustable LM2596, instead of a fixed 5 volt output LM2596, or even using 2 resistor as a voltage divider for output of adjustable LM2596, like it mentioned in TI data-sheet. I am still having problem with my circuit, seeing 1.8 volt only, I ordered anew part from Digikey to try. I appreciate any clue to solve my issue,
ThanksSeptember 30, 2015 at 8:53 am #40400
The zener and resistor is for clamping and will cause the fuse to blow if something goes wrong and output rises to more than 5v. Your design should use a fixed regulator as Ray stated, the design for a variable one is different, check the datasheet again.
IngoSeptember 30, 2015 at 9:09 am #40401
If all we need a 5 volt for RPI and other components, so why he used adjustable LM2956, I can see a benefit of adjustable in this DC/Dc supply:
which has the flexibility of output voltage, as far as fuse we could still have the same fuse on fixed 5 volt output!
P.S. I read the TI data sheet more than onceSeptember 30, 2015 at 9:23 am #40402
Ray asked if you had a fixed or variable one, if you do have a variable one then you need additional components to make it work. The diagram you attached is for a fixed regulator.September 30, 2015 at 9:37 am #40403
Thanks for clarification, I did not pay attention to his OSPi power section design, and you are absolutely correct, it is a fix 5 volt LM2596, and that also explains my issue as well, output voltage being wrong as he also explained it to me in his previous email, my bad, and thank you so much for solving my issue.September 30, 2015 at 11:41 pm #40411
Yes, as Ingo said: the OSPi schematic assumes it’s LM2596-5.0, which outputs a fixed 5V. Sorry that the schematic caused confusion — but it’s good for you to know that most switching regulators come with either fixed voltage version, or adjustable voltage version. If you use adjustable voltage version, you need to supply two resistors which sets the output voltage.
The zener is used for additional protection — if someone puts a high voltage directly on the 5V line, that could damage the circuit. It’s optional but just there for additional protection.October 1, 2015 at 8:32 am #40418
I am totally clear now, and thanks so much for your help. Great web site and very informative forum.
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