Circuit still sends voltage surge through when plugging into Power Supply

[Falcon69]

So, I went with this circuit, as suggested per data sheet for the LM2596s-ADJ chip.

as you can see from the circuit, I added the enable pin circuit which was suppose to be a slow on start that I thought would prevent all my LED's in the circuit from turning on for a split second when the circuit board was plugged in.

FYI, once the initial plug in (and that voltage surge), is done, the entire circuit works as it should!  SO I know this is a plug-in problem.

Would changing C3 to a higher value solve this problem?

Thanks in advance,
Jason

[Seekonk]

Don't you mean C4?  .1uF seems like a very short period.

[Falcon69]

okay

I think I have a 1.0uF I could try. Or would I need to go MORE than that?

Oh, I think the 1.0uF i have is only 16volts.  I would probably need at least a 35volt one, just in case the input voltage is 24volts?

I also have a 4.7uF, it's an 0805, but I think I can still make it work on the 0603 pad on the board.  Just for testing anyway.

[Falcon69]

Well, I tried the 1.0uF  I had, It didn't change anything.

So, Before I place another order to buy components, what are your suggestions, anybody, as to what size I would need to change the C4 to?

I was reading on the data sheet, that changing C4 to be careful, because a too high value will cause the LM2596 to turn on and off at random.

[Seekonk]

For a longer delay I would add a transistor or a FET.  That's getting complicated and I'm not sure I see a need for a delay anyway.

[Falcon69]

There probably is not a need. I was just concerned with the initial plugging in of the circuit board.  For some reason, a spike happens and flashes all the LED's on for a split second. (even ones that should not be on, which of all are controlled via the logic gates...LED's current sinks or sources through the logic gates).

Does this initial spike hurt the components?

This circuit board will be turned on and off, based on what the other items are doing within the control box for a machine. For example, each time the machine turns on and off, so will this circuit board.

Do you think it will still send this voltage spike, if and when the power inputs will be switched on and off by a separate control box switch (which turns off the machine), each time it is turned on and off?

[Falcon69]

well, I fond some almunium caps, so I just placed them parrallel across that 0603 0.1uF cap I had.

a 33.1uF is too long of a delay
a 22.1uF is too long of a delay
a 10.1uF is too long of a delay (but i think should work just fine. It's about a 2 second delay before circuit turns on).
a 4.7.1uF doesn't work

I think maybe a 6.8uF might work, but I don't have one to test

So, I think I will order up some 10uF 35v ceramic capacitors and replace that with the C4 in the schematic.

N/M, they don't make them in an 0603 package. Damn.  hmmm, any ideas? I already got 14 of these boards made.  Would like to try and use the boards I have.  I can redesign the next batch for the 0805 size 10uF 35v

[Falcon69]

would i be safe with a 25volt for C4?  The max input for this circuit is 24volt.  I've just been using a value rated for at least 10volts over for the capacitors.

I ask, because, i found a 10v 0603 XR5 package in 25volt I could use for these 14 boards.

[kxenos]

Q1 is reversed.

[Falcon69]

ya, but for this, I'm not asking about that. I merely drew it wrong in the schematic, but it is the correct way on the circuit board. The main schematic for the entire board, it is drawn correctly. Just not in the picture I grabbed.

[Falcon69]

well, I got in the 10uF 25volt ceramic capacitors to replace C4.

It didn't work. the LED's still get a current spike when the board is initially plugged in and flash for a split second.  I'm worried that by doing this, it is going to damage the components. The board will be turned on and off via a switch, which turns off other boards on the machine.  So, everytime the machine is shut down, so will this circuit.  And then, when it is turned back on, it will have a voltage spike again.

It seemed the Aluminum Cap worked (i had a thru hole one I tried earlier).  Why would the aluminum work, and not the ceramic?  I would have to completely redesign the board to make the aluminum one work.  Don't really want to do that.  I cant afford another order for boards.

Any suggestions?  Would changing the 47k resistors to another value help and go back to a 0.1uF ceramic cap, as in the data sheet?

[Falcon69]

well, I just tried it again with the 10uF aluminum Cap.

I must have overlooked this last time i tried it.

On initial plug in, there's a spike that lights all the LED's, then no LED's are lit until the cap charges and then circuit operates as normal.

So, How do you solve the voltage spike problem then?

Apparently, the suggested circuit as drawn above and in the data sheet is correct, but, I still get a voltage spike on plug in.

I'm thinking that maybe it can't be fixed?  Since on plug in, the logic chips are like, "what do we do?" as the inputs would be floating at intitial plugging in of the circuit, and that is why the LED's flash for a brief second, until the logic chips realize the inputs are no longer floating?  Is this normal operation for them?  I just don't want the circuit to get damage when an on/off switch is hit.

[Circlotron]

Q1 is reversed.

It looks like it is, but in this cct it is the right way around. When normal polarity power is applied the body diode of the mosfet conducts and also the mosfet turns on, helping the body diode conduct. But yeah, the current is in fact going the reverse direction through the mosfet, but mosfets can do that. If you reverse the supply polarity, the mosfet stays switched off and it has "normal" polarity across the source-drain. 

[Falcon69]

Ya, it is a reverse Polarity circuit, using a P-Channel mosFET.

I noticed too, that day, after I made that comment that it IS in the correct orientation.

[Circlotron]

Oh yeah, P-channel. Whoops!

[Falcon69]

I was just thinking, and this would actually be a GREAT idea if it works.

Pin 5 of the LM2596S-ADJ voltage regulator is an ON/OFF control for the voltage regulator.  Normally, it would be connected to ground, or, if you need a delay start (in my case, that doesn't seem to function right for my application), it would be connected as in the schematic above.

My thought was, what if a simple SPST switch was placed between pin 5 and ground.  If the switch was Open, would that NOT allow the voltage regulator to turn on?  If it was closed, that would allow it to turn on?

Is there any problems with doing this?

That would be perfect, because using this method, the circuit board could be switch on and off, based on the position of the switch.  That could be wired to a relay, that could be activated by the main control panel for the machine which turns on all electronics of the machine.

What are your thoughts on this?  ANyone have experience with the LM2596 and has done this before with Pin 5?

Thanks in advance.

[tautech]

Lower the value of your R4, R2 in the datasheet.
This will delay the startup further.

[Falcon69]

Thank You tautech, I will try that, however, I am not sure that IS the problem.

with the circuit just as it is in the data sheet, but changing that 0.1uF cap to a 10uF, I get the delay, however, this is how the circuit works....

Plug in Circuit (connect the walwart)
LED's flash for a fraction of a second
Then nothing, no LED's, no circuit power for like 2 seconds (well, the output of the voltage regulator to rest of circuit)
Now circuit turns on as normal, no flash of the LED's


That is what is happening, so they delay start circuit is working, however, somehow, a bit of current is getting through, turning on the logic gates and supplying power to the LED's for that fraction of a second when the walwart is plugged in.

I'm not exactly understanding what is happening.

[tautech]

Time to get the scope out and see what and why it is happening.
Little else will tell you.
Monitor the pins that matter and study the datasheet.
Set the trigger and use single.
Then you will find it.

[Falcon69]

I think I found my answer searching the net.

The LM2596S-ADJ has a voltage threshold of 1.3V on PIN 5 with a maximum voltage of 25v.

THe LM2596S-ADJ will default to the 'ON' state if PIN 5 is left floating or connected to ground.

If a voltage source above 1.3v is connected to the LM2596S-ADJ PIN 5, then the voltage regulator will revert to an 'OFF' state.

If I understood what I read, then connecting a resistor (i'm thinking a 100k) from the walwart supply to pin 5, and then to the in of a switch and out to ground from there, by turning the switch on and off it should shut the voltage regulator down.

Seeing how my circuit will not see more than 24 volts, I should be okay with a 100K ohm resistor?

FYI, I don't have a scope, nor do I know how to use one....YET. Can't afford one right now anyway.

[Falcon69]

Still having trouble with this.

I had the circuit like the first pic below.  It was according to the Data Sheet.  I tried changing the C4 to 10uF and didn't help.  It gave the delay, but, upon plug in, the LED's still flashed for a fraction of a second, then it did the delay start for the voltage regulator, and then all the circuit turned on correctly.

Now, I tried the circuit this way (second pic) and this way it still does the same thing, but without the delay start.  Although, I think I will go with this idea, because it allows the whole circuit to be turned off via a standard SPST switch.

Now, I've included the debounce circuit I have.  The output of the inverter goes to every logic gate that is supplying power to the red and green LED's I have.  The green LED's are always on (unless one of the hall sensors triggers it, switching it to a red LED) and when the tactile switch in the debounce circuit is pressed, all the LED's light, this is a feature I added so that you can see if all the LED's are turning on as they should, or if one has burned out, etc.

So, I am wandering if maybe changing a value in the debounce circuit will solve the voltage spike that happens on plugging in of the wallwart to the circuit board, or flipping the rocker switch for pin 5 of the voltage regulator?

[Christopher]

The on off pin literally only turns the device on or off. There is no soft start.

You can either change to a chip which does include a soft start pin, which allows you to ramp up the voltage over say 100 ms to stop the device from folding back due to current limit, or add in an external soft start to the feedback pin as shown here


http://m.electronicdesign.com/site-files/electronicdesign.com/files/archive/electronicdesign.com/files/29/6384/figure_01.gif

[Falcon69]

okay, don't understand that one. that's a completely different chip than the LM2596.

So it is the feedback pin of the LM2596 that needs to have a circuit implemented onto it to have the delayed start up?

Do you have a schematic I could look at that is specific to the LM2596 or similar?  That would be very helpful to me.

Thanks

[Christopher]

The way your chip works is increasing/decreasing the frequency of the pulses of current through the inductor. It keeps changing this until the voltage on the feedback pin reaches a specific voltage then keeps doing what it can to the output till the vfb is constant.

You can put a 100 or 1u or 10u ceramic cap 6v rated+ right across r1 to see what I mean. But do not keep this across there forever as this will reduce the transient response and is a bad idea.

U1 in the circuit I linked does this in a more controlled way at startup

[Falcon69]

okay. If I leave the circuit as is, will it damage the components?  If it doesn't, I'll just leave as is, as It's really not a big deal I guess for the function of the circuit.