Arduino - Buckpuck interface little issue

Other Hardware General Electronics
1

Hi.

First of all, let me tell you that, even if this is my first post, I’ve been reading this forums for a long time and thanks to that I achieve so much in my current project. This is my first project with Arduino. I worked with Raspberry PI and Beaglebone before, but usually the Hardware / Microcontroller part was handled by other people.

I’ve been working on this project for a year now, sometimes getting the parts was a little tricky because where I live is not easy to import things, but at the end, I think I got everything I need.
At last, I achieve what I was looking for, I have a little software issues, but I’ll debug them later.

On the other hand, I have an “issue” that I think is not something I can fix with software.

Let me describe you the circuit, I have an Arduino Nano PWM Pin connected to a Buckpuck (3023-D-E-1000) (http://www.luxdrive.com/content/3021-BuckPuck.pdf) Ctrl Pin, and then connected with a LED. Everything is powered by a Li-Ion battery with 7.4v. (I attached a diagram of the circuit)

The circuit, which I ‘design’ reading this forums and with a few google searches, works fine. The problems is when the Arduino is booting, the PWM Pin don’t have the necessary 5V to turn off the Buckpuck. So, when I close the switch, the Buckpuck turn on the LED full power until the Arduino finish booting. I want to avoid this behavior.

Until now, the only way I avoid any kind of light from the LED while turning on the Arduino was by powering the Buckpuck after the Arduino finish booting (putting a second switch between the Battery and the Buckpuck VIN+)

I’ve been reading about transistors, MOSFET. That’s all new to me, I learnt a lot this days about this, but I am sure I just start scratching the surface.

So I decide to ask you for some help. If I didn’t bore you so far with all this, can you help me out with this problem? In a few words, I want to avoid the flash of light while the Arduino is turning on.

Thank you in advance and thank you for all the other questions you answer for me without even noticing.

(I am from Argentina)

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2

A 220 ohm resistor between the BuckPuck control pin and Arduino's 5V pin will fix this.

The BuckPuck has a rather low input impedance requiring this rather low-valued pull-up
resistor to take it above the 4.2V shutdown threshold, but it ought to work. The down side
is 20mA of extra power consumption.

3

Thank you, i'll try it when I get home. I still don't get the theory behind this, but I'll look it up ASAP.

I am using the 5V to power a WT588D-U, but I think I can live with the 20mA extra consumption.

4

The pull-up pulls the pin up to near 5V. However the Arduino can pull harder in the other
direction when the pin becomes an OUTPUT. Arduino outputs have about 30 to 40 ohms
impedance so they win over 220 ohms, and 220 ohms wins over the 1500 ohms of the
control pin of the BuckPuck (if the Arduino pin is an INPUT, as it is at reset).

5

Hi Again.

Sadly I didn't have 220 ohms and ,I know it's not the same, but I tried with a 560 ohms and it didn't make any difference at all.

For what I understand about your response yesterday and some research I did, when the PIN is configured as an INPUT, the impedance is very strong. The pull-up resistor will give almost 5V to the Control pin in the buckpuck and turn it off until the pin is configured as OUTPUT.

I'll try today again just in case I missed something.

6

when the PIN is configured as an INPUT, the impedance is very strong.

No that is the wrong word and it conveys the wrong meaning.

The impedance is very high and so it can be affected by all sorts of things like interference.

The pull-up resistor will give almost 5V to the Control pin in the buckpuck and turn it off until the pin is configured as OUTPUT.

Well not quite.
There is basically a fight between the pull up resistor and the buckpuck. Because the buckpuck requires so much current to turn it off, more than your 560R resistor can supply it is winning and turning on. A lower pull up resistor should do it. If you put two 560R resistors in parallel that will bring the resistance down to 280R so try that first. If you get no luck then you should add a transistor to drive the output harder and use not an external pull up resistor but a pull down on on the Arduino pin.

7

Thanks for the help and the explanation.

I tried with 2 in parallel without luck, however 3 in parallel did the trick (around 180 ohms).

When I use this arrangement I see a very short flash when I close the main switch, I assume that if I want to remove the flash I’ll need to use a transistor to power up the Buckpuck after the Arduino (this is the only scenario in which the flash don’t happen). Any recommendations / comments about this?

8

You could add a transistor or fet to drive the CNTL pin from and arduino output.

9

What an obnoxious device...

Defaults to on, and you can't just put a gentle pullup on it to keep it from turning on, you've gotta use such an aggressive pullup to turn it off that the current needed from the IO pin to turn it on becomes high enough that it's pushing the limits of what an Arduino can provide.

Ugh.

Did you look at the sample schematics in the datasheet? It looks like they give you a few schematics...

10

Good Morning / Afternoon / Evening

I looked at the schematics before buying, I thought that with the PWM was enought but I guess I was mistaken.

The truth is that I start thinking and researching about transistors (and thats when I learnt about the MOSFETs) because of the schematics.

In the schematics, they use a transistor between the REF and CTRL pins of the Buckpuck, but my tests indicate that, even with that arrangement I am going to have a "flash" when the main switch closes. Right now, even when I connect the REF and CRTL directly i have this "flash".

So, as an alternative I was thinking of putting a transistor (I don't know what kind) between the positive of the battery (7.4v) of the battery an the VIN+ of the Buckpuck and use it as a on/off switch. I tried this with a common push button and worked as I expected.

The problem is I am very new to transistors and I don't have a clue about what to buy.

I need a transistor that can output 7.4v and 1A and can be activated (am not sure if this is the right term) with a simple arduino pin (I think 5V and 40mA).

So basically, how bad do I understand all this? And, if what I said before makes sense, what should I buy? I live in Argentina and, afaik, there is no much variety and everything is somehow more expensive than overseas.