My circuit that burnt my BBB needs modify: I wonder if the ground is necessary

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I burnt my BBB, which let me very sad. I ordered a new one online because it’s an urgent project, but until I have fully figured out what happened with my circuit, I dare not connect the BBB to it again.

This circuit, as shown in the attached image, worked all well during the last whole week. I turned it off this Monday, and turned it on this morning, and Bang! The BBB’s burnt. It won’t boot up. When I plug the 5V power cable or mini USB cable to it, only the PWR LED will light up a little bit, and it never boots up again. I didn’t believe it’s my circuit’s fault, so I plugged my backup BBB on, and again, it’s killed. I burned $100 in 10 minutes.

I’m attaching the circuit design here because I need to know if this design is really problematic. Let me explain a little bit: I’m trying to control an I/P converter (SMC ITV0011), which is driven by a 0-5V voltage signal and powered by 12V dc. The problem is It only has three wires, and the ground wire is shared. That’s why I think I have to connect all of the ground wires together.

I’m using a DAC chip (MCP4725) to generate a proper voltage control signal to the I/P converter. This DAC chip is communicated by I2C, and powered directly by the onboard 3.3V from the BBB. I have carefully checked with multimeter that with digital command through I2C, the voltage could be linearly generated, which is very cool.

I’m using a GPIO to control a relay (powered by 5V) to control the on/off of the I/P converter, which means only when I want the I/P converter to be turned on, I will send the GPIO to 1. Otherwise, when GPIO is 0, the I/P converter would not be powered, which is a way of enlarging the lifetime of the $200 I/P converter.

That’s the whole idea. And I also used a Qt GUI to control the output of both the I2C and GPIO. Last week, I enjoyed a lot using the stylus to drag the slidebar on the touch screen to adjust the pressure output by the I/P Converter. I just don’t understand why it suddenly became so dangerous.

But last week I did noticed one strange phenomenon. When the relay was not working, there is a strange minus 17 volt on the relay output port. This is probably 12V + 5V, But I don’t understand neither why this is a sum nor why it is negative, and since everything works, I didn’t pay attention.

Today, after I burned the two BBB, I measured the voltage of the relay output port when it’s off again. The voltage is 1.7V (5V-3.3V?). So strange!

In the image, I think once I remove the red wire, the BBB will be isolated from the whole 5V and 12V dc circuits. But I don’t know if this is the right solution, and also if the voltage output from the DAC will be transfered to the I/P converter. If any of you could explain a little bit about this issue, I would appreciate a lot.

Also, If I want to add some more protection to the BBB (on both I2C and GPIO), what should I do? Thanks!

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Forgot to attach the illustration figure.

circuit.jpg

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Unless you're deliberately isolating components, all the grounds need to be tied together. There are some additional considerations for EMI and accurate measurement, but you should try to bring all your grounds together at a point.

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After just a quick look at the circuit, I suspect that the BBB is trying to source/sink too much current?

Also, the BBB has 3.3V logic and you are driving 5V logic. I would suggest that you use an opto-isolator

for both isolation and logic level conversion.

Bill

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This DAC has a flexable input voltage option. So if I’m inputing a 3.3V voltage to it, by controlling via I2C, I can get a 0~3.3V linear voltage output. That 5V power source is just for powering the relay and the BBB & touch screen. So I don’t think I’m driving a 5V circuit with BBB. On the other hand, I think the BBB can also output 5V, so I can as well give the DAC a 5V directly from BBB, right?

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You MUST have all grounds tied together or you will have very
unpredictable results including smoked BBB boards.
The only thing i can really think of after looking at your circuit is
that when you turn off your project the power supplies
you are using do not all power down the same. Can you be sure that when
the BBB is powered off the relay does not blip on ? as the BBB is
powering off if that relay happens to turn on and there is still 12v
being provided by that power supply you do have a chance to zap the BBB.
Isolation of the BBB using an opto isolator is your best bet to make
this work.

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The circuit looks reasonable in general, but it’s hard to tell because you don’t show all the connections and labels. Like, what are the G and V things on the relay boards? I assume you’re talking about http://www.superdroidrobots.com/product_info/TE-249-070_Schematic.pdf, so probably JP17-1 and -2, but how would we know for sure? can you redraw your schematic with more specific connection diagram, listing the specific connections you make to the relay board and the DAC?

Also, it’s possible to mess up by plugging the things in the wrong sequence, e.g. if you work under power and connect the data lines before connecting power.

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On Thu, 29 Oct 2015 09:49:39 -0400, Yongfan Men
<Yongfan.Men.1@nd.edu> declaimed the following:

This DAC has a flexable input voltage option. So if I'm inputing a 3.3V
voltage to it, by controlling via I2C, I can get a 0~3.3V linear voltage
output. That 5V power source is just for powering the relay and the BBB &
touch screen. So I don't think I'm driving a 5V circuit with BBB. On the
other hand, I think the BBB can also output 5V, so I can as well give the
DAC a 5V directly from BBB, right?

  NO... The BBB only handles 3.3V digital (and the analog input is only
1.8V as I recall).

  You should be using a level shifter between the BBB 3.3V and the 5V
target chip.

  You should probably also be using some sort of block for back-EMF on
the relay (when you shut off the coil, the dropping voltage will feed back
out on the control leads).

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Folks,

I’ve just had a discussion with my colleague, and we realized that the reason that I’m trying to connect all the grounds together, is because the 5V and 12V power 2`1335467i[’
are not co-grounded, and I’m forcing them to have the same ground level to transfer the signal (I’ve measured the voltage difference between the two grounds, and it’s at least 300mV! And it’s not stable), so that there might be a large current (aka ground loop) through the BB because of the ground difference between the two power sources, since the ground of them are ‘floating’. If I use a single power source with double outputs, like this, maybe I can literally eliminate the red line on the schematic, and still have the same ground through all the boards.

How does this sound? Maybe this is the ultimate solution!

Henry