help regarding using Arduino to drive laser device

Hi, I need to use Arduino uno as an interface to drive laser operation from a programable power source. The power source as 0V (as +) and -28V (as -), and the idea is using LM7805 to convert the voltage down to 5V to trigger arduino.
The wiring design is like what shown in the diagram photo. When I tested this system with 12V battery, it seems works fine. But when I connected this system to the real external 0V/-28V power source, it messed up the circuit of external power source and blew the power source I think. The uno is still fine afterward though. This is only happened when uno is connected to laser (the device that uno will drive) when 0V/-28V power source is used, but not when the uno is tested alone with external power source. Any thought for what might be the cause??

hard to say. your sketch should work. the questions could be:
- what was the even of 'blowing up' like? was it something that happened in an instant or took some time (2-3 seconds or longer)
- how is the laser really connected, is the programmable power supply used to power it?
- how much power it draws,
- what is the max voltage programmable power source can provide
- how does the voltage changes (do you have time diagrams for example)
- why arduino has to be powered from that power source
- was the regulator properly installed (heatsink, capacitors, was it insulated etc)
- are there any shields connected to arduino
- is there anything else connected to it (communication cable or whatever)

Hi, thanks for going through potential problems of my system.

panic mode said:

- what was the even of 'blowing up' like? was it something that happened in an instant or took some time (2-3 seconds or longer)

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The power source stops function immediately after it delivered its first pulse of output (we program the output as multiple pulses).

panic mode said:

- how is the laser really connected, is the programmable power supply used to power it?

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The laser driver is powered by an independent 110 VDC. The driver stays on and can receive TTL input to swtich on the laser. The driver is connected to arduino through a BNC connector.

panic mode said:

- how much power it draws,

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since it's TTL, I assume it's 5V

panic mode said:

- what is the max voltage programmable power source can provide

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28V

panic mode said:

- how does the voltage changes (do you have time diagrams for example)

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I didn't measure the voltage change across the connection. We program each output pulse to 100ms to avoid overheating 7805.
When the arduino-laser driver connection is powered by the 12V battery, the voltage difference is 7V between 7805 Input and 7805 output, and 5V between 7805 output and 7805 ground.

panic mode said:

- why arduino has to be powered from that power source

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Arduino is powered by 12V input through a regular adapter.

The power source is programed to drive arduino (which in turn delivers 5V to the laser driver) and other devices at different but specific time points.

panic mode said:

- was the regulator properly installed (heatsink, capacitors, was it insulated etc)

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This is a good point. I noticed that the presence or absence of capacitor in the wiring diagram (in this case) mess up the power source differently.

In the absence of capacitor, the power source stop functioning immediately once the 1st output pulse from the power source is delivered. In the presence of capacitor, the power output was no longer controllable by the program (and won't recover afterward).

panic mode said:

- are there any shields connected to arduino

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yes or no. the arduino is placed in a plastic project case, but the case was opened when during the test.

panic mode said:

- is there anything else connected to it (communication cable or whatever)

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nope. just power source to 7805 to arduino to laser driver.


Again, any thought or suggestion will be much appreciated.

there is your problem, you need power to 7805 and Arduino to be continuous, not delivered in pulses.