Electronics > Beginners
Can you help to explain this waveform of an idle motor?
injb:
Update: I didn't ground the ground clip on the probe correctly.
Hi, I'm seeing something that confuses me and would really like to see if anyone can explain it. My knowledge of electronic is very basic. I recently bought a picoscope and I've used it to look at the signal on this device (see pics) which is the idle stabilizer valve from an 80's Bosch Motronic fuel injection system.
It has 3 pins - the middle one gets battery voltage, and then the computer grounds it via one of the other 2. If it grounds one side, the valve opens, allowing air to pass through. Ground the other side, it closes. I've also attached the circuit diagram of the driver and it looks straightforward enough.
Knowing all of the above, I expected to see a signal of 0 to battery voltage (around 14v with the engine running) with the probe tip on one of the 2 ground pins, and the probe's ground clip on chassis ground. I used 2 channels to watch both grounds at the same time.
But to my surprise, instead of 0 - 14v, what I got was -7 to +7 volts! Everything else was as I expected. Now, I don't think the cars electrical system has a negative supply. I double checked that I was using DC coupling on my scope, and I definitely was.
I'd like to understand why I see -7 to +7 instead of 0 to 14. I thought it might have to do with the inductance of the coil in the device, and/or back EMF when it starts turning. But that's only a vague idea for me - I don't really understand how it would happen.
Can anyone shed any light on it for me? Thanks!
Mr. Scram:
Doesn't it simply depend on your reference or ground? Voltage is always relative, so if you declare 0 to between the two peaks you'll have 7V and -7V. If you declare the lowest point to be 0 volt you'll have 0V and 14V. The potential between the two points is the exact same, you just shift the convention of what you consider neutral.
injb:
--- Quote from: Mr. Scram on November 12, 2018, 01:35:54 am ---Doesn't it simply depend on your reference or ground? Voltage is always relative, so if you declare 0 to between the two peaks you'll have 7V and -7V. If you declare the lowest point to be 0 volt you'll have 0V and 14V. The potential between the two points is the exact same, you just shift the convention of what you consider neutral.
--- End quote ---
Well, yes, but since I set my reference to the chassis ground (shared by the battery and the computer) I thought that this would be the lowest value I would see. But somehow I'm seeing it go 7 volts below that. This is 7 volts below what the probe's ground clip is attached to.
vk6zgo:
--- Quote from: injb on November 12, 2018, 01:46:37 am ---
--- Quote from: Mr. Scram on November 12, 2018, 01:35:54 am ---Doesn't it simply depend on your reference or ground? Voltage is always relative, so if you declare 0 to between the two peaks you'll have 7V and -7V. If you declare the lowest point to be 0 volt you'll have 0V and 14V. The potential between the two points is the exact same, you just shift the convention of what you consider neutral.
--- End quote ---
Well, yes, but since I set my reference to the chassis ground (shared by the battery and the computer) I thought that this would be the lowest value I would see. But somehow I'm seeing it go 7 volts below that. This is 7 volts below what the probe's ground clip is attached to.
--- End quote ---
What does your Oscilloscope show if you put tne probe on the positive battery terminal?
injb:
--- Quote from: vk6zgo on November 12, 2018, 05:04:10 am ---What does your Oscilloscope show if you put tne probe on the positive battery terminal?
--- End quote ---
It shows approx 12v (the engine was off when I did that).
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