Car battery voltage spikes

[Rajinder]

Hi all,
I was after a bit of advice. I want to measure voltage spikes from a car battery using my scope. I am building a car voltage monitor (to show state of battery), I am a bit worried that I might damage my scope. Are there any precautions that I should take?

Do I need some kind of divider with some clamping to protect the scope from transients.

Thanks in advance.

 

[davenn]

I want to measure voltage spikes from a car battery using my scope.

what voltage spikes ??
where do you think these spikes are coming from or being generated by ??

 

[Rajinder]

I think starter motor, inductive spikes etc

 

[Rajinder]

I think starter motor, inductive spikes etc

Perhaps I should have said was monitor any spikes when the car is switched on from overnight i.e. starting up the engine

 

[PETERDECO]

Unless you plan on hooking up your scope to the spark plug wires, you won't have any problems. I used my scope on an older car with an intermittent ignition coil. With the scope I traced it to a sensor on the flywheel. This was done on the 12V side of the wiring. If there were any huge inductive spikes, your radio and other electronics would blow.

 

[Rajinder]

Unless you plan on hooking up your scope to the spark plug wires, you won't have any problems. I used my scope on an older car with an intermittent ignition coil. With the scope I traced it to a sensor on the flywheel. This was done on the 12V side of the wiring. If there were any huge inductive spikes, your radio and other electronics would blow.

Hi,
Thanks for your help. I was going to look at the voltage on the :
1. Car battery - scope across the negative and positive terminals
2. Car cigarette lighter
3. OBD socket through a cable
I was slightly concerned that any transients (that I have read about can be >150V) and the last thing I want is to damage my scope.

Thanks

 

[PETERDECO]

Why do you want to do this? What kind of problem do you have?

 

[Rajinder]

Why do you want to do this? What kind of problem do you have?

Having some issues that through OBD, which blows up IC. I want to build a voltage comparator to indicate voltage levels initially, hopefully the experiments will help me to see if spikes are the issue.

 

[Kiwi]

"Having some issues that through OBD, which blows up IC"
Please explain what you mean by this.

 

[Rajinder]

I have a OBD unit that has a 60Vmax IC, this is getting destroyed. I am not sure if it is due to voltage transients (due to inductive) or current surge. Hence I am trying to find out by firstly monitoring the voltage, when the car switches on etc. I just want to make sure that I do not destroy my digital oscilloscope with any spikes.

 

[Harald Kapp]

When the motor is turned on, the voltage will drop, not rise, due to the impressive current drawn by the starter.
Voltage spikes arise due to load dump, when a load is turned off and the regulator is slow to reduce the generated power from the alternator. From Wikipedia: "This surge may be as high as 120 volts and the surge may take up to 400 ms to decay. It is typically clamped to 40 V in 12 V vehicles".
To clamp the voltage typically TVS diodes are used in conjunction with current limiting components. See e.g. this app note.

 

[hevans1944]

I just want to make sure that I do not destroy my digital oscilloscope with any spikes

Before attaching your probes to any circuit, make sure the range setting (volts per division) is greater than any anticipated pulse input. If necessary, use a X10 probe to reduce the probe tip voltage to one-tenth of its value before the probe signal is introduced to the oscilloscope. Most 'scopes can easily measure a 240 volt sinusoidal line voltage (approximately 340 volts from zero to peak voltage), with or without a X10 attenuator probe, without damage to the oscilloscope.

I suggest you carefully read the operating manual that is specific to your brand and model of oscilloscope. If you did not receive an operating manual, let your friend Google search the web for one.

 

[Rajinder]

Before attaching your probes to any circuit, make sure the range setting (volts per division) is greater than any anticipated pulse input. If necessary, use a X10 probe to reduce the probe tip voltage to one-tenth of its value before the probe signal is introduced to the oscilloscope. Most 'scopes can easily measure a 240 volt sinusoidal line voltage (approximately 340 volts from zero to peak voltage), with or without a X10 attenuator probe, without damage to the oscilloscope.

I suggest you carefully read the operating manual that is specific to your brand and model of oscilloscope. If you did not receive an operating manual, let your friend Google search the web for one.

Hi, thanks for your advice. I was going to use a X10 probe, perhaps also use a voltage divider to reduce the signal further say using 2 x 1M resistors.
I was also going to ring the manufacturer too.
Thanks again.

 

[Rajinder]

When the motor is turned on, the voltage will drop, not rise, due to the impressive current drawn by the starter.
Voltage spikes arise due to load dump, when a load is turned off and the regulator is slow to reduce the generated power from the alternator. From Wikipedia: "This surge may be as high as 120 volts and the surge may take up to 400 ms to decay. It is typically clamped to 40 V in 12 V vehicles".
To clamp the voltage typically TVS diodes are used in conjunction with current limiting components. See e.g. this app note.

Thanks for the valuable advice, much appreciated.