Hi all, this is my first post (of many, I expect). I am trying to monitor a variable voltage signal from 10-20vdc and this signal will have intermittent spikes of up to 200v. What I am trying to do is find a way to ignore the spikes (they have a purpose so I don't want to get rid of them), and use the 10-20v as a trigger for my device. I am concerned that the 200v spikes will damage any measurement circuits or mess up the readings. Is there a relatively simple way to do this? Sorry if it is a bit of an obtuse question.

 

Can you show us the circuit that you currently have for processing this signal?
A schematic of the source with the spike generation would also be handy.

 

Hello,

Can you give some more information on the device to be triggered?

Bertus

 

Maybe just a bit more information.
How long do the spikes last?
Do you just need to know the signal changed from 10 to 20 volts or do you need to know it's level.
What will make the measurement? Hardware or micro?

 

It is a battery charger of sorts. I am trying to see what happens when I put a pulse input to a couple different types of batteries. I'm working on an electric truck. I will have to draw up a schematic (which my cad skills will take a bit of time, haha).

The batteries will, of course, have to start to charge at the lower end and shut off at the higher end. The voltage range is just for testing as the battery pack final voltage is not known yet. There is a lot of range with these motors depending on the motor maker.

 

The spikes i suspect will be at 1khz

 

One obvious way to limit the spikes is with a series resistor and a 25V Zener to ground.

 

I don't want to limit the spikes but I do want to only measure the voltage between the spikes

 

I don't want to limit the spikes but I do want to only measure the voltage between the spikes

Then limit the input to the measuring device. If the measuring device can't survive 200 volts, don't apply them to it. Did somebody tell you that you can't connect a limiter to your meter so it only measures between the spikes???

 

I am trying to monitor a variable voltage signal from 10-20vdc

and use the 10-20v as a trigger for my device.

Are these two separate requirements?

 

I wont' be using a multimeter to measure it because it has to be somewhat automatic. It needs to "see" the 10v (for example) on the battery, turn on the charge circuit and let it charge till it hits X volts. But how to weed out the effect of the spikes and still let the charger read properly?

 

Then limit the input to the measuring device. If the measuring device can't survive 200 volts, don't apply them to it. Did somebody tell you that you can't connect a limiter to your meter so it only measures between the spikes???

Nope, no one said that it cannot be done. I guess I would need to do parallel measurements, one with the spikes removed to measure and control the circuit.

 

Many ways to skin that cat, from some complicated microprocessor that knows about timing to a simple low pass filter that essentially ignores anything that is .001 second long.

How much time between these millisecond long pulses? Several seconds?

 

Thats a good question. Time between pulses is not set yet. I'm moving houses and my scope is packed away in a box for the next month along with much of my other electronic stuff. Stupid me to do that, haha. To be sure I would like to get it out and see what the waveform looks like but, in principle, putting a VM across the signal and grounding out the pulse would allow me to monitor the actual battery voltage, I guess, then create something that can recognize the value and start/stop the charging. What would be wrong with this approach?

 

So half baked. So lack of information. So, no answers.

 

I'll report back when I have more info

 

Unless you are hitting the battery with really high current it will limit the voltage. So I would just run the battery thru a resistor with a zener clamp in case there is some inductance. Then just integrate it to get average battery voltage.

 

integrate: sometimes means the same thing as, "low pass filter"
A resistor and a capacitor might be all you need. $1 to $2
Now, spill the beans. We're tired of trying to imagine exactly what you are talking about.

 

I don't want to limit the spikes but I do want to only measure the voltage between the spikes

I'm talking about clamping the spikes at the input to the measurement circuit so they aren't measured.
That won't affect the spikes in the circuit that's generating them.

 

You guys have given me lots to think about. The ultimate goal is to be able to stop the charging process at the right time with an accurate measurement. The best way to know if a battery is fully charged is by taking a specific gravity measurement of the electrolyte but there is no cheap automated way to do this so this is the next best thing. Sometimes a battery voltage measurement is not accurate until it has been allowed to settle which can't be done when charging so I am starting to think of a correction factor.

Now to make the proper circuit to measure voltage without the spikes. More investigation. Oddly enough, I went to DeVry 30+ years ago, got a technologist ticket and never worked it the industry so forgot much of what I learned. Time to get some of it back.