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Old 08-06-2012, 11:47 AM #1
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Default Oscilloscope Interpretation & Current Spikes: How harmful are they?

Hi all,

I just went to get my driver modded by replacing the sense resistors so that it would output exactly 1.8A.

I bought the SMD resistors, and went to a different shop to get them soldered on because I can't solder for (censored).

I noticed an oscilloscope lying around, and asked the guy if he would let me use it, and he was kind enough to let me have a look through it.

To my horror, there was a LOT of current spikes across my test load, and I don't think it will be too good for an LD.

The currents are:
Med: 550mA
Low: 190mA
High: 1.8mA
Strobe: 8Hz
SOS: N/A

Edit: I've got my hands on the probe now, and here are some shots:
These are taken with a 6.6V 1000uF cap. The inductor is NOT wired series to the output. It came with the driver, and I have no idea where it is connected to.

Medium Power:


Low Power:


High Power:


High Power current output through DMM (dummy load volts):


High Power (larger scale):


Testing Setup:


Circuit board setup:



Please let me know if there is anything to clarify what's going on.

So my question is, is this driver safe to use with a M140 diode? Or will it kill it?

Thanks for your help!


__________________________________________________ _______________________________


Old vids:
(Please disregard these videos as they are outdated. These are taken when the driver did NOT have a cap nor a inductor to filter the output.)

Apologies for the crappy quality; it's taken with an iPod Touch 4G.









Thanks, appreciate it!

Cheers!


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Old 08-06-2012, 02:40 PM #2
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Default Re: Current Spikes: How harmful are they?

Hard to say, it really depends on the peak currents, not whether or not there are spikes. Spikes below lethal current are just noise modulated in to the output. A nanosecond pulse of 1A into a diode which can handle 1A safely is no more dangerous than running the diode at high frequency.

So, how high of peak current were you measuring? That is the question.

You could try choking out the high current with a series inductor, and adding a low pass filter (parallel capacitor) never hurts as it also protects against voltage spikes.
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Old 08-06-2012, 05:10 PM #3
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Default Re: Current Spikes: How harmful are they?

Yeah, on the videos I posted, the spikes seem to go over 1.8A, which definitely will harm the diode. To combat that, I added an overkill cap rated at 6.6V 1000uF. But space definitely can be an issue, as it all needs to fit in a flashlight pill.

An inductor comes with the driver, but I'm not sure to what part of the circuit it is hooked up to. I think I'll remove it and solder it directly to the.... I was about to say the output leads, but that would cause a short circuit.

I'll do some more testing and update you guys as well.

Thanks again!

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Old 08-07-2012, 12:33 AM #4
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Default Re: Current Spikes: How harmful are they?

I think Cyparagon is a good "reader of oscilloscopes" if he is around these parts. I did a similar test on a different driver that had similar results to some of the results you got. There are some ugly drivers out there. I think that flashlight diodes are very robust compared to laser diodes so they get away with some crazy bad circuits. I think adding caps to the output is a good start, I'm not sure what your options are with the inductors though. You might try to stay close to the same value in uH but try to be sure a replacement inductor can handle high frequency well.

I don't know if some of these are worth cleaning up or trying to fix.
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Old 08-07-2012, 02:54 AM #5
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Default Re: Current Spikes: How harmful are they?

Thanks for your reply, Tom.

Did you use any of the drivers that had similar noise and spikes in a laser build? Did you try to add caps and chokes to smooth it out?

I added an overkill 1000uF cap, and I'll check it out with an scope to see how it works out.

I wish Cyparagon saw this thread.

Thanks again!
Cheers!
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Old 08-07-2012, 07:03 AM #6
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Default Re: Current Spikes: How harmful are they?

What was the volts/div and time/div settings in those videos? Going to need that information before it can be of any help.

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Old 08-07-2012, 07:07 AM #7
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Default Re: Current Spikes: How harmful are they?

Quote:
Originally Posted by Things View Post
What was the volts/div and time/div settings in those videos? Going to need that information before it can be of any help.

Dan
Yessir, I'll get new (and better) pics and vids in about an hour or so.

This will be my very first time touching a scope, so please be nice.

Are the settings you mentioned displayed on the screen, or is it something I need to took into the dials or is it something I need to calculate?

Apologies for the crappy vids--I was trying to cause the least amount of inconvenience to the scope owner.

Many thanks!
Cheers!
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Old 08-07-2012, 07:18 AM #8
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Default Re: Current Spikes: How harmful are they?

It will say it on the screen somewhere, usually like 20ms/div or 5V/div

The guy helping/letting you use it will know what and where they are.

Dan
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Old 08-07-2012, 09:19 AM #9
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Default Re: Current Spikes: How harmful are they?

Thanks Dan. I've gotten my hands on the scope and tried a few things out.

The pics are updated on post #1, and please let me know if I can do anything else for ya guys.

Thanks!
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Old 08-07-2012, 09:36 AM #10
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Default Re: Oscilloscope Interpretation & Current Spikes: How harmful are they?

OK, that's better. Does it make those waveforms all the time, or only on turnon?

As it is a booster device, it's going to have some output ripple, but if hat really is 5V/div then that's some insane ripple!
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Old 08-07-2012, 11:39 AM #11
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Default Re: Oscilloscope Interpretation & Current Spikes: How harmful are they?

Quote:
Originally Posted by Things View Post
OK, that's better. Does it make those waveforms all the time, or only on turnon?

As it is a booster device, it's going to have some output ripple, but if hat really is 5V/div then that's some insane ripple!
Nice, thanks for your info.

These waveforms are always present. Mid/Low "ripple" is somewhat expected because it's PWM (pulse width modulation) at 198 and 200 ish Hz.

How am I able to capture startup ripple on the scope?

Cheers!
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Old 08-07-2012, 12:00 PM #12
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Default Re: Oscilloscope Interpretation & Current Spikes: How harmful are they?

The inductor I referenced is not the one that comes with the board! It is an additional one I am saying to add IN SERIES with the positive terminal of the output. A capacitor in parallel and an inductor in series = a low pass filter, which prevents fast changes in both voltage and current. The inductor that came with the circuit is the heart of the driver and does the switchmode voltage change needed.

V/div and t/div are neccessary parameters for determining what each square on the screen represents. For example: in 1V/div a square wave going from center line to one square of the screen up is = 1V, but in 0.05V/div that same image on the display would mean only 50mV of a squarewave.

Your first and third video show mostly a PWM squarewave, and it is hard to tell, but all the spikes appear to be during the LOW portion of the wave. I'll assume the guy who owned the scope set it up correctly and measured the voltage across the sense resistor and not the voltage across the output.

Adding a capacitor across the output should help reduce voltage spikes but it won't make too much of a difference to current spikes. You could try adding a decoupling capacitor across the input, or better yet, across the IC directly. Sometimes parasitic oscillations can cause spikes like this, and most certainly are causing the fuzzyness on the LOWs of the waveform.

If it were my circuit I'd add a decoupling cap and a Pi-network lowpass filter on the output.
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Old 08-07-2012, 12:59 PM #13
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Default Re: Oscilloscope Interpretation & Current Spikes: How harmful are they?

Ah, now I understand what that inductor that came with the board is for. Yup, I won't touch it.
I've added a 6.6V 1000uF cap with a overkill inductor I found lying around to the output as well. Here's a pic of the setup:


However, I'm very confused about something here. In medium mode, the scope says the peak voltage (which translates to amps via the test load) is very very very high (24V), and the average voltage read from the scope is 5.XX volts, which is ridiculous. My DMM, hooked in the same places as the scope, reads 0.5XX volts, which is very reasonable for a medium power setting PWM.

I'll upload a video here: (may be processing for a while)
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Old 08-07-2012, 02:49 PM #14
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Default Re: Oscilloscope Interpretation & Current Spikes: How harmful are they?

Double check all the scope settings are correct. Your DMM should read the average voltage, which for a PWM signal is not the normal 0.707 x Vpk (and Vpk = 1/2 x Vp-p for an ac only signal) that a sine wave uses, so there will be a small discrepancy. I'm not familiar with your scope, I'm only used to old analogue clunkers haha, so I can't tell from the readout what is going on. It does look like the spikes have stopped though, right?
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Old 08-07-2012, 04:34 PM #15
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Default Re: Oscilloscope Interpretation & Current Spikes: How harmful are they?

Sheesh that inductor is bigger than your driver. What's the magnitude of the spike we're trying to filter out?
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Old 08-07-2012, 04:58 PM #16
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Default Re: Oscilloscope Interpretation & Current Spikes: How harmful are they?

Ryan;

Although I am not familiar with that scope,

the voltage across the sense resistor looks too high.

You need someone familiar with the scope,

to measure voltage across the sense resistor, and translate that to peak current.

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