12VDC (0.35A) Motor with 1N4001 Diode?

[TomGeorge]

Hi,
Select,   VERT MODE --   ADD
Select   CH2 POL  --  INV (I think button in)

Place both channels on same V/div

Use CH1 and CH2 as differential inputs.


Result on screen,   CH1 - CH2 = differential voltage.

Tom....

[raschemmel]

I'm measuring across a diode. Which channel goes on the positive side of diode (cathode) and which goes on the negative side of diode (anode) ?

[Archibald]

Raschemmel,

Perhaps I'm totally confused and have not read this thread carefully enough, but I suspect the measurements you have been making of the coil with diode are not valid at all.  I was alerted to this and puzzled a while ago by the oscilloscope traces you posted that showed the voltage going up to about 17V despite the presence of the diode, even allowing for some forward turn-on time.

What I think is happening with your test circuit is as follows.  The coil (choke) you are using looks here as if it will have a resistance of only a few mΩ.  So, even with the pulse duration of only 6μs, I believe you are virtually shorting out your 10V power supply. My hypothesis is that the voltage going up to 17V and then ringing is simply the response of your power supply recovering after the short-circuit.  Does this make sense?

I suggest you use a relay coil instead of the 100 µH radio frequency choke.

I also suggest you temporarily replace the coil and diode by a resistor having a resistance fairly close to the resistance of your coil. You will then be able to check whether you get nice fast transitions displayed on your oscilloscope.  Have you trimmed (compensated) your oscilloscope probes?

I don't understand all the fuss in this thread about measuring across the inductor.  I suggest you continue to measure at the MOSFET drain but also check (using the other channel of your oscilloscope) that your 10V power supply remains essentially constant throughout.  You may need to add decoupling to the 10V power supply (close to the coil and MOSFET).  If your 10V supply is constant, there is no need to make differential measurement across the inductor.

[larryd]

There is no fuss about measuring across the inductor, it was just curiosity.

[raschemmel]

  

I believe you are virtually shorting out your 10V power supply

Here you are mistaken. The power supply current was posted in the first photos and the power supply voltage is rock solid at 10 V. There is no shorting out of anything (unless I connect my scope probe ground clip to any point in my circuit). It's a bench LAB supply with adjustable current limit and both voltage and current displays. The photos posted clearly show the pulse (not a short).
The whole point of this measurement is to compare diode response, hence the measurement across the diode. Previously I didn't know how to do that because I can't connect my scope ground to any point on the circuit.


@LarryD,
It worked.

ch-1 on cathode
ch-2 INVERT on anode
VERT MODE SELECT: ADD
Vcc: 10V
TIME/Div: 10 uS
Amplitude: 2V/Div
Test Pulse: 300uS , 2% duty cycle

1N4001


1N5711


1N5817


1N5822




STTH 3R02

STTH 3R02 DATASHEET


NO DIODE

[larryd]

@raschemmel
Nice.
It appears the transorb and 1N4001 can handle the kick back differently (better?), interesting.
I have never used Schottky diodes for snubbing.

The transorb is designed to recover from high energy transients, they are more expensive.
It does appear they may have application where heating may result.
Over the years, I have also used MOVs in SWC designs.

At $95 you better get a second scope for spares. 

[raschemmel]

The Schottky diode scope shot is proof that there is a back EMF.


I bought the scope at a surplus store and it still has the name of the company that  owned it (IEM SENSORS) stencied on top. They must have had it calibrated because everything works.





From the photo of the inductor response with no diode it looks like the back EMF negative voltage is at least 6V