MPP Step 3 Confirmation of scope data please?

[Fozzy The Bear]

Hi Guys,

I've started my MPP Rev D .... What a nice kit!

step 1 got -4.95V at U8 Pin 4, which is within tolerance as far as I can tell.

step 2 got +5.01V at TP2 again, within tolerance.

step 3 Testing with the scope between 0V (TP10) and TP1 get the following on the screen:


Looks like the frequency is very close at: 1.051kHz the pulse width is bang on target at: 0.058ms 6% duty cycle.

However... figure 5 in the build manual is showing a Vpp of +4.48V I'm showing a Vpp of +12V.

Could somebody confirm for me that this is OK or not please? I've checked that I've used the correct value components in the right places. Can't see any problem. just wanted to confirm that everything is good before I proceed to the next step.

Best regards
Julian.

[Qiaozhi]

Hi Guys,

I've started my MPP Rev D .... What a nice kit!

step 1 got -4.95V at U8 Pin 4, which is within tolerance as far as I can tell.

step 2 got +5.01V at TP2 again, within tolerance.

step 3 Testing with the scope between 0V (TP10) and TP1 get the following on the screen:
[ATTACH]35613[/ATTACH]

Looks like the frequency is very close at: 1.051kHz the pulse width is bang on target at: 0.058ms 6% duty cycle.

However... figure 5 in the build manual is showing a Vpp of +4.48V I'm showing a Vpp of +12V.

Could somebody confirm for me that this is OK or not please? I've checked that I've used the correct value components in the right places. Can't see any problem. just wanted to confirm that everything is good before I proceed to the next step.

Best regards
Julian.

Good point.
As you can see from the schematic, U4 (NE555) is connected between 0V and -VB, so you would expect there to be a pulse output of about 12V.
However, once node B is connected to U6A (MC14538BCP) the pulse will restricted to 5V. I cannot recall, but I must have captured the scope image after the board was complete.
Perhaps in retrospect it would be better to have TP1 attached to U4 pin 3.
The only thing I don't understand here is that your scope shows the peak-to-peak voltage to be a positive value, whereas it should be negative.

[Fozzy The Bear]

The only thing I don't understand here is that your scope shows the peak-to-peak voltage to be a positive value, whereas it should be negative.

That is a good point...... no idea why that is. The ground lead of the scope is definitely connected to TP10 and the probe is connected to TP1
attaching a pic of how it was connected when I took the scope image.


The only other thing was that the scope was set to AC Couple at the time... rather than DC Couple... but I'll try it again tomorrow morning and report back. That said, isn't figure 5 in the instructions also showing Vpp at +4.48V?

Best Regards,
Julian.

[Qiaozhi]

The only other thing was that the scope was set to AC Couple at the time... rather than DC Couple... but I'll try it again tomorrow morning and report back.

OK - that's probably why.

[Fozzy The Bear]

Mystery solved: Metered it with a volt meter this morning and got -11.56V ..... stuck it back on the scope and set it to DC Coupling... Vpp still showing near to 12.00V But that's actually OK.... The thing neither of us registered yesterday, was that Vpp isn't positive or negative. It's just the total potential difference between the highest positive and the lowest negative... It was my fault for putting the plus sign in front of it when I originally posted it, I think that's what threw us.

So all good!! and OK to move on to the next step.

Many thanks for your help... Your thoughts on it helped me resolve exactly where I was at.

Best Regards,
Julian.