Single Vs Stacked PWM Scope Results!

[vornado]

I have some free time, and a nice oscilloscope.
I was reading a thread about a single batter Vs stacked batteries and there was a lot of wondering going on about what the peak voltage looked like. So here are the results!

I do not have any way of testing under load at the moment (but will likely buy some connectors for a proper test box some time soon, especially if requested), so these numbers and waveforms are just from the top of the Vamo without an atty attached.

All configurations fired at the typical 33.3 Hz PWM so I won't mention that further.
All tests were done with 18350 batteries fully charged.

I also forgot to measure the duty cycle for these tests but if anyone is really curious for the exact numbers I can remeasure it, though I admit it would be much more interesting to see it under load.

First up is a single 18350 battery at it's 3.0 V setting-

It is firing at around 7.6 volts peak to peak (IE it's max).
It is firing at 3.1 volts RMS. At it's 3.0V setting with no load this seems plenty accurate. I expect it would drop closer to 3v under load.

Next up is a single 18350 at the 6.0 V setting-

It is firing at around 7.4 volts peak to peak (IE it's max). This is slightly less then the 3V setting, but this shouldn't matter much.
It is firing at 6.1 volts RMS. At it's 6.0V setting with no load this seems plenty accurate. I expect it would drop closer to 6v under load.
Also worth noting is that the pulse width modulation is near a 100% duty cycle, likely indicating it's at the maximum voltage it can push with a single battery. With a partially drained battery I would suspect it has a much harder time maintaining this 6V setting.

Next up is the 2 18350 mode (stacked) at the 3.0 V setting-

This is where it gets interesting.
It is firing at 9.4 volts peak to peak (IE it's max). This is much more than the single battery mode.
It is firing at nearly exactly 3.0 volts RMS. It seems to do a better job at voltage regulation with a stacked battery.
The duty cycle is also much reduced to compensate for the higher peak voltage.

Next is the 2 18350 mode (stacked) at the 6.0 V setting-

This is where it gets really interesting.
It is firing at 9.4 volts peak to peak still (IE it's max).
It is firing at 6.1 volts RMS.
The duty cycle is also much reduced from the single battery 6v setting. This seems to me indicative that it would do better at pushing a higher voltage (and do it more accurately) and possibly current load with stacked batteries.


Summary-
I typed a lot. There are pretty pictures.
The single battery mode seems to operate at the voltage booster's limits and it may struggle with higher voltage / power loads.
The stacked battery mode provided a higher overall starting voltage and more wiggle room with at it's maximum settings so I assume it would be more accurate with a heavier load.
The stacked batteries also provide a larger peak-to-peak voltage that gets PWMed so the coil will experience a higher voltage for a shorter duration for the same RMS voltage. I'm not sure if this is good or bad. I would think it's somewhere between "mehh" and "not good".

If you have any other tests/measurements I'd be happy to take any requests, and will consider building a proper load testing rig if anyone really wants those numbers too.

I'm an engineer by hobby, not trade so if anyone has any actual qualifications or more experience feel free to correct me if I got anything wrong

I hope this clears up some confusion with the VAMO's various battery modes.

 

[nahoku]

It appears that Vamos differ by quite a bit. From what PBusardo showed as the p-p voltage on the Vamo he reviewed, it sat at about 6 volts. Here yours sits at 7.4 - 7.6 volts. The stacked battery p-p voltage being higher doesn't surprise me as this is what I already suspected, but sitting at 9.4 volts is interesting.

For sure, with a higher peak voltage, PWM has more room to play with for a given setting, so all your assumptions seem very plausible.

Thanks for this! By the way, I am an engineer by trade, but I no longer have access to an o-scope! Sad but true... then again, being retired and waking up whenever I want to has it's pluses too!

 

[UncleChuck]

I'm most interested in the high peak voltage. I could have sworn I remember watching more than one vid (Pbusardo might have been one) where the peaks showed about 6v.

If you have any way possible to do so, could you run this test again, but under load? I'm wondering if maybe under load the peaks drop down to around 6v. That would seem like a large sag in voltage, but I'm not exactly sure on how PWM and voltage sag work together. I'm thinking since the voltage is boosted much higher than the voltage of the battery, the voltage sag is also amplified, which would result in roughly 6v peaks under load.

If this is the case, repeating this test under load would show more drastic differences in peak voltage between a single battery, and dual battery setup. With no load the single cell can be boosted a lot higher than it would normally, where as using dual batteries the voltage is already very high, so it might be better at staying there.

Something like a single batt going to roughly 6v peaks, but dual batts going to the same peaks we are currently seeing (over 9 volts peak)

Thanks for the time put into the tests!

 

[nahoku]

Yes, I would also like to see additional waveforms, but request VW mode waveforms. I find it strange that if my Vamos are set to 4 watts and higher, I can measure approximately peak voltage with just a DMM. At 3.5 watts the meter blinks like it can't read due to PWM.

If you can show VW mode waveforms for the following settings, it would be much appreciated. Single battery, unloaded is fine.

3.5W, 4W, 9W, 15W

TIA!

Dang! I need an O-scope!

 

[UncleChuck]

I'm not sure how numbers for wattage would be very useful, depending on the resistance of your heads the voltage will be totally different. You could easily calculate the voltage that SHOULD be supplied for a given resistance at a given wattage, but all this would tell you is that the device's ability to calculate voltage in wattage mode is working properly, which has never been in question AFAIK.

Say you have a 2ohm carto, and have it at 9 watts. The device should be producing about 4.2v. At 15 watts the voltage should be around 5.5v. The scope will likely look exactly like it does when the voltage setting is used, as you could simply scope the device while the voltage is set at 4.2, or 5.5 v.

Basically like I said, I just don't think there is any additional info to be gained by checking it again in Wattage mode, since the signal will be identical to whatever the signal was for the voltage the device is supplying.

 

[nahoku]

The reason I would like to see VW waveforms is because the Vamo appears to act differently in VW vs VV. As I stated, my DMM will read close to peak voltage at 4 watts and higher in VW mode. However, in VV mode, the only time it reads peak voltage is when the Vamo is set to 6 volts... other than that, the DMM appears to be hit with PWM. Mind you this is open circuit Vamo measurements but why the VV vs VW mode difference in DMM reaction? You would think the DMM would act the same relative to VV voltage for the relative VW settings. Now whether or not there is an difference in the waveforms for VV or VW, I don't know, but would like to see if there is.