My EspoTek Labrador was delivered early, so I got some tests in with the scope today. I don’t really know what I’m trying to accomplish with this particular test. Just curious how the diode and driver behave.
Disclaimer: I am not an expert on the operation of these drivers or diodes. I’m just observing and hypothesizing for my own knowledge. If I state something incorrectly or if you can provide more insight, please let me know.
Setup:
- BlackBuck 8M
- 5A current setting
- PWM driven from an arduino
CH1 = Diode Voltage (yellow)
CH2 = PWM Out (cyan)
At low duty cycles, it isn’t strictly an on/off at full power. I hope this will alleviate my issue of still burning the work piece at low power while aiming with the stock driver. That one really had a hard on and off since it was just switching the full voltage on and off.
About 2-3%:
About 7-8%:
About 35%: This one is interesting becuase it’s overshooting a fair amount. The diode Voltage at 5A continuous is 4.7V. This is peaking at 6.2V.
About 80%: This one is not over shooting as much as the prior test. Peaks at 5.8V and averages out around 5.3V before the cycle starts over. This pattern goes away as you approach 100%.
I was curious what the current was doing so I put a 0.1ohm sense resistor in line to see what that looks like. It’s super noisy, but you can kind of see the same pattern. Triggering wasn’t working well due to noise, so I widened the time a little to 20ms across the screen. The scope shows the voltage drop across the sense resistor. V*10 = A
I think this overshoot is a characteristic of the diode, rather than the driver. Using a purely resistive load, there is no overshoot. However, a higher voltage is needed to get to 5A, and closer to the 12V input to the driver, so that might have something to do with it.
I would hypothesize that most constant current power supplies overshoot a little when they first come on. The ones I have looked at use a sense resistor at the output to adjust the voltage being delivered to achieve the desired current. Being relatively analog in nature that will take time to reach equilibrium.