Introduction: DSO138 USB Power: No Boost Converter!
The JYE DSO138 is an excellent little oscilloscope for audio work and would make a great portable signal tracer. The problem is, it's not really portable because it needs a 9V power adapter. It would be better if it could be supplied from a standard USB power bank or any USB source. For some reason JYE did not take the opportunity to make the original DSO138 fully USB powered, which is strange as it is very easy to do. The PCB even includes a USB connector, but it does nothing! (There is an updated DSO138 MINI that is USB powered, but most people seem to own the original version).
There are instructions on the internet showing you how to add a boost regulator to convert USB power into 9V, but that is inefficient and unnecessary. In this instructable I will show you how to use the on-board USB connector directly for power input. I've also included an optional mod to get rid of the annoying waveform glitches that happen on some gain settings.
I've drawn the necessary mods on the original JYE schematic together with photos of my modded PCB.
Some hook-up wire
100uH 100mA (or more) inductor
470pF to 1nF capacitor
3k resistor (optional)
1.8k resistor (optional)
1.2k resistor (optional)
Remove D2. This is a safety precaution to disable the old DC power jack. This will ensure nothing goes wrong if you accidentally plug the old DC power supply in after you've done the USB power mod.
Solder a wire from the pad labelled VBUS to the pad labelled +5V. (These are labelled TP33 and TP21 on the schematic). This connects the USB power pin to the circuit's 5V power rail. The +3V rail is derived from this voltage by U3 which needs no changes.
Now remove U5 and solder a wire between the two outer pads, to jumper across where it used to be.
This takes care of the positive power rails, in the next step we'll mod the negative rail so it works off 5V USB too.
The DSO138 circuit uses a simple switching inverter to generate a rough negative voltage which is then regulated down to -5V by U4. Even though there is a feedback network to the CPU through R41/42, it appears JYE never implemented it in the firmware, the CPU simply produces a continuous 17.6kHz signal at R40. This means we have to modify the circuit to work from the 5V USB supply.
Replace the L2 with a 100uH inductor (rated for 100mA or more). I had one in my junk box. I had to bend the legs a little to make it fit in the same spot.
Add a snubbing network across D1 consisting of a capacitor in series with a 2.2k ohm resistor. It doesn't matter what type of capacitor you use, but the value should be between 470pF and 1nF. I used a 1nF plastic cap because that's what I had. This will clean up the switching waveform.
You're done! You can now plug in a USB cable and measure the voltages on the PCB test pads which should still be -5V, +5V and 3.3V. The next step is optional.
When viewing large signals you may have noticed glitches on the waveform. This is caused by excessive loading of U2B by the potential divider R6/7/8. The solution is easy:
Replace R6, R7 and R8 with resistors ten times greater in value, i.e. R6 = 3k, R7 = 1.8k, R8 = 1.2k.