CRSNB Mini Pure Sine Wave Inverter

       When I saw this, I wanted to get my hands on one as I have not seen one this small which is a pure sine wave output. I have one from 2008 but it is 120V and MSW output, this one is PSW and is 220V.

       This was a broken one so this is an excuse to tear apart to see the parts, circuit design, build quality and attempt a repair.

The inverter as I got it (packaging not shown)

The input plug has been chopped off and reverse polarity was applied which caused the damage.

Specs are on the sticker underneath.

Plug can accept US, EU, AU plugs so it is virtually universal.

It can take a 3 prong plug with a little bit of work.

Under the sticker is the button switch.

The PCB cannot be pulled out of the case unless the green button cap is removed.

Input side end plate removed.

There is a blower fan! It is also used to press down on the main switching devices. The aluminum extrusion case bottom is used as the heatsink.

Fan pulled out. PCB is on its way out.

Board taken out of the case.

Bottom side of the PCB. This was actually filled with flux residue but I cleaned it off with IPA.

Circuit is pretty dense. A microcontroller is on a sub board which plugs on the main board.

Top of the sub board.

Yeah, the chip is not mounted exactly spot on.

It contains the 7 segment display and the PIC 16F882 uC with a 16MHz clock.

This uC provides the switching control for the DC-DC converter switches. It also uses ADCs to sense temperature, read input and output voltages and control the display. Basically, housekeeping duty.

Bottom side is a pair of 595 shift registers for segment driving.

Output H-bridge devices are Unisonic 5N60, 5A 600V MOSFETs.

Output inductor is glued on for mechanical support.

Brown dipped caps with the inductor forms the output LC filter.

The left device is a 7805 which is used for the USB 5V supply. Not the most efficient design but would work in a pinch. The SMD device on the right of the USB jack is a 78M05 which supplies the logic circuitry. Main switches are IRF1404 which is very overkill for this job. Nice! SO8 device is a IR4427 dual low side driver for the main switches. Nice!!!

This bunch of components are the sine wave oscillator and drivers. A PIC16F716 generates the sine wave modulation, a pair of IR2110 high/low side drivers are used for the output H-bridge. Nice!!! I can see the circuitry is nicely designed with application specific IC's. It would be a pain to fix if those broke, they don't come cheap! I have no idea what the trimmer pot is for.

Transformer has a thermistor siliconed onto it for thermal sensing.

After a little troubleshooting, 5V logic supply (78M05) is dead, tried connecting it to USB5V as temporary check. Display works, unit beeps but does not produce output. Turns out 7805 is also dead as it gives 3.7V only. Luckily, I had both in stock and replaced them. Would it work?

There is also a blown trace which is fixed by a blob of solder.

Is it working?

Beeped and volts reading is 1.1 to 1.5V low but appears to be working.

And it works!

Idle power consumption is 590mA at 12.5V input.

Output is regulated so idle current drops at higher battery voltages, and increases at lower battery voltage.

I checked that the output is galvanically isolated from the input so I connected the scope probes on the outlets. The waveforms are similar to the SPWM trick used by EG8010 based inverters, but this one used a PIC microcontroller to emulate it.

And here is a scope pic, taken from a transformer secondary output.

This inverter had no trouble running an AC stand fan, I have not tried other loads yet.

       Circuit looks well designed but the electrolytic caps are all no-name which will blow up eventually. I tested it with a small induction motor and it works well so this should be a nice portable use inverter.

30 Aug 2016:

The fan runs full speed all the time even when the inverter is off so it is kind of wasteful. I wondered what the pad was for and realized it is a temperature controlled output which runs the fan when it is hot.

I modified it by adding a 22R resistor between the original fan negative and temp controlled negative and connecting the fan to the controlled output. What happens is that the fan runs at low speed for sufficient ventilation even when not hot enough then runs full speed when it gets too hot.

I added a trimmer to mod the voltage sensing divider to make the display more accurate. Trouble is, it made the inverter shut down on normal battery conditions. The inverter shuts off below 10.0V and above 14.3V. The lower limit is fine but the upper limit is a little low as lead acid batts can easily get to 15V when charging.

After I left it running the whole night off of the secondary off grid system powering cellphone and laptop chargers, It turned off in the morning as it has the over voltage protection tripped by the charge controller in absorb mode at 14.7V. Looked at the inverter and the display lost 1 segment.

The buzzer is very loud so I taped it. It does not serve other purposes as it doesn't seem to beep when any protection is triggered. It only sounds when the inverter is turned on or off.

I tested it running a rotary tool. startup surge pulled about 18A from the battery pack and 6A running at no load. It worked pretty well, got only a little warm.

Page created and copyright R.Quan ©29 Aug 2016.