[thingstodo]

I decided to try one of the inverters that I use, that works OK so I can get an idea of the signals. I can always go back and investigate the surplus stuff. The manufacture date is 2010.

The reason that I'm taking apart DC/AC inverters is to see if I can use the DC/DC converter part ( the first part) to boost the 12V up to 150 VDC, then chain them together in series to build up 300 VDC (for a 208VAC three phase test inverter) and eventually 900 VDC (for a 575VAC three phase inverter). Along the way, I'd like to do some testing on the inverters to determine the efficiency (estimated around 85%), what cooling is required (is a fan and a heat-sink enough?) and what sorts of things can be done to deal with a DC/DC converter failure while operating the electric vehicle. I have some crazy ideas I'd like to test out.

Back to the topic at hand

The ends come off after the screws are removed, then the bottom plate slides out the end to reveal a board mounted on standoffs from the 'top' of the heat sink.

The setup is remarkably similar to the Wagan, on a smaller scale. Perhaps this is an optimal design. I'd guess that some of the same people were involved in the design.

There are only four 40A fuses instead of 16. They are not associated with each of the 6 transformers, but are all in parallel. There are diodes visible beneath each of the transformers. There are two power transistors for each transformer. The output section has 4 power transistors, with only three larger capacitors in that area.

The electronics to switch the transistors appear to be distributed around the board, in areas that are not required for the power section. The control signals would be much more difficult to separate, since they are part of the multi-layer PC Board instead of routed through ribbon cables.

The power transistors are electrically isolated from the heat sink with a barrier that must be pretty good at thermal transfer. The transistors do not use the tabs, but rather have a mechanical clamp to hold them firmly against the barrier.

There are number of places on the board that are marked as resistors that have 16 gauge copper jumpers soldered in place.

Silly things which appear to be mistakes:
- the cooling fan has a cable pinched between the fan case and the bottom of the enclosure. The fan needs to be rotated 90 degrees and the cable would not be pinched.
- there is a pinch in the insulation of the output for outlet 1. There is clearance, but it was pinched anyway due to no cable management.

Connect the power supply to the DC to AC inverter. Turn on the inverter. The inverter powers up, then turns off? Check the voltage output from the power supply. 17.7V. Check the alligator clips, voltage at the inverter input terminals - all good. 17.7V was within the 10 - 18VDC for the other dc to ac inverter. The voltage range on this one is not printed on the outside of the case. Try lowering the voltage to 16. The inverter powers up, then off, then powers up, then off. There is an audible alarm that I think means the inverter input voltage is low. Check the voltage - it is dropping to 10V, the inverter powers down, then back up when the voltage rises above 13V, and the cycle continues.

The inverter appears to require more current than the power supply can put out. 1.1A is not enough.

Change power supplies. This one will do 5A at 20V max. The DC current in is around 2.5A at 16V to power up the inverter. I may have to revisit the other inverter. It is larger and likely needed more current to 'turn on'. Perhaps later.

Measure the output voltage - 0VDC, 0 VAC. What? It turned on. There should be voltage present. Verify good voltage in. The power LED is lit. The run LED is lit.

Try plugging in the WattsUpPro (a power, current and voltage measuring device for 120 VAC pluggable tools). It draws very little power, but it is a load and it will read voltage input. No display. How did I break this?

Turn off the power switch to the inverter. Turn it on again. Why? It was something that occurred to me, just because. The inverter powers up, the LEDs are both lit, and the WattsUpPro display lights up. 122.2VAC is what it shows. What gives? Dig out the manual - the inverter goes into a sleep mode if there is no load for 2 minutes. I guess I wasn't very fast getting the meter connected to check out the voltage.

Now that I know it needs a load, leave the WattsUpPro plugged in. The voltage at the output stage (the DC that is switched to generate the modified sine wave) is not easily measured with probes as large as the ones on a meter. I did get it done, eventually. The voltage is 135 VDC, which is lower than the 150 VDC I got from a smaller inverter that I had taken apart previously. I should check with a larger load on the inverter - perhaps it changes with load? Try a light - 1A at 120VAC is over 10A at 12VDC - overload and shut down. Same with the electric drill. I found a load small enough to plug and and not draw down the power supply. It's a 12V trickle charger, 12V at 1A output. The DC current rose from 2.5A to 2.6 or 2.7A. The DC Bus voltage drops to 134.5VDC. Unplug both the WattsUpPro and the charger. The voltage rises to 138 VDC. It's not regulated well, but I'm still at very low output power. I'll verify with a larger load when I get a battery connected. It's somewhere between 130 and 150 VDC.

The 'DC bus' is a bit hidden on this inverter. The transformers that step up the voltage go through a diode, and the output goes through the board. It may be to the other side or even to an internal layer, since this is a multi-layer board. It is not accessible from the top. To get access to the bottom of the board, I'd have to remove the clamps that tie the power transistors to the heat sink - all 16 of them. I'd rather not go there. The 'DC Bus' does come to the top of the board where it connects to the output transistors. That's the only location that I can see. It would not be easy to solder to that pad.

If I were to use this inverter as a DC/DC converter, the 'on/off' switch could easily be replaced by a relay. That would 'turn off' the load on the batteries. I should check what the current is when the switch is off. It drops to 0, then wanders a bit, from -0.1 to 0.1A - but I'm using a clamp-on meter since my regular meter does not measure currents above 100 mA. I expect that the inverter has no load when turned off, since the manual describes permanent mounting in a vehicle. If there was current draw when the switch was off, the vehicle battery would drain. That would not be a popular feature.

I'm not sure how I'd fake a 'load' for the 120 VAC outlet if I were to use this unit as a DC/DC converter.

Another thing that I have to verify when I get a battery connected. I guess that'll be it for tonight.