Open Inverter - Part 5, More Experimentation

Today was a day in the lab to try out the various H-bridge modules I have access to, plus the toroidal transformers, which are more efficient than the E-core types that Trystan and I were using last week.

Low Cost IBT-2 Modules

Some months ago, I  bought a pair of the IBT-2  H-bridge modules from an ebay seller. These are advertised as 43A and 24V. They use the now obsolete Infineon BTS7960 half H-bridge IC, that I discussed in Part 3.

The IBT-2 Module uses the Infineon BTS7960 half-bridge module

These boards are 50mm x 50mm and the holes (3.2mm) are on 45mm centres - this is an ideal size to take advantage of the low cost 5 x 5 cm pcb services being offered.  I paid about £7 each for these - including free shipping from China!  There are several variants of this module around, as it has been widely copied in China by various manufacturers.

It uses two of the Infineon BTS7960 half bridge modules, and a 74HC244D buffer-driver IC to provide some isolation between the "Arduino" and the H-bridge.

Not entirely visible is the anodised aluminium heatsink that is screwed to the underside of this module - to take the heat from the BTS7960 devices.

The BTS7960 is quite easy to drive - it has a single PWM pin an an /INHibit pin - active low. Only 4 port lines are required from the Arduino to drive this module.

At first I was getting some very unusual waveforms from the output tab of the ICs.  I soon found out that they are limited to a 25kHz pwm signal - and I was using 62.5kHz. I made a quick change to the PWM timer control register - to reduce the PWM to 7.8125kHz and all was well - I was getting good clean sinusoidal signals from the IC tabs - with the scope set to 1kHz low pass filter mode

Connecting the Transformer

I happen to work for a company that uses a lot of toroidal transformers of various VA capacities. Today I selected our smallest and cheapest - which is 120VA with a nominal 24V rms secondary and 5A current.

Our standard 120VA toroidal transformer has a nominal 24Vrms secondary winding @ 5A

The transformer is intended for both 115V and 230V operation - so it is a case of connecting the split primary windings in series in order to get 230Vrms output.  If you get them the wrong way, the phases wil cancel out and you will see no output.  If you want 115V, you have to get the primaries the correct way around in parallel - otherwise you will short them out - which is bad   :-(

In initial tests, I found that the "Magnetising Current" for this particular transformer was 0.22A from a 24V dc supply.  So the inverter burns 5.28W when idle - before an ac load is attached.

The BTN8960 H-Bridge

My company makes products that uses brushed dc motor drives - between 100W and 600W.
Earlier this year I designed an experimental board o use the newer Infineon BTN8960 half-H driver IC. This replaces the BTS7960 - which is now end of life - and becoming harder to find.

On the left is an "Arduino" providing the pwm drive signals to the BTN8960 H-bridge board on the right.

The "Arduino" board is a ATmega328p-AU with 16MHz crystal, reset circuit and FTDI connection. Most of the I/O is brought out to connectors for easy plugging - this allows simple generation of 50Hz complimentary pwm sine waveforms using Timer 2.

The board on the right is an experimental motor drive board I cooked up earlier this year to evaluate the BTN8960 devices for dc motor control. The BTN8960 devices, IC1, IC2 are located in the middle of the upper and lower edges of the pcb - with the large dc-link capacitor located between them. A thermistor (thin yellow wires) allows the temperature of the upper BTN8960 to be measured.  This board has no external heatsinking -  but relies heavily on large "flag" copper areas both on the upper and lower surfaces of the pcb to dissipate the heat.

The 24V dc input to the board is on the lower left, and the output to the toroidal transformer is on the right edge of the pcb.  The orange device is a relay which allows the toroid to be disconnected from the transformer.  The board also includes a LM2576  5V "simple switcher"  voltage regulator - for powering the microcontroller.

Here we see the full set-up.

The Driver Board, Toroidal Transformer and switched socket outlet complete the prototype Inverter.

So that was the state of play at 6pm this evening. I had the opportunity to connect my Weller soldering iron station to the output of the transformer. Off load the mains ac output was 240V dropping to 238Vrms when the soldering iron was plugged in. It used 24V dc at 1.15A from the 24V bench supply to power the iron.

More Testing Tomorrow.

Today I was lacking in  suitable 230V loads to try. Tomorrow I have a bunch of 240V 60W incandescent lightbulbs to try out and slowly push this inverter up in power output to characterise its performance and efficiency.