Beware modified sine inverters!
Posted: Thu Jul 24, 2008 10:36 am
I'm looking to take back my 600W inverter to the shop I bought it from a couple of weeks ago.
It works ok (in so much as modified sine ones can) but while the lights and the laptop work on on it, the LCD TV doesn't care much for it. The TV works but makes a loud mechanical 100Hz buzzing from it's power unit. On mains it works silently.
Then today I tried out the plug-in kWh meter I bought at another car boot sale last week. It too made a loud buzzing noise and gave some weird results on its meter. With no load plugged in it was saying that there was a 3W load (or 25VA) at 120mA and the power factor was 0.00 instead of 1.00. It also said that the mains frequency was "DC"! Plugged into the house mains it read normal (no current, no power, 1.00 PF, 50Hz).
Worst was the burning smell it made! Taking it apart I found a badly designed power supply to get from 230VAC to about 17VDC to run the electronics. It had a 100R 3W power resistor in series with a u68 cap on the live rail going into a couple of diodes and then some more resistors. On the mains, the cap limited the 50Hz power that could pass through the rectifiers and the resistor sat at 40 degrees C. On the modified sine supply the squarish wave HF harmonics went straight through the cap and started to fry the resistor - I measured 180C before the insulation on the thermocouple started to melt!
Who knows... maybe that could be happening in my TV (although I've not noticed any burning smells from that yet).
There's a reason why modified sine inverters are cheap...
I've found a seller who has a new range of Cotek pure sine inverters that are better than older types and quite a bit cheaper. These ones are 1kW continuous with 2kW surge rating and have a load seeking standby mode to reduce their idle current to about 0.25A. When you turn "on" a load the inverter wakes up from standby to live. Otherwise it would sit there drawing about 1.2A DC while doing nothing. Of course you can still turn it off manually as well. It's been a bit of a pain to have to turn on the light switch on the lamp and remember to turn on the DC relay for the inverter as well (and remember to turn it off before going to bed or else leave it sitting there pulling 0.8A all night).
It works ok (in so much as modified sine ones can) but while the lights and the laptop work on on it, the LCD TV doesn't care much for it. The TV works but makes a loud mechanical 100Hz buzzing from it's power unit. On mains it works silently.
Then today I tried out the plug-in kWh meter I bought at another car boot sale last week. It too made a loud buzzing noise and gave some weird results on its meter. With no load plugged in it was saying that there was a 3W load (or 25VA) at 120mA and the power factor was 0.00 instead of 1.00. It also said that the mains frequency was "DC"! Plugged into the house mains it read normal (no current, no power, 1.00 PF, 50Hz).
Worst was the burning smell it made! Taking it apart I found a badly designed power supply to get from 230VAC to about 17VDC to run the electronics. It had a 100R 3W power resistor in series with a u68 cap on the live rail going into a couple of diodes and then some more resistors. On the mains, the cap limited the 50Hz power that could pass through the rectifiers and the resistor sat at 40 degrees C. On the modified sine supply the squarish wave HF harmonics went straight through the cap and started to fry the resistor - I measured 180C before the insulation on the thermocouple started to melt!
Who knows... maybe that could be happening in my TV (although I've not noticed any burning smells from that yet).
There's a reason why modified sine inverters are cheap...
I've found a seller who has a new range of Cotek pure sine inverters that are better than older types and quite a bit cheaper. These ones are 1kW continuous with 2kW surge rating and have a load seeking standby mode to reduce their idle current to about 0.25A. When you turn "on" a load the inverter wakes up from standby to live. Otherwise it would sit there drawing about 1.2A DC while doing nothing. Of course you can still turn it off manually as well. It's been a bit of a pain to have to turn on the light switch on the lamp and remember to turn on the DC relay for the inverter as well (and remember to turn it off before going to bed or else leave it sitting there pulling 0.8A all night).