No Load Cut off circuit

Thread Starter

Jahangir Alom Sumon

Joined Jun 13, 2019
4
Hello. I am using a inverter which driven by 12v DC battery and provide 500w. Now I need a circuit which automatically turn off the inverter in No Load condition and again turn on when a load is connected with the inverter.
Please help me.
Thank You.
 

danadak

Joined Mar 10, 2018
3,578
One way would be a controller that disconnects load (if its there or not), with a relay or SSR, then drives a V looking for a current flow. Possibly done as a Z measurement. Use of relay then makes design impervious to way inverter output was designed.

Using Z as measurement makes load presence measurement independent of load type, eg. capacitive or inductive, it will simple take a measurement and compare that to values known for no load connection.


Regards, Dana.
 
Last edited:

kubeek

Joined Sep 20, 2005
5,603
How do you propose to detect if the load is connected or not, when the inverter is off? Turn it periodically on to test?
 

Kjeldgaard

Joined Apr 7, 2016
378
Could a solution be to have a push button to turn on the inverter with a self-holding relay, then measure the consumption on the inverter and release the self-hold on the relay at idle?
 

danadak

Joined Mar 10, 2018
3,578
@kubeek, interesting idea, one additional consideration would be
safety, eg. having an inverter turn on while user is wiring to it. Maybe
add a safety switch user turns off any auto start.

Regards, Dana.
 

MisterBill2

Joined Jan 23, 2018
3,884
How do you propose to detect if the load is connected or not, when the inverter is off? Turn it periodically on to test?
For at least 20 years there have been systems that use a small DC voltage to monitor the output and when something is switched on a relay engages and switches on the inverter while disconnecting the monitor circuit. That was even used with rotary inverter systems back in the 1950s. It works well. A current version would use an opamp and a power FET to switch on the inverter, although a mechanical relay to do the switch over, I think.
 

MisterBill2

Joined Jan 23, 2018
3,884
For load sensing a pair of 10 Amp diodes in anti-parallel in series with the inverter output lime will allow an external DC source to provide a bias voltage across the AC line wiring. When any appliance is switched on current will flow and the voltage will drop. That can trigger a sensitive relay to initiate a short start-up sequence: latch the start relay, disconnect the sense circuit, connect the inverter to the AC circuit, and power up the inverter, and finally, connect the load-disconnection sensor to the diode pair.
When the appliance load is switched off, the voltage across the diodes drops to zero and a timer starts. After some short number of seconds the timer causes the latched on start relay to be released, and the inverter switches off. Presently most of the functions can be done with rather simple circuits, and really only one relay is needed, the one to connect the inverter to the AC line to feed the appliances.
 

Reloadron

Joined Jan 15, 2015
4,517
Before I would worry about turning the inverter on or off I would measure the current draw under idle or no load conditions. A 500 watt inverter, under a full load will draw about 42 amps of current (500 watts / 12 volts = 41.66 amps plus the inefficiency of the inverter. That same inverter under no load likely draws less than 1 amp. I would start by measuring the no load 12 VDC current draw, then worry about turning the inverter on or off.

Ron
 

MisterBill2

Joined Jan 23, 2018
3,884
Before I would worry about turning the inverter on or off I would measure the current draw under idle or no load conditions. A 500 watt inverter, under a full load will draw about 42 amps of current (500 watts / 12 volts = 41.66 amps plus the inefficiency of the inverter. That same inverter under no load likely draws less than 1 amp. I would start by measuring the no load 12 VDC current draw, then worry about turning the inverter on or off.

Ron
Even a 1 amp static draw will pull down a storage battery eventually, and so needs to be avoided. And the rotary inverters that I mentioned drew about 20 amps at 48 volts, no load. And around 30 amps with the load switched on. So it was more important there. Yes, that was a few years back. And many inverter designs do not sense the load and so do have some serious no-load draw. So really, the switching control is important.
 

ScottWang

Joined Aug 23, 2012
6,827
At 12 volts 599 watts is a lot of amps, far beyond what I would want to be switching with any sort of button switch. I have already described the solution.
Thanks.
I have thought that if the output is 12V(it looks doesn't make sense) or 24Vdc or 48Vdc then just use the button switch maybe isn't a good idea, but it is the way to let our helpers to think that no needs to use any detector, but it still have some other ways to do.

Actually the situation doesn't clear yet, since it is a 12V Inverter 500 Wout, how if the output is 110Vac or 220Vac then the output current is complete different.
 

MisterBill2

Joined Jan 23, 2018
3,884
Thanks.
I have thought that if the output is 12V(it looks doesn't make sense) or 24Vdc or 48Vdc then just use the button switch maybe isn't a good idea, but it is the way to let our helpers to think that no needs to use any detector, but it still have some other ways to do.

Actually the situation doesn't clear yet, since it is a 12V Inverter 500 Wout, how if the output is 110Vac or 220Vac then the output current is complete different.
ALL of my comments have been presuming that the inverter is powered from 12 or 24 volts DC, and that the inverter supplies either 120 or 220 volts AC to wiring within the vehicle. Of course, those inverters were all in some sort of emergency vehicles, usually EMS of some variety. The rotary inverters were used in towboats that had dual engine drives and 48 volt systems.
 
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