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Load "sensing"

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Rando86

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post load sense

I am doing a project for a module that controls the switching of a few different loads. A microcontroller will be controlling most likely a MOSFET to switch the current to the load. The module must be able to "sense" the load, in that the microcontroller must know if a a load open-circuits - such as a bulb has burned out, or if there is a break in the wires to the load. I was wondering what the least complicated and most efficient way of doing this is. Any ideas are appreciated, Thanks.
 

Sense the current through the load. If the current is too low or zero - the load is open circuit. If the current is too high - the load is shorted. Are you driving DC into your load, or AC? How much current will the load draw? What's the maximum current your system can deliver?
 

Thanks for the reply. Yeah, if I could sense the current through the load that would definitely work, but I can't think of a good idea to do it. All loads are driven by DC. There is about ten loads and the current for each ranges from 50mA to 20A. Each load would be driven by an individual solid state device, most likely MOSFET. I need to conserve space, and with ten different loads, if the circuit is too complicated, it would require a lot of parts and space. I was trying to find a spot on the load driving circuit that I could check the voltage of with the uC to determine if the circuit opened.
 

Sensing load current is a common method, applying a small test current another. I don't think that there many additional options. You can check your specification what is more suitable.
 

Rando86,
The most common method of current sensing is to place a small resistance in series with the load. The voltage across the resistor is then applied to a comparator to produce a logic level for the microcontroller. This works well if the current sense resistor can be placed in the low side of the load. If the current sense resistor has to be placed in series with the high side of the load, then you would need to provide a ground referenced voltage for the comparator. This can be done with a differential amplifier.
Regards,
Kral
 

I don't think there are many ways to differentiate between wire failure or load failure. Specifically for bulbs you could detect higher current in the begining when filament is still cold, but there is no way you could tell if circuit is broken because of bulb or wire. Not unless there is some electronics at the load itself to signal back status of load. You could maybe put "ID" resistor in parallel with load to allow some current when load fails as indication of wires functional status. Current draw of this resistor should be easy to detect and if lower consumption is needed more softisticated circuit should send signal of load status.
 

The suggestion by Kral is most likely the most reliable way.
The digtal only way (might require more inputs
or use a multiplexer) Have one input on your cpu with a weak input resistor. It could be the one built into the cpu or an external one. If you use the external one you might want extra circuitry to disconnect it to prevent leakage over time. If this input is connected to your lamp and the lamp is off then if the lamp is open circuit you will get a hi on the input or else it will be low because of the lamp pulling it to ground.
 

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