10 years, 4 months ago.

AnalogOut gives only ~100mV when connected to a 18ohm electromagnet

Hi, I am connecting the analogOut (p18) of mbed-NXP-LPC1768 to an electromagnet. I set the analogOut to max ie.1.0 (3.3V) but the electromagnet has no effect. I checked the voltage across the electromagnet and it shows 100mV. However if I only connect the voltmeter and disconnect the electromagnet the output voltage is as expected ie. 3.3 V. Note: I am basically trying to control the input voltage and hence the magnetic field of the electromagnet with mbed.

Please help.

3 Answers

10 years, 4 months ago.

You're trying to source 183 mA from the mbed. It cannot drive this load.

Lerche

10 years, 4 months ago.

You are right but I am trying to figure out that how the voltage across the Gnd and VOUT is able to provide 3.3V across the 18ohm magnet but the Gnd and p18(analog out) is unable to do the same. When i connect the electromagnet to the analogout of mbed and check the voltage across the magnet its 100mV only but when i connect the magnet to Vout its 3.3 V across the electromagnet.

Could you draw a schematic?

posted by Christian Lerche 05 Feb 2014
10 years, 4 months ago.

Per the datasheet on the LPC1768 the load specified is 1Kohm which infers an output current of 3.3mA (per ohms law). The D/A output may be able to supply a little more current than that but certainly not 183mA. The reason your power supply will power the load is that it is capable of providing the 183mA the load requires. Think of trying to start your car with 8 AA cells in series. You have 12V but not nearly enough current to turn over the starter motor. The reason you are seeing only 100mV is you are essentially shorting out the output of the D/A.

You will need a buffer following the D/A output, perhaps a power op amp or a transistor based circuit. Please see the link below for some information on doing that.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&sqi=2&ved=0CCQQFjAA&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fml%2Fsloa086%2Fsloa086.pdf&ei=NWTyUv7kDIifyQGE7oDYAw&usg=AFQjCNGsK_kCiZrIcPN-S--DOEzydwTYCw