AP1: Setting up PWM for EHMx-Lxx

 

EHMx-Lxx are emitters of alternating electric ("E-field") and alternating magnetic ("H-field") fields, which have no integrated microcontroller. Therefore users should take care of generating necessary signals to control the emitter. E- and H- parts of the device are independently controlled through the CMOS-level PWM inputs (E-PWM and H-PWM). Both lines are pulled down to ground. For setting the PWM control signals, the milliamperemeter, oscilloscope, laboratory power supply (0-30V or 0-40V) and a signal generator (or a microcontroller) are necessary.

 

Preparation:

 

  1. Connect "VCC" of the EHMx-Lxx (red wire) through milliamperemeter to "+" of the power supply unit, "GND" (black wire) to "-". Set the voltage on 6V, leave both PWM input unconnected. The milliamperemeter should measure a current <1mA.
  2. Connect "E-PWM" (green wire) to a signal generator, set the frequency 15kHz, duty cycle 50%, square waves, power supply - 6V. Leave "H-PWM" unconnected. The milliamperemeter should measure a current ~4mA.
  3. Connect "H-PWM" (blue wire) to a signal generator, set the frequency 15kHz, duty cycle 20%, square waves, power supply - 6V. Leave "E-PWM" unconnected. The milliamperemeter should measure a current ~15mA.

 

Generating PWM

 

  1. The control signal is a square wave of frequency between 15kHz and 25 kHz.
  2. Low-frequency modulation (1mHz-1kHz) of this currier signal defines the frequency of corresponding AC field.
  3. Set first duty of the main currier signal (15kHz-25kHz) to 10%-20% and the power pupply to 6V. Slowly increase the voltage and monitor the current through the emitter. For heat-free mode, it should not overstep 25mA for E- and H-parts of this device.
  4. The intensity of the field can be controlled either by duty of the main carrier signal or by the voltage level of the power supply. The following table shows approx. value for at 25kHz/100Hz signal at different values of duty, voltage and current (for EHM5-L8R module).
    Voltage (V) Duty (%) Current (mA) H field (µT) at 100Hz E-field (V/m) at 100Hz
    6 50 15.5 186 ---
    12 50 31.2 213 ---
    18 50 47.2 255 ---
    6 20 5 150 ---
    12 20 10 530 ---
    18 20 15 689 ---
    25 20 21 754 ---
    30 20 27 813 ---
    6 50 1.7 --- 774
    12 50 6.6 --- 5220
    18 50 10 --- 6600
    25 50 15 --- 4540
    30 50 18 --- 4228
    6 20 0.9 --- 100
    12 20 2.4 --- 1400
    18 20 4 --- 3200
    25 20 5.9 --- 5740
    30 20 7 --- 6400
    30 29 9.6 --- 7700
    30 30 10 --- 7000
    30 35 12 --- 5740
  5. When using laboratory power supply, we strongly recommend to keep the duty by 20% and to regulate the intensity of fields by setting the voltage level.    

 

Test:

 

  1. Functionality of the emitter can be easily tested by using a small magnet (on H-Side, AC magnetic field) and a piece of paper (on E-Side, AC electric field). See this video (mp4).
  2. For tests and measurements put a E/H field meter closely to the surface of the emitter. The following diagrams are produced by "Spectran NF 5010" at 15kHz/100Hz, duty 20% signal, power supply 30V for E-side and 25kHz/100Hz, duty 30% signal, power supply 30V for H-Side of the EHM5-L8R module.
     

 

Heat-free and heat-generating modes

 

The emitter has a high efficiency and normally does not produce essential heat during operation. However in several applications a heat should be generated. To achieve this effect, increase the duty to 50% or higher first at a low voltage level and by monitor rise of temperature (both E- and H- parts can be used to produce a heat). Setting different voltage at high duty of the main carrier signal it is possible to control the heat generation. It should be however noted, that a peak current of 60-70 mA and a temperature of 55°C should be not overstepped.