Reduce EMI by driving high-power LEDs
High-power LED modules for infrared radiation in water quality monitoring systems operate using high-current square wave signals that cause strong electromagnetic interference (EMI) on the system's bioelectrode sensors, resulting in compromised water quality data.
Figure 1 shows one method of reducing EMI, and an example is described below.
Reducing EMI block diagram Figure 1 is a block diagram of a method for reducing EMI caused by the input square wave signal of the water quality monitoring system.
In this example, a 30W high power LED module will be considered, which consists of six 5W LED packages and requires more than 12 VDC and 2A under full load conditions. The threshold voltage (VH) of approximately 11 VDC is the voltage required for the LED to emit light. The DC power supply provides an initial voltage of V1 = 10.5 VDC, which gradually increases to just below the critical voltage VH.
A second DC voltage (for this example V2 = 3.7 VDC) is then placed in series with the first DC voltage so that the sum of the applied voltages V1 + V2 is greater than 12 VDC (resulting in light emission).
Read More Design Concept V1 and V2 both operate in series with solid state relays. When the input square wave is at VH, the relay opens and V1 (the critical voltage just below VH) is applied to the LED package; when the input square wave reaches VH, the relay closes and the combination of V1+V2 is applied. The square wave input effectively converts the amplitude from 12 VDC to 3.7 VDC, thereby reducing the biosensor's EMI levels. Diode D1 is used to prevent current from flowing back to the V1 source when the combined V1 + V2 voltage is applied.
The LED module is driven directly by a 12 VDC square wave and the noise of the biosensor averages over 500? V, the peak value is significantly higher, as shown in Figure 2 below.
Biosensor Noise Figure 2 The noise level of the biosensor with an LED module driven by a 12 VDC square wave averages over 500V, with peak levels being significantly higher.
Figure 3 shows that the EMI of the LED module driven using the above method is significantly reduced, with an average induced noise level of approximately 400? V, and there is no obvious peak.
Reducing electromagnetic interference Figure 3 Using the above method to drive the LED module reduces the noise level of the biosensor.
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