此LED电流应为多少?

最近在做一款出口指示灯,LED用的是∮5mm 高亮度白发红色,电路如下图,120VAC经变压器降压,DB整流,后给LED供电,由于后面电路的要求整流后不可以加滤波电容.用示波器测LED限流电阻上的波形如图2所示.峰-峰值1.64V, 均方根值0.968V,  用万用表(FLUKE17B)DC档测电阻两端电压为:0.709V, LED厂家规格书上标示的电流为20mA, 请问如何计算LED的使用电流? 是按峰峰值,均方根值,还是按万用表的DC电压计算呢? 谢谢!
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