The schematics

Click on the picture to get a larger view. The picture shows the schematics of the amp. Of course you can't use it for anything except for an overview. Please download the pdf-file instead if you want to see the details.

Circuit description

Reliable circuit

This design have I used now since 1989 in the QRO-amp. So the design is well proven. The C1, C2 design is used in time relays and manufactured in millions of copies. I have personal experience of this circuit. More than 10000 units are out and none of them have had any failure of the C1, C2 cap.

Lossless resistor

The power for the electronics is derived through a capacitor which acts like a "lossless resistor". This technique is possible to use if the wave shape of the mains is good enough (sinus with no harmonics). The size of the C1 and C2 is rather critical. If you are going to use 110-120 VAC the capacitance must be double, but this isn't tested yet, the exact value.

For this application you can rather easily calculate how big C1 and C2 should be on order to get desired brightness of LED. The calulation below will get you the total current.

I = Umains*2*3.14*f*C

C = I/(Umains*2*3.14*f*) = 10E-3/(230*2*3.14*50)=138.5 nF for 10 mA current.

f=50 or 60 Hz

Since the incoming voltage is 230 V and the voltage you want is very low, like 5-10 volts you can consider the caps as a current generator. It means that you will get the same current regardless of load.. and... you must not break the circuit!

Trimming brightness of the LED

The voltage for the LED is fullwave rectified by B1. Max voltage is set by DZ1, which is chosen rather low, 6.8 V.

R1 is the main resistor for the led. Choose it for getting max 6.8 V, the DZ1 voltage. (6.8-1.7)/0.01 = 510 ohms. The 1.7 is the voltage of a normal red LED. Other colours have different voltages and they are all higher than red. R2 could also be used for trimming the LED current and brightness. Maybe your C1 andC2 are bit too big you can shunt away some current by using R2.

DZ1 is for transient protection and you should always have it installed.

Avoiding unpleasant surprises

R3-R5 are for avoiding unpleasant surprises, to discharge the C1 and C2. Three resistors is used to ensure resistance against transients. The total value should be between 220 kohms to 3 Mohms.

Transient protection

R6-R9 limit the max inrush current for C1 and C2. Four resistors are used simply because they are small and cheap.

Other mains voltages

The C1 and C2 must always be calculated for the current and the mains voltage.


Note that this circuit must be properly insulated and ..... very important... the LED must not be mounted directly in some metal hole in the front panel or similar. The insulation is too little for this purpose.