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<\/center>\nThis is a typical reverse voltage protection circuit used on many guitar effects pedals. I have used it at times myself and it is effective within limits.
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\nUnder normal conditions when a power supply with the properly polarity is being used, the D1 diode is not conducting. A few milliamps are being drawn through the series resistor and the power dissipation is less than the max rating of the resistor, which is typically 1\/4 watt.<\/p>\n
However, if a reverse polarity voltage supply is plugged into this circuit, the D1 diode will conduct and current passes through it and back to the power source through the 100 ohm resistor. This is when the problems start.<\/p>\n
The diode has about 0.7v drop across it but the rest of the voltage will be across the resistor and Ohm’s law can tell us how much current will flow: I = V\/R<\/p>\n
The AMZ LED calculator<\/a> is a quick way to get the answers that are needed. The Vf of the diode is 0.7v and it is entered in the LED forward voltage box (an LED is a diode after all). So if you plug in the voltage supply, forward voltage and resistor value, the calculator will reveal how much current is used and also how many watts have to be dissipated by the resistor.<\/p>\n In this case, with 9v supply, 0.7v diode and a 100 ohm resistor, the current is 83 milliamps and almost 0.70 watts. This is clearly in excess of the rating for the 0.25 watt resistor and it will get quite hot after a few seconds. Usually the resistor melts down and becomes open shortly thereafter. No current will flow then since the resistor has acted like a fuse.<\/p>\n The diode doesn’t melt down because it is usually rated for 1.0A or more and this circuit is only pulling 0.083A through it.<\/p>\n A very well-known commercial pedal uses 47 ohms for the resistor and if you substitute that value in the calculator, it shows that the resistor will have to stand almost 1.5 watts! I have replaced the fried 47R power supply resistor in many of these pedals.<\/p>\n One solution is to use a 2w rated resistor for the series input from the power supply. This works but the resistor will get hot if the power is reversed but at least it won’t go up in flames.<\/p>\n A better solution is to add a new diode in series before the resistor. This protects the circuit and in reversed power conditions, no current flows. The downside is that there will be a voltage drop across the diode and the voltage that makes it to the circuit will be less; about 8.0 – 8.3 volts, depending on the diode and how much current the circuit draws.<\/p>\n The voltage drop can be minimized by using a low Vf diode such as the 1N5817 Schottky part. This is the best solution and the diode can typically be added where the positive voltage wire connects to the pcb.<\/p>\n If you have a pedal that is not working and there does not seem to be any voltage getting to the transistors or chips, check the series resistor on the plus power line.<\/p>\n Share this article with your Twitter followers: Tweet<\/a>![\"Reverse](\"http:\/\/www.muzique.com\/lab\/images\/protect2.gif\")
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