Hi, I came by accident on a big fault in linear voltage regulators in designing a circuit. I decided to use a fixed 24 volt regulator for my circuit. I was previously using a simple 24v Zener power supply but decided I needed better regulation.
I picked the L7824CV. The specifications for this show that Ro=28 million ohms. Looking up what this means on the internet is that it is the resistance between the output and ground of the chip. However on checking this resistance directly on the chip it is approx. 20k ohms. Because of this there is a connection of 20k between the 24v line and ground on the circuit. Therefore any resistor connected across the power line is in parallel to this resistance.
This has completed destroyed the functioning of my circuit as compared with the Zener power supply. If the connection was 28 million ohms this would not cause any problem in my circuit as this is close to infinity.
Either I don't understand the meaning of Ro or the chip is faulty. It is also easy to see that adjustable linear regulators would also be loading the circuit with this parasitic resistance. I have tried a 4.7uf across the power lines but this made no difference.
Does anyone know of a regulator that does not have this problem?
I picked the L7824CV. The specifications for this show that Ro=28 million ohms. Looking up what this means on the internet is that it is the resistance between the output and ground of the chip. However on checking this resistance directly on the chip it is approx. 20k ohms. Because of this there is a connection of 20k between the 24v line and ground on the circuit. Therefore any resistor connected across the power line is in parallel to this resistance.
This has completed destroyed the functioning of my circuit as compared with the Zener power supply. If the connection was 28 million ohms this would not cause any problem in my circuit as this is close to infinity.
Either I don't understand the meaning of Ro or the chip is faulty. It is also easy to see that adjustable linear regulators would also be loading the circuit with this parasitic resistance. I have tried a 4.7uf across the power lines but this made no difference.
Does anyone know of a regulator that does not have this problem?