# How to compensate for voltage drop across a diode?

• dano1005
In summary, you want a switch that has 2 discrete inputs, 4 outputs, and is (on-off-on), (on-off-mom) or (mom-off-mom).

#### dano1005

Hi, I am working on a project where I am using a LCD voltage display to measure voltage at 2 points in the circuit. The first is the voltage of the batteries providing the power, and the second is to measure the output of the circuit. It is in a very small box and I don't have room for a complex circuit. Basically I want to have 2 momentary switches- one will activate the circuit and the voltmeter and display the output of the circuit. The other switch would simply activate the voltmeter and display the voltage of the batteries(they are lithium and I don't want to over-discharge them). I thought about using a diode to isolate them but I need a way around the voltage drop that it will create. Thanks in advance for any help, and let me know if I need to clarify anything.

There are circuits that replace diodes with MOSFETs because they have a much lower voltage drop.
But you could do it all with a miniature toggle switch.
Description could be DPDT, centre off = (on-off-on). Or momentary (mom-off-mom)
One side is used to select the input. The other is wired to power the meter when not off.

Thanks for the reply. I am now thinking I can use a momentary (on)-on switch. It would need to have 2 discrete inputs to be switched and 4 outputs. Would that be dpst?

Lookup analog switch or analog mux. There are 2 input devices like the 74LVC1G3157 that are smaller than your diodes.

I am not sure if this concept is useful in your application or not, look up "superdiode" it's just an opamp feeding a diode. The op amp feedback is taken from the output of the diode and fed back to the negative pin. In this way, the op amp will output whatever is necessary (input + diode volt drop) to make the pins equal to one another. So the output of the circuit exactly resembles the input, gain of 1, but the diode prevents negative half cycle.

The switch you need is probably initially described as “DPDT, centre off”.

“DP” is double pole. One pole is used to provide power to the meter when the switch is not in the centre position. The second pole is used to select the input to be measured.

“DT” is double throw. There are two active positions.

“centre off”. Gives a middle position that disconnects all terminals.

“mom = momentary”. You can select if the switch will be remain in position or must be held in position while reading the meter.

Search for a switch that is (on-off-on), (on-off-mom) or (mom-off-mom).

You need to better describe the states required to get better suggestions..

## 1. How does a diode cause voltage drop?

A diode is a semiconductor device that allows current to flow in only one direction. As current passes through the diode, it encounters resistance due to the material properties of the diode. This resistance results in a voltage drop across the diode.

## 2. Why is it important to compensate for voltage drop across a diode?

Compensating for voltage drop across a diode is important because it affects the overall performance and efficiency of a circuit. If the voltage drop is not taken into account, it can lead to incorrect readings, reduced power output, and potential damage to the diode.

## 3. How can I calculate the voltage drop across a diode?

The voltage drop across a diode can be calculated using Ohm's Law: V = IR, where V is the voltage drop, I is the current passing through the diode, and R is the resistance of the diode. The exact value of the voltage drop will vary depending on the type of diode and the current passing through it.

## 4. What are some ways to compensate for voltage drop across a diode?

One way to compensate for voltage drop across a diode is to use a voltage regulator, which can maintain a constant output voltage despite variations in input voltage. Another method is to use a zener diode in series with the regular diode, which can provide a more stable voltage drop. Additionally, using a higher voltage power supply or multiple diodes in parallel can also help to compensate for voltage drop.

## 5. How can I test for voltage drop across a diode?

To test for voltage drop across a diode, you can use a multimeter in diode mode. Place the positive lead on the anode (positive) side of the diode and the negative lead on the cathode (negative) side. The multimeter will display the voltage drop across the diode. If the voltage drop is within the expected range for the diode, it is functioning properly.