# Placement of ESD resistor

We have a circuit that has a 6V input for charging. We want to protect the device from ESD on the input, but have some discussion on where to place a resistor. The two differences we are looking at are below.

In the first case, we have the argument that the series resistor will slow the input charge somewhat, making life easier for the ESD diode and thus making it more effective.

In the seconds case, we have the argument that the ESD diode is left alone to do its job, and the series resistor can help absorb whatever the ESD diode doesn't handle.

Is there a common practice where to put the resistor and if so, why?

simulate this circuit – Schematic created using CircuitLab

simulate this circuit

The series resistor indeed does "slow down" the ESD pulse but that's not the complete story. The function of a series resistor is also to dissipate the energy of the ESD pulse and also limit the current. Limiting that current can make the ESD pulse longer which is good as the pulse will be "less intense" (longer in time at a smaller current) and therefore easier to handle by the ESD diode. For this, the first solution is the best.

The second solution is worse as a severe ESD pulse can break the diode as there is no extra series resistor to lower the current. Also here the resistor relies on the ESD protection present in U1.

Another/additional solution for this charging input, provided it will only be used for DC, is to add a capacitor. For example in parallel with D1 you could place an (electrolytic) capacitor of 1 uF to 10 uF or any other value you have available. In combination with the first circuit (diode at the left) that would really slow down the ESD pulse and provide very good protection.

• Thank you for your excellent answer! Is the choice of electrolytic capacitor due to higher voltage ratings? Can we use ceramic capacitor, or are they 'useless' for this? Commented Mar 7, 2018 at 14:37
• You can just use the voltage rating for the maximum DC voltage, meaning that 6 V. I'd use some margin so I would use a 10 V or 16 V rated capacitor. A ceramic capacitor can also be used. I suggested an electrolytic as these have a higher value which lowers the RC time constant so slowing down the ESD pulse even more. Commented Mar 7, 2018 at 14:42
• @Bimpelrekkie: Electrolytic capacitors have a much higher inductance then ceramic capacitors. ESD pulses have a lot of high frequency components in them For ESD protection I would not use electrolytic capacitors. Commented Mar 7, 2018 at 15:15
• @oldfart I would agree with "most electrolytic capacitors have a higher inductance than most ceramic capacitors". I am sure there are certain models of electrolytic capacitors which have less inductance than certain ceramic capacitors. Also the actual value of the capacitor matters. Some very small SMD ceramic "chip" capacitors can have quite significant inductance. Also I am unsure how much inductance is OK to have, I would have to investigate (simulate) how much inductance would be acceptable. Commented Mar 7, 2018 at 16:59
• Can you please explain the function of the resistor in the second figure? Does it have to be 100 ohms or larger, or is 10 ohms good enough? Commented Dec 4, 2018 at 2:39

I would suggest the second circuit, where the resistor is placed after the TVS/ESD diode. The idea of placing the resistor there is to limit the current going into your circuit as the TVS/ESD diode clamping voltage is way higher than the breakdown voltage of the diode. You would think that the ESD cannot survive the brunt of multiple ESD events so you would place the resistor before it, but my experiments revealed that ordinary or commonly used resistors cannot survive ESD. Their resistances dropped after being zapped several times. And.... as you might have guessed it, the TVS/ESD diode survived hundreds of powerful direct zapping. That is because it is built for such purpose.