Active Capacitor Discharge  - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: April 20, 2021, 01:26:43 am
content: The high capactance is for an on going experiment.I need an active capacitor discharge for an application where active discharge resistors and/or relays are not practical. A resistor across the capacitor bank either changes the load voltage to much if resistance is to low or won't discharge the bank fast enough is resistance is to high.Considering the current requirements , the points on a relay don't last after many cycles.So I can up with an idea of using a P-channel MOSFET to shunt energy from capacitor bank across a low value resistor.This is an adaptaion of similar circuits with the exception of the origin of the gate voltage.In this version I decided to take the gate vlotage from before the rectifier because a series diode after the rectifier would not be practical considering the voltage and current requirements.The half wave rectifier charges the gate upon start up which should hard switch the Mosfet to an on state which disconnects the shunt resistor allowing the capacitor bank to charge. Inrush current is damped by a limiter so that is not a concern in the final application. Also the shunt resistor will have to be much larger value for the final application. I havn't calculated the energy for the 210000uf capacitor bank just yet , but can imagine it will be quite high.In the test I used a 2200uF capacitor and a 24V 350VA transformer without inrush limiting .I needed to test without inrush limiting for resiliency over several on/off cycles.In practice the operation does work but notice that the capacitor discharge was slow to start because the gate needed time to discharge. Also if the zener is to low the MOSFET won't completely switch on. A 39V zener allowed for this but still the discharge was slow . So I added a 20Meg resistor from gate to ground in parallel with the zener which sped up the discharge.This worked for several cycles with out noticeable issues until I removed the zener assuming it wasn't doing anything. This worked for a couple On cycles and the Mosfet popped.The only thing I could think that would cause this is the zener diode acted as a transient voltage suppressor. Would that be an accurate analysis for the reason for the failure ?

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 20, 2021, 03:23:41 am
content: I don’t like that idea of bridging the diode bridge with another diode, it creates some unexpected voltages, and that is why the zener is necessary, to clamp the gate.I would use an optocoupler connected to the AC to turn on the mosfet, it would “light” only on the + cycle but who cares if you filter the output.Also I would place some kind of NTC or PTC arrangement to the gate of the mosfet, to limit its current and avoid self-destruction of itself and the resistors in case the logic fails and the mosfet stays turned on 100% of the time.

author: emmawarner96
author_rank: Newbie
timestamp: April 26, 2021, 06:36:14 am
content: Diodes are used as series components in a diode bridge to allow current to flow forward during the positive part of the AC cycle and as shunt components to redirect current flowing in the reverse direction to the opposite rails during the negative part of the AC cycle.A bridge rectifier's main drawback is that it needs four diodes, two of which conduct in alternating half-cycles. As a result, the overall voltage drop in diodes doubles compared to a centre-tap rectifier, losses increase, and rectification efficiency decreases slightly.

author: Jwillis
author_rank: Super Contributor
timestamp: April 26, 2021, 06:55:06 am
content: Quote from: MasterTech on April 20, 2021, 03:23:41 amGreat thanks . I only have a analog scope and it's impossible to see voltage spikes let alone the voltage of those spikes . I suspected the zener is acting as a voltage suppressor but wasn't sure.I finally caved and ordered a DSO so I can see whats going on .

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 26, 2021, 09:21:26 am
content: Quote from: Jwillis on April 26, 2021, 06:55:06 amIts always nice to have a DSO, gives you more capabilities.However in this case just learn LTSpice, and simulate the circuit so you can see what is going on.