Driving a capacitive load - Page 1
Forum section: electronics / projects

author: e_ln
author_rank: Newbie
timestamp: April 18, 2024, 02:06:43 pm
content: I'm trying to switch a capacitive load (~10's of nF) using voltages of ~0-70V and frequencies of ~5KHz. Because this is a still a small project I'm trying to keep complexity and cost to a minimum.I've built the control circuit the generate the signal I want using low voltage electronics (555's etc.) but for the output stage I'm unsure what to do.I've used several 9V batteries in series as the high voltage supply.Since I've some n-MOSFETS lying around, the simplest thing I remember from my studies is a low side switch.Measuring the voltage on the load shows severe distortion.- Is using a low side switch really that bad for these purposes?- Say I would like to invest in some dedicated equipment, would piezo drivers do the trick?- I'm currently not using more than 5 batteries in series due to danger of electrocution. How worried should I be when going up to 8 batteries in series and what's the best way to protect myself? I thought of a low amp circuit breaker in series.Attached are a schematic of low-side switch with n-mosfet and the signal that I'm trying to recreate. The signal shown is sinusoidal but it doesn't really matter and its amplitude is between 0 and 70v.

author: moffy
author_rank: Super Contributor
timestamp: April 18, 2024, 03:04:21 pm
content: Please provide some sort of schematic of what you are trying to do a photo of something hand drawn is fine. Please include whatever load the capacitors are switching into.

author: Retirednerd2020
author_rank: Regular Contributor
timestamp: April 18, 2024, 09:20:52 pm
content: The internal resistance of a 9V Alkaline battery varies but might be about 1-2 Ohms. If you model the circuit be sure to put the total series resistance of your batteries in the model. On the positive side, this will limit the current spikes when supplying the capacitive load.More information about your circuit goals would help. What are you trying to accomplish?

author: e_ln
author_rank: Newbie
timestamp: April 19, 2024, 07:46:48 am
content: I've attached a schematic.The goal is to drive a Polymer-dispersed Liquid Crystal (PDLC) film. As far as I know, it's electronic properties are similar to those of a capacitor.

author: moffy
author_rank: Super Contributor
timestamp: April 19, 2024, 08:07:59 am
content: Without knowing the values and what sort of wave shape you are trying to obtain, a half bridge would be of some advantage, you would have a high side switch in series with R1, which would be your charge cycle, you then turn off the high side switch, have a deadband where both switches are off, then turn on the low side switch and this is your discharge cycle. This way you are only discharging the capacitor you are not also having to pull down your HV supply.

author: e_ln
author_rank: Newbie
timestamp: May 04, 2024, 09:55:47 am
content: Quote from: moffy on April 19, 2024, 08:07:59 amFor the sake of simplicity, let's say I'm trying to create a square wave. Is there a commonly used circuit that combines a H-bridge with some sort of timing/PWM mechanism? Thats sounds similar to driving motors to me.