Howland Current Source Applied to Magnetic Field Generation in a Tri-Axial Helmholtz Coil

Abstract

This work describes the design and testing of a power Voltage-Controlled Current Source (VCCS) based on the Howland Current Source (HCS) topology. This source is part of a system that contains a three-axial Helmholtz coil and is used to generate a controlled magnetic field environment for aerospace applications. Initially, the paper presents the HCS theory and practical limitations on the premise that the system must be built using low-cost off-the-shelf components. The paper also carefully address on how to design and plan the VCCS matching physical and electrical parameters/limitations of a specific Helmholtz coil. All project details as well as the built VCCS electronics and its results are shown. These include linearity and a first order calibration test, stability and low-frequency error measurement, step response and a frequency limitation analysis. The built source is capable of sourcing up to ±1.5 A at ±25 V, maintaining its linearity and achieving an error smaller than 0.5% with a first order calibration. The final HCS prototype together with the Helmholtz coil allows an excellent capability regarding magnetic field generation for both open and closed-loop applications.

Stability analysis and design of floating load voltage‐controlled current sources

“…The analysis of the literature and technical documentation showed the existence of two basic types of circuit configurations that work with floating or grounded load. In previous works, [5][6][7][8][9][10][11][12][13][14] different circuits of voltage-to-current converters with grounded load are presented, in which various useful structural modifications and frequency compensations with external passive components are proposed for improving the stability in a given frequency range, as well as increasing the value of the output resistance. Moreover, the majority of the electronic circuits [3][4][5][6][8][9][10][11][12]14 are based on the Improved Howland Current Pump, which uses one op-amp and five (or six) resistors.…”

“…In previous works, [5][6][7][8][9][10][11][12][13][14] different circuits of voltage-to-current converters with grounded load are presented, in which various useful structural modifications and frequency compensations with external passive components are proposed for improving the stability in a given frequency range, as well as increasing the value of the output resistance. Moreover, the majority of the electronic circuits [3][4][5][6][8][9][10][11][12]14 are based on the Improved Howland Current Pump, which uses one op-amp and five (or six) resistors. For the Improved Howland Current Pump, with an accurate selection of the ratios of the resistors and proper selection of the op-amp, the independence of the output current from the voltage drop over the load can be achieved, as well as the bandwidth can up to about 100 kHz 6,10,[12][13][14] and in some cases up to 1 MHz.…”

“…Moreover, the majority of the electronic circuits [3][4][5][6][8][9][10][11][12]14 are based on the Improved Howland Current Pump, which uses one op-amp and five (or six) resistors. For the Improved Howland Current Pump, with an accurate selection of the ratios of the resistors and proper selection of the op-amp, the independence of the output current from the voltage drop over the load can be achieved, as well as the bandwidth can up to about 100 kHz 6,10,[12][13][14] and in some cases up to 1 MHz. 5,8,9,11 For the circuits given in previous studies, 3,4,[15][16][17] topological structures are used in which the load is connected between two terminals that are not directly connected to the analog ground.…”

Analysis, Simulation, and Development of a Low-Cost Fully Active-Electrode Bioimpedance Measurement Module

“…The voltage-mode approach is characterized by the current generation through the voltage in one or more of the active circuit's nodes. Such topologies include the well-known Howland current pump (HCP) [14][15][16][17][18][19], the Tietze topology [20], the load-in-the-loop topology [21], and differential-difference topologies [22][23][24]. Current-mode approach makes use of operational transconductance amplifiers (OTAs) [7,11,25,26] or current conveyors (CCII) [15,27,28] for the current excitation.…”