Help wanted with portable current source design - Page 1
Forum section: electronics / projects

author: motocoder
author_rank: Unknown
timestamp: January 19, 2016, 06:28:40 am
content: I've been working on a small project recently. It started out as a simple zener diode tester, but once I had something working there, I started thinking how nice it would be to have a little adjustable current source. The project has since morphed beyond the diode tester into a small, battery-powered "precision", configurable current source. I'm attaching my hand-drawn schematic. Everything in the schematic has been breadboarded already, and appears to work great, but I would like feedback, particularly on -Design options to minimize drift (temperature or otherwise)Op-amp selection (especially as it pertains to #1)Anything related to stabilityThe project is a small, battery-powered, high-side current source, with high precision (at least 0.1%, but ideally 0.01%, or better). Current will be selectable via a range switch (0.1mA, 1mA), and via a value switch (1 - 10).I did quite a bit of reading on high side current sources before starting to work on this. I read a great post here on EEVBlog from Dr. Frank:https://www.eevblog.com/forum/projects/precision-dc-current-source-diy/. I like Dr. Frank's design, except I want a high-side current source.Here is a brief description of the circuit:Voltage Reference (Lower section of schematic)I am using a REF102 to provide a relatively stable 10V output (I know there are better references out there, feel free to recommend one). Because this is a high-side current source, I need to convert the low-side reference voltage, Vrn, to a high-side version, i.e. one that is based off the upper power rail. I played around with a circuit I found in an app note for this, but it did not work well. So what I have done instead is to take the 10V reference and use it to drive a (low-side) current sink, which in turn drives a resistor connected to the upper power rail. The voltage drop across this resistor creates our high-side reference voltage, Vrp. The low-side reference voltage, Vrn, drives the non-inverting input of an op-amp. The op-amp has a MOSFET (2n7000) in the feedback loop, and a low-side sense resistor. This configuration implements a low-side current sink. The 1mA passing through the MOSFET also passes through the resistor tied to the upper power rail, creating a 1V drop off the positive power supply voltage. The current value can be trimmed using the REF102 trim feature, implemented with a 20K pot. Note that because this drop is only 1V, the high-side current source (below) is operating up near the positive rail. This does place some restrictions on the op-amps we can use for the main high side current source, due to the need to be operating with a common mode voltage up near the positive rail.High Side Current Source (Upper left section of schematic)The high-side reference voltage drives the non-inverting input of another op-amp. This op-amp is the high-side version source version of the low side current sink in the voltage reference section. The sense resistor Rs, can be swapped out to change the current range (see below).Note that for this portion of the circuit, I experimented briefly with a circuit that "floats" an op-amp off the high side rail using a shunt regulator. The shunt regulator is the crossed-out section in the lower left of the schematic. This worked well, and allowed me to use a lower voltage op-amp, but because I am using relatively modest voltages (power is supplied by 2x 9V batteries, for a total of 18V), I decided the extra complexity wasn't needed.Voltage Buffer and Volt meter (Upper right section of schematic)I'd like this circuit to do double-duty as a zener diode tester. Towards that end, I've acquired some small voltmeter off of eBay. It provides a multi-digit 7-segment display, and is reasonably accurate. I'll probably replace this with one of my own design at some point, but for now it's a quick an easy enhancement. The voltmeter should be connected across the load to sense the load voltage. However, the input resistance of the voltmeter isn't as high as I would like, so I've added another op-amp configured as a voltage follower to buffer the voltage across the load and drive the voltmeter.I'd also like an out-of-range indicator. Rather than add an LED for this, I am using the gate voltage of the high side current source to drive an N-channel MOSFET that gates power to the voltmeter. This works really well. Because the circuit does not have any hysteresis, it is possible to pick a load resistor that causes the voltmeter to flicker. However, I like the fact that this part of the circuit is really simple, so I don't feel a need to replace the MOSFET with something more elaborate (like a comparator with a little positive feedback).The op-amps used are all LT1637, which have great rail-to-rail characteristics, but perhaps not as good as they could be in terms of offset and drift.The schematic doesn't show value switching, and possible needs a small change to get to 100mA. To implement range switching, the sense resistor, Rs would be switched between different values for the different ranges - 100R for 1 - 10mA range, 1K for 0.1 - 1mA range, etc. To implement value switching (1 - 10), I would add a switch controlling a voltage divider on the output of the REF102 - very similar to what Dr. Frank has in his design. To get to higher currents, I would possibly need to replace the small MOSFET I am using with a PNP + MOSFET pair - basically a Darlington type configuration but using the MOSFET on the input, driving a PNP that then sources the bulk of the current.Sorry for the long post, but I wanted to explain the circuit and the design choices.Thanks

author: Marco
author_rank: Super Contributor
timestamp: January 19, 2016, 07:20:35 am
content: If you use a separate opamp for the high side and give it a linear regulator for V- it can works for higher supply voltages.Unless you already have the FETs pick something modern with gate protection (DMP10H4D2S for instance).What's with the 1KOhm//1nf in the feedback to the inverting input? It's basically a short at all frequencies (relative to the MOhm//pf range opamp input) so why not just use a short?

author: motocoder
author_rank: Unknown
timestamp: January 19, 2016, 07:42:20 am
content: Quote from: Marco on January 19, 2016, 07:20:35 amThe 1k is there so the op-amp input bias currents see a similar resistance on both terminals - this helps minimize voltage offset caused by these currents. The 1n capacitor is a mistake. It should actually go from the output of the op-amp to the inverting input. With that correction, it is there to provide AC bypass so when the FET is cutoff or saturated, you have 100% feedback (still have a closed-loop) for AC signals.Thanks for the recommendation on the MOSFET with gate protection. The ones in the diagram are those I had laying around. I'll have to double-check that the gate protection doesn't adversely impact gate leakage, though.

author: motocoder
author_rank: Unknown
timestamp: January 19, 2016, 07:15:04 pm
content: I received some advice from Dr. Frank via PM. I am passing that along here so the info is captured in the thread.Quote

author: motocoder
author_rank: Unknown
timestamp: January 19, 2016, 08:42:28 pm
content: It looks like the the current source circuit in the 34401A is using an AD706JR op-amp. Notable features of this op-amp:100uV max offset voltage1.5uV/C max offset driftVery low input bias current and high input impedance

author: motocoder
author_rank: Unknown
timestamp: January 20, 2016, 01:42:37 am
content: After reviewing the similar circuit in the Agilent 34401A reference manual, I realized I can eliminate the need for the two current source/sink op-amps to support input signals near the rails. For example, I can insert a zener diode (or a few diodes, or a Vbe multiplier - anything to drop some voltage) between the sense resistor and the rail. Let's say I add a 2V zener there, the op-amp still has 18V as its rail, but the voltage below the sense resistor would be <= 16V (approximately). The same thing can be done for the current sink, floating both the REF102 and the sense resistor on a 2V zener.The only downsides to this are that I lose about 2V of compliance voltage, and I need to worry about power dissipation of the zener (or whatever) on the upper rail at higher currents.

author: motocoder
author_rank: Unknown
timestamp: January 20, 2016, 02:56:12 am
content: Quote from: motocoder on January 20, 2016, 01:42:37 amLooks like I'm just talking with myself here, but for the sake of posterity I thought I would document that I put 2V zener above the source current sense resistor, and a zener below the current sink sense resistor (also attached to the REF102 GND pin, and the trim-pot ground connection). This is all working perfectly, and I didn't even need to re-trim the current. This makes a HUGE difference in terms of flexibility in what op-amps I choose. I can just focus on one that has low offset drift characteristics.