Jim Williams current calibrator - Page 1
Forum section: electronics / projects

author: chilternview
author_rank: Frequent Contributor
timestamp: August 04, 2025, 12:32:51 pm
content: I thought I'd knock up a current probe calibrator based on Jim Williams' design here:https://www.analog.com/en/resources/design-notes/current-probe-for-lcd-backlight-measurement.htmlUsed all components as described with the exception of the 2N4338 which seems to be unobtanium these days, so picked a 2N3819 from several, selecting one with a Vgs ~= -1v @ Ids=0 to be in the range of the 2N4338.The circuit works, and oscillated about 58kHz as expected, but the sine wave is clipped, e.g. at A2 pin 3 the output is from about -14v to +14v, and the full wave rectified output of A5 is about +14v p/p.Is this expected? While it's only the absolute amplitude that matters - and the article says to tweak the reference voltage for a nominal 1.000V across the 100ohm output resistor, I was expecting more of a pure sinewave.I have checked all opamp supplies are correct (+15v and -15v from a bench PSU), adequate supply decoupling, the gate of Q1 is at about -0.6v. But I'd be interested to hear if anyone's made one of these themselves.Edit: of course the output is 10mA RMS across the 100ohm resistor, so the output of A2 must be 10mA * (100+619), convert from RMS to peak to peak and it should be 10v. Mine's more so the amplitude control isn't working.- CV

author: Grandchuck
author_rank: Super Contributor
timestamp: August 04, 2025, 01:53:40 pm
content: I have built several of those FET AGC controlled oscillators and found them to be tricky to get low distortion and reasonable start up time. Try fiddling with the resistors around the FET ... starting with the 500 ohm device.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: August 04, 2025, 02:00:59 pm
content: Additionally, can you place a trim pot in place of the 15k resistor and see what the effect is? I would use a 10k with a fixed 10k resistor in series, to start with.

author: chilternview
author_rank: Frequent Contributor
timestamp: August 04, 2025, 03:15:06 pm
content: So... it would seem the way to handle the variable threshold of the jfet is to change the 100k resistor that forms a divider with the 10k to ground. A value of 47k gave a very reasonable sinewave.Next replaced the 15k resistor with a 50k trimpot (because I had one to hand!) and a 10k resistor. That could be then used as a fine adjustment of the amplitude. However, anything which sets the amplitude over about 7V rms causes a lower frequency oscillation, effectively an amplitude modulation of the wien bridge frequency.Attached are a few scope traces.1. (with vref below critical point) has CH1 = A2 output, CH2 = across 100ohm resistor, CH4 = current probe.2. (vref beyond critical point) has the vref trimmer adjusted so that amplitude modulation has started.3. (critical slower tb) is the same but sweep at 10mS/divSo there's obviously another oscillation loop round the amplitude control which can be prevented from keeping the gain low, but unfortunately that's too low to give the desired output amplitude of 10mA RMS - above about 7.3mA and oscillation started. Sounds like some filter / compensation is needed in the feedback loop?

author: chilternview
author_rank: Frequent Contributor
timestamp: August 04, 2025, 03:50:51 pm
content: Tried changing the 1uF cap that forms the integrator with A3, and reduced it to 0.47uF. This immediately stopped the amplitude oscillation.Also needed to change the 15k + trimpot to 4.7k + trimpot, the desired amplitude of 1V across the 100ohm resistor was achieved with a total value as measured of 4.727k+2.915k = 7.642k. So all working to spec now. I don't think it's the most accurate current probe calibrator but given that current probes aren't that accurate anyhow, it's good enough, and handy for a quick check.