Archive for August 14th, 2013
A bag of 50 cheap Hall effect sensors arrived from the usual eBay vendor, who was different from all previous eBay vendors (if in name only). Passing 124 mA through the armored FT50 toroid with 25 turns of 26 AWG wire, we find this distribution of bias points, measured as the offset from the actual VCC/2:
The bias point is actually referenced to the negative terminal (usually ground) with a ±0.25 V variation around the nominal. SS49 sensors run about 0.5 V below VCC/2 (2.25 V with a 5 V supply), SS49E sensors at 2.5 V with a tighter VCC limit that suggests you better stay pretty close to 5.0 V.
Allowing for the fact that I really don’t have good control over the actual magnetic field, the gain distribution seems tight:
You’ll recall the Genuine Honeywell sensor specs:
- SS49 – nominal 0.9 mV/G, limits 0.6 to 1.25 mV/G
- SS49E – nominal 1.4 mV/G, limits 1.0 to 1.75 mV/G
The gain is roughly half that of the previous “49E” sensors, confirmed by sticking one of them this field. I don’t know which is more accurate, but these have a much prettier distribution.
So this lot resembles 49E sensors in both bias and gain.
Given the bias variation, though, it’s obvious that a DC application must measure the zero-field output and apply an analog offset to the amplifier, because a twiddlepot setting won’t suffice. Most likely, you’d want to update the offset every now and again to compensate for temperature variation, too.
Tossing the outliers gives an average gain of 1.17, which would give results within 10% over the lot. Given that you don’t care about the actual magnetic field, you could calibrate the output voltage for a known input current and get really nice results.
If you were doing position sensing from a known magnet, you’d want better control of the magnetic field gradient.