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| LCR Tweezers test and analysis (HP-990, DT71, ST5S...) |
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| bateau020:
Nice quote from another thread running now: --- Quote from: bdunham7 on February 15, 2021, 06:13:08 pm ---If the circuit in question can't take an ampere, then we likely don't need to measure it in microohms or even milliohms. Measuring uV or nV is a lab experiment, not a something you casually check with a DMM. A 2 degree (K) temp difference between identical copper alloys will give you a microvolt, the same difference between a stainless steel probe and an ENIG PC trace is probably in the 10s of uV. This dedicated low ohm instrument uses up to a 1A test current. Some use even more. https://www.chromausa.com/pdf/16502-E.pdf --- End quote --- Now that voltage (from the Seebeck effect if I remember correctly) is DC or close to DC, so measuring in AC would allow mitigation. However, its noise adds to the noise in your input stages. Add that to the varying quality of the contact (which is the main problem I think), and the proximity of a heat source (hand), and you got a situation that is not favourable. |
| Shannon:
--- Quote from: bateau020 on February 16, 2021, 10:48:49 am ---Nice quote from another thread running now: --- Quote from: bdunham7 on February 15, 2021, 06:13:08 pm ---If the circuit in question can't take an ampere, then we likely don't need to measure it in microohms or even milliohms. Measuring uV or nV is a lab experiment, not a something you casually check with a DMM. A 2 degree (K) temp difference between identical copper alloys will give you a microvolt, the same difference between a stainless steel probe and an ENIG PC trace is probably in the 10s of uV. This dedicated low ohm instrument uses up to a 1A test current. Some use even more. https://www.chromausa.com/pdf/16502-E.pdf --- End quote --- Now that voltage (from the Seebeck effect if I remember correctly) is DC or close to DC, so measuring in AC would allow mitigation. However, its noise adds to the noise in your input stages. Add that to the varying quality of the contact (which is the main problem I think), and the proximity of a heat source (hand), and you got a situation that is not favourable. --- End quote --- Yep, You reminded me, it is possible to obtain higher signal-to-noise ratio base on using AC excitation signal to measure impedance, Some high-precision Wheatstone bridges use AC excitation to avoid low-frequency noise. But if power consumption and temperature rising of large current are acceptable, DC large current is a more easy choice. |
| bateau020:
--- Quote from: Shannon on February 16, 2021, 03:14:02 pm ---Some high-precision Wheatstone bridges use AC excitation to avoid low-frequency noise. But if power consumption and temperature rising of large current are acceptable, DC large current is a more easy choice. --- End quote --- Depends on the use case. If you want to find a short on circuit board, you will need to do it in DC or a low frequency. Otherwise you will measure the caps, plus the board layout will add inductance, so you will be mislead. Just keep the voltage low enough to no to fry components. Well, you could go fancy and test through pulsed excitation, but then you are in the area of the higher end tools. If you have 1 component that you want to test, then AC could be better. By the way, for testing on boards in the milli-ohm range it is best to avoid tweezers: contact surface and pressure have too much influence on the measurement, and are relatively hard to control with tweezers. Use pointy kelvin probes, and they are hard to find (the battery test kelvin probes are easy to find, but they are way too large). For those who want to see how it's done, look what Robrenz did. Deep respect for his skills. |
| serg-el:
I already wrote earlier in another topic, and attached photographs. Kelvin probes with a short distance are easy enough to make from felt-tip pens and sewing needles. Shielded wires. Closed relay contacts and tracks between contacts are clearly distinguished on the boards. Relay resistance is higher;) And my tweezers are HB-1. The first of the HB series. We have already written about them above in the topic. (NV-14, NV-15). |
| fabriciofx:
Shannon, Please, can youy let me know when you start sell it? Best Regards. |
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