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EEVblog #1336 – DT71 LCR Tweezer Destructive Teardown + Lab Update

Destructive teardown o nthe Miniware DT71 LCR meter Tweezers Plus a lab move update. Forum: ...


  1. I don’t know if it’s just me or not, but I noticed lately that I seem to have become easily fatigued when listening or watching your videos, despite the subject and material being very interesting to me. I am considering the audio as a potential instigator. I think I am perceiving high dynamically compressed audio of your voice. Prolonged high dynamic compression can be very fatiguing to a listener. I am not sure what post-processing you implement on your files, but I am wondering if this is something you purposely implement. If so, it may explain how I usually become this way after 10-15 minutes of watching. Again, it has nothing to do with the material or the presentation. It is very interesting to me, and your enthusiasm and vigor is a definite plus. I could be way off base though. Maybe it’s some other factor on my part. I am just throwing this out there to see if anyone else has possibly noticed a similar effect. Keep up the awesome videos.

  2. Hello, on the occasion to talk about DP832 I would like to present an interesting think. Look at the picture. This is a voltage on the DP832’s output when I’ve used an OVP feature. OVP protection was on 5V level and set voltage was on 24V. When I switch on the output, a voltage rises to 24V and after 250ms PSU drops it down. Notice that voltage above 5V is present on the output for more than 400ms. Do you think that is quite enough to kill 5V digital circuit? Rather yes. I wrote to Rigol with this problem but I received only silence… :-).
    The PSU’s control unit “knows” that I’m going to switch on voltage higher that OVP setting so I think it should display a warning message only without switching on the output.
    ANYWAY please beware to use OVP function – it’s not fully protect your delicate expensive circuit.

  3. I’ve blown up regulated supplies due to inductive kick back (even with leads – if there is inductance, you might be asking for problems!) which may be exacerbated by relay contact chattering – but you’d think that this would be suppressed with elementary snubbers. (Did I spot green snubbing caps in the video? Are any of those monster caps located some distance away from the output actually in parallel with the output?) On a homebuilt power supply I placed a high-speed diode backwards across the pass transistor (a BJT in that case) to prevent not only to help shunt such spikes around the pass transistor, but to prevent that transistor’s junctions from conducting should a higher voltage appear on the output than input (e.g. powering down with a battery being charged connected.)

    Difficult to tell from the video, but is there any evidence of a Zener between the gate and source? If there is even the briefest of voltage spoke, the Gate-to-Source voltage could exceed 20-ish volts and short out the gate: I add that Zener (typically 15 volts) when I use MOSFETS for power regulating/switching applications for exactly that reason!

    It would also be interesting to put a shunt in series and look at the current waveform across it when the power supply’s output was shorted: I can imagine a brief, high-current pulse flowing before the overcurrent circuitry had time to take effect. This could not only stress the series pass transistor, but also increase the magnitude of inductive kick (if any) as well since the short-term current could well be many amps.

    As for that little BJT on the heat sink that was getting hot: Does this supply have output sinking capability? Like many lab grade supplies I would expect it sink excess voltage on the output. If so, it may have been doing its job trying to sink when the shorted pass was giving it >30 volts.

    If it is like an HP supply that I have the “sink” current will show up on the ammeter to indicate to the operator that current is flowing the “wrong” way that that you (the user) should probably figure out why it’s happening – such as might occur if one is inadvertently back-feeding voltage into it when charging a battery and lowering the output voltage below the terminal voltage – and this might explain a current reading even when there was no connected output.

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