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| Wish to scope the secondary diode voltage on our offline Flyback SMPS |
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| ocset:
Hi, We wish to scope the ringing voltage waveform on the secondary diode of our Offline isolated 240VAC Flyback SMPS LED driver. (The Flyback works in DCM, but we also want to try it in CCM with lower voltage LED strings.) The LED string voltage is around 75V, and the Transformer NP/NS = 26/16. I usually use one of those scope probes with a “spring probe” on the end, which allows me to avoid using the “dangling ground lead”. However, we do not have any here, and I can’t seem to find them on Farnell any more. Do you know where I can get one of these “spring probes”? For lower voltage signals (eg sense resistor waveforms), we just use a piece of coaxial cable and strip one end and solder it to the sense resistor…and connect the BNC at the other end into the scope. –That gives great results but our coaxial pieces can’t be used across the Flyback secondary diode, as the voltage is too high. As you know, the secondary diode voltage can ring up to { 2 x [v(pri)_reflected + vout]}. |
| capt bullshot:
I usually use a piece of wire as the ground spring. One can build quite well working 1:10 or 1:100 "low impedance" probes out of a 450R or 4450R resistor soldered to one end of a coax cable and connect the other end into a 50Ohm terminator or 50Ohm scope input. |
| ocset:
Thanks, ill bear that in mind, ...for scopeing scross the secondary diode, with its high frequency ringing, we wish to use a high Z probe, so as not to interfere with the actual waveform. I think the best way seems to be to buy a 1:100 scope probe and just wrap wire round the ground barrel as Capt Bullshot says? We are wondering about actually cutting through the coax of the 1:100 probe and soldering it directly across the diode, so we dont have to fiddle with wrapping wire round the ground barrel....since we still have the problem of how to connect it across the diode after we have done the "wire wrapping". Has anyone tried this? |
| Phoenix:
I've used thin tinned copper wire instead of the probe spring clips (D=0.5mm ish). You can form it however you want; use it for the ground connection; use it for the tip connection; and it will hold the probe in place. |
| capt bullshot:
--- Quote from: treez on September 20, 2018, 05:15:28 am ---We are wondering about actually cutting through the coax of the 1:100 probe and soldering it directly across the diode, so we dont have to fiddle with wrapping wire round the ground barrel....since we still have the problem of how to connect it across the diode after we have done the "wire wrapping". Has anyone tried this? --- End quote --- No, haven't tried that, but it won't work anyway. There are substantial components inside the probe body to make it work as a 100:1 probe. If you cut the coax, you'll end up with some lousy kind of a 1:1 probe. One can buy adaptors from "probe tip" to BNC (sometimes they are supplied with the probe), we've abused these here (@work) sometimes to make an soldered probe connection to a circuit. |
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