HP3478A Questions - Page 1
Forum section: electronics / testgear

author: PixieDust
author_rank: Frequent Contributor
timestamp: November 04, 2022, 09:37:51 am
content: Hi guys!I'm reading the service manual for the HP3478 and I'm having some difficulty understanding a few things.http://www.arimi.it/wp-content/Strumenti/HP/Multimetri/hp-3478a-Service.pdfOn page 11 of the pdf, or page 1-2, we see Table 1-1 Specifications.Questions:1.) Under DC Voltage, the "Maximum Input Voltage" is stated as "Hi to Lo: 303V rms or 450V peak". Why is it just +ve 450V? In the line below, we see +/-500V. Typo? In the "Input Characteristics" we see +/-303.099V as the maximum reading, so it seems like it should +/- 450V no?2.) What does the second line "Maximum Input Voltage" mean? What is this "Hi or Lo to Earth Ground"?

author: HKJ
author_rank: Super Contributor
timestamp: November 04, 2022, 09:55:24 am
content: Quote from: PixieDust on November 04, 2022, 09:37:51 amI do not see the problem, the input can handle up to 450V peak or DC before damage, and the meter can show readings up to 303V, i.e. from 303 to 450 the meter will not show correct (It will probably either show 303 or overload), but the meter will not be damaged.The isolation between the electronic and the box (i.e. Earth Ground) can handle up to 500V and that is the same from either input terminal.

author: PixieDust
author_rank: Frequent Contributor
timestamp: November 04, 2022, 11:29:28 pm
content: Quote from: HKJ on November 04, 2022, 09:55:24 amYep, I understand the second bit. I just wasn't sure why they left out the +/- sign in front of 450V whereas they used it everywhere else. Surely probing things back to front wouldn't result in a blown meter! Would happen so often!Quote from: HKJ on November 04, 2022, 09:55:24 amThis I still don't understand.

author: HKJ
author_rank: Super Contributor
timestamp: November 05, 2022, 08:43:35 am
content: Quote from: PixieDust on November 04, 2022, 11:29:28 pmThe meter is rated to show +/- voltage, i.e. the sign is already implied in the range.Quote from: PixieDust on November 04, 2022, 11:29:28 pmNot all voltages are measured relative to ground, sometimes you need to measure a voltage on top of another voltage and in these cases the total voltage must not be above 500V, i.e. you cannot measure a 5V supply is on top of a 600V supply like: 0V / 600V / 605V, but you could if the connections was: -600V / 0V / 5V (It is the same voltages, but ground is connected different).All meters has this rating, on handheld it is often the same as maximum input voltage, but bench meters are different.

author: PixieDust
author_rank: Frequent Contributor
timestamp: November 10, 2022, 03:51:55 am
content: Quote from: HKJ on November 05, 2022, 08:43:35 amUnderstood, thanks.Quote from: HKJ on November 05, 2022, 08:43:35 amThanks. I haven't bumped into things like this yet, so I'll keep in mind as I continue on my electronics journey.

author: HKJ
author_rank: Super Contributor
timestamp: November 10, 2022, 07:45:08 pm
content: Quote from: PixieDust on November 10, 2022, 03:51:55 amYou will find it in many switch mode power supplies. The control IC is on the mains side, but working at around 8-16 volt. To measure this voltage you need a meter that can handle the mains voltage to ground (Basically all meters can do that as long as input is from normal mains and not higher voltages), even though the voltage is fairly low (This is even more an issue with oscilloscopes because their input is connected to ground and do not handle any voltage difference).

author: PixieDust
author_rank: Frequent Contributor
timestamp: November 11, 2022, 02:09:02 am
content: Yep I really need to read up on how multimeters and oscilloscopes work so I don't blow anything up.

author: Brumby
author_rank: Unknown
timestamp: November 11, 2022, 07:38:49 am
content: As with any exercise when working with test equipment, care and understanding is essential. In that respect, multimeters aren't too bad - but oscilloscopes have some extra twists and turns......

author: PixieDust
author_rank: Frequent Contributor
timestamp: November 11, 2022, 08:35:14 am
content: Thanks Brumby, I've seen it, but my electronics know how isn't up to scratch to understand the principles involved. Still need to do a lot of reading.