Differential Probes or "Float the Scope" ??? - Page 1
Forum section: electronics / beginners

author: bsas
author_rank: Regular Contributor
timestamp: February 15, 2018, 07:15:36 pm
content: Quick dumb/noob question, but well, I am learning:I understand the reason of Differential Probes, but I was wondering...What if you have those big battery packs that provides 110-120V outputs (those are very common for camping) and you plug your scope to it, so, it potentially makes the scope "float" right?If so, does that makes the diff probes unnecessary? Does that will explode my probe? And I going to be teleported to the hall of shame of EE?Thanks!

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2018, 07:27:27 pm
content: There are many threads on this forum (and elsewhere) about this topic. They are easy to find, so in that sense you are in the hall of shame. But then youdidask the question...With topics like this, you have to consider not how things mightwork, but how they mightfail. If you float a scope that isn't specifically designed for that then you risk damaging the scope internally and yourself. In particular, consider that the scope's case will be at the same voltage as whatever you connect the probe's shield to.Read, learn and inwardly digesthttp://info.tek.com/rs/tektronix/images/60W_6053_13_LR_Letter.pdf

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 15, 2018, 07:30:08 pm
content: Short answer: if you have to ask, don't float. Ever.

author: PA4TIM
author_rank: Super Contributor
timestamp: February 15, 2018, 07:59:06 pm
content: Use Diff probes, Do not even think about floating a scope that is not made for it.You can float the DUT via an isolation transformer.I use an isolated scopemeter, an isolation transformer or diffprobes. Whatever fits best for the job

author: bsas
author_rank: Regular Contributor
timestamp: February 15, 2018, 08:06:44 pm
content: What is the most affordable way for a newbie to get a diff probe? Some cheap alternatives, DIY, etc? What do you guys suggest?Thanks!

author: woody
author_rank: Frequent Contributor
timestamp: February 15, 2018, 08:13:07 pm
content: For example:https://www.amazon.de/Micsig-DP10013-Differential-Attenuation-Tektronix/dp/B074K4XPW3#

author: tautech
author_rank: Super Contributor
timestamp: February 15, 2018, 08:22:01 pm
content: Quote from: bsas on February 15, 2018, 08:06:44 pmTwo in particular are worthy of note but also dependent on your needs.Pintek DP-25 is the entry level standard (NOT Pintech !)Daves probe if you want/need something a bit better:https://www.eevblog.com/product/hvp70/

author: Benta
author_rank: Super Contributor
timestamp: February 15, 2018, 08:26:46 pm
content: Rules are:Never float your test equipment, you'll be touching it.Float the DUT (device under test) if necessary (this is what the differential probessimulate).

author: bsas
author_rank: Regular Contributor
timestamp: February 15, 2018, 08:31:00 pm
content: Quote from: Benta on February 15, 2018, 08:26:46 pmSo, basically, if I use the same idea of the battery pack but to float the DUT instead of the probe, I am fine? (obviously the only "issue" is if the DUT can be floated, I got that part).That is a fine way to think about it.If so, why all DUTs (that can be floated) aren't "floated" on lab by default?Thanks!!!

author: danadak
author_rank: Super Contributor
timestamp: February 15, 2018, 08:39:48 pm
content: Some ref material -https://www.tek.com/document/technical-brief/floating-oscilloscope-measurements-and-operator-protectionhttp://in.tek.com/dl/3AW_19134_2_MR_Letter.pdfRegards, Dana.

author: Benta
author_rank: Super Contributor
timestamp: February 15, 2018, 08:56:40 pm
content: QuoteBecause it's not always possible, for instance with mains connected equipment. That being said, in a proper lab DUTs are connected through an isolation transformer to the mains. But at high power levels it's not always feasible.Don't take my two rules as the complete story, but it's a basic starting point for not killing yourself.

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2018, 10:03:12 pm
content: Quote from: bsas on February 15, 2018, 08:06:44 pmDon't spend more than your life is worth, or the life of someone else in the room is worth.You don't know enough to roll your own probe.

author: Vic20
author_rank: Contributor
timestamp: February 16, 2018, 08:44:09 pm
content: Quote from: bsas on February 15, 2018, 08:31:00 pmMost DUTs can be floated. In fact most power supply outputs are floated and can be grounded on the positive or the negative terminal.Any circuit powered by the supply will also float if you don't ground it.Of course, when you connect the DUT to the scope, a path to ground will be provided from the scope.Typical DUTs that cannot be floating are mains connected ones.You can use an isolation trasformer to connect them to an scope but, although an isolation transformer gives some protection, it doesn't need to be hazard free.Last point: Scope grounding is for YOUR protection. Never mess with it unless you sure know what you are doing.

author: jh15
author_rank: Frequent Contributor
timestamp: February 17, 2018, 08:20:20 am
content: I used a scope floater by Tektronix. Had a battery pack, nicad i'm sure in the '80s.Specifically made to address this issue.Freaked out some Facilities people when walking down the hall and a 60 watt tungsten trouble lamp running in hand.

author: Zero999
author_rank: Super Contributor
timestamp: February 17, 2018, 09:29:18 pm
content: Some mains switched mode power supplies will run at much lower voltages, than the rated AC voltage. The Rigol DS1054Z will work down to 36VDC, before it starts to become unstable, making it ideal for powering off a 48VDC power supply, such as a battery. Awhile ago, I contemplated making a battery pack for my DS1054Z, but I lost interest in the project, because I have the OWON SDS7102, which has an internal battery. Recently someone else revived the thread and made their own battery pack, from 18650 cells.https://www.eevblog.com/forum/testgear/battery-pack-for-rigol-ds1054z/I echo the points made earlier on about safety: floating an oscilloscope can be dangerous, if it's used on a circuit running at a hazardous voltage. It's much safer to float the device under test, using an isolation transformer. Another issue is connecting other things to the floating 'scope which may earth it, such as a computer or laptop, running from the mains.Only float the oscilloscope if the device is running at a non-hazardous voltage <60VDC or 25VAC. If you really must do it, when it's connected to a device running at a higher voltage, don't touch the oscilloscope or connect it to anything else, such as a laptop!

author: bitseeker
author_rank: Super Contributor
timestamp: February 18, 2018, 02:26:03 am
content: bsas, IIRC you're interested in working on vintage audio gear, so differential probes would serve you well for safety, high voltage capability, and measurement convenience. Alternatively, you can keep your eyes peeled for a used handheld scope with isolated inputs and high voltage capability, but they're not inexpensive.Check out the PinTek and MicSig probes mentioned earlier as they're good and affordable. I have the EEVblog HVP-70 probes (made by Sapphire) and they work great, but are more expensive. Some differential probes have a very high "low" range (e.g., 50X), which can be a bummer if you're trying to measure small signals. If you can get 10X on the low range, it makes the probes more convenient for general use, but it really depends what your particular requirements are.For reference, I've thus far used my differential probes for debugging power supplies where, internally, some points are in the hundreds of volts and it's useful being able to measure the voltage across components, not just relative to ground.