Foolproof probes - Page 1
Forum section: electronics / general

author: rr100
author_rank: Frequent Contributor
timestamp: April 03, 2012, 04:14:41 pm
content: Inspired by the "Anti-Static Mat Myth" video I started to think about some foolproof voltmeter probes. As we all know the input impedance of multimeters/voltmeters is usually huge and I was wondering if anybody makes some probes with "high resistance tips" which can't short out your circuit no matter how clumsy you are (useful to probe for example inside presumably energized connectors for example). If you short something out the resistance on 0.5mm of "probe tip" should be in the 1kohm range, this should be good enough for low voltages and some tens of kohms shouldn't influence your measurements much (unless you short something...).So, anybody makes such beasts? Alternatively any recommendations for some thin probes where only a tiny bit of the tip is "naked"?

author: robrenz
author_rank: Super Contributor
timestamp: April 03, 2012, 04:21:51 pm
content: Just put heat shrink your probe tip and cut off the heat shrink to expose the amount of tip you want showing.

author: Neilm
author_rank: Super Contributor
timestamp: April 03, 2012, 06:13:21 pm
content: Probes that meet the new version of IEC61010-030 should not allow adjacent conductors to short out.Neil

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 03, 2012, 07:21:16 pm
content: when you change to ohm range, this will be "foolfull" tip.

author: Monkeh
author_rank: Super Contributor
timestamp: April 03, 2012, 07:27:34 pm
content: Quote from: robrenz on April 03, 2012, 04:21:51 pmLike so:

author: onlooker
author_rank: Frequent Contributor
timestamp: April 04, 2012, 01:37:07 am
content: I think OP had an excellent idea and may be he can apply a patent for it. I believe I had destroyed a (malfunctioned) camcorder recently due to short by probe lead.

author: robrenz
author_rank: Super Contributor
timestamp: April 04, 2012, 01:41:52 am
content: Its not mine or a new idea, I have Pomona extended thin tips that have heat shrink on all but the tip that are at least 20 years old.

author: onlooker
author_rank: Frequent Contributor
timestamp: April 04, 2012, 02:05:29 am
content: Sorry for the confusion, I meant the OP's "resistive probe tip for voltage measuring" idea.

author: G7PSK
author_rank: Super Contributor
timestamp: April 04, 2012, 08:21:11 am
content: The problem with high resistance tips is what material fits the bill. most of the materials used in resistances are either too soft or too brittle for probes which have to start off at 2 mm or less and taper down to 0.2 or something like that. It is possible that a carbide type compound might do the job but all the carbides I have come across have much lower resistance than 1 meg.ohm per mm which is the least that would be required, Further more the resistance on one axis would have to be much less or the probes would not work due too voltage drop lengthwise.Scope probes often come with a kit of shields which can be used to test outputs on chips without shorting them, So fitting similar to test probes for meters would be the answer.The probes that came with my Amprobe meter are shielded right down to the tip as standard, You could also solder needles to the tips of probes to make them smaller and so less likely to be able to short out the pins due to width.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 04, 2012, 09:27:19 am
content: You don't need 1 meg. 1 kohm or so would limit the current to a small enough value. This would be for things like probing leads on TQFP packages where you don't want to short Vcc to ground -- not avoiding faults when probing line voltages. Heat shrink tubing or the new IEC61010-030 compliant probes are better suited for that, but don't do anything when the adjacent conductor is 0.5 mm away.With moderate resistances you don't have to worry as much about voltage drop. Ideally, the bulk of the resistance is spreading resistance, so the probe doesn't see dramatically higher resistance than the adjacent conductor, possibly less. 1 kohm only gives you an 0.02% error with a typical 10 meg meter.I still think finding a good material is going to be difficult.

author: Unknown
author_rank: Unknown
timestamp: April 04, 2012, 10:01:01 am
content: This is one reason why Bob Pease suggested bench meters with > 10 Gohm impedance on the lower DCV ranges, although he was more worried about the meter injecting noise into the circuit. Putting 100 kohm in series with 10 Gohm introduces a negligible error.

author: onlooker
author_rank: Frequent Contributor
timestamp: April 04, 2012, 01:02:41 pm
content: The material may be a problem (I do not know enough). But the mechanical design can be very simple: one just needs a kind of coating/wrap/(removable) cap on top of existing probe leads. Hence, the cost can be very low too.