Power adapter with 6K earth reference - Page 1
Forum section: electronics / beginners

author: Alex30
author_rank: Regular Contributor
timestamp: December 11, 2014, 07:55:32 am
content: So I was just checking out a power adapter and I measured 6Kohm from the negative to earth. As this is not a dead short and the supply is 16V DC (16V across 6K = 2.7mA) can I still probe around safely with my oscilloscope and put my ground lead on the probe wherever I like?

author: rs20
author_rank: Super Contributor
timestamp: December 11, 2014, 09:01:59 am
content: First off, I can't help but wonder if this supply is working properly; and if it's not working properly it mightn't be safe. I'm open to being corrected here, but I feel like 6k is an awfully high impedance for an earth-referenced supply, and an awfully, dangerously low impedance for a nominally isolated supply. But maybe lots of adapters are like this for some weird reason and I'm just ignorant.That aside, there are plenty of real-life circuits where injecting or pulling 2.7mA into/out of certain nodes can totally modify the behaviour of the circuit, and even destroy devices. You'd have to use your judgement and think before clipping the earth lead onto any particular node, will this be fine if I connected it to the negative supply with a 6k resistor? That's not something I want to be thinking about when debugging my circuits. And assuming that that's exactly what's going on here (simply a 6k resistor from negative to earth and nothing else) could be missing something more dangerous.Everyone else, am I dramatically overreacting? Just seems shady to me.

author: Alex30
author_rank: Regular Contributor
timestamp: December 11, 2014, 09:40:22 am
content: I believe it is working fine. If you watch EEVblog #279 david probes an adapter and measures 1k between earth and negative

author: rs20
author_rank: Super Contributor
timestamp: December 11, 2014, 10:17:30 am
content: What is this power supply nominally for, just out of curiosity?Nevertheless, I think the video answers the question for you. He saw 1k, and said "avoid/beware/be very careful". 6k is less than an order of magnitude away, the story's no different. Just measure the voltage between your negative supply and earth to ensure that it really is earth-referenced on the negative rail, and if that reads zero, stick to connecting your ground clip to that negative rail only.

author: Alex30
author_rank: Regular Contributor
timestamp: December 11, 2014, 10:40:52 am
content: It is normally a laptop charger. I know he said beware, but surely I should only care if it was high voltage? Like at about 2.7mA will I really change the measured voltage by that much? Normally I won't even be using my cro to measure voltage but just to preview transient response. I'm just wondering whether I should care that much because most things I'd be powering with this would be able to stand up to 10mA or more.

author: rs20
author_rank: Super Contributor
timestamp: December 11, 2014, 11:01:43 am
content: Quote from: Alex30 on December 11, 2014, 10:40:52 amDepends on the node. 2.7mA will most likely have very little measurable effect on the positive rail, but it will blow up a sensitive diode or MOSFET on a very high impedance node. Best practice is to connect the alligator clip to the negative rail of the circuit (IF the negative rail is what's connected to earth by a 6k R). You haven't explained why this isn't an acceptable solution. Or, even safer, save yourself the trouble and use an isolation transformer or differential probe and probe/clip whatever you want.

author: Alex30
author_rank: Regular Contributor
timestamp: December 11, 2014, 11:13:37 am
content: Well, I have been hunting a power adapter for a while that isn't a cheap chinese knock off and got this adapter that I wanted to power a project with. I am just a hobbyist so I don't have access to an isolation transformer or differential probes and would probably never want to spend hundreds of dollars for one. Usually just attaching the negative clip to the negative rail is fine but I just wanted to leave my options open that's all. I suppose you have answered my question anyway, I will just have to use some common sense if I ever wanted to probe between two nodes. I suppose I could just clip the earth terminal off the outlet or attach my own plug on the end of the cable if I really wanted to too. Thanks mate!

author: rs20
author_rank: Super Contributor
timestamp: December 11, 2014, 11:27:34 am
content: Quote from: Alex30 on December 11, 2014, 11:13:37 amAlso, another option is to use two probes (both grounded to the negative rail) and the subtraction function of your scope to give your differential trace on-screen. Or, AC coupling for transients in some situations.No worries!