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October 27, 2021, 11:56:04 23:56


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Author Topic: Logic probe with external power supply. Ground loss and reverse power connection  (Read 1779 times)
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Signal
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« on: February 19, 2021, 06:00:21 18:00 »

Thinking about logic probe.

Classic ones had two clips to connect to GND and VCC directly on tested board and it was fine! Even if ground clip disconnects or ground of probe mistakenly is connected to power no problem at all. The worst you can make is to load tested pin by 10-20 kOhm.

Another story if you test equipment that has several (tiny) voltages inside by tool that get power supply from the same external source with higher voltage like automotive 12V. In case you lost ground a tested pin will be fried by greater voltage and some pins will not survive. The same result if ground clip will be connected to power.

One way to somehow manage the problem is to use asymmetrical connectors like lighter socket, but wires have tendency to eventually brake even connector is ok, so still not satisfactory.

Any ideas? It seems that I am reinventing the wheel..
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Vineyards
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« Reply #1 on: February 19, 2021, 06:14:23 18:14 »

Can't you just drive the tested circuit through a diode and use current limiting resistors before it?
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pickit2
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« Reply #2 on: February 19, 2021, 09:08:09 21:08 »

how about using a power bank.
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Signal
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« Reply #3 on: February 21, 2021, 11:28:38 11:28 »

Can't you just drive the tested circuit through a diode and use current limiting resistors before it?
Sorry. I do not understand what you suggested exactly or do not see how it could help.

Posted on: February 21, 2021, 07:12:43 19:12 - Automerged

how about using a power bank.
The idea to use power bank is quite interesting and actually differs from simply using isolated power supply or battery.

From isolated power supply as DC/DC converters (there are some small and low-cost devices) it differs by absence of power wire. Having the only ground wire we at first do not affraid of ground loss and much less (but still present) probability to connect this ground to power. The remaining could be managed by placing separate ground wire far from any power on desk.

From battery (alcaline or liion) it differs by readily availability of ready to use devices with integrated charger.Some of them are rather small.

So I like what you suggested, but curious what is the mainstream solution or such caveats are the reasons for logic probes dissapearing in labs?
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« Reply #4 on: February 21, 2021, 01:04:30 13:04 »

see https://www.swansontec.com/sprobe.html
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Vineyards
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« Reply #5 on: February 22, 2021, 10:42:34 10:42 »

I didn't completely understand what you were trying to do. Assuming that you need to separate the grounds due to ground loop issues there have been analog and digital signal isolator IC's in the market for ages. It all depends on your circuit topology. I used both kinds. There must even be some types with onboard DC-DC power isolators. If you are adventurous you can build them using discreet components too.
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mexpcb
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« Reply #6 on: February 26, 2021, 05:14:55 05:14 »

why don't you just do use a fast AND Gate with Schottky diode to self power the gate.
you may need to pick one that will work for your voltage and current needs...
if you need to just test a true or false, it will be fine, if its self powered it may be just withing the range of your devices, but not sure what you are trying to accomplish...
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« Reply #7 on: February 28, 2021, 05:08:20 17:08 »

I didn't completely understand what you were trying to do. Assuming that you need to separate the grounds due to ground loop issues there have been analog and digital signal isolator IC's in the market for ages. It all depends on your circuit topology. I used both kinds. There must even be some types with onboard DC-DC power isolators. If you are adventurous you can build them using discreet components too.
Maybe the attached picture explains better than many words the problem with ground loss. I do not need to separate ground. Common ground is used as common reference to measure potentials.


Posted on: March 01, 2021, 01:06:40 01:06 - Automerged

why don't you just do use a fast AND Gate with Schottky diode to self power the gate.
you may need to pick one that will work for your voltage and current needs...
if you need to just test a true or false, it will be fine, if its self powered it may be just withing the range of your devices, but not sure what you are trying to accomplish...
Regards

Could you explain a little bit more the idea about gate and diode?
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