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Inductor test circuit.

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dr pepper

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I had a weird thing happen to my inductor test circuit I built from ronald dekkers nixie clock site.
The 'scope trace was opposite to what I expected on larger inductors, the current sense resistor in ronalds design is in the + rail, at larger currents the current limit on my bench supply was being hit dropping the voltage on the + line, which was 'scope ground, so obviously the 'scope saw this as a major drop in signal voltage causing the trace to go down not up.
I dont spose many will beuild this circuit but if you do put the sense resistor in the ground leg between the switching fet drain and the - rail, then the 'scope ground is test circuit ground and then everything will work.
With the 4 820uF 25v low esr caps I can get over 50 amps through a test inductor.
 

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Hi there,

It's quite common to connect the scope ground to the positive supply rail instead of ground. That's so that we can look across a part that is common with the positive rail instead of the ground rail.

Many scopes have a switch that you can use to invert one of the channels so you can see it normally even if the scope is connected 'backwards'.

I dont think it is a good idea however to stick the ground lead of the scope to a node in the circuit that changes voltage by a large amount. A DC level or ground is better.
 
What Mr Al said, however, make sure your scope is "isolated" from ground (we use what we call suicide plugs for this) if you're going to connect the 'ground' to a positive rail.
 
Mr al is correct, the + in this situation isnt stable due to high currents, what confused me a little at the time was the fact that the dip was large enough to invert the trace on the screen making an inductance look like a capacitance.

I spose things like this sort the men from the boys.

Wheres my beano comic?
 
Hi,

My scope plug has the "middle finger" cut off (ha ha) so it is never connected to the power line ground, but it's still not a good idea to connect the scope ground to a changing voltage. It's simple enough to see if it is changing before connecting by simply measuring that point with the scope with the ground connected to circuit ground first.
Think of it this way...if your scope ground connects to the metal chassis and metal case of the scope and the ground lead is connected to a 20v peak to peak 10 MHz signal, your scope chassis is being modulated at 10Mhz (chuckle). If it is a small plastic scope then the ground plane is being modulated at least and any circuit shielding is being modulated too. Well at least that is what the circuit under test will try to do, and that may mean a lot of extra capacitance introduced into the circuit or a lot of extra noise as the scope chassis can easily pick up noise too. So sensitive inputs in the circuit under test could go nuts.
 
apart from if say a wire came off inside my scope and touched the metal case, is there any reason why i shouldn't cut the middle finger (lol) of my scope? i cant afford a isolation transformer.
 
Hi,

If all you are worried about is the power line ground, then you can use one of those small three prong to two prong adapters. They cost less than a dollar USD each and you only need one.

An isolation transformer for a scope should not cost too much because the scope probably does not draw too much power. Check the power consumption of the scope to find out.

Also a good choice which is probably the best all around is a differential scope input amplifier ie "differential probe". Probably not that cheap these days though.
 
My philips has a metal case but only 2 wires to the mains plug, no earth.

I sometimes wish it had isolated inputs.

Yes I see what you mean about the chassis radiating noise.
 
Depending upon the design of your scope, you may be able to isolate the secondary ground (that runs all the circuitry) from the mains ground, while still keeping the case grounded for safety. It might be nice to fit a switch to ground or lift as needed.

Of course you can always float the circuit you're working on in stead of the 'scope. That way you can retain a ground to your scope for safety and shielding, and avoid the possibility of getting a shock to ground from the circuit under test.
I built an isolation box last year - it was a bit of work, but has been worth every minute I put into it, very handy indeed.
 
can you explain the isolation box please. i normally use atx psu for general stuff and its gnd is tied to ground earth
 
I think what he's on about lg is an isolation transformer, just a tranny with a 1 to 1 ratio, 240 in 240 out.
This as you can see isolates the connection between your supply and the mains grid, if you came into contact with one of the transformer terminals there isnt a path to ground.

My workshop has 110v for power tools, this comes off a transformer with a centre tap to ground, so theres only 55v to earth on either supply connection.
 
Yep, that's the one (sorry for the delayed response). I used two 55-0-55 transformers as it was the most economical option. The ratio isn't exactly 1:1 but, snce I also use a Variac (variable autotransformer) it doesn't really matter.
 
a variac is on my must get list :D but they cost alot! so it will be ages before i get one! although i am currently in discussion's at school with a couple of my class mates who want a 3x3 LED cube each (they did want 5x5 but i pointed out the cost would be pretty high) need to find other ways of making money! my weekend sandwich job dosnt pay much
 
Do a saved search on ebay they come up now and again.
 
yeah 5A ones are around £70-£100 for enclosed type(only allowed enclosed one's :meh:) and that is alot of cash to me
 
Me too, I'm watertight.
I come across them now and again, allthough with the advent of igbts and switching technology you dont see them much in industry now.
 
would be really handy for when i try and fix stuff! so far ive only fixed 2 oscilloscopes and a function generator but i made a little money selling them on and that cash went on my signal generator
 
apart from if say a wire came off inside my scope and touched the metal case, is there any reason why i shouldn't cut the middle finger (lol) of my scope?
While there are those here who will state that a scope with an earthed case can under certain circumstances be dangerous, I am firmly of the opinion that all equipment with a metal case should be correctly earthed.
If you need to "float" the scope case, you are using a bad measurement technique.

An unearthed case will probably float at around 120v (in the UK).
The Y capacitors in the mains filter are connected from neutral to case and from live to case, and make a nice potential divider.

When the case is touched, it will feel "tingley", it is unlikely to hurt you, but is could easily kill any semiconductors in the circuit under test.

JimB
 
I've had that tingle before, more than once.

Lg, if you've fixed a couple of 'scopes and a sig gen and then made cash out of it for 13 thats not bad at all.

I have a 'scope that I still havent repaired even after numerous attempts, however that is more due to the fact theres an hours assembly/disassembly to see if what I've done works or not.
 
While there are those here who will state that a scope with an earthed case can under certain circumstances be dangerous, I am firmly of the opinion that all equipment with a metal case should be correctly earthed.
If you need to "float" the scope case, you are using a bad measurement technique.

An unearthed case will probably float at around 120v (in the UK).
The Y capacitors in the mains filter are connected from neutral to case and from live to case, and make a nice potential divider.

When the case is touched, it will feel "tingley", it is unlikely to hurt you, but is could easily kill any semiconductors in the circuit under test.

JimB
ok i am convinced :D the earth stays on
 
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