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# Isolating Oscilloscope Channels

E

#### EdV

Jan 1, 1970
0
I have a dedicated production test system that uses a Tektronix
oscilloscope which does not have isolated channels. I have to measure
the "lag time" between the falling edge of one IGBT H bridge drive leg
and the rising edge of another. It is specified to be 3 to 5
microseconds. Of course these signals are referenced to to different
"grounds".

I thought a simple opto isolator circuit would do the trick but as I
look at parts I get the feeling I am going to stack up enough
propagation delay that measuring 3 to 5 microseconds is not going to
have the needed preciscion.

Maybe drive a resistor and then use diff amps?

Your thoughts as always are welcome.

Thanks,
Ed V.

G

#### GregS

Jan 1, 1970
0
I have a dedicated production test system that uses a Tektronix
oscilloscope which does not have isolated channels. I have to measure
the "lag time" between the falling edge of one IGBT H bridge drive leg
and the rising edge of another. It is specified to be 3 to 5
microseconds. Of course these signals are referenced to to different
"grounds".

I thought a simple opto isolator circuit would do the trick but as I
look at parts I get the feeling I am going to stack up enough
propagation delay that measuring 3 to 5 microseconds is not going to
have the needed preciscion.

Maybe drive a resistor and then use diff amps?

Your thoughts as always are welcome.

Nice to have the good old scopes with differential plug-ins and

10 m probes. That would work.

Are any of the grounds, grounded?

greg

E

#### EdV

Jan 1, 1970
0
Nice to have the good old scopes with differential plug-ins and

10 m probes. That would work.

Are any of the grounds, grounded?

greg- Hide quoted text -

- Show quoted text -

No the "grounds" are the emitters of the upper leg and lower leg of
the H Bridge. Maybe I should just arbitrarily assign "ground" the
upper emitter which is also the lower collector invert one of the
signals and tweak the scope vertical. I am still waitng for my test
fixture to get built so I can snoop these signals.

Thanks for helping me start thinking again.

Ed V.

J

#### John Larkin

Jan 1, 1970
0
I have a dedicated production test system that uses a Tektronix
oscilloscope which does not have isolated channels. I have to measure
the "lag time" between the falling edge of one IGBT H bridge drive leg
and the rising edge of another. It is specified to be 3 to 5
microseconds. Of course these signals are referenced to to different
"grounds".

I thought a simple opto isolator circuit would do the trick but as I
look at parts I get the feeling I am going to stack up enough
propagation delay that measuring 3 to 5 microseconds is not going to
have the needed preciscion.

Maybe drive a resistor and then use diff amps?

Your thoughts as always are welcome.

Thanks,
Ed V.

Get one of the Tek isolated scopes? They're fabulous.

If it's just an edge, transformer couple it.

John

P

#### Paul Mathews

Jan 1, 1970
0
I have a dedicated production test system that uses a Tektronix
oscilloscope which does not have isolated channels. I have to measure
the "lag time" between the falling edge of one IGBT H bridge drive leg
and the rising edge of another. It is specified to be 3 to 5
microseconds. Of course these signals are referenced to to different
"grounds".

I thought a simple opto isolator circuit would do the trick but as I
look at parts I get the feeling I am going to stack up enough
propagation delay that measuring 3 to 5 microseconds is not going to
have the needed preciscion.

Maybe drive a resistor and then use diff amps?

Your thoughts as always are welcome.

Thanks,
Ed V.

See Tektronix P5200 series of HV diff probes and similar products.
That's the right way to do it.
Paul Mathews

J

#### Joerg

Jan 1, 1970
0
EdV said:
I have a dedicated production test system that uses a Tektronix
oscilloscope which does not have isolated channels. I have to measure
the "lag time" between the falling edge of one IGBT H bridge drive leg
and the rising edge of another. It is specified to be 3 to 5
microseconds. Of course these signals are referenced to to different
"grounds".

I thought a simple opto isolator circuit would do the trick but as I
look at parts I get the feeling I am going to stack up enough
propagation delay that measuring 3 to 5 microseconds is not going to
have the needed preciscion.

Maybe drive a resistor and then use diff amps?

Your thoughts as always are welcome.

As John mentioned transformer coupling can achieve that. Mini-Circuits
has lots of those. Possibly you could also get away with LAN
transformers from an old Ethernet card. But make sure nobody needs that
card anymore ;-)

F

#### [email protected]

Jan 1, 1970
0
Joerg said:
EdV wrote:
As John mentioned transformer coupling can achieve that. Mini-Circuits
has lots of those. Possibly you could also get away with LAN
transformers from an old Ethernet card. But make sure nobody needs that
card anymore ;-)

Wont work I.M.O.

The current through the primary - secondary capacitance will screw your
measurement over AND if the (on the day they used the right goop for
encapsulating) 500 V (or so) isolation level in the cheap(!) Chinese LAN trafo
from "Electronics Factory No 5, Huawei" ever goes away then the H bridge -
off-mains since the O.P. needs isolation - might then make lots of colours with
your scope (before the muffled Boom from the sub-basement fuses cuts the show
;-).

The O.P. should buy the proper differential / isolated probes for the voltage
level. They will cost money. But so will the Health and Safety incident, the
trip through court, and compensation payments should some worker get injured.
The word "production" ups the stakes quite a bit.

The Chinese will print whatever safety labels are requested from the user of the
part - it does not follow that any of that actually applies; as f.ex. HP found
out the hard way with a run of power supply "Y" capacitors ...

T

#### Tim Williams

Jan 1, 1970
0
Possibilities:
- Turn off power (to the bridge) and measure directly
- Disconnect gate drives from bridge and measure at ground
- Couple with C, R and transformer (you're looking for the edges, right?) --
with care you can also measure the edge height
- Watch carefully at high voltage (15V of gate drive does kind of get lost
in 600V or whatever though)
- Build your own differential probe ;-)
Etc.

Tim

E

#### EdV

Jan 1, 1970
0
Possibilities:
- Turn off power (to the bridge) and measure directly
- Disconnect gate drives from bridge and measure at ground
- Couple with C, R and transformer (you're looking for the edges, right?) --
with care you can also measure the edge height
- Watch carefully at high voltage (15V of gate drive does kind of get lost
in 600V or whatever though)
- Build your own differential probe ;-)
Etc.

Tim

--
"Librarians are hiding something." - Steven Colbert
Website @http://webpages.charter.net/dawill/tmoranwms

- Show quoted text -
This link looked interesting:

http://www.libinst.com/Difprobe.htm

I am going to experiment with these circuits. Unfortunately the
suggested transformers are not meant to withstand some of the voltages
that might appear
if something went wrong. :-(
rate.

Hawker

J

#### Joerg

Jan 1, 1970
0
EdV said:

Just don't use audio mic transformers for microsecond stuff. Also, their
breakdown voltage isn't very high, or there is no rating for that at all
on some.

I am going to experiment with these circuits. Unfortunately the
suggested transformers are not meant to withstand some of the voltages
that might appear
if something went wrong. :-(

You can always roll your own or buy from a reliable source where there
are breakdown specs. If you roll your own you could get some Rubadue wire:

I think spending $1K each for two good active diff probes from Tektronix is going to make the most sense. Certainly makes sense, especially if you have to do this often or if you must conduct documented test. Saves a lot of ECO writing for test jigs which probably costs more than$1k in EE hours.

E

#### EdV

Jan 1, 1970
0
I do a pile of consulting for a company that often needs the same thing.
They have some fancy large Tektronix 8 channel isolator they use.
Me I us an old THS series battery powered Tektronix scope for this.
It has two channels, each is isolated from ground and from each other.
It's not fancy but it would pretty good for that purpose.
They run about $800 used and I think$2500-$3500 new depending on sample rate. Hawker - Show quoted text - I am stuck with the oscilloscope that is in the tester because it used to test many different baord level assemblies. Kind of remeber asking "why don't we have isolated channels"? I believe it was cost. Oh well. E #### EdV Jan 1, 1970 0 Just don't use audio mic transformers for microsecond stuff. Also, their breakdown voltage isn't very high, or there is no rating for that at all on some. You can always roll your own or buy from a reliable source where there are breakdown specs. If you roll your own you could get some Rubadue wire: http://www.rubadue.com/products.html Certainly makes sense, especially if you have to do this often or if you must conduct documented test. Saves a lot of ECO writing for test jigs which probably costs more than$1k in EE hours.

--
Regards, Joerg

http://www.analogconsultants.com- Hide quoted text -

- Show quoted text -

A shame about the microsecond thing. The signal should always be 20
Vpp now that I review the circuit, when it is working correctly. I
could protect the transformer with clamp diodes but not much point if
the timing would not be preserved.

Thanks,
Ed V.

J

#### Joerg

Jan 1, 1970
0
EdV said:
A shame about the microsecond thing. The signal should always be 20
Vpp now that I review the circuit, when it is working correctly. I
could protect the transformer with clamp diodes but not much point if
the timing would not be preserved.

You can get fast transformers at places like Mini-Circuits. I don't know
how high LAN transformers go and where they saturate but a datasheet of
one should tell. Reason I don't know is because I always keep a pound of
#43 ferrites in my lab so I can quickly wind one up if in need. Faster
than filling out a Digikey order ;-)

T

#### Tim Williams

Jan 1, 1970
0
Joerg said:
You can get fast transformers at places like Mini-Circuits. I don't know
how high LAN transformers go and where they saturate but a datasheet of
one should tell. Reason I don't know is because I always keep a pound of
#43 ferrites in my lab so I can quickly wind one up if in need. Faster
than filling out a Digikey order ;-)

Heh yep. I've had excellent response from even really crappy transformer
designs. 100 turns of 30AWG on a high-permeability core (toroid or
etcetera) will get you there. Primary then secondary, don't really even
need to interleave them. Use *two* layers of masking tape between windings
if insulation spec is really high. ;-) My experience gives bandwidth into
the 10s of MHz (risetime comparable to the driving circuit: 100-200ns).

I would use a series capacitor to block DC and a (parallel) resistor to
dampen oscillations. With small C, you can get away with pretty wimpy
windings, since the pulse ends up really narrow. With large C (and L!), you
can see the whole square wave with maybe just a little droop along the way.

Tim

J

#### Joerg

Jan 1, 1970
0
Tim said:
Heh yep. I've had excellent response from even really crappy transformer
designs. 100 turns of 30AWG on a high-permeability core (toroid or
etcetera) will get you there. Primary then secondary, don't really even
need to interleave them. Use *two* layers of masking tape between windings
if insulation spec is really high. ;-) My experience gives bandwidth into
the 10s of MHz (risetime comparable to the driving circuit: 100-200ns).

I would use a series capacitor to block DC and a (parallel) resistor to
dampen oscillations. With small C, you can get away with pretty wimpy
windings, since the pulse ends up really narrow. With large C (and L!), you
can see the whole square wave with maybe just a little droop along the way.

Looks like Ed is only interested in the transition times. My staple for
that looks like this: About 6" PTFE wire, 6" of a CAT-5 strand or
whatever, twist the two together in 1/2" turns or so, wrap four times
through a 1/2" OD toroid of #43 material. I get sub-10nsec transitions
through those. The PTFE insulation is only needed where there is a lot
of bzzzt between primary and secondary.

J

#### Joerg

Jan 1, 1970
0
Wont work I.M.O.

Oops. I've done it a few hundred times by now. Or maybe thousands. Do I
have to go to confession for that now?

The current through the primary - secondary capacitance will screw your
measurement over AND if the (on the day they used the right goop for
encapsulating) 500 V (or so) isolation level in the cheap(!) Chinese LAN trafo
from "Electronics Factory No 5, Huawei" ever goes away then the H bridge -
off-mains since the O.P. needs isolation - might then make lots of colours with
your scope (before the muffled Boom from the sub-basement fuses cuts the show
;-).

The O.P. should buy the proper differential / isolated probes for the voltage
level. They will cost money. But so will the Health and Safety incident, the
trip through court, and compensation payments should some worker get injured.
The word "production" ups the stakes quite a bit.

Well, you have to do it right. Transformer coupling can be very helpful
especially in production because space it usually cramped and it doesn't
exactly help to cram two or more diff-probes in there. That's one reason
why there is PTFE cable available where they test and certify every foot
of it for dielectric strength.

Anyhow, that's also how we design patient interfaces. Lots of them out
there in the field. So if you really don't trust such transformers it
would be best to eat healthy, exercise, and absolutely not develop a
coronary artery problem because chances are they'll use one of those
machines on you. But rest assured, the ones I designed are all
defibrillator proof and tested for that. I wouldn't do them any other way.

The Chinese will print whatever safety labels are requested from the user of the
part - it does not follow that any of that actually applies; as f.ex. HP found
out the hard way with a run of power supply "Y" capacitors ...

Yep, that sure can happen. Know thy sources ;-)

H

#### Hawker

Jan 1, 1970
0
I am stuck with the oscilloscope that is in the tester because it used
to test many different baord level assemblies. Kind of remeber asking
"why don't we have isolated channels"? I believe it was cost. Oh well.

I have often told clients "if you want the work done you need the
correct tools" if I can't get the correct tools I can't get the work
done. If they need this tested then they need to get the correct tool
for the job no two ways around it.

It amazes me how often a client is willing to pay me $5,000 -$25,000
for consulting work, but won't spring an extra $500-$1000 for the
specialized tools I need for the job. I understand that basic tools are
my responsibility to supply (a standard scope is a basic tool) but since
every job is unique there are often a few special tools required (esp
ICE tools for whatever processor I am using or HV tools for HV products).

Hawker

J

#### Joerg

Jan 1, 1970
0
Hawker said:
I have often told clients "if you want the work done you need the
correct tools" if I can't get the correct tools I can't get the work
done. If they need this tested then they need to get the correct tool
for the job no two ways around it.

It amazes me how often a client is willing to pay me $5,000 -$25,000
for consulting work, but won't spring an extra $500-$1000 for the
specialized tools I need for the job. I understand that basic tools are
my responsibility to supply (a standard scope is a basic tool) but since
every job is unique there are often a few special tools required (esp
ICE tools for whatever processor I am using or HV tools for HV products).

From a business point of view this is often the reason: Our consulting
services are billed just like materials. They get purchased and then
consumed. A tool becomes inventory and must be entered into the
amortization tables. Depending on cost and type it has to remain there
for several years. Inventory Dollars is a number that needs to be as low
as possible so the CFO can leave the next board meeting a happy camper.
Payroll Dollars is the other number that needs to be low and that's why
US companies like consultants. Which is a good thing ))

This is also one reason why businesses often spend more money on long
term rentals than we would, beyond the point where an outright purchase
would have been a better deal.

E

#### EdV

Jan 1, 1970
0
Looks like Ed is only interested in the transition times. My staple for
that looks like this: About 6" PTFE wire, 6" of a CAT-5 strand or
whatever, twist the two together in 1/2" turns or so, wrap four times
through a 1/2" OD toroid of #43 material. I get sub-10nsec transitions
through those. The PTFE insulation is only needed where there is a lot
of bzzzt between primary and secondary.

--
Regards, Joerg

http://www.analogconsultants.com- Hide quoted text -

- Show quoted text -

Thanks! I'll try this one.

Ed V.

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