EMI on scope probe

R

Roland Andrag

Guest
Hi everyone,

I have a circuit which contains a ULN2803, which is used to switch a 12 VDC
relay (standard 8 pin, around 100 mA). The relay, in turn, switches a
contactor at 230 VAC (a couple of mA at 230 VAC). The resulting arcing over
the contacts of the relay is generating a lot of EMI, which is at the moment
wreaking havoc with the rest of my circuit.

In trying to solve the problem, my first problem is that I can't accurately
measure the noise anywhere in my circuit, since my scope probe picks up the
noise (up to 5 V peak-peak) even when the probe tip is clipped directly onto
its own ground connection, and to nothing else.

The scope is a Tektronic TDS1002, with its standard probe. If I disconnect
the probe at the co-ax connector, the noise dissappears, so I'm sure that
the nois is not entering via the scope power supply from the AC line.

Any help very much appreciated.

Thanks,

Roland
 
Roland Andrag <randragnews@gmail.com> wrote:
Hi everyone,

I have a circuit which contains a ULN2803, which is used to switch a 12 VDC
relay (standard 8 pin, around 100 mA). The relay, in turn, switches a
contactor at 230 VAC (a couple of mA at 230 VAC). The resulting arcing over
the contacts of the relay is generating a lot of EMI, which is at the moment
wreaking havoc with the rest of my circuit.
Click to expand...
Arcing of relay, or contactor?
In trying to solve the problem, my first problem is that I can't accurately
measure the noise anywhere in my circuit, since my scope probe picks up the
noise (up to 5 V peak-peak) even when the probe tip is clipped directly onto
its own ground connection, and to nothing else.

The scope is a Tektronic TDS1002, with its standard probe. If I disconnect
the probe at the co-ax connector, the noise dissappears, so I'm sure that
the nois is not entering via the scope power supply from the AC line.

Any help very much appreciated.
Click to expand...
First.
I'd connect the device to an isolation transformer, (or use a battery
operated scope (disconnecting the earth of the scope is an option used by
some, but is a really bad idea safety-wise)) to avoid current flowing in
the shield, which it otherwise will want to.

How is the contactor wired, on a PCB, or ...
Ideally the contactor switch wires should be twisted together, to
reduce any current loops.

Cling-film wrapped round the circuit, with a few wraps of foil over the top
can tell you if shielding might help.
 
Hi Ian, thanks for the quick reply.

Arcing on the relay contacts - the contactor contacts are not connected at
the moment.

I've had the scope on a UPS (which was unplugged completely to isolate it),
still have the same problem at the scope side - fairly sure that the noise
is getting induced in the probe, since the noise becomes much smaller as I
move the probe further from the relay. I'll get hold an isolation
transformr tomorrow - it is definately a lot easier than usning a UPS.

I can't twist the 230 V live and neutral that switch the contactor together,
since the live signal passes through a couple of relays - I have kept fairly
close together though.

Thanks,

Roland



"Ian Stirling" <root@mauve.demon.co.uk> wrote in message
news:41a0e566$0$43585$ed2e19e4@ptn-nntp-reader04.plus.net...
Roland Andrag <randragnews@gmail.com> wrote:
Hi everyone,

I have a circuit which contains a ULN2803, which is used to switch a 12
VDC
relay (standard 8 pin, around 100 mA). The relay, in turn, switches a
contactor at 230 VAC (a couple of mA at 230 VAC). The resulting arcing
over
the contacts of the relay is generating a lot of EMI, which is at the
moment
wreaking havoc with the rest of my circuit.

Arcing of relay, or contactor?

In trying to solve the problem, my first problem is that I can't
accurately
measure the noise anywhere in my circuit, since my scope probe picks up
the
noise (up to 5 V peak-peak) even when the probe tip is clipped directly
onto
its own ground connection, and to nothing else.

The scope is a Tektronic TDS1002, with its standard probe. If I
disconnect
the probe at the co-ax connector, the noise dissappears, so I'm sure that
the nois is not entering via the scope power supply from the AC line.

Any help very much appreciated.

First.
I'd connect the device to an isolation transformer, (or use a battery
operated scope (disconnecting the earth of the scope is an option used by
some, but is a really bad idea safety-wise)) to avoid current flowing in
the shield, which it otherwise will want to.

How is the contactor wired, on a PCB, or ...
Ideally the contactor switch wires should be twisted together, to
reduce any current loops.

Cling-film wrapped round the circuit, with a few wraps of foil over the
top
can tell you if shielding might help.
Click to expand...
 
Roland Andrag <randragnews@gmail.com> wrote:
Hi Ian, thanks for the quick reply.

Arcing on the relay contacts - the contactor contacts are not connected at
the moment.

I've had the scope on a UPS (which was unplugged completely to isolate it),
still have the same problem at the scope side - fairly sure that the noise
is getting induced in the probe, since the noise becomes much smaller as I
move the probe further from the relay. I'll get hold an isolation
transformr tomorrow - it is definately a lot easier than usning a UPS.
Click to expand...
Ok.
Does the problem go away if you disconnect the relay contacts?
You do have snubber components across the contactor coil?
 
"Roland Andrag" <randragnews@gmail.com> wrote in message
news:cnqns2$pfs$1@ctb-nnrp2.saix.net...
Hi everyone,

I have a circuit which contains a ULN2803, which is used to switch a 12
VDC relay (standard 8 pin, around 100 mA). The relay, in turn, switches a
contactor at 230 VAC (a couple of mA at 230 VAC). The resulting arcing
over the contacts of the relay is generating a lot of EMI, which is at the
moment wreaking havoc with the rest of my circuit.

In trying to solve the problem, my first problem is that I can't
accurately measure the noise anywhere in my circuit, since my scope probe
picks up the noise (up to 5 V peak-peak) even when the probe tip is
clipped directly onto its own ground connection, and to nothing else.

The scope is a Tektronic TDS1002, with its standard probe. If I
disconnect the probe at the co-ax connector, the noise dissappears, so I'm
sure that the nois is not entering via the scope power supply from the AC
line.

Any help very much appreciated.

Thanks,

Roland
Click to expand...

Hi, Roland -

Contactor coils are notorious for generating noise. I can't remember the
exact values we used, but something like .01 uF or .1 uF in series with 47
Ohms around the contactor coil may help. Vary the values and look for
minimum noise. You might get better values for this snubber from a search on
the Web or from the contactor manufacturer's Web site.

All our new circuit designs were placed next to a contactor (sans snubber)
which was alternately energized and deenergized and the response of the
circuit was examined. The line cord to the contactor was coiled up and laid
on the circuit. Sometimes we saw strange stuff, like an opamp scooting off
to the rail. We then added snubbers, filters, or bypasses to the circuit
until it was clean. The boards were usually installed in DC or AC motor
speed controllers operating thyristors on the 460VAC, 3-phase mains. So, the
boards were required to be as insensitive to noise as possible.

Here is a site by Douglas Smith, consultant, with lots of helpful probing
ideas and with information on EMI... http://emcesd.com/. But, it will take
some time to read through all his stuff.


I understand the problem you're having. Good luck.

John
 
"Roland Andrag" <randragnews@gmail.com> wrote in message
news:cnqns2$pfs$1@ctb-nnrp2.saix.net...
Hi everyone,

I have a circuit which contains a ULN2803, which is used to switch a 12
VDC relay (standard 8 pin, around 100 mA). The relay, in turn, switches a
contactor at 230 VAC (a couple of mA at 230 VAC). The resulting arcing
over the contacts of the relay is generating a lot of EMI, which is at the
moment wreaking havoc with the rest of my circuit.

In trying to solve the problem, my first problem is that I can't
accurately measure the noise anywhere in my circuit, since my scope probe
picks up the noise (up to 5 V peak-peak) even when the probe tip is
clipped directly onto its own ground connection, and to nothing else.

The scope is a Tektronic TDS1002, with its standard probe. If I
disconnect the probe at the co-ax connector, the noise dissappears, so I'm
sure that the nois is not entering via the scope power supply from the AC
line.

Any help very much appreciated.

Thanks,

Roland
Click to expand...

Here is a mention of the shorted scope probe problem you have. He gives no
solution in this brief, however.

http://emcesd.com/tt070199.htm

John
 
Roland Andrag wrote:
Hi everyone,

I have a circuit which contains a ULN2803, which is used to switch a 12 VDC
relay (standard 8 pin, around 100 mA). The relay, in turn, switches a
contactor at 230 VAC (a couple of mA at 230 VAC). The resulting arcing over
the contacts of the relay is generating a lot of EMI, which is at the moment
wreaking havoc with the rest of my circuit.

In trying to solve the problem, my first problem is that I can't accurately
measure the noise anywhere in my circuit, since my scope probe picks up the
noise (up to 5 V peak-peak) even when the probe tip is clipped directly onto
its own ground connection, and to nothing else.

The scope is a Tektronic TDS1002, with its standard probe. If I disconnect
the probe at the co-ax connector, the noise dissappears, so I'm sure that
the nois is not entering via the scope power supply from the AC line.

Any help very much appreciated.

Thanks,

Roland
Click to expand...
What's the characteristic of the noise?
I have the same problem on a TDS540. If I do the FFT, I can clearly
distinguish all the radio and TV stations in the area. Don't know if
the TDS1002 has fft, but if it has a channel output, you can run that
into a spectrum analzyer. Sometimes it's much easier to deduce the
problem in the frequency domain.

Metrology in a high noise environment is always a problem.
Lose the ground lead and use a probe socket grounded at the source.
You'll have to use a probe with the ground all the way to the tip.

Probe cable shields have been known to break.

mike

--
Return address is VALID.
500MHz Tek DSOscilloscope TDS540 $2200
http://nm7u.tripod.com/homepage/te.html
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
http://www.geocities.com/SiliconValley/Monitor/4710/
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top