Quadrac circuit woes

[garyr]

I've made a 120 V AC motor speed controller using a Quadrac that doesn't
work the way it should. Is it safe to look at the waveforms at various
places in that circuit with the scope in the CH1 - CH2 mode and using two
probes? Of course, without connecting the probe ground clips to anything on
the circuit. I assume this would be OK but I'd hate to fry my scope so I'm
looking for some expert advice. I don't have an isolation transformer.

 

[tm]

I've made a 120 V AC motor speed controller using a Quadrac that doesn't
work the way it should. Is it safe to look at the waveforms at various
places in that circuit with the scope in the CH1 - CH2 mode and using two
probes? Of course, without connecting the probe ground clips to anything
on
the circuit. I assume this would be OK but I'd hate to fry my scope so I'm
looking for some expert advice. I don't have an isolation transformer.

Can you run it at reduced voltage, say 24 or 48 volts AC? An isolation
transformer is also a safety factor.

Using X10 probes with the Ch1 + Ch2 inverted should be pretty safe as far as
the scope goes. Just watch out that the ground leads don't touch anything.

tm

 

[tm]

And make sure neither channel is clipping.

We have a TPS2024, a fully-isolated 4-channel scope. It's great for
this sort of thing... clip any probe ground lead anywhere you like.

Is the prob ground lead isolated? You can connect it to line hi?

It sounds like the OP is controlling direct line voltage.

 

[tm]

Yes. All four channels are individually isolated. The trigger input
floats, too.

I can probe the s-g voltage of a mosfet that's 400 volts off ground. It's
really
cool for power stuff.

Very nice. I'll look at it.


tm

 

[Martin Riddle]

Very nice. I'll look at it.


tm

Tek had a portable with isolated probe grounds too. THS710 and THS720,
60 and 100mhz

Cheers

 

[Phil Allison]

"garyr"

I've made a 120 V AC motor speed controller using a Quadrac

** That is not a common part, I had to look up the name to find it was a
triac with integral diac.

So the speed control is for a brush ( AC/DC or universal ) motor ???

that doesn't work the way it should.

** A perfectly information free comment.

Is it safe to look at the waveforms at various
places in that circuit with the scope in the CH1 - CH2 mode and using two
probes?

** The gate circuit can be referenced to the neutral conductor - which is
normally only a fraction of a volt away from safety ground.


.... Phil

 

[Klaus Bahner]

I've made a 120 V AC motor speed controller using a Quadrac that doesn't

Well, you haven't given much information about neither your circuit nor
your problems.
But common problems with Quadracs are a rather limited dV/dt rating and
that they in general cannot be used in the forth quadrant (or require
excessive gate current). The first one leads to unintended triggering,
the second one may make your circuit to a kind of rectifier ...

Klaus

 

[Phil Allison]

"Klaus Bahner"

Well, you haven't given much information about neither your circuit nor
your problems.

** Bet the OP is using an induction motor with no snubber.

But common problems with Quadracs are a rather limited dV/dt rating

** Snubbers fix that.

that they in general cannot be used in the forth quadrant

** Not even faintly relevant.



... Phil

 

[garyr]

"garyr"

** That is not a common part, I had to look up the name to find it was a
triac with integral diac.

So the speed control is for a brush ( AC/DC or universal ) motor ???



** A perfectly information free comment.



** The gate circuit can be referenced to the neutral conductor - which is
normally only a fraction of a volt away from safety ground.


... Phil

Thanks for your interest in my problem.

I wasn't asking for help debugging my board, I just wanted to verify that it
was safe to poke around on it with the scope probes since it's connected
directly to the 120 volt AC line. The subject of my post was perhaps
misleading. The speed controller is for a brush AC motor taken from an old
vacuum cleaner.

An overview of Littlefuse quadrac devices is found here:
http://www.littelfuse.com/products/switching-thyristors/quadrac/qxxxxltx/q4008lth.aspx


In the circuit I'm using the duty cycle is set by a 250K pot. The problem
was that the resistance had to be reduced to around 20k before there was any
triggering at all. The duty cycle was suddenly changing from zero to around
50 %. I replaced the Q4008LTH with a Q4008LT (that I received from Digi-Key
today) and now it appears to be working correctly, the duty cycle
can be smoothly varied from near zero to near 100 %. So I guess the LTH
device was damaged though how that occurred is a puzzle. I've worn a
grounded wrist strap when handling the device. It seems unlikely that it
was bad when it was shipped from Digi-Key...

 

[Phil Allison]

"garyr">

"Phil Allison"
Thanks for your interest in my problem.

I wasn't asking for help debugging my board, I just wanted to verify that
it
was safe to poke around on it with the scope probes since it's connected
directly to the 120 volt AC line.

** You cannot control the thinking of other people like that.

The details you left out were important to the safety issue - eg: induction
motors can produces dangerous spike voltages when phase controlled.

You did not even say if you had a pair of 10:1 probes.

Your no information attitude in order to control replies is a sign of
serious incompetence and THAT was alarming.

The subject of my post was perhaps
misleading. The speed controller is for a brush AC motor taken from an old
vacuum cleaner.

** Such motors can draw very large currents when running.

A wise person would not use an 8 amp triac with one.

In the circuit I'm using the duty cycle is set by a 250K pot. The problem
was that the resistance had to be reduced to around 20k before there was
any
triggering at all. The duty cycle was suddenly changing from zero to
around
50 %. I replaced the Q4008LTH with a Q4008LT (that I received from
Digi-Key
today) and now it appears to be working correctly, the duty cycle
can be smoothly varied from near zero to near 100 %. So I guess the LTH
device was damaged though how that occurred is a puzzle.

** No puzzle to me.

This is the WRONG newsgroup for beginners like you.


.... Phil

 

[Klaus Bahner]

"Klaus Bahner"

** Bet the OP is using an induction motor with no snubber.

Seems so.

** Snubbers fix that.

Yep, but we don't know whether the OP knows that. Even if there's a
snubber, if it is dimensioned according to rules of thumb for the
standard TRIAC circuit, it may not work for Quadracs.

** Not even faintly relevant.

In my experience it happens quite often that people don't pay attention
to the polarity of A1 and A2. With a TRIAC based circuit you may get
away with this, but not with a Quadrac.

Regards,
Klaus

 

[amdx]

Thanks for your interest in my problem.

I wasn't asking for help debugging my board, I just wanted to verify that it
was safe to poke around on it with the scope probes since it's connected
directly to the 120 volt AC line. The subject of my post was perhaps
misleading. The speed controller is for a brush AC motor taken from an old
vacuum cleaner.

When I was in my teens I disassembled a vacuum cleaner, I attached a
rubber sanding pad to the motor. Within about 3/4 second after plugging
it in, I had a large welt on my chest where a piece of the exploded
rubber pad hit me. I was lucky, the dammage could have been worse.
I took the motor to my high school shop class where we had a device
to measure rpm. Full speed no load was over 28k rpms.
Mikek

 

[Phil Allison]

"Klaus Bahner"

Yep, but we don't know whether the OP knows that. Even if there's a
snubber, if it is dimensioned according to rules of thumb for the standard
TRIAC circuit, it may not work for Quadracs.

** You are full of mays and maybes - but no facts.

In my experience it happens quite often that people don't pay attention to
the polarity of A1 and A2.

** Because of the diac, the issue is non existent.


...... Phil

 

[Phil Allison]

"amdx"

When I was in my teens I disassembled a vacuum cleaner, I attached a
rubber sanding pad to the motor. Within about 3/4 second after plugging it
in, I had a large welt on my chest where a piece of the exploded rubber
pad hit me. I was lucky, the dammage could have been worse.
I took the motor to my high school shop class where we had a device to
measure rpm. Full speed no load was over 28k rpms.

** Yep - vac motors are scary things when the compressor fan is removed.
Plenty of stories on the web about them exploding like hand grenades as the
windings and metal bits come apart at 32,000 Gs.



..... Phil

 

[Spehro Pefhany]

"amdx"


** Yep - vac motors are scary things when the compressor fan is removed.
Plenty of stories on the web about them exploding like hand grenades as the
windings and metal bits come apart at 32,000 Gs.



.... Phil

The classic example of a series-wound motor. Potentially causing
parallel wounds.


Best regards,
Spehro Pefhany