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'[EE:] Commutating diode selection and snubber circ'
2004\08\23@094036 by Roberts II, Charles K.

picon face
Hello All

I have a Electro-Mechanical Brake on a motion stage that is emitting a
lot of noise when it's coil is opened. I understand how a commutating or
snubber diode works, but what criteria must the diode meet? And what
should I consider in designing a circuit to make this coil quiet. It
operates at 24V .46A.
I guess it would just have to have a reverse bias brake down voltage
grater than 24V and be able to handle about half an amp of current. Will
the coil have enough internal resistance to dissipate the power or
should I have a resistor in series with the diode? I am very green at
this and unsure of myself so don't thrash me too hard and thanks in
advance.
Chuck

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2004\08\23@111551 by Russell McMahon

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> And what
should I consider in designing a circuit to make this coil quiet. It
operates at 24V .46A.
/>


Has to pass max current and withstand max voltage. 1N400X probably OK there.
Low speed but unlikely to be a problem as long as this is off and on for
longish periods at a time (eg >= 0.1 second) and not PWMd. . Want to get
fancy use eg BVY26 or equivalent - but probably no need.


>I guess it would just have to have a reverse bias brake down voltage
grater than 24V and be able to handle about half an amp of current.
/>

Yes

> Will the coil have enough internal resistance to dissipate the power or
should I have a resistor in series with the diode? I am very green at
this and unsure of myself so don't thrash me too hard and thanks in
advance.
/>

Coil may well be OK.
24V at 0.5 amps or so suggests 48R coil.
If so power dissipation will be I^2R = 12W at max current.
No coincidence that this also eqwuals 24V x 0.5A.

Adding extra R will increase flyback voltage of coil.
Maximum disispation PROBABLY about when Rexternal = R coil (past bed time,
brain protests, it says that  transfer theorum probably applies, ask it in
the (this) morning.

HOWEVER all the above may not work. Description is a little uncertain. If
the brake is a generator then adding the diode may make it self brake when
you don't want it to. When you say "electromagnetic brake" do you mean eddy
current, or electromagnet drives a brake show pair or ?????


       Russell McMahon

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2004\08\23@112554 by Roberts II, Charles K.

picon face
The Brake is engaged when no voltage is applied, the break is disengaged
when there is a voltage. When I took it apart it looked as if it used a
spring to hold the brake shoe onto the shaft and a solenoid to release
it.
That's all I got, there is not much information on the manufactures
site.

Thanks Russ
Chuck

{Original Message removed}

2004\08\23@113838 by Dave VanHorn

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At 10:25 AM 8/23/2004, Roberts II, Charles K. wrote:

>The Brake is engaged when no voltage is applied, the break is disengaged
>when there is a voltage. When I took it apart it looked as if it used a
>spring to hold the brake shoe onto the shaft and a solenoid to release
>it.
>That's all I got, there is not much information on the manufactures
>site.

The diode has to have enough PRV to withstand the on-state.
Then the energy stored in the inductor needs to dump through it's own resistance, so that defines the maximum one-shot energy the diode needs to sustain.
You don't often see the math for this presented, I suppose it's because generally speaking, a 1N400X diode will handle most loads.

I've tried to get this sort of info out of stepper chip makers.
They say they have on-chip diodes, but when you press for details, they are often unable to tell you how much the diodes can handle.

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2004\08\23@154304 by Martin McCormick

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       One thing to watch out for when putting a commutating diode on
a solenoid or relay coil is the fact that it will not loose its
magnetic field as quickly as it did before the diode was added.  This
is because the commutating diode for all practical purposes, feeds
that energy back in to the coil such that it keeps itself energized
for a fraction of a second while the magnet dumps its stored energy
through the now forward-biased diode.

       I was made aware of this as a small boy when I connected a
relay with normally-closed contacts such that it buzzed when energized
from a battery.

       The buzz or chatter was normally fierce with nothing across
the coil.  If you touched the coil, the shock was painful and there
was enough voltage released in the field collapse to light a neon bulb.

       If I put a diode in reverse across the coil, the chattering
turned to a faint buzzing sound and the high-voltage spikes along with
AM radio static were gone.  It was obvious that the relay couldn't
release quite as fast as it used to.  If split-second timing is
important, remember that the solenoid will energize as quickly as
before, but it may hang up for a split-second before releasing.

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2004\08\23@161822 by Wouter van Ooijen

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>  If split-second timing is
> important, remember that the solenoid will energize as quickly as
> before, but it may hang up for a split-second before releasing.

If timing is important you can use a zener in series with the diode, or
even a resistor. The voltage at the 'low' side of the coil will be
higher, so you must decide what the maximum voltage there can be.

Wouter van Ooijen

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2004\08\23@162443 by Olin Lathrop

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Wouter van Ooijen wrote:
>> If split-second timing is
>> important, remember that the solenoid will energize as quickly as
>> before, but it may hang up for a split-second before releasing.
>
> If timing is important you can use a zener in series with the diode,
> or even a resistor. The voltage at the 'low' side of the coil will be
> higher, so you must decide what the maximum voltage there can be.

Another trick to increase the response time of a relay is to put a parallel
capacitor and resistor in series with the relay coil.  Make sure the time
constant of R*C is less than half or so of the minimum relay on or off time,
then set the R so that the steady state current thru the coil is just the
"holding" current, but not the "pull in" current.  The capacitor initially
allows more current to flow when the relay is switched on, but the current
will fall to a lower level once it has been engaged.  This lower current and
therefore lower magnetic field is quicker to switch off.


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2004\08\23@171326 by Russell McMahon

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> The Brake is engaged when no voltage is applied, the break is disengaged
when there is a voltage. When I took it apart it looked as if it used a
spring to hold the brake shoe onto the shaft and a solenoid to release
it.
That's all I got, there is not much information on the manufactures
site.
/>

Marks assigned:   100
 Time allowed:    No limit
Answer ALL questions

Is the brake mostly either fully on or fully off or is it modulated in some
way to be partially off or on.
eg do you turn it off (by applying power), roll the stage somewhere and then
turn the brake on (by turning off power) or is it used to apply partial
brakes by either varying the DC level to the brake or by PWM switching it?

If it is on/off only then I can't see why it should be noisy for electrical
reasons (unfortunately). In this case what may be happening is that the
brake shoes aren't clearing the drum or disk or whatever and 'chattering"
for mechanical reasons. In this case, attention to the mechanical design of
the mechnism or maintenance may be what is required.

If it is switched by eg PWM and electrically noisy then is it noisy when the
brake is OFF but the power is ON.

Is it "noisy" when the brake is off (power on) but with the stage not moving
OR does the noise occur only when the stage is moving?

What is the nature of the noise?

Can you provide a fuller description of how the brake works and what signals
are supplied when it is noisy?



       Russell McMahon

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2004\08\23@225922 by Rich

picon face
One of the common mistakes engineers make is using a diode that is slower
than the transient.  Schottky diodes are a good choice for most
applications.


{Original Message removed}

2004\08\24@083723 by Olin Lathrop

face picon face
Rich wrote:
> One of the common mistakes engineers make is using a diode that is
> slower than the transient.  Schottky diodes are a good choice for most
> applications.

That's a blanket statement that is misleading at best.  Schottky diodes are
indeed fast and have about half the forward drop of a silicon diode, but
they are limited to relatively low voltages and can have large reverse
leakage currents, especially at high temperatures.  Even the common 1N5818
can leak around 20mA if I remember right.

Schottky diodes are a good fit for some applications, like low voltage
switching power supplies, but hardly for "most".


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2004\08\24@084139 by Roberts II, Charles K.

picon face
The motion stage moves a radiation detector that has some very sensitive
electronics. The stage is 2 Parker 404XR's one in the X direction (side
to side) and one in the Z (Up and Down). The original design of the
stage only locked the Z direction when power was shut off to the motor
drives, which also cuts power to the motor controller (Parker 6K). The
detector electronics are picking up noise from the motor so they use 6K
to cut off the motor drives and manually disconnect power from the
brake, as the brake was designed to hold the stage in place when there
was no power to the drives. So in the process of disconnecting the coil
they get the huge emission of noise from the coil collapsing that resets
there detector electronics.

So the brake is only on and off, not PWM. It uses a coil to hold back a
spring that clamps shoes to a shaft, at least that's how I remember it.
When the coil is opened you can see a huge spike, now the other noise
created by the stage, which disconnecting the power to the drives and
using the brakes is to fix, is still being chased :)
I hope you follow the radiation detector and electronics I don't want to
talk about too much. All those patent issues, trade secrets and such.




Chuck


{Original Message removed}

2004\08\24@092028 by Russell McMahon
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> The motion stage moves a radiation detector that has some very sensitive
electronics. The stage is 2 Parker 404XR's one in the X direction (side
to side) and one in the Z (Up and Down). The original design of the
stage only locked the Z direction when power was shut off to the motor
drives, which also cuts power to the motor controller (Parker 6K). The
detector electronics are picking up noise from the motor so they use 6K
to cut off the motor drives and manually disconnect power from the
brake, as the brake was designed to hold the stage in place when there
was no power to the drives. So in the process of disconnecting the coil
they get the huge emission of noise from the coil collapsing that resets
there detector electronics.

So the brake is only on and off, not PWM. It uses a coil to hold back a
spring that clamps shoes to a shaft, at least that's how I remember it.
When the coil is opened you can see a huge spike, now the other noise
created by the stage, which disconnecting the power to the drives and
using the brakes is to fix, is still being chased :)
/>

OK - much clearer. The diode is an excellent step in the right direction.
Given the nature of the application you can afford to splurge a little on
the diode. An ultra high speed diode may be very slightly better in such a
one off application, although a standard 1N400x for a few cents may well do
just as well.

"Slugging" of the release response (ie extension of release time) due to the
circultaing currents in the diode are almost certainly not going to be a
problem here. If they are you could add a series resistor to the diode but
just try a diode as is first.

An alternative, probably not needed here, is to tailor the driver turn off
so that it reduces the coil current slowly and eliminates any spike. This is
liable to be substantially more complex to implement (but should still be
reasonably cheap and simple).

In your case I'd firstly just try a 1N400x and see what happens. It's highly
likely to work.


       Russell McMahon

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2004\08\24@193836 by steve

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Those are pretty impressive stages and the motors to drive them aren't
small either. The cause of your original noise problem will stem from the
motors being driven with a PWM drive. With motors of that size, you
would need to dissipate a huge amount of heat to get reasonable
performance with a non-PWM drive method. That would almost
certainly solve the original noise issue, but simply isn't practical.

You essentially have a low impedance driver, switching large currents at
one end of a long piece of cable. ie an antenna.
Because the operation of the motor controller needs some ac signal
component, you are limited in the amount of filtering you can apply to
the noise source, so your next best option is to add more and more
shielding to the cable and keep it out of reach of the sensitive parts.

Back to your original question on inductors and diodes. Put simply, an
inductor will do what it takes to maintain the current flow.
In other words, when you remove the external driving force, the voltage
across the inductor will rise to the point where that current will continue
to flow. From there the current will decay according to the equation with
"e" in it.
So, if you put a diode across  the inductor, current will start to flow as
soon as the terminal voltage reaches 0.6V. According to the rule above,
the current will be the same as was being driven through it before it was
turned off and decay from there. If you used a zener arrangement, then
the current would start to flow at the zener voltage. The initial current
would still be the same value, but will decay much quicker.

You also have to consider the generator effect of any mechanical
movement like a solenoid plunger or motor intertia. You can work on the
assumption of a spinning motor with the power removed is now a
generator. Using that as a guide, assume that the voltage generated will
be on the top of the supply voltage and the current could be up to the
amount that you applied.

So for a pure inductor, voltage rating is determined by the supply and
peak current rating is whatever the steady state current is. For anything
with a moving part, both peak current and peak voltage could be twice
that (as a guide).

There is also a turn on delay with any diode and since that is when the
inductive voltage is peaking, it is something worth minimizing. You can
do that with a faster diode or an RC snubber.


On 24 Aug 2004 at 8:41, Roberts II, Charles K. wrote:

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2004\08\25@075808 by Olin Lathrop

face picon face
steve@TLA.CO.NZ wrote:
> From there the current will decay according to
> the equation with "e" in it.

Not necessarily at all.  The "e" equation is a specialized solution for the
inductor current only when the load is resistive.

> There is also a turn on delay with any diode and since that is when
> the inductive voltage is peaking, it is something worth minimizing.
> You can
> do that with a faster diode or an RC snubber.

Most diodes have nearly instantaneous turn on speed for our purposes.  The
difference between fast and slow diodes is primarily an issue of turn off
speed.


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