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Best Snubber for Nasty Spikes?

P

Peter Kiproff

Jan 1, 1970
0
I have a problem " In another city " with some electronic controls on a
brake press.
The brake's ram is hydraulic with 3 electric valves & micro switches to
control the positions for
Top of stroke, Pinch point where the ram slows, & Bottom usually a 90 deg
die for sheet metal fabrication.

Most basic controls for this function are 115VAC, This one is 110VDC, When
the valve is turned off there is some kind of high voltage discharge,
adversely affecting other logic.

The only other info I can supply is that it is fused for 5 amps total & 2 of
the valves can be on @ the same time, so I suspect approx 1Amp+ for each
valve. Also I've noticed if a relay is used in place of the micro switch a
spark can be seen!

What would be your best guess for a RC snubber to keep this from affecting
the logic?

I remember reading a .1uf & 47 ohm in series across the coil would work, so
in a pinch I paralleled two 100 ohm 1/4W resisters & used a .1 uf @ 600v
cap. but without being able to measure I don't know if that is enough.

I'd rather over kill somewhat to make sure, the cycle rate would be every
few seconds @ most.

What would be your recommendation?

Thank you
Peter
 
F

Fritz Schlunder

Jan 1, 1970
0
Peter Kiproff said:
I have a problem " In another city " with some electronic controls on a
brake press.
The brake's ram is hydraulic with 3 electric valves & micro switches to
control the positions for
Top of stroke, Pinch point where the ram slows, & Bottom usually a 90 deg
die for sheet metal fabrication.

Most basic controls for this function are 115VAC, This one is 110VDC, When
the valve is turned off there is some kind of high voltage discharge,
adversely affecting other logic.

The only other info I can supply is that it is fused for 5 amps total & 2 of
the valves can be on @ the same time, so I suspect approx 1Amp+ for each
valve. Also I've noticed if a relay is used in place of the micro switch a
spark can be seen!

What would be your best guess for a RC snubber to keep this from affecting
the logic?

I remember reading a .1uf & 47 ohm in series across the coil would work, so
in a pinch I paralleled two 100 ohm 1/4W resisters & used a .1 uf @ 600v
cap. but without being able to measure I don't know if that is enough.

I'd rather over kill somewhat to make sure, the cycle rate would be every
few seconds @ most.

What would be your recommendation?

Thank you
Peter


If this valve is juiced up by DC, then all you should need is a diode
antiparallel to the valve coil. Something like a 1N4004 should be fine.
Adding this diode will add a little bit of turn off delay, but it doesn't
sound like that would be much of a problem.
 
F

Frithiof Andreas Jensen

Jan 1, 1970
0
What would be your recommendation?

Buy some of those encapsulated RC networks that clip into the mounting
system (DIN Rail ... Clip_on_top_of_Relay ... WhatEver) that the controls
are using; it looks much neater and the site technician can get/install new
ones, should the need arise. Discrete components are subject to abuse,
vibration, moisture e.t.c.

The values are not that important - it seems to be a "one-size-fits-all"
device - shouldn't surprise me they are 47 Ohm, 0.1 uF as you say.

PS:

Diodes across relay coils are Evil:

They are *easily* blown out by one of the many spikes in an industrial
environment; Especially the wimpy 1N4148 diodes that some designers are so
fond of. When that happens they will fail short, and then the the wires or
the controls will be damaged too! RC-Networks, MOV's or Tranzorbs (if one
absolutley MUST have a diode) will survive much longer!

I prefer to put the voltage limiter across the *driver*,
contacts/transistor, not the coil, because it is the *driver* and not *the
coil* that needs overvoltage protection (and some site techies might forget
it if it is not soldered in ;-)!!
 
J

John Popelish

Jan 1, 1970
0
Peter said:
I have a problem " In another city " with some electronic controls on a
brake press.
The brake's ram is hydraulic with 3 electric valves & micro switches to
control the positions for
Top of stroke, Pinch point where the ram slows, & Bottom usually a 90 deg
die for sheet metal fabrication.

Most basic controls for this function are 115VAC, This one is 110VDC, When
the valve is turned off there is some kind of high voltage discharge,
adversely affecting other logic.

The only other info I can supply is that it is fused for 5 amps total & 2 of
the valves can be on @ the same time, so I suspect approx 1Amp+ for each
valve. Also I've noticed if a relay is used in place of the micro switch a
spark can be seen!

What would be your best guess for a RC snubber to keep this from affecting
the logic?

I remember reading a .1uf & 47 ohm in series across the coil would work, so
in a pinch I paralleled two 100 ohm 1/4W resisters & used a .1 uf @ 600v
cap. but without being able to measure I don't know if that is enough.

I'd rather over kill somewhat to make sure, the cycle rate would be every
few seconds @ most.

What would be your recommendation?

Thank you
Peter

You posted something like this in sci.electronics.basics, but I don't
think you mentioned, there that this coil is driven with DC. RC
networks are normally used only for AC inductive loads. For DC you
can do much better with a diode in the network.

I am not sure of your supply situation, but here is the concept:

supply positive----------+----------+
| |
C |
| |
+--+ |
| | C
| - C (coil)
R ^ C
contact | | |
supply negative---||-----+--+-------+

The resistor, R limits the capacitor current when the contact closes.
The resistor can be a higher value than with the AC version, because
it can take the entire actuation time to precharge the cap. I would
probably go with 470 ohms, 1 watt to handle the 21 watt peak power at
closure.

But the diode allows the coil current to detour directly from the
contact to the cap, C when the contact opens with only a volt across
the contacts, preventing the arc. Then the cap charges smoothly as
the coil transfers its energy to the cap, so the rate of change of
voltage is limited. Since the inrush current can be kept low with a
high value of resistor and the cap has to precharge only once per
actuation (not every half cycle as with AC) the cap can practically be
larger than with the AC version. I would look for a 1 uf class X1 cap
(for across the 240 volt AC line service, about 1000 volts peak
capability). The one ampere coil current will start the charge ramp
at 1 volt per microsecond, which should be slow enough to radiate very
little.

The diode is a 1N4004.

This kind of snubber will not slow the turn off of the hydraulic valve
as much as putting a diode directly across the coil.

The comments I made about wire routing in the other group still apply.
 
R

Rolavine

Jan 1, 1970
0
Subject: Re: Best Snubber for Nasty Spikes?
From: "Frithiof Andreas Jensen" >



Buy some of those encapsulated RC networks that clip into the mounting
system (DIN Rail ... Clip_on_top_of_Relay ... WhatEver) that the controls
are using; it looks much neater and the site technician can get/install new
ones, should the need arise. Discrete components are subject to abuse,
vibration, moisture e.t.c.

The values are not that important - it seems to be a "one-size-fits-all"
device - shouldn't surprise me they are 47 Ohm, 0.1 uF as you say.

PS:

Diodes across relay coils are Evil:

They are *easily* blown out by one of the many spikes in an industrial
environment; Especially the wimpy 1N4148 diodes that some designers are so
fond of. When that happens they will fail short, and then the the wires or
the controls will be damaged too! RC-Networks, MOV's or Tranzorbs (if one
absolutley MUST have a diode) will survive much longer!

I prefer to put the voltage limiter across the *driver*,
contacts/transistor, not the coil, because it is the *driver* and not *the
coil* that needs overvoltage protection (and some site techies might forget
it if it is not soldered in ;-)!!

Good points. How is a 1N4004 as a flyback eater diode? Is it fast enough? When
I look up the specs they are not very specific about speed, since they are
intended to be rectifiers, however when I test them they seem fast enough, my
100 meg scope shows no transient. If not a 1N4148, what is the stronger
cheapest option? How about a BAW78? Do you think a coil driver really benefits
from a faster clamping device?

Rocky
 
P

Peter Kiproff

Jan 1, 1970
0
John Popelish said:
You posted something like this in sci.electronics.basics, but I don't
think you mentioned, there that this coil is driven with DC. RC
networks are normally used only for AC inductive loads. For DC you
can do much better with a diode in the network.

I am not sure of your supply situation, but here is the concept:

supply positive----------+----------+
| |
C |
| |
+--+ |
| | C
| - C (coil)
R ^ C
contact | | |
supply negative---||-----+--+-------+

The resistor, R limits the capacitor current when the contact closes.
The resistor can be a higher value than with the AC version, because
it can take the entire actuation time to precharge the cap. I would
probably go with 470 ohms, 1 watt to handle the 21 watt peak power at
closure.

But the diode allows the coil current to detour directly from the
contact to the cap, C when the contact opens with only a volt across
the contacts, preventing the arc. Then the cap charges smoothly as
the coil transfers its energy to the cap, so the rate of change of
voltage is limited. Since the inrush current can be kept low with a
high value of resistor and the cap has to precharge only once per
actuation (not every half cycle as with AC) the cap can practically be
larger than with the AC version. I would look for a 1 uf class X1 cap
(for across the 240 volt AC line service, about 1000 volts peak
capability). The one ampere coil current will start the charge ramp
at 1 volt per microsecond, which should be slow enough to radiate very
little.

The diode is a 1N4004.

This kind of snubber will not slow the turn off of the hydraulic valve
as much as putting a diode directly across the coil.

The comments I made about wire routing in the other group still apply.

John I think your comment about routing is important, the relays are in the
same box as the logic!
I'll relocate.

I was unsure that the .1uf @ 600V cap was large enough to absorb the valves
coil energy.
the solution will be used on AC as well as DC valves.

Also I was unaware that a snubber was available, and thought I would have to
make up something.

Peter
 
Z

Zak

Jan 1, 1970
0
Rolavine said:
Good points. How is a 1N4004 as a flyback eater diode? Is it fast enough? When
I look up the specs they are not very specific about speed, since they are
intended to be rectifiers, however when I test them they seem fast enough, my
100 meg scope shows no transient.

I think 'fast' with diodes applies to the opposite direction: how fast
they go from conducting to no current again. How much this bites you
depends on switching speed to the on state (off state for the diode) and
the frequency.


Thomas
 
R

Rich Grise

Jan 1, 1970
0
Good points. How is a 1N4004 as a flyback eater diode? Is it fast enough? When
I look up the specs they are not very specific about speed, since they are
intended to be rectifiers, however when I test them they seem fast enough, my
100 meg scope shows no transient. If not a 1N4148, what is the stronger
cheapest option? How about a BAW78? Do you think a coil driver really benefits
from a faster clamping device?

Personally, I'd use a 1N4007, but only because the 4004 is only a 100V
diode. From what I recall from the OP, the duty cycle was in seconds, so I
don't think speed is a consideration here - all you're doing is dumping
some charge, after all.

Whoever recommended a 1N4148 as a solenoid diode needs to read the book
again. ;-)

Cheers!
Rich
 
J

John Popelish

Jan 1, 1970
0
Rich said:
Personally, I'd use a 1N4007, but only because the 4004 is only a 100V
diode.

Make that a 400 volt diode.
http://www.fairchildsemi.com/ds/1N/1N4004.pdf
From what I recall from the OP, the duty cycle was in seconds, so I
don't think speed is a consideration here - all you're doing is dumping
some charge, after all.

I agree that the coil would very likely have an L/R time constant well
under a second, since it sounds like a fast response hydraulic valve.
Whoever recommended a 1N4148 as a solenoid diode needs to read the book
again. ;-)

For a 100 volt 1 ampere coil, I agree. For a 50 volt or less 250 ma
or less coil, they work fine.
 
G

Guy Macon

Jan 1, 1970
0
I prefer to put the voltage limiter across the *driver*,
contacts/transistor, not the coil, because it is the *driver* and not *the
coil* that needs overvoltage protection (and some site techies might forget
it if it is not soldered in ;-)!!

See http://www.guymacon.com/SUN/INDEX.HTM section
titled "Industrial washing machine control."
 
T

Terry Given

Jan 1, 1970
0
Zak said:
I think 'fast' with diodes applies to the opposite direction: how fast
they go from conducting to no current again. How much this bites you
depends on switching speed to the on state (off state for the diode) and
the frequency.


Thomas

Correct - this is called reverse recovery. Diodes also have the
equivalent forward recovery time, usually unspecified. a 1n400x has a
reverse-recovery time of tens of microseconds. That just means dont use
them in switchmode power supplies

Cheers
Terry
 
T

Terry Given

Jan 1, 1970
0
John said:
Make that a 400 volt diode.
http://www.fairchildsemi.com/ds/1N/1N4004.pdf




I agree that the coil would very likely have an L/R time constant well
under a second, since it sounds like a fast response hydraulic valve.




For a 100 volt 1 ampere coil, I agree. For a 50 volt or less 250 ma
or less coil, they work fine.

To further elucidate: The coil limits the maximum current, which the
diode needs to be big enough to cope with. 600A 3-phase contactors with
12Vdc coils that draw 10A tend not to work with 1N4148 free-wheeling
diodes, whereas pcb-mounted relays are usually around the 100-200mA
range, and will work fine.

The voltage rating of the diode needs to be greater than the supply
voltage - including any evil spikes that might appear briefly. A 100V
diode on a 75Vdc bus might not be a good idea. In an industrial plant,
I'd just use 1N5408 - 3A, 1kV, damned cheap & in the toolbox.

Cheers
Terry
 
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