freewheeling diodes to speed up solenoids?

[oregonsam]

Our current design includes freewheeling diodes on speed critical DC pneumatic solenoids. The purpose is supposed to be to speed up the response. We generally use Contrllogix 24VDC output cards to fire the solenoids.

I am of the opinion that the diodes will have no effect on the solenoid energizing time, and actually slow down the shut off. Does anyone have any practical experience where diodes have been used to successfully improved DC solenoid speed?

Thanks

 

[rdrast]

Well, they shouldn't do anything for firing time, but they will definately speed up the shut off time by providing a circuit for the collapsing magnetic field to discharge through.

As an additional benefit, they will help protect the solid state outputs.

 

[bernie_carlton]

Actually they'll slow down the turn off - we found out the hard way in some supposedly fast acting circuits.

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

 

[curlyandshemp]

Our current design includes freewheeling diodes on speed critical DC pneumatic solenoids. The purpose is supposed to be to speed up the response. We generally use Contrllogix 24VDC output cards to fire the solenoids.

I am of the opinion that the diodes will have no effect on the solenoid energizing time, and actually slow down the shut off. Does anyone have any practical experience where diodes have been used to successfully improved DC solenoid speed?

Thanks

Going back to my stone age electronics training from the '70s, anything in parallel will either have a capacitive or inductive effect. A diode is displays capacitive traits, two electrodes separated by a dielectric. So in theory a diode acting like a small value capacitor, would resist changes in voltage, where an inductive coil resists changes in current, so a diode would actually reduce response time. I cannot believe I actually remember this stuff after 30 years.

 

[CaseyK]

If it is firing that fast or as often as I think, then you will probably want the diodes for the sake of eliminating a lot of downtime waiting for the module to be swapped out periodically!

 

[oregonsam]

speed is the only reason we use them, none of our other solenoid outputs use them.

 

[Gerry]

Going back to my stone age electronics training from the '70s, anything in parallel will either have a capacitive or inductive effect. A diode is displays capacitive traits, two electrodes separated by a dielectric. So in theory a diode acting like a small value capacitor, would resist changes in voltage, where an inductive coil resists changes in current, so a diode would actually reduce response time. I cannot believe I actually remember this stuff after 30 years.

Interesting theory - first time I've heard of semiconductors doubling as dielectrics.

The free-wheeling diode will delay the drop-out of the solenoid because it provides a path for current generated by the collapsing magnetic field to circulate through the coil in the same direction as when the circuit is energised.

 

[bernie_carlton]

Varicaps - diodes used as variable capacitors for frequency tuning - have been used for a long time. A variable voltage changes the gap changing the effective capacitance.

 

[keithkyll]

Actually they'll slow down the turn off - we found out the hard way in some supposedly fast acting circuits.

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

That's a great link, Bernie. My first thought was also that the diode would 'short out' the colapsing field, and speed up release. The energy has to go somewhere. A diode holds it in the coil, keeping it alive!

For others, look at the chart at the lower left of the page. It shows a diode with a 24V zener as the best solution. The Zener will dissipate the excess energy as heat.

 

[Gerry]

Varicaps - diodes used as variable capacitors for frequency tuning - have been used for a long time. A variable voltage changes the gap changing the effective capacitance.

OK, I should have remembered that. However, I don't think they are appropriate for use as a free-wheeling diode, or that the junction capacitance has any influence in this application.

Varactors are operated reverse-biased so no current flows , but since the width of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary. Generally, the depletion region width is proportional to the square root of the applied voltage; and capacitance is inversely proportional to the depletion region width. Thus, the capacitance is inversely proportional to the square root of applied voltage.

All diodes exhibit this phenomenon to some degree, but specially made varactor diodes exploit the effect to boost the capacitance and variability range achieved - most diode fabrication attempts to achieve the opposite.

 

[JesperMP]

Why do you think it is called "freewheeling" ?

To achieve fast turn off and also avoid voltage spikes, use RC snubbers in stead of diodes.

 

[Sparkz]

A diode in parallel over any coil will result in an increased deenergizing period!

It's the energy stored in the coil that needs to be discharged: The higher the emf voltage is allowed to raise at drop off, the shorter the period the current can be maintained. W = U.I.t -> Nothing will beat a spark when it comes to reducing drop off times...

As far as I know, 'magnetic spark extinction' relays are still being used in cranes with an electrical magnet: The sparks inside the spark chamber are gigantic, but the load is released within an acceptable period.

I've also seen customers going back to relay outputs (but then of course capable of switching inductive DC loads) to reduce drop off.

A more reasonable solution for the pneumatic solenoids is like Jesper mentioned, or place a (fast Schottky) diode over the output device. This way the emf voltage will not raise above the supply voltage, so at least no damage will occur AND drop off time will be shorter than when using a freewheeling setup.

 

[oregonsam]

What type of circuits do output cards use for surge supression (Control logix for example)? And do they also tend to increase the drop off time of solenoids versus a contact that allows the spark?

 

[Sparkz]

I'm not that familiar with AB material, but Siemens transistor output modules seem to be equipped with a zener between output and supply voltage:

Most likely, Control Logix modules have a similar surge protection...but it might be wise to check the specs first.

If they have a similar setup, get rid of the freewheeling diodes in your design! You'll notice a significant reduction in drop off. Don't take my word for it, just take a look at Bernie's PDF file (post #3): You'll be amazed by the test results...

 

[micahg]

A few years ago I worked for a company with several large bridge cranes. The hoist had a magnetic brake to stop it from drifting down with the load on it. The brake coil was a 500 dc coil. There was a diode across it that was blown apart. I was asked to provide a large diode to handle the current and voltage, which I did. We found that once the coil was de-energised it took a full 10 seconds for the magnetic field to collapse enough to drop out the brake. We discovered it required a 500 volt zener diode which allowed the magnetic field to drop out and the voltage across the coil to go from +600 vdc to -500vdc during the drop out period. We had some discusion of what if we had a superconducting magnet and diode where there would be no resistance loss in it, you could effectly charge an electromagnet and have a permenant magnet as long as the circuit was closed. So back to the question, the diode delays the dropout of the magnetic field, as previously stated several times.