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Pulse Width Modulation, coil application

Freemasm

Well Known Member
I'm ME but understand PWM and it's applications, or so I thought. What are the reasons, advantages/disadvantages, etc. of a PWM coil in a DC contactor?

Comparing the coil currents, something that's gonna be important for my application, the PWM is 1/2 that of the dual coil counterparts; 0.13 versus 0.23 @ 12VDC. This is a fraction of the legacy silver can contactor draws which can approach an amp for a new and healthy one.

Sorry for the amateur question but I'm flat not conceiving the related hows and whys.
 
The PWM coil has the same contact performance, with lower hold current. It uses the inductance of the coil to maintain sufficient magnetic field to hold the contact since lower magnetic field is required to hold the direct contact gap compared to the field required to “pull in” the armature. The PWM frequency is typically at a frequency > 10KHz or even higher.

Integrated electronics that performs the PWM function can fail, electronic noise can be induced if the PWM circuit is low quality of has a failure.

Generally speaking these issues are low likelihood, but just pointing out the additional failure mode if the contractor is used in a critical application
 
I shifted over to marine master solenoids years ago to reduce the “silver can” holding current.

I use Blue Sea 9012 solenoids, one for each battery. Holding current is 0.16 amps. Momentary current to change state (shut) is 3.6 amps.

Carl
 
The Blue Sea L9012 is on the list. Going to assume it's also a PWM controlled coil sole based on comparative specs. I'm considering that one and the GigaVac GV200MA. As mentioned, one of my prime criteria is coil amp draw as it relates to reserve capacity; safety and reliability being equal of course. These two devices have very similar specs. A quick summary below. Pardon the funky format. Original delimiters aren't recognized here:

Original table deleted. Format incompatibility. See attachment


@Blytle. Much thanks. The application(s) would be between parallel batteries and alt/back-up gen/non vital bus. Both solenoids would have to fail and I'd have to be out of battery capacity for the PWM application to be a real contributor. That architecture and the low probability of related failure and I think my mind would be OK with this application. Someone else my chime in.

@Carl. I assume you jumpered the + coil power and control by switching on the coil ground? I can't think of a reason why not.
 

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PWM

The PWM coil has the same contact performance, with lower hold current. It uses the inductance of the coil to maintain sufficient magnetic field to hold the contact since lower magnetic field is required to hold the direct contact gap compared to the field required to “pull in” the armature. The PWM frequency is typically at a frequency > 10KHz or even higher.

Integrated electronics that performs the PWM function can fail, electronic noise can be induced if the PWM circuit is low quality of has a failure.

Generally speaking these issues are low likelihood, but just pointing out the additional failure mode if the contractor is used in a critical application

PWM control for coil hold functions is a well know electronic advantage, reducing HOLD current. But it MUST be done in a way that doesn't propagate excessive electrical noise in other aviation equipment. High frequency PWM that has very fast rise time and fall time (dV/dT) can cause a lot of interference in radio signals in the panel. A simple PWM lighting controller, operating at 120 Hrz, with very fast rise/fall times, can kill AM radio and GPS receiver abilities. The rise time, and fall time of the PWN control has to be limited such that it doesn't cause this interference. A PWM signal operating at 10Khz would definitely cause interference.

Ask me how I know!!!!!!
 
If you look up the specs for a LM 1949 peak and hold drive chip, it is by default configured for linear drive of a solenoid with a peak to hold ratio of about 4:1.

Since the linear drive requires excess voltage to be dissipated through the switching transistor, this is not an efficient system with potentially plenty of heat dissipation. The same chip can also be used with a high speed switching instead of the linear method. Thus you use a very efficient switching FET with low losses and then control duty cycle by the % of time the FET is conducting. The frequency does not have to be very high, it just has to be high enough that the hysterisis of the main contactor will keep it closed when the duty cycle is in the desired range. The main contactor would have a lower impedance for faster response and current is then limited by reducing the hold duty cycle. Peripheral components control the duration of the peak pulse which is 100% duty cycle then once it has pulled in, a reduced duty cycle limits the power dissipation. It also ensures a fast release of the armature when you turn it off.
 
I’ve been using the Gigavac MX11-S for many years with no issues.
 
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Tell us noobs https://www.gigavac.com/sites/default/files/application-notes/AN-002-Two-Coil-Economizer-Principles.pdfabout dual coil contactors. How do they work? One to move and one to hold?

Don't see an "MA", but Walt's "S" is listed here:

The MX11 is a 2 coil design with no PWM, here’s the application note with a description:
(Yes pull in and holding coils)

https://www.gigavac.com/sites/default/files/application-notes/AN-002-Two-Coil-Economizer-Principles.pdf
 
Tell us noobs about dual coil contactors. How do they work? One to move and one to hold?

Don't see an "MA", but Walt's "S" is listed here:

https://www.gigavac.com/sites/default/files/catalog/spec_sheet/mx11.pdf

Strange. It was there and suddenly wasn't. Can still order them from GV proper. Here is a spec sheet. I did get the weight wrong in my previous quick compare table. It makes more sense. The form factor and other factors are close enough between manufacturers that I'm not seeing an unacceptable risk for my planned application. As mentioned, SS coil current draw is a strong design consideration for me, all other factors being equal-ish.
 

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My reference and question is to the GV200 series with the M coil option (PWM control and low holding current). The dual coil options in this series would have better characteristics than the MX series Walt lists. Talking about my application only.
 
GX-11 also works

This doesn't address the OP's question but for anyone considering a GigaVac the GX-11 also works fine.

The MX-11's were back ordered when I went to buy one so I got the GX-11 instead. I've been using it since early 2019 and it works great.
 
Misspent creative juices

Interesting thread, when I built, I used a solid state relay thinking how wonderful it would be to reduce the current. Ran one for 5+ years problem free. What I failed to consider was the whole system, using a solid state relay or a tapped coil contactor added to cost and complexity. It also makes it harder to fix if you have a failure when traveling.


To power budgets, do I care if the contactor draws 250ma when my alternator’s field is drawing a factor of 10 or more? Do I care if it’s drawing 250ma while I am flying? No really, if I have an alternator failure, I will shed load by turning off the master and using the E-bus as the most efficient way of reducing power usage.


If you want to build a PWM it’s cheap and easy but until you are 30-40 miles out and the radio sensitivity is ruined by noise or the contactor burnt up because the hold current was just a bit too low (once it got some age on it) you don’t know the real cost. It’s not so easy to build a quiet PWM or to figure out the specific hold current and margins needed to make a contactor do something it was never intended to do. (Buy the GigaVac instead)

I’m sure there are dozens of corner cases that can argue against my approach, and I’m one to talk. I have a complete CAN bus running all the power, fuses and secondary flight controls on my own pet project. But I still have a simple master contactor and E-bus on the front end of it!
 
Interesting thread, when I built, I used a solid state relay thinking how wonderful it would be to reduce the current. Ran one for 5+ years problem free. What I failed to consider was the whole system, using a solid state relay or a tapped coil contactor added to cost and complexity. It also makes it harder to fix if you have a failure when traveling.


To power budgets, do I care if the contactor draws 250ma when my alternator’s field is drawing a factor of 10 or more? Do I care if it’s drawing 250ma while I am flying? No really, if I have an alternator failure, I will shed load by turning off the master and using the E-bus as the most efficient way of reducing power usage.


If you want to build a PWM it’s cheap and easy but until you are 30-40 miles out and the radio sensitivity is ruined by noise or the contactor burnt up because the hold current was just a bit too low (once it got some age on it) you don’t know the real cost. It’s not so easy to build a quiet PWM or to figure out the specific hold current and margins needed to make a contactor do something it was never intended to do. (Buy the GigaVac instead)

I’m sure there are dozens of corner cases that can argue against my approach, and I’m one to talk. I have a complete CAN bus running all the power, fuses and secondary flight controls on my own pet project. But I still have a simple master contactor and E-bus on the front end of it!

Not sure I follow but trying. As more aircraft become electrically dependent, wasted "parasitic" loads become a bigger deal IMO. Sure, I could throw the standard silver cans in the architecture. A whole lot of the certified stuff is hiding behind their PMAs and would quickly, quietly die in an open market. My hangar neighbor just put a $900 certified battery in his Cirrus. I could buy many different solenoids from a variety of sources that have the identical form, fit, function as the ones I'm considering. No risk there.

If a somewhat newer tech gives better performance and doesn't add any risks, why would it not be considered? There is no complexity increase. They are internally controlled. If any noise escapes, the swap is easy. It's cheap margin and doesn't affect any fabrication. A combined 4Ahs is a relative lot. Would cost a lot more in weight and recurring money to make that up in batteries.

The more I consider dual, redundant electrical architecture, the more it makes sense for most aircraft though I'm sure some will disagree. In time, this will pay for itself. I don't plan on having any device dedicated back-up batteries. Garmin prices ain't coming down and will surely continue to out pace other price escalation(s).

If I misunderstood something, let me know. I asked about the differences and any potential downsides. The community has gotten me smarter yet again.
 
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I get the solenoid reliability need, but the original question sounded like PWM vs switch for activation. It is really hard to get more reliable than a switch, vs an electronic component not validated for the application, and it still has a switch to activate.

Apportioned and summed reliability (MTBF, or Failure frequency) would favor the elimination of a PWM drive on a reliability basis.
 
A different viewpoint?

If a somewhat newer tech gives better performance and doesn't add any risks, why would it not be considered? There is no complexity increase. They are internally controlled. If any noise escapes, the swap is easy. It's cheap margin and doesn't affect any fabrication. A combined 4Ahs is a relative lot. Would cost a lot more in weight and recurring money to make that up in batteries.

The more I consider dual, redundant electrical architecture, the more it makes sense for most aircraft though I'm sure some will disagree. In time, this will pay for itself. I don't plan on having any device dedicated back-up batteries. Garmin prices ain't coming down and will surely continue to out pace other price escalation(s).

If I misunderstood something, let me know. I asked about the differences and any potential downsides. The community has gotten me smarter yet again.


I was under the impression you were considering making a unique version of an existing device. (relay with low hold current). I see two different types of builder on this forum, one wants to fly around the country and one who loves developing and trying new things. I am a bit of both, with a healthy eye towards what systems could ground me far from home. I did the new tech thing with a SS relay promoted by a leading aircraft supplier. It worked well but the product was expensive and eventually discontinued making it a liability (I replaced it with a basic contactor).
I also love new tech and trying new things but sometimes I just end up re-inventing the wheel or getting too deep into a cool idea to see the bigger picture.
BTW, I was not aware that PWM was a term used on any contactor. I have seen dual coil, tap coil and time delays but not 'PWM'
 
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