Hey guys! I was planning on doing a fun lil project though not sure where to start.

Objectives
-Design a circuit for an efficient economizer that will operate between 9-32V.
-Peak and hold
-Provide a constant current source.
-Use Digikey, LCSC, and/or Mouser, electronics distributors for parts.
-The PCB must fit around the contactor in a small, confined space. It must have a very small form factor.
-Have the inputs only be power and ground for minimal wiring.

Contactors
-Contactor 1: 2272229-1 Minimum 550ma pull in and 170ma hold MINIMUM, may vary from unit to unit
-Contactor 2: 2138622-1 Minimum 333ma pull in and 160ma hold, these are minimums and may vary from unit to unit
-Both contactors: 25ms pull in time minimum Operating voltage for input circuit is 9-32v, 9v min is contactor maximum pull in voltage 32v is highest expected supply voltage Must regulate current
- not just PWM

Background information
Contactors used in the car are galvanically isolated and are only activated with the force of a magnetic field pulling the switch closed. The magnetic force is generated when current (not voltage!) flows through a coil of wire. Often to activate a contactor the inrush current is fairly high, but once activated the current to sustain the state is much lower. Without an economizer the contactors will dissipate power as heat, weakening the magnetic force and resulting in a flickering on/off state. On the car all boards are designed to handle between 9-32V, the contactor economizer is no exception. There will be two contactors used to switch the high and low side of the battery pack.

 

Welcome to AAC.

Sounds like an interesting project. I am wondering why you are focused on constant current, though. Did you do some analysis that suggest this is an optimum approach?

To be clear, while it is true the B-field is a result of current flow, so too current flow is the result of applied voltage and circuit resistance. My first impulse would be to focus on power rather than the either I or E independently.

A constant power circuit might well look different from a design perspective, and I feel it might also be more efficient overall as the focus on delivered power would allow for adjustments of either voltage or current in reaction to changing demands of the solenoid as it heats up during an operational cycle (resistance of Cu increases with temperature) or changes during the solenoid’s lifecycle (an increase in resistance, though probably not of the magnitude of the heating-based change).

It is also possible that using power required for pull-in and hold phases would be a more durable metric than just current (or voltage). I don’t know, so I am wondering if you considered this. I also wonder if you intended to do self calibration on start-up rather than rely on static measurement that could change over time, or vary based on production runs of components involved.

One more thing, I wonder if you can tell—based on measurement—if the solenoid is going to release. That is, could you increase power until it pulls in (easy to tell, since the battery gets connected) then reduce power dynamically until it is being safely held in with minimal current, based on something (e.g. current rise, voltage drop, ???). This way you wouldn‘t rely on fixed values but instead runtime sensing of the actual effect of the applied power.

Sounds like you have a pretty good handle on the design in general, I am interested to see you get to the prototype stage.

 

It must have a very small form factor.

“Very small” is not a specification. Maximum dimensions is.

 

Hi Ya’akov!! Thank you for replying! Unfortunately it was the restriction/requuirement I got assigned to have a constant current so I must follow that. But I will definately look into a power version as well!

My apologies, I am very new to this and this is my first project that I got assigned so I may not understand a couple of concepts. This is part of an extra-cirricular project at my university, it is responsible to drive contactors used in a solar car.

Welcome to AAC.

Sounds like an interesting project. I am wondering why you are focused on constant current, though. Did you do some analysis that suggest this is an optimum approach?

To be clear, while it is true the B-field is a result of current flow, so too current flow is the result of applied voltage and circuit resistance. My first impulse would be to focus on power rather than the either I or E independently.

A constant power circuit might well look different from a design perspective, and I feel it might also be more efficient overall as the focus on delivered power would allow for adjustments of either voltage or current in reaction to changing demands of the solenoid as it heats up during an operational cycle (resistance of Cu increases with temperature) or changes during the solenoid’s lifecycle (an increase in resistance, though probably not of the magnitude of the heating-based change).

It is also possible that using power required for pull-in and hold phases would be a more durable metric than just current (or voltage). I don’t know, so I am wondering if you considered this. I also wonder if you intended to do self calibration on start-up rather than rely on static measurement that could change over time, or vary based on production runs of components involved.

One more thing, I wonder if you can tell—based on measurement—if the solenoid is going to release. That is, could you increase power until it pulls in (easy to tell, since the battery gets connected) then reduce power dynamically until it is being safely held in with minimal current, based on something (e.g. current rise, voltage drop, ???). This way you wouldn‘t rely on fixed values but instead runtime sensing of the actual effect of the applied power.

Sounds like you have a pretty good handle on the design in general, I am interested to see you get to the prototype stage.

 

It seems to be missing a description if what it actually does. We can guess, but that is not good enough.

Something like:

When power is supplied to the coil inputs, the circuit supplies the minimum pull-in current + (margin) fir x msec, then reduces the current to the min hold current + (margin) until power is removed.

Also, if only 9V is supplied and the current does not reach the requirement, are you planning in boosting it? What happens if 6V is applied?

 

Hi Ya’akov!! Thank you for replying! Unfortunately it was the restriction/requuirement I got assigned to have a constant current so I must follow that. But I will definately look into a power version as well!

My apologies, I am very new to this and this is my first project that I got assigned so I may not understand a couple of concepts. This is part of an extra-cirricular project at my university, it is responsible to drive contactors used in a solar car.

Ah, well—first, because this is coursework I am going to move this message to the Homework Help forum. AAC rules limit the assistance members may give to students to guidance in solving the problem themselves. That is, not direct help but pointers concerning where the solution they have chosen to follow may work—or not. This means always “showing your work” is important.

The help is real, despite these limitations, it’s just more a sort of support than what some students might like—the complete answer packaged and ready to go. So, you can always ask specific questions about the work you are doing and get good answers.

Second, can you ask why CC was a given? It might suggest a particular strategy. I assume the “not just PWM” constraint is also part of the assignment?

In any case a good source of information about CC supplies and their design is from LED drivers. Looking for application notes on the ICs used in them will probably help some.

My suggestion is to breadboard something throw-away to get oriented to the possible pitfalls. While you have a size constraint, it probably doesn’t pay to make that a requirement at the very start—instead, focus on the strategy you will employ and what circuits might answer for that. Consider the various scheme for CC, and for voltage reduction, which is, of course, unavoidable if you need reduced current.

Good luck, do ask any specific questions that come up. Hands on, trying things, I find is a great way to get a start on something like this where you lack practical experience. Then you can try simulations, etc.

 

Correct its roughly like that. I need to design and product a circuit that makes a contactor more efficient. Essentially the contactor is opened with high current originally, but this is not needed - as high current (333 or 550ma) is only needed to latch-on or PULL the contactor closed, and then the current can be dropped (to a reasonable point) where it is sufficient enough to HOLD so that the contactor can still operate (160 or 170ma).

It seems to be missing a description if what it actually does. We can guess, but that is not good enough.

Something like:

When power is supplied to the coil inputs, the circuit supplies the minimum pull-in current + (margin) fir x msec, then reduces the current to the min hold current + (margin) until power is removed.

Also, if only 9V is supplied and the current does not reach the requirement, are you planning in boosting it? What happens if 6V is applied?

 

-Contactor 1: 2272229-1 Minimum 550ma pull in and 170ma hold MINIMUM, may vary from unit to unit
-Contactor 2: 2138622-1 Minimum 333ma pull in and 160ma hold, these are minimums and may vary from unit to unit
-Both contactors: 25ms pull in time minimum Operating voltage for input circuit is 9-32v, 9v min is contactor maximum pull in voltage 32v is highest expected supply voltage Must regulate current

Generally concern for Inrush current is applied to AC coil relays, not really an issue with DC.
The voltage for DC coils can often be lowered after pull-in due to lower voltage required to retain the armature.
As opposed to the AC version where it must be maintained.

 

Use a magnetically latched contactor. Power consumption in the ON state is zero!

 

One issue to be aware of with the magnetically latched version, is in the event of a unscheduled power-off, the relay/contactor will remain retained.'

 

One issue to be aware of with the magnetically latched version, is in the event of a unscheduled power-off, the relay/contactor will remain retained.'

That's also an issue with the reduced power version. If the power to the contactor were to be removed and reinstated, without the control system being aware of it, then the contactor coil would be supplied with reduced power, insufficient to pull it in, and it would be off when it should be on.
Switching the "economiser" in with a pair of auxiliary contacts overcomes this.

 

That's also an issue with the reduced power version. If the power to the contactor were to be removed and reinstated, without the control system being aware of it, then the contactor coil would be supplied with reduced power, insufficient to pull it in, and it would be off when it should be on.
Switching the "economiser" in with a pair of auxiliary contacts overcomes this.

Or, just source power for it from the load side of the contactor and design it to fail in bypass.