Coil economizer

[johu]

I bought some chunky relays as charge port contactors. As it is with chunky relays they take a lot of current into their coil to make contact. Two of these take in a hefty 8.2W. So at the risk of being told "but Johannes this was invented by Nicola Tesla in 1870" I want to propose my simple economizer: Just put some resistors in parallel to a capacitor in series with the relay coil.
In this particular case 1000µF (10V model is sufficient) in parallel with 40 Ohms. Cap takes long enough to charge to energize the coil and make contact, after that the coil voltage drops to 3.5V via the dropper resistors. The relay drops out at about 2.4V so enough margin.

Power consumption drops to 2.5W for both relays.

[Jack Bauer]

Nice. Done that few times in the past but always found the resistors got too hot.

[johu]

Yes the 4x10 Ohm 2W models get reasonably warm. You can touch them without burn marks

[tom91]

Any more details on what the relays are?

Also any idea on how they behave when switching off in terms of delay or bounce?

What sort of turn of EMF is there on the control line?

[SciroccoEV]

Look at automotive injector drivers.

[johu]

Any more details on what the relays are?

Also any idea on how they behave when switching off in terms of delay or bounce?

What sort of turn of EMF is there on the control line?

So switch-off is not affected by this at all since it provides no current path across the coil. What does slow down switch-off is a flyback diode across the coil. The relay is no name Chinese from ebay (WM686 RL/280-12 12V/200A). It is not meant to interrupt current flow, it will only switch when current is already 0. So if you stick your fingers in the charge port you won't get electrocuted.

Look at automotive injector drivers.

Because they use the same technique?

[MattsAwesomeStuff]

This is the kind of partsbin solution I love

[SciroccoEV]

Look at automotive injector drivers.

Because they use the same technique?

Yes, PWM peak and hold drivers. There are even some solenoid drivers that can detect the core position.

[larsrengersen]

How about using the DRV103?
I thought about using those in my EV-Peripherals controller for the coils I'm controlling, but in the end decided that in my case it was not worth the added complexity for the gain (I'm controlling small coils).
But I thought it is an interesting chip.

[joromy]

Have used the DRV102, on tesla GigaVac contactors a lot, very simple.

viewtopic.php?f=13&t=131&p=4140&hilit=drv102#p4140

[johu]

I also made a boost variant to switch a 12V relay from 5V. Just boost for a few ms then stop the booster - 5V will keep the relay closed and you don't have switching noise going on.

[m.art.y]

I also made a boost variant to switch a 12V relay from 5V. Just boost for a few ms then stop the booster - 5V will keep the relay closed and you don't have switching noise going on.

How to do that?

[johu]

How to do that?

You can use a boost mode IC with enable input, like MIC2288YD5

When you enable you get your 12V, otherwise 5V just flow via the inductor