Driving Relays

This page hopes to go over the relay driving requirements for the different relays that we will be using.

As with any part of our car, we hope to limit the parasitic power use that the car uses. Part of this loss comes from driving the relay coils necessary to keep the high voltage energized. Some relays (the EV200HAANA) have a coil economizer built in that provides a high inrush current to close the contacts quickly, but then decreases the current to hold the contact in place once closed. This is possible and effective because of the magnetic fields inside the coil. When the coil is driven, it creates a magnetic field that pulls the contact and closes the circuit. At the start, the contact is far away so this magnetic field must be strong in order to pull it in quickly. Since the magnetic force is proportional to the distance between the objects (the coil and the contact), once the contact is closed, we require less force to keep it in the same place and thus less current to drive the coil.

A few handy links that explain it better than I can are found here:

https://www.electronicdesign.com/power-management/article/21805068/reap-the-benefits-of-economizers-for-solenoidrelay-drivers
https://e2e.ti.com/blogs_/b/industrial_strength/archive/2017/03/16/liberate-your-solenoid-or-relay-from-the-bonds-of-supply-voltage
http://www.ti.com/lit/ds/symlink/drv120.pdf

EV200HAANA: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=EV200_R_TBD_KILOVAC_EV200_Ser_Contactors&DocType=CS&DocLang=English
Inrush Current: 3.8A
Hold Current: 0.13A

Inrush to hold current ratio = 3.8 / 0.13 = 29.2

The relay was tested with the BK Precision power supply, and drew 1.6W when holding the contact. With a driving voltage of 12V, we get a hold current of 1.6W / 12V = 0.133A

These relays have a built-in economizer that reduces the holding current of the coil. The main chips on the board are a PIC12C672 microcontroller, an IRF7341 Dual N-FET, and a 5V LDO. So likely PWM across the coil with the FETs, and controlled with the PIC to provide a delay, the PIC being powered from the LDO.

G9EJ-1-DC12: https://www.mouser.ca/datasheet/2/307/en-g9ej-1-e-371299.pdf
Inrush Current = Coil Drive Voltage / Coil Resistance = 12V / 120Ohm = 0.1A
The datasheet does not give a value for the hold current, so we will go with a similar ratio to the EV200HAANA as our minimum hold current, and add a safety factor.
Hold Current = 0.1A / 29.2 = 3.5mA.
This seems extremely low, so lets go with a value of around 25mA - roughly 1/4 of the inrush current.

Economizer frequency: The faster the better (aside from switching losses in the driving circuit). The coil has an inductance of:

Coil Resistance (tested): 71.4Ohms