3
\$\begingroup\$

I need to interface a 12VDC or 24VDC relay into a 48VDC (nominal voltage) battery pack for switching applications in my automotive design.

Directly driving the relay coil with the 48VDC battery pack is not feasible, so I'm exploring the best method to interface the relay.

There are two potential ways to achieve this:

Voltage divider circuit (using resistors). Step-down SMPS (Switch-Mode Power Supply). Please advise on the optimal method to interface the 12VDC or 24VDC relay into my application. Thank you.

\$\endgroup\$
8
  • 1
    \$\begingroup\$ Why not use a 48 V DC relay? \$\endgroup\$
    – vu2nan
    Commented Jul 21, 2023 at 7:07
  • \$\begingroup\$ Few reasons. Many relay manufactures do not develop PCB type 48VDC. Second one is cost. \$\endgroup\$
    – HANS Dpmco
    Commented Jul 21, 2023 at 7:16
  • \$\begingroup\$ Given that telephone installations are very frequently 48V, are you sure you can't find good PCB 48VDC relays? A quick check found dozens under EUR 5 from Omron, TE, Axion, Panasonic with many contact choices up to about 16 A. From a reliability standpoint, a designed-for-purpose relay is likely to be much better than something with a (possibly quite hot) adaptation. What are the other requirements for the relay that makes you rule this out? \$\endgroup\$
    – jonathanjo
    Commented Jul 21, 2023 at 8:13
  • \$\begingroup\$ Actually, I am also taking vibrations into consideration. I'm not sure how the relays used for telephone applications respond to vibrations. We primarily use 24VDC relays, with only a few 48VDC relay requirements. If we could purchase all the relays from one type, it would save money. That was my main concern. \$\endgroup\$
    – HANS Dpmco
    Commented Jul 21, 2023 at 8:43
  • \$\begingroup\$ Could you post a part number or datasheet for a typical 24VDC relay that you'd like to use? \$\endgroup\$
    – jonathanjo
    Commented Jul 21, 2023 at 10:12

4 Answers 4

1
\$\begingroup\$

Using resistors, if there are good conditions for cooling

\$\endgroup\$
3
  • \$\begingroup\$ Yah, resistors can be used as voltage divider. But I am not sure whether this is an best practice for automotive application \$\endgroup\$
    – HANS Dpmco
    Commented Jul 21, 2023 at 6:24
  • \$\begingroup\$ @HANSDpmco, if you use wire wound resistors, Vasily's solution is, from the point of view of functional safety, possibly the best not only for automotive but even for railway applications. The failure mode of this components are simpler and better understood than the ones of any AEQ certified IC, and moreover their ambient operating temperature exceed the maximum one allowed for semiconductors. \$\endgroup\$ Commented Jul 21, 2023 at 7:16
  • \$\begingroup\$ Actually, I am seeking information about the standard practices used for this kind of application. Thank you very much, Daniele, for your response. \$\endgroup\$
    – HANS Dpmco
    Commented Jul 21, 2023 at 7:21
1
\$\begingroup\$

Using a resistor in series with the coil is a simple and economical solution.

enter image description here

Standard automotive relays are available in a range of coil voltages (6, 12, 24, 36, 48, 60 and 72 V DC) and have a coil consumption of 1.8 W.

The 24 V coil has a resistance of 324 Ω.

A 324 Ω - 3 W voltage dropping resistor should suffice.

\$\endgroup\$
0
\$\begingroup\$

Drive the coil with a current not a voltage. There are families of semiconductors designed specifically for this application.

\$\endgroup\$
1
  • \$\begingroup\$ Did you mean relay economizer circuits right ? As I am aware of it, they control the contactor using PWM signal. Am I correct? If your suggestion is that, cost is the problem again! \$\endgroup\$
    – HANS Dpmco
    Commented Jul 21, 2023 at 9:20
0
\$\begingroup\$

Sure the resistor is simple and easy when the power is low .Efficiency can be low when considering a 12VDC relay coil running of 48VDC via a resistor that drops 36VDC .25% is expected in this case .PWM would have losses of less than 10% and is not difficult to do these days.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.