Questions to Ask

If you are at the beginning of the design cycle of a new product, or the re-work of an existing one, you have certain questions to ask, and answers to collect, about the components in your circuit or system. If one or more of the components in your design are relays, you usually concern yourself with things like load voltage ratings, type of voltage (AC or DC), load current rating, number of poles to be switched, etc.

But ultimately there are other questions that have to be asked, and more importantly answered, before you, or your procurement department, can move on in the relay sourcing process. Some of these questions are easy to answer, some not, but they all factor in to the specific relay components that will function correctly over time and deliver value to the end-user.

These questions concern not only the electrical specs of the product, but also the environment in which the product will function. We’ve huddled with our relay specialists here at Master Electronics to put together this suggested list of what needs to be answered.

Our Relay Specifying Qualifier Questions include (but may not be limited to) the following:

1. Has the relay been tested in a real circuit with a real load?
2. Has the relay been tested in the atmosphere in which it will be used?
3. Does it make design sense to specify a standard relay with more than the required number of contacts?
4. Has care been taken in the circuit design to ensure that the movement of the relay contacts is perpendicular to the direction of any vibration or shock in the final application?
5. Do the relay ratings meet the relevant international standards which apply to the application?
6. Does the specified relay life match the life of the device in which it will be used?
7. Will the specified relay operate correctly when subjected to the shock and vibration expected to be present in the application or environment?
8. Has consideration been given to the effects of line voltage sag when selecting the relay, especially when the equipment incorporates or may be connected to the same circuit as heaters, motors, transformers, or other high in-rush current loads?
9. Have the relay contacts been tested or specified for reliability when used in systems with infrequent switching (<1 per month) or switching low level signals?
10. Has a relay been selected whose temperature rating is not exceeded by the ambient temperature of the installation?
11. Will the relay be used in an installation involving silicon-based or other corrosive substances (which may cause contact failure)?
12. Will the relay be used in a situation of high humidity, possibly causing condensation and subsequent deterioration of contact performance?
13. Will the relay be protected from oil or water splashes, or excessive airborne dust or dirt?
14. Has consideration been given to relays placed near magnetic field sources, which may affect operation?
15. Is the relay contact material composition appropriate for the type of load?
16. Has consideration been given to the relay type (particularly SSRs) when used in a circuit prone to arcing generated at a high frequency?
17. In the case of switching large DC current or high in-rush loads, has a relay been selected which has a contact composition resistant to material transfer?
18. Has the effect of DC relay coil resistance variations due to ambient temperature and internal heating been considered?
19. Has attention been given when selecting relays for circuits where different voltages are applied between electrodes in a multi-pole relay (particularly when switching power supply circuits)?
20. Has consideration been given for the effect of induction developing in long wire runs for control circuit and power lines?
21. When a latching relay is being used, will it be used in a circuit that initializes the relay to the required state whenever the power is turned on?
22. Is the coil circuit protected from malfunction when the relay is driven by transistors or ICs?
23. Can switching of the relay contacts, particularly with SSRs, synchronize with the phase of a non- power factor corrected AC power source, causing possible problems?

Now, this list may be somewhat long, but it is not necessarily exhaustive. Depending on your relay application, there may be other questions you need to ask up front, but don’t even know about. The best advice is to reach out to the product experts at Master Electronics, describe your product, application, environment, expected life-cycle, etc., and get the answers you want and the relay products you need.

Getting the right questions answered up front can save you countless hours of design time, needless weeks of re-work, and precious dollars in unplanned costs. As the experts say, do it right and you won’t have to do it over.

Relay usage in a typical electric vehicle application may seem straightforward, but critical questions on performance related to heat, moisture and vibration from the vehicle must be addressed.