# Relay selection for 48V loads

I want to use a relay to remotely switch a 48VDC 1A load (3A peak on startup with capacitive load), however, nearly all relays I have looked at specify a 30VDC maximum contact voltage. When searching Digikey, many parts state 125VDC, but the datasheet strongly derates the current at this maximum. The Omron G5LE for example specifies 8A at 30VDC, but derates to 3A at 50V in the Maximum Switching Capacity graph.

This relay might suite my needs at 48V, but my question is more general. If I need to switch a DC load up to 100V or so, how do I find a relay. There are high-voltage relays designed for >400V, but this seems to be a different specialty that the industrial control regime I am interested in.

Is a mechanically switched relay the right tool for switching loads of a few amps from 30V to 100V, or is there a better solution such as solid state relays?

In my specific case the load is a DCDC supply with a 1A 48V input and peak inrush current of 2.8A. However, this question generally applies to switching of intermediate voltage loads. Since 24V and 48V power supplies are common in industrial control, I assume that switching these is a pretty common requirement.

• shopping questions are off topic here Nov 30, 2021 at 1:42
• My intent is not to shop for a specific part. Since there seems to be a gap in catalog capabilities, I am asking about appropriate technologies that fill this gap.
– Mike
Nov 30, 2021 at 2:12
• Simply follow the derating, there is good reason for it. And avoid components with no such derating in teh datasheet. Nov 30, 2021 at 15:01

The derating is done to protect the layers of plating materials with silver, platinum and other materials from the > 5000'K temperatures and duration of the inductive on open and capacitive arc on close and worse with any potential contact bounce at low coil current. Bigger power relays are called Contactors and are much more expensive.

It's better to design a snubber for the reactive loads you are switching if these can be well-defined.

For Caps, ICL's are used to reduce inrush and then a secondary relay can bypass the ICL which operates > 85'C to extend lifespan. For inductive loads, there are many snubber designs such as power Zeners in series with reverse diodes and RC snubbers with metal film caps for low ESR and series R with carbon resistors to absorb high transient powers.

MOV's & TVS diodes are other solutions but the lower the DCR the long the arc quench time from T = L/R which can create other issues. The same is true with the coil flyback diode, that its low ESR diode resistance also slows the open response time constant and extends the burn time of the contacts. Which can be improved also with a Zener > Vcc and diode. (1*)

But rather than generic formula which takes time and effort, ask a specific worse case problem and desired MTBF. There are lots of tradeoffs with complexity and reliability for switching stored reactance energy. $$\E=0.5CV^2 ~or~ 0.5 LI^2\$$. The holdoff current and voltage limits limit the max DC voltage across the contacts, but these can be attenuated by AC coupled snubbers and coil and contact heat also increases activation (copper PTC resistance reduces acceleration current and force) thus greater switching time and damage from rapid cycling.

This supports my comments (1*) and adds more details on what not to do, but says little about how to do it better.

https://omronfs.omron.com/en_US/ecb/products/pdf/auto-dc_precautions.pdf

Here's a good power relay but its \$47 https://www.mouser.ca/ProductDetail/Omron/G9EJ-1-E-UVD-DC24?qs=sGAEpiMZZMtJbfcMcIM8CI%252BTM1izMQFwx3guShln9Zo%3D and still would improve reliability with Zener+ diode on coil and power Zener + diode on contact with RC snubber perhaps as well-chosen to be underdamped.

You can expect to obsolete all these relays and replace them with protected smart solid-state switches when the lifecycle costs justify this.

• Good advice about the snubber circuit. My load is a DCDC converter that has this functionality built-in, its input surge current is limited to 2.8A. Do you think a solid-state relay is a better choice for this kind of power switching?
– Mike
Nov 30, 2021 at 2:57
• can you access the internals of the DC-DC converter to pull on the "shutdown" pin? Nov 30, 2021 at 4:05

Automotive type DC relays, with the following ratings, would suit your purpose.

Working voltage: 6V / 12V (14V) / 24V (28V) / 36V / 48V / 60V / 72V.

Working current: 40 A Ith