Relays for switching DC loads - Page 1
Forum section: electronics / projects

author: JohanH
author_rank: Frequent Contributor
timestamp: February 09, 2025, 09:01:41 am
content: I'm trying to research a bit (as hobbyist) what is available in this area. Regular relays that are commonly used in electric appliances or circuit boards that we are familiar with, often specify e.g. max 30V DC, even if they are typically 250V AC, 10/16A or similar. Automotive relays are limited to 12/24V DC. You won't find DC relays or contactors for higher voltages and currents in your typical electronics store, except for large and expensive ones.It's known that switching high DC loads is much more difficult, compared to AC, with risk of arcs and pitting of relay contacts. So for higher DC voltages and current, you need better contact mechanisms. This is typically done with coils and contacts in a linear fashion (like solenoids), with dual coils in 2NO or 2NC configuration that break or close both connections at the same time. The smaller size SPST DC relays likely have a single coil for both contacts.Such relays or contactors (as the larger ones are called) have been available, but they have been large and expensive. Nothing smaller that could be afforded for hobbyist projects.But with the arrival of all kinds of electrical vehicles, there is a need for switching off and on varying sizes of DC loads. Thus smaller and more affordable DC relays are now available.This is a new area to me, but I was able to find affordable (for the hobbyist, meaning in the $10 range) relays with the following specifications (no product promoting here, this is a "research" example). You can already find these types of relays on Ebay and typical marketing places, but they are often listed only by their part numbers, so they are hard to find. I'm sure there are more, if you know what to search for.Example of a relay in SPST configuration:Hongfa HFE80V-20CContact rating 20A DCMax. switching voltage 750 VDCMax. breaking current 30A(450 VDC) 5 opsMax. switching power 13.5kWCoil voltages: 12, 24 or 48 V DC, 3WThe size of the relay is like a regular automotive relay, about 30x30x30 mm. What is especially impressive is the switching power. The coil current is larger than typical AC relays, so this has to be considered.With regards to coil current, one thing that the manufacturer states in the documentation:"In order to suppress the reverse electromotive force of the coil, it is recommended to use transient voltage suppressor(TVS),varistor(ZNR) or zener diode(ZN). Please take care that if the freewheeling diode is used, it may cause the release time gets longerand the deterioration of breaking performance."This has been known that a simple diode is not always enough to protect from the reverse spike of the coil. Often a zener in series with a diode has been proposed (zener voltage same as the coil voltage). A TVS is probably as good as any solution.Conclusion so far (from my limited perspective): There is a new class of small size, affordable DC relays that could be used also in hobbyist projects to safely switch larger DC loads. This if you need any higher DC voltage than the traditional 24/30V.Does anyone have any experience or input to this topic? Did I miss something obvious?

author: EEVblog
author_rank: Unknown
timestamp: February 09, 2025, 09:19:17 am
content: The solar and EV/battery industries have driven the need for high voltage DC relays, Hongfa are one those that are well known.High voltage reed relays are the traditional type.Not sure about low cost though, as gas filled is common.

author: Gyro
author_rank: Super Contributor
timestamp: February 09, 2025, 10:20:07 am
content: Quote from: JohanH on February 09, 2025, 09:01:41 amThe telling specification there is the "Max. breaking current 30A(450 VDC) 5 ops", which illustrates the difficulties in breaking a DC arc as you discussed. The 5 operations limit demonstrates the amount of energy that needs to be quenched and the toll it takes on the contact material and wherever the vapor of the evaporated contact material deposits (without creating a partial short that bypasses the contacts).Such relays are used in a very specific way - there is usually a 'precharge' relay that uses a series resistor to charge up the capacitors in the EV inverter before the main relay engages. The inverter also turns off its current draw before the relay is opened. The breaking current of 30A is only used in an emergency situation, eg. A shorted MOSFET or BJT in the inverter which prevents it from being able to turn off, hence the 5 operation limit.In many respects, these relays are similar to (good quality) standard automotive 20A relays, probably just with improved contact open spacing, maybe different contact material (Tungsten?) and better insulation. There is no real way to avoid the DC arc problem in a small relay, they can only be used in a very specific way, i.e. the need to turn off the load current by other means, before opening them, which will limit their general application as higher voltage DC switching relays.Edit: Looking at the Hongfa website, the HFE80V-20C is described as a "precharge relay" ie, the one with the series resistor for initially charging the inverter caps. The HFE82 series look more interesting with their hermetically sealed Hydrogen atmosphere - that's where the advanced technology looks to kick in.

author: JohanH
author_rank: Frequent Contributor
timestamp: February 09, 2025, 10:31:27 am
content: Quote from: Gyro on February 09, 2025, 10:20:07 amThank you, very good information.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 09, 2025, 07:09:04 pm
content: DC power causes big sparks.Sparks really damage contacts.I have seen a relay that welded its contacts together from high inrush current, and that was 120VAC.What about some solid-state switch?Could you perhaps limit the breaking/making current?

author: garrettm
author_rank: Frequent Contributor
timestamp: February 09, 2025, 11:40:12 pm
content: For DC applications, a hybrid relay/contactor that parallels a semiconductor switch with a classic mechanical commutator is ideal. The semiconductor switch is used during turn-on and turn off-events. This allows for both the semiconductor and mechanical contacts to be greatly reduced in size, increases reliability and prevents contact bounce. It also allows for much higher DC voltage switching, as the semiconductor can safely open the circuit without arcing (unlike DC, the AC zero voltage crossing naturally extinguishes arcing). There are a lot of articles and research papers on this subject. You can make your own too, just need to correctly time the turn-on and turn-off events. Using latching type commutator (with two coils for "on" and "off") greatly increases efficiency but can complicate the design to ensure the mechanical commutator is always in the correct state when a fault occurs.

author: Poroit
author_rank: Frequent Contributor
timestamp: February 10, 2025, 04:29:22 am
content: ditto....Solid State Relays as other members have suggested.https://www.sensata.com/products/relays/solid-state-relays

author: JohanH
author_rank: Frequent Contributor
timestamp: February 10, 2025, 05:07:45 pm
content: Quote from: Gyro on February 09, 2025, 10:20:07 amGood find! Indeed, the HFE82 series was more what I was looking for. They are a bit more expensive, but not too much!HFE82V-20Electrical endurance Switching:3x10^4 ops(1000 VDC,20A)