Relay for high voltage switching

Other Hardware General Electronics
1

Hi,

I would like to switch 450VDC @ 5mA load max using a 5VDC (coil) relay.

I am finding lots of relays but with a switching voltage of around 240VAC.

I have found a 6V relay that can switches 1000VDC but it's expensive @ £94 and 20A rated.

Is is possible for me to use the 240VAC switching relay or is there a risk of the contacts arcing?

Any guidance would be most appreciated!

Thanks

2

There is no surprise that you have not found the desired relay. What is the purpose? Please show a schematic of the circuit you need to switch with a relay. Perhaps there is a better way.

Relay contacts will always arc, AC or DC. DC will give bigger arcs. An inductive load at DC may arc forever if the inductive reverse voltage is not controlled.

Switching a high DC voltage may require a vacuum relay. None will work at 5 volts.

Paul

3

balgill021:
Hi,

I would like to switch 450VDC @ 5mA load max using a 5VDC (coil) relay.

I am finding lots of relays but with a switching voltage of around 240VAC.

I have found a 6V relay that can switches 1000VDC but it's expensive @ £94 and 20A rated.

https://uk.rs-online.com/web/p/non-latching-relays/8113520/

Is is possible for me to use the 240VAC switching relay or is there a risk of the contacts arcing?

Any guidance would be most appreciated!

Thanks

The problem you will have is you are looking to switch 450 VDC with what amounts to a negligible current. The relay contacts will need something like a silver plate. Then a 5 volt coil adds to finding something reasonably priced. On the bright side you are switching a very low current. What is your load, inductive or resistive?

Maybe you can find a DC SSR but even a 500 VDC 7.0 amp SSR is overkill and with an SSR you need to consider leakage current.

http://www.crydom.com/en/products/catalog/1-dc-series-dc-panel-mount.pdf

If an SSR will work for you.

Ron

4

The load is a 90k resistor across 450VDC. This is to create a load of 5mA max, controlled on/off with a relay.The purpose is to simulate a current drain on a electric car battery. There are a bank of resistors with values greater than 90k also to simulate 1mA to 5mA.

Thanks

5

balgill021:
The load is a 90k resistor across 450VDC. This is to create a load of 5mA max, controlled on/off with a relay.The purpose is to simulate a current drain on a electric car battery. There are a bank of resistors with values greater than 90k also to simulate 1mA to 5mA.

Thanks

So a resistive load only! Are you switching the ground end or the hot end?

Look for an OPEN FRAME relay with the armature isolated from the contacts so you can wire directly to the contacts, not using any built-in wiring. That way you can keep your HV wiring from anything else.

How often do you need switching and are you opening or closing the circuit?

Paul

6

The relay can be in the posituve or ground side. I'm only interested in creating the load by opening and closing the circuit every minute or so.

Thanks

7

balgill021:
The relay can be in the posituve or ground side. I'm only interested in creating the load by opening and closing the circuit every minute or so.

Thanks

In that case, you can use a cheap 5 volt relay to control a 120 volt industrial type open frame relay to do that HV switching.

Paul

8

balgill021:
The relay can be in the posituve or ground side. I'm only interested in creating the load by opening and closing the circuit every minute or so.

Thanks

Why not use MOSFETs?

9

Yes there's no reason why a mosfer can't be used. I've never used one to switch 450Vdc before. With a relay at least the coil and contacts can have 1KV+ isolation. I'm unsure what isolation can be acheived with a mosfet unless an opto isolator is used on the gate.

Thanks

10

Continuous arcing of DC switches and relays is not related to whether the load is inductive of resistive (although inductive loads cause more damage due to kickback).

The important factor is how much DC voltage and power is available to sustain the arc and how much physical separation is between the contacts. Its easy to make a switch rated at 10A 240Vac that self-quenches, much
harder to do one for 10A 240Vdc - that's a specialist piece of gear called a DC contactor.

450V / 5mA means only about a watt is available to sustain the arc current through the resistor, not enough to be a problem I think.

That 10A at 240Vdc switch/contactor has several kilowatts available to sustain an arc, a completely different realm.

Very low voltages are usually safe even with very high currents as a certain amount of voltage is needed to ionize air reliably.

Most standard mains switches rated for 240Vac will typically be rated for only 24Vdc as the safe current limit drops rapidly with increasing voltage. I've seen a single solar panel hold a significant sized arc in bright sun for instance, at only about 36V.

11

There is a video:
AC versus DC load breaking comparison with a knife switch

I think the video does a nice job using AC and DC with a simple resistive heater load. While the thread starter is hardly concerned with something like this for some it may be worth saving.

Ron

12

Do you really need isolation? You said (implied?) that one side is grounded. I'm assuming that someone who is messing with car batteries already has a good handle on safety. The resistor automatically limits the amount of current from +450V, and is mostly non-inductive so there's not much back EMF.

13

Interesting video Ron.Can see the sustained arc well with DC voltage.

@aarg. When I say isolation, I don't want the control side (Arduino) of the relay to have any electrical connection with the 450Vdc switching.

Thanks

14

Have you looked at SSR's?