Can someone recommend a relay for me?

[EmmyBamidele]

TL;DR

Relay recommendation for controlling power supply using DAQ

I have a 30V - 100A power supply that I want to control using a -/+ 10V NI DAQ. I want a relay between the current output of DAQ and the power supply. Please advise the specification of relay to use. I'd appreciate a comment on how to connect it also.

 

[Baluncore]

Welcome to PF.

What is a DAQ ?

Given a DC control voltage, DC solid state relays (DC SSR) are available that will switch 100 amp.
https://www.ebay.com.au/itm/313719499575?

 

[berkeman]

I have a 30V - 100A power supply that I want to control using a -/+ 10V NI DAQ. I want a relay between the current output of DAQ and the power supply.

Why use a relay? Why not use National Instruments DAQ software to control the power supply directly? Does it not have a control input port (serial, Ethernet, etc.)?

 

[Rive]

I have a 30V - 100A power supply...

It's not trivial to build a reliable switch solution for high current DC. I would try a different approach: either to switch the input side of the PSU (supposedly that'll be some AC) or find a PSU with control input.
If none works, then SSR. But definitely not anything with contacts.

 

[Tom.G]

Please supply links to the datasheets for both the National Instruments DAQ board and for the Power Supply.

That way we can evaluate the details of them to come up with a reasonable answer.

As Rive pointed out:

It's not trivial to build a reliable switch solution for high current DC.

I have had good results using an automotive starter solenoid though. Of course the NI DAQ can not directly control that either! They require several amps for the coil.

A quick Google search showed pricing from USD $170 to $9,477, some of which were for 3-phase power line use.

For power electro-mechanical relays, Allen Bradley is probably the major USA manufacturer.
https://www.rockwellautomation.com/en-us/products/hardware/allen-bradley/relays-and-timers.html

Cheers,
Tom

 

[DaveE]

But definitely not anything with contacts.

Yep. More likely called a contactor at these currents. They typically take a lot of power to drive the coils, so you'll probably also need a drive transistor (amplifier, buffer, etc.) or similar. That's why people are suggesting a SSR. So, yes to the previous suggestions of controlling the PSU input, not switching the output. It's usually not as simple as it sounds.

Also, if your going to play the game at this level, you need to be able to search for parts yourself at distributor sites like digikey, allied electronics, https://www.newark.com/, https://www.mouser.com/, https://www.arrow.com/, etc. Choosing parts isn't something that works well via social media. We don't know all of your requirements. There's a reason that the datasheets have lots of writing on them.

 

[Windadct]

OP is MIA -- but "current output of DAQ and the power supply" makes no sense:

Data AQuisition unit would not have a current output. He may mean DAC - Digital to Analog - but in NI and typical control verbage that is just an Analog output- which may be what he is indicating with the - /+ 10V.

And then - he states a relay (on/off) which would not really "control" the power supply.

So ... to many questions.

 

[gary350]

TL;DR Summary: Relay recommendation for controlling power supply using DAQ

I have a 30V - 100A power supply that I want to control using a -/+ 10V NI DAQ. I want a relay between the current output of DAQ and the power supply. Please advise the specification of relay to use. I'd appreciate a comment on how to connect it also.

Buy a 100 amp industrial motor starter us it for a relay.

 

[Tom.G]

Buy a 100 amp industrial motor starter us it for a relay.

Well... maybe at the 30 Volts being switched. Motor Starters tend to be for switching AC. When switching high DC currents they can not quench the arc as the contacts open, you end up with an arc welder at the contacts.

The problem is much reduced with AC because the arc is quenched around the AC zero crossings.

I don't know how true it is, but I seem to recall that one solution for DC switching is to put a permanent magnet next to the contacts. The magnetic field will blow the arc away from the contacts by interacting with the arcs magnetic field.

Cheers,
Tom

 

[Baluncore]

I don't know how true it is, but I seem to recall that one solution for DC switching is to put a permanent magnet next to the contacts.

"Blowout" magnets are used in some situations.
The use of a solid state relay, such as a MOSFET with a snubber, can eliminate the arcing problem when switching DC.
SSRs also have reduced switch control drive requirements. 3 volts, 5 mA.

 

[DaveE]

I don't know how true it is, but I seem to recall that one solution for DC switching is to put a permanent magnet next to the contacts. The magnetic field will blow the arc away from the contacts by interacting with the arcs magnetic field.

Yes, that is one solution. But you'll probably only see it for HV circuits. It's complex and expensive. A more normal approach, if you have to have contacts separating to stop the current is to just use good, tough, materials and separate them quickly (i.e. high coil drive/power). It's really about the voltage compliance (inductance) of the circuit, which in low voltage circuits can be solved with snubbers to provide a temporary shunt path for the current.

In short, you're right, but only when the easier solutions won't work. Switching 100A at 30V only requires normal good EE skills. Amateurs can't do it, but the solution isn't exotic. IMO, it's MOSFETS, clamp diodes, snubbers, and about a week of testing in a good lab.

OTOH, contactors can also do it, even in most DC circuits. I've just developed a strong bias against electro-mechanical solutions when an electronic solution is also practical. Mechanics wear out, electronics (mostly) don't.

Still, I like the idea of just telling the PS to turn off. Less conflict there.

 

[gary350]

If you think mechanical silver contacts will arc weld them self together then try mercury contacts. I have worked in industry 40 years, I have used mercury contacts for very high amp circuits they are impossible to arc weld the contacts. I used 3 of these on a 480 volt 300 amp 3 phase electric furnace once it ran good for 20 years almost zero maintenance. Fuji temperature controllers cycle on/off all day to keep oven temperature differential accurate to 5°F. The most common Mercury contactors on/off voltage is 120 volts AC but you can buy these in many voltages. They also some as, single contactor, double contactor and triple contactors. You can buy 300 amp triple contactors connect them in parallel to get 900 amp contactors. You can use mercury contactors to turn spot welder on/off. Notices picture #2 shows the 3 phase contactor rated 60 amps, put all 3 in parallel you get 180 amps.

There are a lot of mercury contactors for sale on ebay, I did not look at every one for them but the ones I looked at all have a control voltage of 120 vac. The contact voltage can be any voltage you like up to 600 volts. Most common contact voltage rating is 480 volts it will work good for your 30 volts. I have seen max ratings of 240v, 480v & 600v. Do ebay search for Mercery Contactor I found about 20 of them. You can also buy new contactors from any industrial electric supply.

I just now noticed picture 2 shows 3 contactors are already connected in parallel for 360 amps.

 

[Rive]

120 volts AC

I don't really know mercury contacts, but in general switching high current AC and DC is really different beasts.
A small cut from a random^* relay datasheet:

600V AC relay is rated only for 60V DC. ...

Some manufacturers specify the guaranteed cycle number instead, which is usually 'plenty' for AC and 'few' for DC with similar current/voltage. ...

So, motor starters and other industrial AC stuff are absolutely decent stuff, but without detailed and careful considerations they cannot be recommended for DC applications.

Generally, it's the best if high current DC can be done without any contacts at all.

(*) well, not that random: I had to dig through a dozen datasheets till I could find one rated for both AC and DC...

 

[gary350]

Call an Industrial electric supply in your area they can look up mercury contactor specs in their book.

Lot me see your circuit drawing?

 

[anorlunda]

The discussion made me wonder about circuit breakers used for HVDC power transmission. The are used to break 100 kA at voltages more than 1 MV.

They use extra snubber circuits as mentioned earlier to force zero current in the main breaker at the moment of opening. Here is one variation.

But the sizes of the breakers are impressive.

 

[gary350]

We live in a town with a large industrial park with about 200 factories. We also have a scrap yard where industry throw out old used equipment. Every day after work I stopped at the scrap yard for 5 minutes on my way home. You will not believe all the things I bought from the scrap yard. Industrial electronics is fun to play with.

The diodes in my hand are 800 amp 600 volt pancake diodes. The 2 connectors on the side of each pancake diode are the trigger. Pancake diodes probably came from a spot welder. After spot welder closes and clamps down on metal parts then it fired the diodes to do the spot weld. The aluminum heat sink weights 16 lbs each.

The 2 antique diodes in my hand are rated 350 volts 300 amps. I use to have diodes like these with trigger wires I forgot correct name, I think they were called, triacs. Someone in another country wanted the triacs so I sold them. I built a nice 12 volt car battery charger on wheels with the 350 amp diodes its 12v 60a its a good jump starter for dead battery. Industrial parts are bullet proof you need to try very hard to destroy them.

If your circuit has AC that goes through a rectifier to make DC you can put the mercury contactor in the AC section of your circuit before voltage becomes DC. Turn AC on/off DC will go on/off too.