Switching on 30A relays

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chipset

Senior Member
Im looking at the ratings and itll switch with 6v so you should be ok. On a side note these relays are pretty damn hefty, they weigh 86g or for Americans like me, 1/5th a pound!! Id think for what your doing you should start looking at Bosch automotive relays. If you can post your electrical requirement I could probably help you find a relay thats small and would work great, has a high life expectancy and they are dirt freakin cheap. They can handle most any DC voltage supplied by a battery (within reason) and they can survive in what Id consider the worst conditions possible. Im talking fire, high shock loads, vibration ect. They are rated at 30A @ 12v and can switch 24v loads.

How are you planning to trigger the relays? Are you pulling them to ground or with a positive trigger? Id recommend pulling them to ground but I dunno what your plans are. If you are using (2) 12v batteries to get to 24v Id suggest powering the coil at 12v (trigger side of relay) and pulling them low via a transistor. Then you can switch the load to your motors at 24v. I wouldnt suggest using a positive trigger for the relays and although the coil on the relay you linked to will switch at 6v, its better to do all your relay switching at 0v since everything has common ground, regardless of the voltage levels. Might wanna give it some thought though of using an auto relay as they can switch 24v. There are DPST variants also just not as widely used.
 
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westaust55

Moderator
The 24V dc relays will draw about 68mA and peak would be around 360mA to 400mA until the armature closes the magnetic circuit.

ULN2003 is rated 50Vdc at 500mA continuous and 600mA peak per driver.

ULN2003 has inbuilt flywheeling diodes so that combination will work
 
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chipset

Senior Member
those relays are almost industrial and in the use of a battle bot may be a bit overkill. Id rethink the use of them. Considering that your running at 24 volts your amp load is gonna go down quite a bit and you can get away with using some fairly small relays. Please list what the peak amp draw at 24v is for your motors. I may be able to suggest a relay much much smaller than what youre running. Then again I may be totally wrong and your running a motor out of a drill press. At that point Ill shut my mouth lol.
 
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Andrew Cowan

Senior Member
Given up on the 5000W motors, have you?

With those FETs, since it says the absolute maximum gate voltage is 20V, I would not put 20V into the gate. That's asking for trouble.

Try 18V - that should be fine.

Those relays will not be suitable. While the normally open contacts are rated for 30A, the normally closed contracts are rated for only 2A.

NO : 30A 120VAC/277VAC; 20A 28VDC
NC : 3A 28VDC/277VAC; 2A 480VAC, 1A 600VAC

If you use that to reverse your motors, you will weld the relay open/shut. For a robot wars robot, you MUST have reliable equipment.

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westaust55

Moderator
chipset said:
those relays are almost industrial and in the use of a battle bot may be a bit overkill. Id rethink the use of them. Considering that your running at 24 volts your amp load is gonna go down quite a bit and you can get away with using some fairly small relays. Please list what the peak amp draw at 24v is for your motors. I may be able to suggest a relay much much smaller than what youre running. Then again I may be totally wrong and your running a motor out of a drill press. At that point Ill shut my mouth lol.
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Note that those "30AMP" rated relay contacts are rated 30Amps for 120VAC/277VAC and a resistive load.
At 24V dc the contact rating drops to 20 AMp for a resistive load since there are no zero current crossing to help extinguish any arcing on contact opening.

For the proposed drill motors, the load is inductive and although not stated in the datasheet, the maximum switching current will be further reduced.
 
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chipset

Senior Member
westaust55 said:
Note that those "30AMP" rated relay contacts are rated 30Amps for 120VAC/277VAC and a resistive load.
At 24V dc the contact rating drops to 20 AMp for a resistive load since there are no zero current crossing to help extinguish any arcing on contact opening.

For the proposed drill motors, the load is inductive and although not stated in the datasheet, the maximum switching current will be further reduced.
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the have 20/30 rated relays, 30/40, 40/50 and 40/60 in the same housing. They also have 75 amp relays in a different design but equally lightweight and reliable but those are NO only. depending on amp needs I guarantee theres a bosch relay to handle the job. And just to add to the fact that I am certain they will work, the blower motor in your car for the HVAC on full blast is straight voltage to the motor via a bosch style relay. Ill have to check but last time I checked most every DC motor works pretty much the same way at full whirl ie:no resistance need. But then again a motor inheritantly has resistance so I dont see what the problem is.

What Im getting at is that those relays are cheap, reliable, small, lightweight and have been put through every rigour known to man. I think they even have Mil spec variants and ones certified to go to space and if they didnt Id be surprised lol. Give them a second look they are a wonder piece of hardware that has a million uses in electronics of all types
 
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JTepper

Member
thanks for the help guys, yes the 5000w motors are still going to happen, this board is going in a development bot, the relays are in addition to the pwm mosfets, the relays are actually dpdt versions of those ones i just couldnt find the data sheet, so there will be no current flowing while they are switching, so no arching they are just to swap directions. you said you could find a bosh realy for me? im looking for a 100A minimum rated dpdt relay to go with those 5000w motors, if you could put me onto one of those that would be great.. thanks in advance

oh and ive decided to go with a123 lithium cells for those big motors
 
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gengis

New Member
The evil suppression diode

It is almost ubiquitous to add flyback suppression diodes to damp the induced voltage that relays produce when switched off.

If you are serious about switching large currents with relays, a bare flyback suppression diode is not a good idea. Just like a shading coil added to AC relays to stop the armatures from buzzing, a diode shorts the induced current. The short causes the magnetic field to drop more slowly and the relay armature to fall out slowly - promoting arcing and shortening its life.

Most of us use the diode and don't think twice about it. Most of us never push relays to their current limits, and get away with it.

Nice explanations:
http://www.kilhamaig.com/Joomla/resources/24-relay-driving.html
relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf
 
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chipset

Senior Member
if you need a relay that can handle a MINIMUM of 100A at 24v then the relays you posted arent gonna cut it. Might wanna look into a constant duty solenoid instead. They are the same form factor as a Ford style starter relay and can handle a continuous load of 200A at 12v. They are about 1/3 lbs and are only SPST. They are used to switch power on motorhomes and buses so they are more than up to the task. Finding a DPST version will not happen though.
 
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JTepper

Member
ok in order to be clear, from now on all comments in this thread are for my 30A max bot.

ok now what was i gonna say...............
FIRE!!!!!!!!
i think i may have been underestimating the sheer amount of power of 20v at 30A!!!!!!!!
thats 600W!!!!!!!!
One of my motors stalled at startup (i think a screw may have fallen into one of the cooling slits any tips for getting it out?) and the 20v track on my mosfet board istantly caught fire, and now a 5mm section is missing from the pcb track!!! is there any chance this has damaged the picaxe or my mosfets? rather idiotically, i only made the track a 1mm one, if i go heavy duty with the solder all allong the track do u think this will solve the issue? or is it back to the etching tank for a more heavy duty board? what i dont understand is how do my rc car and plane escs get away with power tracks half this width?

OH hang on can somone give me an estimations of a 18v cordless current? i have too of them so if my estimations of 30A stall is correct, that 1200W at stall.............. i think i need bigger tracks
 
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SilentScreamer

Senior Member
JTepper said:
what i dont understand is how do my rc car and plane escs get away with power tracks half this width?
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Don't forget it resistance that causes heat to be produced not the width. If you have a better quality board the resistance will be lower and you can have the track thinner. Thicker copper and better quality copper on the PCB means thinner tracks. I don't know shorter tracks would help it also?
 
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Andrew Cowan

Senior Member
I'm pretty sure your FET is toasted - not sure about the PICAXE. A stalled 540 RC motor can be over 150A for a top end motor - I would expect the stall current of an 18V drill motor to be around 100A.

If the resistance of the tracka and wire connections is 0.02 ohms, that means 200 watts of heat are being disipated in the system. This will cause a fire.

If you look at your rc model ESCs, the tracks are very thick. What thickness of tracks were you using?

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manie

Senior Member
In my experience anything much more than +-10 Amps require quite heavy gauge wire. I'll get the current into wire as soon as possible after the switching is done, even if it means soldering the Mosfet pins DIRECTLY to the wire. then the wire acts as a HEATSINK also....
 
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chipset

Senior Member
You need a circuit breaker to protect the mosfet and the rest of the circuit itself. It will instantly open up in the event a motor locks up and pulls full current. As soon as it can rotate normally it will close up and it will resume function. This is the same idea with a power window motor in your car and why you can hit the switch when the window is all the way up (or down) and it doesnt damage the wiring or the machanical parts. More or less its a self resetting fuse and shoulda been used from the start...

10amps at 12v will light up a 16 gauge wire in seconds if shorted to ground. 30A @ 20v will turn a 10 gauge wire into a heating element in the same amount of time. Be careful as youre playing with potentially harmful voltages. It wont kill you but as you noticed a fire can start quite easily. Every circuit hooked to the battery should have a fuse of some sort to protect it. It saves your components from damage and save your hard earned money and time as well. I almost burned a 1932 Ford roadster to the ground because I neglected to fuse a trigger wire for a fan relay. I had to literally grab the glowing wire and rip it from the power before the cars interior caught fire. I had a horrible burn from it and narrowly adverted destroying a $60k+ car. Needless to say I havent made the same mistake.
 
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