Solid State Relays (SSR)

[Nyx]

My robot might be consuming up to ~50A of currents at any given time. A problem I'm having is that it's hard to find switches or relays with 24V coils that are rated for 50A.

I am thinking of using an SSR instead. However, it seems that most SSR are rated for AC currents. Can SSRs also be used to switch DC loads? For example, this SSR is rated 60A at 380VAC:
http://cgi.ebay.ca/Solid-State-Relay-SSR-60A-Output-24V-380VAC-Input-3-32V_W0QQitemZ350052414247QQihZ022QQcategoryZ36328QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Also, do SSRs heat up alot?

[benji]

My robot might be consuming up to ~50A of currents at any given time

u sure of that???  what are the things draggin all that?

[airman00]

My robot might be consuming up to ~50A of currents at any given time. A problem I'm having is that it's hard to find switches or relays with 24V coils that are rated for 50A.

Check out industrial relays and autmotive relays. Are you sure it is drawing 50A? My butler robot which is wheelchair motor based draws 22A maximum

[Nyx]

Well I said up to 50A. There are two very large motors in the wheelchair base, rated 15A each, so 30A, but then I'm not sure if 15A is the stall current. Then there's an onboard computer, which is based on a PC, so 5A is not so far fetched, that puts us at 35A. I also plan to build a robot arm, and I would like to use my wiper motors (gearheads)... This could possibly consume up to 10A... Total 45A so far... Then there are all the other gizmos that I could put on there. I think a 50A limit is high, but it's not crazily unrealistic if everything is running at once, at full power.

[benji]

in case you are draggin 50 amps from the local power system in your house,, i dont think you can drag that much
the cables should be big,, the power in your house should be able to provide 50 amps,, u sure of that?
whats the motor thats draggin 15 amp? how much torque is it?

[Nyx]

The wheelchair's current power module has a 30A breaker in it. The motors have a torque rating of 84 in-lbs.

[kd5kfl]

in case you are draggin 50 amps from the local power system in your house,, i dont think you can drag that much? the cables should be big,, the power in your house should be able to provide 50 amps,, u sure of that?
whats the motor thats draggin 15 amp? how much torque is it?

50 amps of 12 VDC is 600 W.
50 amps of 24 VDC is 1200 W.
Feeding those from 120 AC would take about 5 amps and 10 amps. Plus what the power supply wastes as heat.

[benji]

check these

http://www.mercuryrelays.com/shoppingcart/agora.cgi?&product=60amp

[Nyx]

check these

http://www.mercuryrelays.com/shoppingcart/agora.cgi?&product=60amp

Those are insanely expensive... I found some 12V 65ma relays on ebay... Magnetic latching, 60A 250VAC. About $7 each, shipped. Also got a 50A push-button breaker.... $15 shipped. And finally, bought a 50A bridge-rectifier for $8... I plan on using it as a polarity safety.

[benji]

good stuff,, in general, in controling motors the contactors are used as switches cuz they can handle large currents,, i guess thats what you should be lookin for , not a relay, relays are used to control the contactor.
unless you call the contactor a relay

[paulstreats]

What are you wanting the relays for?

All of the systems can be powered independantly such as the motors. They only draw 15amps each. Now if each motor was driven using power mosfets (probably more than 1 wired up in parallel to spread the load) Then the mosfets and the motor will be on a seperate circuit to everything else which only draws 15amps.

The logic part probbly wouldnt consume more than 1 amp (or youve got problems) and again is on a seperate circuit. Yeah the logic circuit is also connected up to the power mosfets to make them work but it still acts as a seperate circuit. The power mosfets if chosen correctly will not turn on until a signal from the logic is given so there is no need for any other power control (you could maybe add a physical breaker switch to the motors as a safety precaution).

If you are wanting to use relays to turn the motors on from logic then you wont really get any speed control for this you would definately need to be looking at power mosfets

[Nyx]

I want one big relay to use as a big "on" switch for everything. So I can connect/disconnect the whole thing from the batteries using a little switch and a key (the key thingy can only take 1.5A).

The motors will be controlled using this gizmo:
http://www.trossenrobotics.com/store/p/5466-Sabertooth-dual-25A-motor-driver.aspx

Still, it's useful to be able to disconnect everything from the batteries (including that), because gel cells, if you discharge them too much, can die, just like SLA batteries (except they tolerate more discharging). The wheelchair's original controller has both power mosfets and a relay in it.

[airman00]

dude

you're building the same thing that I made!!! ( my chives butler robot)
erobots.blogspot.com 

How do you plan to connect the laptop to the batteries???

I was thinking get an automotive inverter from the battery to the laptop charger adapter .

btw I used automotive relays ( rated 40 A each) on Chives

[Admin]

A 'solid state relay' is just a fancy way of saying 'MOSFET'.

No one really uses mechanical relays any more except for AC.

A lot of the battlebot motor drivers can handle 50A+

[Nyx]

How do you plan to connect the laptop to the batteries???

I was thinking get an automotive inverter from the battery to the laptop charger adapter .

I have a 175W 12V inverter (350W peak). I'm not gonna use a laptop, but a small form factor computer (HP Slimline).

btw I used automotive relays ( rated 40 A each) on Chives

That's what I ended up getting, some 12V 60A automotive relays. I'll use one for "on" power switch, but I'll use the dual 25A driver for the motor.

I'm gonna run the batteries in series... More convenient for charging. I'm just not sure what to do for 12V power. The 12V circuit might need up to 10A... If I use lots of 12V voltage regulators, will those dissipate as many watts as is being used by my 12V circuit as heat?

[HDL_CinC_Dragon]

"I'm gonna run the batteries in series... More convenient for charging"
Dont charge batteries at the same time in series, if im not mistaken, that could damage your batteries very badly.

"will those dissipate as many watts as is being used by my 12V circuit as heat?"
it will dissipate as many watts as youre not using, and if im not mistaken, amperage doesnt count for much...until you go over its rated current. If youre trying to regulate 12v from lets say 36v (three 12 volt batteries), thats 24 volts that your going to be turning into heat. Which would probably mean you need some liquid nitrogen to cool that vreg down lol. I would suggest getting some switching regulators instead. They do the same thing except instead of converting unused volts to heat, they simply turn off their inputs until a certain threshold so they can still output the necessary voltage without sucking up voltage unnecessarily.

I have some SSRs that I got for free from a friends dad. The logic side is rated at 3v to 32v DC and the load side is rated at 25A 24v to 280v AC. I hooked up a 12 volt motorcycle battery to a 12v DC motor with the load side of the relay in series to the circuit. I used a 9v battery on the logic side for testing purposes and away the motor went-a-spinning. However, when I removed the 9 volt battery from the logic side, the motor kept spinning. The relay never reset. It only resets when I remove the load(aka, manually disconnecting one of the wires to the motor). Im assuming this is because I was running a DC load on an AC rated device or that I was running 12v on a 24v minimum rated device. I have some more experimenting to do
Thats just my personal experience with Solid State Relays. I figured it wouldnt hurt to share it

[Nyx]

"I'm gonna run the batteries in series... More convenient for charging"
Dont charge batteries at the same time in series, if im not mistaken, that could damage your batteries very badly.

Well, the charger that came with it is a "24V automatic charger"... So it must be meant to charge the batteries in series.

"will those dissipate as many watts as is being used by my 12V circuit as heat?"
it will dissipate as many watts as youre not using, and if im not mistaken, amperage doesnt count for much...until you go over its rated current. If youre trying to regulate 12v from lets say 36v (three 12 volt batteries), thats 24 volts that your going to be turning into heat. Which would probably mean you need some liquid nitrogen to cool that vreg down lol. I would suggest getting some switching regulators instead. They do the same thing except instead of converting unused volts to heat, they simply turn off their inputs until a certain threshold so they can still output the necessary voltage without sucking up voltage unnecessarily.

Following your advice, I just ordered this:
http://cgi.ebay.com/New-Car-Power-Supply-Convert-Devicer-DC-24V-to-12V_W0QQitemZ320248461459QQihZ011QQcategoryZ73335QQtcZphotoQQcmdZViewItem

It says 30A, but since it's made in China, I guess that means 15A

[HDL_CinC_Dragon]

Well that page doesnt give you much information on the device which is pretty sketchy. It says 24v to 12v but it doesnt say what method it uses (switching, resistance, etc) so lets hope its a switching regulator

[benji]

anyways , supposing there is 50 amps runnin somewhere ,the circuit should be made to be able to allow that,,very big cables,,not sure if a pcb can handle that too
by the way, how are ya goin to feed this thing? i dont think you can drag as much as 50 amps from the wall ...

[Nyx]

anyways , supposing there is 50 amps runnin somewhere ,the circuit should be made to be able to allow that,,very big cables,,not sure if a pcb can handle that too
by the way, how are ya goin to feed this thing? i dont think you can drag as much as 50 amps from the wall ...

Batteries. 24V * 50A = 1200W... About as much power as a hair dryer. I have pretty large cables that were destined for car amplifier power which I think will do just fine.

[HDL_CinC_Dragon]

8 gauge should suffice.

[Nyx]

Do you guys know a good site to find cheap wire? Strangely, I find it's sometimes hard to get it for cheap.

[benji]

Batteries. 24V * 50A = 1200W... About as much power as a hair dryer

the hair dryer opertaes at 110 or 120 volts, not 24,, there is no trans in there so the current would be much less
,,but when u drag 50 amps from the wall,,not sure whats gonna happen

[Nyx]

Like I said, I don't use home AC power, but two 12V batteries. 50A at 24V is 1200W. The same wattage in a home AC circuit (120V) is only 10A.

[Nyx]

Like I said, I don't use home AC power, but two 12V batteries in series. 50A at 24V is 1200W. The same wattage in a home AC circuit (120V) is only 10A. 50A at 120V would be 6KW.

If you tried to draw 50A from a wall socket, you would burn a fuse/set off a breaker, that's what would happen.

[benji]

do you have a battery that supplies 50 amps???

[Nyx]

do you have a battery that supplies 50 amps???

Indeed I do. Two of them.

[paulstreats]

 lot of batteries can. But not for very long..........

[HDL_CinC_Dragon]

Car batteries are capable of dishing out a lot of amps for a very decent amount of time. Especially with a few of them in parallel

[airman00]

as well as wheelchair batteries