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Old 01-05-2010, 08:47 PM   #1
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Default Electrical help needed! DC Relay

Fellas,

I have an Omron SSR (solid state relay), SPST, #GNA-D210B, digi-key #Z168-ND. The print on the top says "5-200VDC 10A". Does this mean it can handle up to 166A @12VDC? Seems unlikely, but I hope it can handle about 40amps. I want to use a SSR because I plan to use a cheap (ebay?) PWM speed controller to control a 12VDC dump truck tarp motor. I'm thinking a standard bosch style relay wouldn't be ideal in PWM applications, clicking like crazy. Because this Omron SSR is SPST, I will still probably run the power & ground thru a standard DPDT relay to change directions.

I guess I could try it and find out but I know there's a lot of you who are qualified to answer this. Or perhaps you know of a cheap PWM speed controller that can handle ~40amps @ 12VDC w/o a relay.

As a side note, as i was typing "painless wiring" into google to see what they have for relays, the first entry in google's autocomplete was "painless ways to die". Sign of the times, eh.

Thanks...
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Old 01-05-2010, 11:25 PM   #2
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it can probably handle 10A at any voltage from 5 to 200VDC.

and i could be wrong, but PWM isn't DC. It's AC.

edit: I'd check into the motion control industry. there's probably milling machine motor controllers or somethign that would work
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Old 01-05-2010, 11:37 PM   #3
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PWM is a technique for approximating a variable voltage without using linear regulation. PWM can be applied to either AC or DC supplies. The output of a PWM circuit resembles an AC waveform in some ways, however in a simple PWM system with a single-ended positive power supply, there is never a negative voltage at the output. AC supplies, by definition, swing between positive and negative.

And you're correct on the rating. 10A at any voltage from 5 to 200.
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Old 01-06-2010, 09:13 AM   #4
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Thanks guys. I figured it was 10amps max regardless of volts, but hoping it was rated at a specific power, not current. I'll figure something else out if I need speed control, but I had this relay already and if it had the rating I'd try to incorporate it from the beginning.

It's my understanding (assumption) that a simple PWM speed control is nothing more than a high speed switch, turning on & off a circuit at XXX Hz, and by varing the duty cycle (% on time per cycle) you get effective speed control. Just like the IAC in our cars, or boost control solenoid, etc. Those are all DC, obviously.
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Old 01-06-2010, 12:11 PM   #5
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the PWM essentially uses the motor as a lowpass filter.
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Old 01-06-2010, 12:13 PM   #6
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You can probably find what you're looking for (bi directional PWM motor controller) marketed as an HHO controller.

I was thinking model railroad stuff at first, but your current demands are too great.
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Old 01-06-2010, 12:15 PM   #7
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So you're finally building an NA automatic soft top? Sweet! I'll buy one once you've worked out the kinks and get it to raise/lower the top in less than 10 seconds.
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Old 01-06-2010, 12:29 PM   #8
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Quote:
Originally Posted by Ben View Post
You can probably find what you're looking for (bi directional PWM motor controller) marketed as an HHO controller.

I was thinking model railroad stuff at first, but your current demands are too great.
Right, hence me trying to use a SSR with enough current capability.
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Old 01-06-2010, 02:21 PM   #9
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You can get MOSFETs which are rated for ludicrously high drain-source currents. They need a fair bit of heat-sinking to keep the case temp at sane levels, but they'll do it.

Examples:
http://www.onsemi.com/pub_link/Colla...NTB5412N-D.PDF
http://www.st.com/stonline/books/pdf/docs/8260.pdf
http://www.fairchildsemi.com/ds/FD/FDA8440.pdf

Now, obviously things like reverse transfer capacitance and switching time start to become issues when you're thinking at this power level, and will need to be considered in part selection and switching driver design. I haven't specifically researched any of the devices I'm throwing up here, just giving ballpark examples. A co-worker of mine is pretty much a minor deity when it comes to switching supply design, if you'd like I can show him the details of your application and see what he suggests.

What sort of circuit are you planning to use for the PWM drive?
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Old 01-08-2010, 12:26 PM   #10
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Thanks for the offer Joe. I am building a powered trailer dolly so I can move my boat around easier and into my garage which is pretty tight (a few in clearance width wise). I am a man and am pretty damn good at backing up trailers, but my garage is behind my house on a single lane back alley, and the driveway is basically the length of the boat. Now with this landyacht of a subaru outback as my tow vehicle it takes a while and a lot of slipping the clutch to straighten out the boat into the driveway while the car is always at a sharp angle to the boat in the alley. The simplest solution would be to just use the miata to back the boat into the driveway & garage but that's not as fun as building something instead and I'd still be blocking the alley with the car. I try to be a nice neighbor.

I am trying to do it on a budget and not go too overboard. Here is a commercially available one for around $1000, I'm trying to stay at 1/4 cost but that ain't gonna happen. All Wheel Drive Battery Powered Trailer Dolly

I decided on DC power for no good reason other than it's simple & cheap in my head to reverse direction & control speed, plus I have a not-so-good optima from the miata that I think will work fine powering the motor.

The motor I ultimately purchased is this one: DUMP TRUCK TARP SYSTEMS GEARMOTOR, MOTOR, GEARBOX & CVR:eBay Motors (item 220418742691 end time Jan-15-10 10:41:09 PST) I got it for $100 shipped. It's basically a wiper motor scaled up 5x. Huge & heavy. There's no specs on it, but similar units online say roughly 30amps & ~800in-lbs @ 90:1, this one is 60:1 so I'll assume ~500 in-lbs. It also says 210W on the case, so that's 17.5a. Very well built with SS hardware too. This will give me about 400lbs of pulling force @ 44ft/min with the drivetrain I'm planning, should be ok. If not, I will play around more with gear ratios. Anyway...for now I was/am planning on using 4 automotive (12vdc, high inrush current) SPST relays to control direction, as I cannot find any DPDT 12vdc high current relays. Going to use a simple momentary SPDT on-off-on toggle switch to actuate the relays/control direction.

I don't think I'll need to control speed but if I do then I'll worry about that later. There's a few unknowns that I won't know until I build it and try so I might as well do it.

The budget looks to be about $300 so far, not including the optima which no longer starts the miata but holds ~11.5volts fine. I am also reusing an old gocart tire/hub assy but switching to 35-2 chain/sprockets.
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Old 03-01-2010, 07:45 PM   #11
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Quote:
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I am building a powered trailer dolly so I can move my boat around easier and into my garage which is pretty tight (a few in clearance width wise).

The budget looks to be about $300 so far, not including the optima which no longer starts the miata but holds ~11.5volts fine. I am also reusing an old gocart tire/hub assy but switching to 35-2 chain/sprockets.
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Old 03-01-2010, 07:51 PM   #12
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awesome....new name? Trailer Tim?
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Old 03-01-2010, 07:56 PM   #13
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Ha! What an awesome little gadget.

So, what did you end up doing (if anything) for motor control? I see a lot of wires, but can't make out much detail. There appear to be two standard SPDT automotive relays and a terminal strip, with a toggle switch at the handle- so I'm guessing just forward, reverse, and off, with no speed control?
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Old 03-01-2010, 08:20 PM   #14
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Quote:
Originally Posted by Joe Perez View Post
Ha! What an awesome little gadget.

So, what did you end up doing (if anything) for motor control? I see a lot of wires, but can't make out much detail. There appear to be two standard SPDT automotive relays and a terminal strip, with a toggle switch at the handle- so I'm guessing just forward, reverse, and off, with no speed control?
Thanks Nick & Joe More pics just uploaded onto the site.

It's 4 automotive/50amp relays, each pair switches the leads on the motor. toggle switch turns on each pair of relays, for forward and reverse (and off). I could have done it with 2 relays by using the NC portion of the relays and 1 relay to turn on and off the power, but the normally closed side of the relay was only rated to ~20amps or so. So one pair closes the power and ground circuit for one direction, the other pair closes the power and ground circuit for the opposite direction.

No speed control. It goes pretty slow as is, I think about 40feet/min. I don't have the calculations in front of me on speed, torque, and linear pull, but it should me more than enough to move my boat around and up into my driveway, assuming I have enough tongue weight. I lowered tongue weight to around 200lbs so my impreza wagon could tow it, I don't know if that'll be enough. I might have to have meg stand on top of the ball I'm currently using a 14/60 tooth combo and can probably get away with a 9/60 if I need more torque.
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