"Output Voltage: 48-440 VAC "
I'm pretty sure that they can't do DC, something to do with the method of solid state switching.
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I know the Jaycar AC ones can only switch AC, and only with a load on it.

Features
- 4000V dielectric strength
- Photo Isolation
- Built-In snubber
- TRIAC AC output
- Panel Mount
- DC Control, switches AC current only.

Thats the jaycar one, so presumably same with the other relay. Triac's only switch AC iirc.
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I tried a few AC ones I got second hand. On DC they switch on but wont turn off.
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yeah triacs need the AC waveform crossing zero volts to switch off.
Once you turn them on they stay on as long as there's a load on them.
It's a pity, because triacs are cheap and handle heaps of power
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Well you can do funky tricks with a capacitor and another triac, but it usually isn't worth the effort. The capacitor has to be large enough to stop the current flowing through the triac for a little while.

This is done in industry, but I would imagine it wouldn't happen on too many designs.

Heres a quick idea I just thought of, but I'll need
to think a bit more before I try designing something
as Im a bit short on cash and spares for it.

How about using the SSR to generate a PWM "power rail",
then use Triacs as the h-bridge switches ??
Then the low-time of the PWM rail will allow the triacs to switch off.
You would have to only switch the triacs on when power is low though.

Are the cheap Triacs powerful enough to manage that?

Any suggestions or pitfalls in this idea that anyone can see?
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Last edited by Grotto on Fri May 18, 2007 8:10 am; edited 1 time in total

Another reason Im asking the above is because Ive had an
idea rattling around the back of my skull for a single motor controller.

Using mosfets or an SSR to create a 0 to Full PWM power rail that
is commected to the motor via one double pole (contacts) relay for
forward/reverse, with the relay only switching between pulses to avoid contact-welding, with pulses being paused long enough for contacts to
settle first.

My question here is, do I understand correctly that the major cause of
contact-welding is due to heavy current WHILST contacs are closing?
Will pausing the PWM until contactbounce is (hopefully) finished going
to largely eliminate welding, assuming I stay inside the relays Amp rating?
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........CEO Cyberdyne Systems

Honestly, just buy an ESC, it will be easier. As Jake pointed out earlier, the SSR will only switch at a very low frequency and PWM modulation will be very poor or nonexistant. Doing all this is just putting one bandaid over another. The other thing with triacs is they have a relatively large internal resistance and voltage drop so at the low voltages and high currents we use, they are going to waste a good portion of your battery power.
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I have a set of home made controlers that use servo boards driveing fetts to switch the relays on and off i use caps across the relays to stop them arcing and also caps on the fet out put tothe coils to hold the voltage stable and stop them buzzing .
the fets switch the relays as soon as the controler is moved then the pwm out put from another bank of fetts goes through the relay contacts .

i used standard 30 amp relays and have pulled 70 + amps acros them with out any welding .


relays mainly arc when they are switched off as the current flow starts to jump the gap and they instantly weld .

if a suiatable capacitor is acros the relay points then the current has a temporary place to flow as the points open by the time the curent stops flowing into the cap the points are open far enough that the arc doesnt occur

the only real problem i ever had with these controles is seting a zero point between foward and reverse

I have used them frequantly in t2m driveing a pair of 300 watt winch motors whilst sitting rideing on the robot


I use drill triger fetts to switch the relay coils and the same type fetts used in the ibc for the heavy currnet although i use 4 in parralel for each controler they are fed to the gate through resistors and have drain resistors for the gates to discharge when pwm ing

And they work bloody good till we went for 24v! But I think we can fix that with a transistor trick
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@Grotto: Arcing is caused by the inductance in the motor.

[Long_Post]
[Educational]

Inductance is the basis of a switch mode power supply (don't leave home without one.) An inductor is a current storage device, just like a capacitor is a voltage storage device. Unfortunately, unlike voltage on a capacitor, the current is always moving. The current can NOT be changed instantaneously.

In a single direction speed controller (+ve - motor - Switching device - -ve) the switch is closed to get a current flowing through a motor. The current slowly ramps up in through the inductor, because it cannot change instantaneously. Then the switch opens. The current in the inductor WILL FLOW, until all the energy stored in it dissipates. There is no way to stop it. If there is nothing else in the circuit, then the current will cause a voltage increase until the current flows. This means that it will arc over the nearest thing to make a circuit. This could be your motor terminals, or your switch. If it's a physical switch then it will cause arcing across the contacts, with possible welding. If it's a a solid state switch, then it could be destroyed by the voltage spike (It blows a hole in the di-electric, which could short it out permanently). Bad .

So in the simple uni-directional controller, they place a diode in anti-parallel with the motor. This doesn't conduct when the motor is powered, since it is put in backwards. What it does is provide a path for the current to circulate, when the switch is opened. Therefore there is no arcing, and the switch survives to switch another day.

Now back to the speed controller with relay reverse. You can't use the diode trick, because, when the poles are reversed, the diode is a dead-short across the motor, causing excessive current to flow, destroying your switch.

Here you have two options: a few diodes in a different arrangement, or some capacitors. Diodes first because thats what I use . Instead of putting the diodes directly across the motor, they need to be put backwards (where the don't usually conduct) from the negative supply, to the motor terminals (one to each terminal). Then another set from the motor terminals to the positive rail. This will allow the current to flow around the motor, no matter what state the contacts are in, (even if they're switching).

The other way, with the capacitors, is to place them over the pole / contactor junctions. This allows a temporary current to flow through the contacts, as they are opening. This leaves a charge on the capacitor. When the contactor closes again the capacitor is discharged.

Both methods have their uses in industry, and their converts. The capacitor method can be seen in cars that still have distribution caps. It wouldn't last long without one. The diode method can be seen on most H-bridge configurations, ie the IBC. Also the mosfets in H-bridges have inbuilt diodes which aid in this process. (External ones are used to reduce heating.)

As you can see Aaron and Andrew like the capacitor method, and I like my diodes. In both cases, the components need to be sized correctly. If they are then your laughing

[/Educational]

Any questions?
As you may have noticed, I love this topic

[/Long_Post]

ah Motor Control - the art of suddenly moving large chunks of current around and dealing with the effects thereof - its like trying to contain dynamite. the bigger the boom, the harder you have to work to keep it where you want it.

It is a fascinating subject, its like Nuclear physics.. when you move into the realm of megawatts in microseconds, a whole new world of previously unsuspected effects start appearing to trip you up and make the magic smoke escape.

Good summary of the problem. The last time I delved into it, the tuned-snubber circuit is where my eyes started to glaze over - shunting the ringing frequency energy of the motors field collapsing, whilst not interfering with the primary PWM freqencies.

Problem is, it started to look like it was motor-inductance dependant which makes it very difficult to design a controller to work on a wide variety of motors of variable inductance The mix'n'match nature of non-production drivetrain combo's was like trying to design a tuned intake system that works no matter what exhaust you put on the car - impossible.

Any takes on that ?
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The controller we are making is sans snubbing. We are putting the grab the spiky bit part out at the motors. IE you run motor + and - and battery + and - out to the motors.
On the motor itself we are going with a diode setup to vbat + gnd and as many smt capacitors as will fit on the back of the can ;->

I don't think there is much need for tuning. Really what you are trying to do is make a low pass filter with the lowest frequency possible. (until you get into the region where your motor reacts faster than the caps but thats unlikely even with a thingap) Basically I think the goal is to provide the smoothest power with the fewest kicks to and from the motor.
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Trust the Plannerz to take driving a simple motor to new and extremely complicated levels
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