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Jaycar SSR and servo print.
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leo-rcc



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Jaycar SSR and servo print.

Hello all.

Yesterday during testing of my 6kg spinner robot I blew the speed controller that was controlling the disk motor. So we switched out the speed controller for one of the Jaycar solid state relays. But since I didnt have a rc switch available to me, we took the electronics of a servo, and put a resistor on it so it would power the steering side of the ssr only in one direction, and tested it again. After fiddling a bit with the servo travel on the transmitter we got it to work.

But what I noticed is that now the SSR is responding like a speed controller. because i used the electronics of a servo, it now pulses the solid state. Is this a problem for the ssr? So far it works fine bu I would like to hear others input on this.

the motor attached to it is a Graupner speed 700 Turbo C 9,6V overvolted to 12V.

Post Wed Mar 28, 2007 7:27 pm 
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dyrodium
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I don't think anyone has tried this before. Jake suggested it might be bad for the SSR but if it works? Shocked
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Post Wed Mar 28, 2007 7:55 pm 
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Nick
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Switching a FET SSR would probably lower it's current capacity but if you are drawing way less than 100A and you have a flyback protection diode, then it might just work - the proof is in arena testing and you may have a cool new bot controller on your hands Cool
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Post Wed Mar 28, 2007 8:00 pm 
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Spockie-Tech
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PWM'ing any variable resistance electronic switch will reduce its overall current rating

Instead of just switching into "saturated mode" (where its resistance is the lowest) and staying there, you are switching back and forth through the "partially conductive" region several thousand times a second.

Partial conduction = heat generation. The longer you take to "transit" this partial conducting mode, the hotter your switch will get. This is why high power controllers are all about the "gate drive" of your mosfets. Since Mosfet gates are capacitive, forcing them to suddenly swing from one voltage (off) to another (on) very quickly takes a *lot* of "push" from the drive circuit to change the charge state of the gate capacitance.

Servo-Motor electronics are designed to PWM a tiny little motor that would probably be lucky if it managed to suck 1 amp of current. So the drive to the SSR's input is not likely to have a lot of oomph behind it, meaning fairly long switch times are likely, which will mean the SSR will get hot at a rating much lower than its "saturated on" rating. How much, depends on what the input capacitance of the SSR is.

Generally high-power Fets have higher gate capacitance (larger channel to effect with the field) and are thus harder to switch quickly (requiring more gate drive) than low power fets. Just like throwing a huge 50 amp switch takes more effort than a little 1amp toggle does. Paralleling multiple fets increases the gate capacitance just like parallel capacitors does, with the same slow-down effect.

So, thats the theory behind why PWM'ing a switch with a low-drive capable circuit will reduce its maximum current. How much so depends on how fast the drive can switch the load capacitance.

The only way to tell for sure how much without a boat load of data sheets and a calculator is to get out a *fast* oscilloscope and look at the "slope" on the edge of the square waves PWM'ing the input. Squarer, nice vertical edges is good, slopey gentle curves is bad. (long time in the transitional zone).
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Post Thu Mar 29, 2007 2:14 am 
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Valen
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the ssr is optically isolated (i believe) and requires something like 4mA of drive current.
They have their own fet drive circuit.

I believe that the maximum PWM frequency of the SSR is something in the 100hz region or less according to the specs on their website.
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Post Thu Mar 29, 2007 10:29 am 
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Valen
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actually i can see another problem. The servo will drive the motor both ways IE it will reverse the polarity of its outputs. I don't think the SSR will like that. You might need to add a diode or something there "just in case"
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Post Thu Mar 29, 2007 10:43 am 
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prong
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Dumhed and I talked about using these as a dodgy speed controller, PWM them at like 20hz, while its going to be a little jerky at low speedy and sound bad but it should still work ok.

Post Thu Mar 29, 2007 5:16 pm 
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dyrodium
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you'd need two per side, making that four per robot. At $40 ea, that's a $160 not very good speed controler! Laughing
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Post Thu Mar 29, 2007 5:19 pm 
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prong
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could use relays to reduce that, but yes, not that cheap, but wokring at Jaycar they are a lot cheaper for me Wink

anyway, it wasnt worth doing, but an interesting idea

Post Thu Mar 29, 2007 5:21 pm 
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leo-rcc



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quote:
Originally posted by Valen:
the ssr is optically isolated (i believe) and requires something like 4mA of drive current. They have their own fet drive circuit.


Thats what we measured as well. We measured the power consumption to open the SSR and it was so low we figured we might get away with it. We also made sure the current input is only one way so we never reverse the input. And it is indeed optically isolated according to the pdf with specs.

The pulsing was something we did not count on (that was an accident) but I am not complaining if it works.

From what i gathered from other websites depending on the various types of servo, the pulses are 1 to 2 mSec, and spaced between 2,5 and 20 Msec apart. According to Jaycar opening and closing the SSR takes 0.5 mSec so unless I am mistaken this should work. If not, please tell me, i have an event coming up in 2 weeks and I would like to know in advance.

Post Thu Mar 29, 2007 5:49 pm 
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leo-rcc



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btw, i am not using this for drive, but to spin a disk, so it only needs to go in 1 direction anyway.

Post Thu Mar 29, 2007 6:04 pm 
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prong
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The 5ms switch time is maximum time, so it might actually switch faster

Post Thu Mar 29, 2007 6:40 pm 
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DumHed
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I think it will work fine, but do some testing and keep an eye on how hot it gets.
If you're worried about it, just make sure you push the stick to 100% quickly so it's not doing too much PWMing Smile
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Post Fri Mar 30, 2007 12:51 am 
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leo-rcc



Joined: 04 Aug 2006
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During testing on the bench during spindown the dreaded thing happend. diskmotor would not start up again. Naturally I assumed it was the SSR of the servoprint, but it turned out it was the motor itself that failed.

Post Fri Mar 30, 2007 6:40 pm 
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Knightrous
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Since I couldn't find the original SSR thread, I'll post it here.

In my quest to find some components I'm looking for, I stumbled across some different version of the SSR (Which is becoming popular now). Futurlec have a bunch of different versions for fairly good prices. They have a Aussie branch located in Sydney too Wink

I found a 70amp version. It's not a DC/DC one like the 100v/40amp Jaycar one, but is suppose to be for 440v/70amp AC switching. I'm not to sure what the difference would be, but I thought I'd post it up anyway.

The SSR70A
(http://www.futurlec.com/Relays/SSR80A.shtml)


quote:

Input
Input Voltage: 3-32 VDC
Input current: 20 mA(@32VDC)

Output
Output Voltage: 48-440 VAC
Maximum Load Current: 70 A
Maximum Surge Current: 1000% of rated current *** 700amps peak!
Maximum on-state Voltage Drop: 1.7 VAC
Maximum Off-state Leakage Current: 10 mA
Minimum off-state (dv/dt): 500 V/µS
Frequency Range: 47-63 Hz
Max turn-on time zero-cross Turn-on: 1/2 cycle+1ms
Maximum Turn-off Time: 1/2 cycle+1ms

General
Dielectric Strength: 4,000 VAC 1 min
Minimum Insulation Resistance: 1000 Mohm@500 VDC
Ambient Operating Temperature Range: -30°C to +80°C



This might be good for you Nick, since your running 2 x 40amp versions to be safe, you could probably replace it with a single 70amp version, saving a bit of space and weight Smile Anyone see a problem with it? Designed for AC Switching an issue?
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Post Thu May 17, 2007 10:04 am 
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