Author Topic: Solid State Relays, Uses, and Trigger Circuits  (Read 7586 times)

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plasmahunt3r

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Solid State Relays, Uses, and Trigger Circuits
« on: December 10, 2016, 09:05:43 PM »
I wanted to start a discussion on the possible uses for Solid State Relays, and Trigger Circuits.  Especially, DC-DC SSR's. Off hand I can see them used as LOW VOLTAGE DISCONNECT, SHUNT REGULATOR, DUMP LOAD CONTROLLER. and Battery Isolator.

First of all, the DC side has a built in circuit with Opto Isolator, and built in resistors.  Watch the following youtube video on a tear down of a DC-AC SSR.  Pay particular interest to the DC side of the circuit.

https://www.youtube.com/watch?v=DxEhxjvifyY

I wanted to set up a DC-DC SSR as a Low Voltage Disconnect.  I do not want to destroy a $500 battery by leaving a light on.  Since I use AGM batteries, I have to deal with the following requirements.

Voltage for AGM batteries:

Flat Battery is 11.8V
Less than 10.8V voids warranty

Fast charging rate up to 14.8V
Float charging rate 13.4V - 13.8V


Relative AGM Battery Capacity:

100%         12.7V - 13.2V
 75%         12.4V
 50%         12.2V
 25%         12.0V
Discharged/Flat   0V-11.8V

I need to make sure I never go below 11.8v, so I created to following circuit to read the voltage being controlled by the SSR and turn off the SSR when the voltage drops below the cutoff voltage.  The circuit is small enough to mount on top of the SSR.

I first tap the positive Vin from the "DC Load" side of the SSR.  When the Voltage in greater than 11.9v, the SSR is on, and passing voltage/current to the load. 

The way the circuit works is:  The positive Voltage passes through the 2K Resistor, the Zener(11.09v), and the Schottky Diode(.22v).  If the Vin was 13V, then the Zener Diode would conduct, and 11.31v would show across the Zener Cathode and the Negative Rail.  The Voltage across the 2K resistor would Drop to 1.69v.  This 1.69v is passed to the PNP transistor (Base-Emitter voltage, 1.69 and 13Vin).  The 1.69 is more than 2 volts less than the Vin, and the PNP transistor turns on and passes positive voltage to the DC Controller side of the SSR and turns the SSR on.  Trigger Voltage is combined:  Zener Voltage (11.09), plus Schottky Diode (.22v), plus PNP Voltage (.6v), totaling 11.9v.  The Schottky Diode was added to get the Trigger Voltage where I needed. 

I could have used a TL431 Precision Zener to get an adjustable Trigger Voltage, and I will use it for the Dump Load Controller and the Shunt Regulator Circuit.  This first circuit was a prototype to get the bugs out.  An adjustable circuit would allow for changes between Gel Cell, AGM, and Lead Acid Batteries, while using the same circuit. 

In a future post, I will show the Dump Load Controller, which triggers when the batteries reach Float Charge of 13.8V.  The SSR would cycle on/off may times a second to maintain the Float Voltage.

george65

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #1 on: December 11, 2016, 08:05:48 PM »
This is something I have been thinking about for a couple of weeks to use on my play solar setup.
I have ordered Some various SSR's  and just waiting for them to arrive.  I'd be real interested to see more of what you have come up with so I can learn from it and do something for my own needs.

plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #2 on: December 14, 2016, 02:39:00 PM »
Just a tech note on DC-DC SSR's:

A DC-DC relay uses a MOSFET to control the load.  This MOSFET will have a DIODE to protect the MOSFET from back emf when handling an induction load, like a motor.  You can use this DIODE to verify which terminal connects to the battery.  The battery needs to connect to the CATHODE end of the DIODE.

First Test:  To test the SSR for correct polarity, hook up a volt meter with the negative lead to the battery negative and the positive lead to one of the two SSR load terminals.  Then hook a wire and a 2K resistor from the battery positive terminal and touch the other SSR load terminal. 

If the voltage shows on the meter, then the battery positive is connected to the DIODE ANODE.  If you hook up the battery to this terminal, then the SSR will be "ALWAYS ON", which is not correct.  You want the SSR, to only be on, when the control circuit turn it on.

Second Test:  Now swap the battery wire and the positive volt meter lead on the SSR.  You should not see any voltage (or maybe a few hundredths of a volt, due to diode leakage).  If no voltage shows, then this is the correct terminal to hook up the battery.  The SSR will only be on when the control circuit turns it on.

OperaHouse

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #3 on: December 14, 2016, 02:52:41 PM »
Some care should be taken, it is pretty easy to turn on a FET with with just the finger resistance from the metal heatsink tab to the gate due to the very high impedance.

plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #4 on: December 15, 2016, 11:55:47 PM »
Here is the circuit for using SSR as Dump Load Controller.

You need to set the voltage of the TL431 before adding the adding the transistor to the circuit.  Trigger voltage, is voltage setting at cathode of TL431, plus .6v PNP Transistor.  Once the Transistor is included in the circuit, you cannot get a good voltage reading between negative and Tl431 cathode.

The Pot and the Resistors for the Voltage Divider take up a little more real estate on the PCB than a Zener, so I am not sure I like it better.  I could just use a 13v Zener, plus Schottky Diode (.22v), and do the same circuit as the Low Voltage Disconnect circuit, just make the load the Dump Resistors.



I am having a problem with this circuit.  I am not sure to add the .6v for PNP Transistor.  I have a second meter on the battery, and it looks like, that as soon as the battery reaches the TL431 Voltage setting, the PNP turns on.  For Example, If I had TL431 set to 13.2V, then it looks like, that at 13.3V the PNP Turns on.  So, the TL431 voltage may the only trigger voltage.   Anyone have any suggestions???

« Last Edit: December 16, 2016, 09:39:40 AM by plasmahunt3r »

OperaHouse

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #5 on: December 16, 2016, 01:50:36 PM »
Zeners have a rather sloppy transition, the TL431 is quite sharp. That transition could probably be made sloppy by adding a resistor to common from the 431 to add some negative feedback.  The B-E resistor should be made smaller, all the current goes through the transistor now. Frankly, there is far too much gain in the system now.  The transistor is not needed as the 431 can easily drive the SSR.

I have grave concerns about using these FET blocks.  They avoid mention of it, but these are quite slow.  Internally they use some variation of the VOM1271 to provide drive and isolation.  These are pretty slow with transitions  longer than 50uS.  And that is before driving some large gate capacitance of 2,000pF or more. With a drive capability of only 15uA that could get quite slow.  FET heating could be severe with a lot of transitions.

It took a long time before I appreciated the GHURD controller. On a quick look I thought the IC was just a house number 431.  I had someone want to give me a box of 2,000 of a similar part.  I couldn't come up with a possible use for them and refused a free item.  Of course I never imagined at that time anyone would ever want a shunt regulator.

plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #6 on: December 16, 2016, 02:48:08 PM »
Thanks Operahouse.  I am a little confused about the resistor comment.  The R3-2K resistor is necessary to handle the voltage drop.  It serves two functions:

First, when the Tl431 is not conducting, it pulls up the PNP base to keep the Transistor off. 

Second, it handles the voltage drop when the Tl431 is conducting.  This voltage drop is what turns on the PNP and it is also the base resistor that limits the base current to the PNP. 

Can you show a sketch of how a TL431 could drive the SSR directly.
« Last Edit: December 16, 2016, 03:05:20 PM by plasmahunt3r »

OperaHouse

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #7 on: December 17, 2016, 09:23:48 AM »
I've designed circuits for 50 years.  You know what, I'm tired of it. Tweeking resistors,
adjusting pots, compensating for variability of parts, it used to be fun. Now just about
anything you could build is available dirt cheap fron China in some sub assembly. Anything
really custom can be built with these little micros. It's the size of my thumb to my knuckle.
One runs my entire house, refrigerator two PWM water heaters with temp controls, pump,
transfer relay, and two MPPT charge controllers. The charge controllers are just a FET,
diode, inductor,capacitor, and opto isolator driver. Run an entire house for $2.50. These
boards have a built in power regulator, USB communications port and the assembly can be used
as a breadboard, just solder resistors right onto it. If you have a laptop and can find a
USB cable the total investment is under $3.  If you can load it in, winner winner chicken
dinner. That costs nothing.

Take a look at what you want to do. The battery is read in and converted to a voltage in
three lines of code. Set the PWM frequency to something slow like 20Hz that the block FET can
handle. That voltage is fed into one line of code called MAP. At 14.2V and under it outputs
a PWM of 0%. At 14.6V it outputs a PWM of 100%.  Anywhere in between the it is proportional.
And you didn't need to know nuttin to make that calculation.  Now I don't like short pulses.
They just cause heating. Another two lines of code makes any PWM under 10 a zero and over
245 100%.  Don't like how it runs, change a number or two. Once one is running, you'll never
go back to the old way of doing things.

Anyone who makes a cad drawing is tech savvy enough. I need some help on this board.  For seven
months of the year I have no way to do any real world tests on this stuff. Just think you would
really enjoy working with these and the projects would have really sophisticated features. A
couple boards is the price of a good cup of coffee.


http://www.ebay.com/itm/1PC-MINI-USB-Nano-V3-0-ATmega328P-CH340G-5V-16M-Micro-controller-board-ArduinoO9-/302163571894?hash=item465a5a34b6:g:VzgAAOSwM4xXaeio

With the three phase project, triggering didn't need to be sophisticated.  Any error was quickly
averaged out.  The FET with DC can't take an analog signal.  That will just cause heating. Something
has to pulse it on and off.  A TL494 switching regulator chip could do that.  It even has a built
in reference and can be set to a low frequency. A 100A FET only has the power dissipation of about
25W even with a good size heatsink due to thermal resistance. Multiple FET can be placed in parallel
if precautions are used to make them share current. An analog step function could drive multiple
FET with individual resistors.  That all gets into a lot of parts, the reason why PWM is so popular.
The SCR blocks are a nifty device that is easy to implement in AC. I just haven't found anything
useful that can be done with the FET counterpart.

On further contemplation I think your circuit needs to oscillate.  This can be done with the addition
of a single resistor in the common line of the opto in the original . Power the opto with a resistor in series and determine what value will produce 10-20mv. This will produce positive feedback in the reference divider of the 431 making the turn off voltage lower. That will insure the FET turns on for a period of time. Temporarily using a pot will help determine the best amount of feedback. Resistor is likely 4.7 to
10 ohms.

Christmas is coming up. If you don't have one I suggest buying yourself a USB scope. A client
purchased one for a project a few years ago that was later abandoned. Small, portable and
electrically isolated when used with a laptop.  For $65 it comes with two probes, suggest buying
a 100X probe as it is easy to overload the input at moderate voltages. It prevents you from accidentally switching from 10X to 1X.  Anytime you see a perfect square wave, the scope is likely overloaded. With this you can get time and voltages easily of a waveform. This willl allow you to share data with others by creating jpg files. I love using mine at the camp.


plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #8 on: December 17, 2016, 08:47:08 PM »
I found the problem in the Dump Load Controller circuit.

I hooked up a .75a battery charger to the battery, which charged the battery slowly, and I monitored the Battery Voltage and the PNP collector voltage with separate volt meters.  This is what happened.

As soon as the TL431 started conducting, the PNP turned partially on.  As the battery voltage increased, the PNP collector voltage increased until it was fully on.  So the .6v PNP requirement is still there, but on a sliding scale.

I need to redo the circuit.  I have ordered some TIP41C (NPN) as a compliment to the TIP42C (PNP) transistor.  When they arrive, I will have to use the PNP as a trigger to turn on the NPN, and the NPN transistor will turn on the SSR.  I should not have to care about the .6v.  Whenever the PNP turns partially on, it will trigger the NPN.  So I should be able to rely on the TL431 setting only.

While I am waiting, I might try a P-channel MOSFET, to see if the results change.

Kind of reminds me of a joke.  You know how an engineer diagnoses a flat tire.  He moves the tire to another wheel and see if the symtoms change.


UPDATE:
Had some P-Channel MOSFETs in my parts box (IRF9540N).  They seem to work in the existing circuit replacing PNP. but Gate Threshold Voltage is -2v to -4V max.  I just lowered the TL431 setting until I got desired result (13.78V).  With less than 100NS turn on/off time, the switch seemed instant.
« Last Edit: December 17, 2016, 11:20:50 PM by plasmahunt3r »

plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #9 on: December 19, 2016, 11:25:29 PM »
I tested the absolute minimum requirements to activate an SSR.  You can use a Zener Diode to drive the controller side of the SSR, by passing positive voltage when Zener breakdown voltage is achieved.  No Transistor is needed.

Here is a sample Dump Load Controller circuit using a 12v Zener.  The way the circuit works is, when battery voltage increases past 12v, Zener Breakdown voltage is reached, and the Zener starts passing positive voltage to the SSR.

Example, if the battery voltage is 13v and the Zener is 12V, 1 volt will pass to SSR.  An SSR can start to activate with as little as 2V, so when the battery reaches 14V, the SSR is activated.

This is based on an ideal Zener.  Zeners are rated at +/- 5%.  My 12V zener was actually 12.4v.  If you use this circuit, check your Zeners and pick one you can live with.  Or you can use TL431 and get a more accurate setting.

All parts are different, so your results may vary
« Last Edit: December 20, 2016, 10:45:12 AM by plasmahunt3r »

DamonHD

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #10 on: December 20, 2016, 07:51:43 AM »
I'd caution that SSRs are unlikely to behave exactly the same for this purpose, so you'd need to test the combination IMHO.  But simple can be good; there's always time time add complexity later!  B^>

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plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #11 on: December 21, 2016, 08:20:52 PM »
Just a note on using TL431 (Precision Adjustable Zener).  There is leakage.

I did some more testing on the Dump Load Controller circuit using the TL431 and PNP Transistor. I preset the TL431 to 13.00v.  When using TL431 as an Adjustable Voltage Regulator, it works just fine.

Then I soldered in the Transistor, and while the battery voltage was 12.55v, the PNP Transistor Collertor Voltage read 1.46V.  While the battery voltage was below the TL431 threshold, no voltage should have shown up on the PNP collertor, but there was some.  Leakage

When the Battery Voltage increases to 13.07 V (only 7/100 past the TL431 setting), the PNP collector voltage increased to 2.00V.  Since the SSR conducts with as little as 2V, the SSR is on. The PNP never achieved full voltage, just an increasing voltage as battery voltage increased.

I suppose I could live with this, and disregard any voltage requirements for the PNP Transistor.  Just set the TL431 to the desired level and be close enough to the actual trigger voltage. 


I did not experience this with regular Zener Diodes.  At least it was different.  When using Zener Diodes, the PNP Collector Voltage Starts increasing when the Zener is within 10/100 of a volt of the Zener setting and at full voltage on PNP Collector when about 10/100 past the Zener setting.


You can't just swap out Zeners with TL431.  They don't work the same.

I have been doing some research.  In TL431, when the Battery Voltage (set by voltage divider) is higher than the reference voltage (2.5V), the TL431 Cathode should read around 2V (my 1.46V is close).  When the Battery Voltage (set by Voltage Divider) drops below the TL431 Reference Voltage (2.5V), the TL431 should stop conducting and the TL431 Cathode read full voltage, and turn PNP off.  There should be some way to make TL431 work as a trigger.  Challenging problem. 
« Last Edit: December 21, 2016, 09:10:28 PM by plasmahunt3r »

JW

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #12 on: December 21, 2016, 09:20:51 PM »
Not sure if this will work with your application

SCR's need a sinusoidal wave (sine wave) to un-latch.

I use a Mosfet rated at 40 amps for my switch control.

NTE  RS3-1040-41M
 
SSR-40A.DC.MOSFET

there kind of expensive thou


joestue

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #13 on: December 21, 2016, 10:18:19 PM »
The simplest way to go about this in my opinion is a 100K resistor, from gate to source, a Zener of arbitrary voltage from drain to gate, and you've got yourself a shunt regulator.

Mosfets are now being sold with said zenar internal to the mosfet itself to enhance avalanche capabilities (doing away with avalanche failure altogether, because the zenar turns the fet on, causing it to operate in the linear region)

To get effective current sharing with other dump loads or other mosfets you will need a resistor in series with the source. most of the heat will go into the fet. If the resistor is significantly large then the "stiffness" of the dump load will be weak because the voltage across the source resistor is subtracted from the zener voltage that turns the fet on.


a different but slightly more complex circuit is as follows:

An additional zener to protect the gate from exceeding 10 volts is connected directly from gate to source, a resistor in series with the zener from gate to drain(10K would be about right), and your dump resistor in series with the drain makes a more conventional dump load. Most of the heat will go into the drain resistor in this case and you can parallel as many as you want. different voltage set points can be selected to achieve a different "stiffness" or load profile.

make sure you put a snubber across the mosfet in this case. you will need to find the right zener voltage to get what you want, as no adjustment is possible.


if you try something like this with a solid state relay i would make doubly sure that it is not operating in any linear region where its power dissipation will be far higher than it was designed for.
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plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #14 on: December 22, 2016, 02:37:01 PM »
Hey JW.  I was not planning on using DC-AC SSR's, which do have SCR's.  I am using DC-DC SSR's, which use MOSFETS.  You can buy 100 Amp DC-DC SSR's from places like Missouri Wind and Solar for around $26 and it comes with a huge Heatsink.

The video was just for understanding on how SSR's are put together, and in particular, how the DC controller side works.  My goal is to combine enternal components with the internal circuitry to control the SSR. 

If I combine an external voltage trigger with the existing internal circuitry, it can accomplish many different tasks, such as Low Voltage Disconnect and Dump Load Controller.  It is easy with Zener's and a bit more complicated using TL431.  Still working on TL431.

Using SSR's as Shunt Regulator and Battery Isolator would need external Diodes because of the reversed Diode imbedded with the MOSFET on the DC load side.

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #15 on: December 22, 2016, 06:40:18 PM »
Here's what I use in parallel with my mosfet output,  its a Zeiner diode but it is classified as a transient suppression diode.

http://www.mouser.com/ProductDetail/Littelfuse/5KP100A/?qs=HR2RnyOI4E7d2eNVd0xJcw%3D%3D

plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #16 on: December 23, 2016, 09:39:26 PM »
Took a while, but I finally learned the secret to using TL431 as a trigger for SSR.  I searched the internet and found an article on configuring the TL431 as a Schmitt Trigger.  The process involved setting up the TL431 as a Shunt Regulator driving a MOSFET as the Shunt.  The Load is a 100K resistor so the circuit draws little current on it's own.

I built my own version of the circuit and it worked beautifully.  There are some tricks to configuring the circuit.

Trigger Voltage has to be set before use, by hooking up the External Circuit, to a higher voltage (I used 14.3v), and with a temporary 180 Ohm resister between Battery Positive and the External Circuit Positive Vin.  Then adjust the POT until you achieve the desired trigger voltage.  The voltage reading is taken at P-ch MOSFET Source Pin.   This 180 Ohm Resistor is used to handle the voltage drop for the TL431.

You cannot leave the 180 Ohm Resistor in the circuit.  This is temporary and used only to set Trigger Voltage.  If this is left in, the circuit will not provide enough current to drive the SSR and Vout will not be full voltage.       This is the Trick to getting TL431 to work.        With the 180 Ohm resistor removed, the MOSFET will turn on at Trigger Voltage and Full Voltage will be applied to the SSR.

I am not using any Current Limiting Resistors in the Circuit.  I am relying on the Internal Resistors within the SSR to limit the current within the SSR.  I also went to a 10Uf Capacitor.  I Tried a 1Uf Capacitor and the Trigger Voltage fluctuated a few hundreths of a volt but 10Uf kept it stable.

Once the Trigger Voltage is set, you can hook up the circuit to the SSR and use it for either the Dump Load Controller or the Low Voltage Disconnect circuit.  Just set appropriate voltages and configure the SSR's as required.  For Example, Set Trigger Voltage to 11.80V and configure as LVD or Set Trigger Voltage to 13.80V and configure as Dump Load Controller.

This was a challenging problem.  Special thanks to the Internet and that anonymous person who figured out how to use TL431 as Schmitt Trigger.


« Last Edit: December 24, 2016, 09:13:24 AM by plasmahunt3r »

OperaHouse

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #17 on: December 24, 2016, 01:21:15 AM »
That is a variation of what I told you, adding positive feedback.  You have the 100K resistor going to the wrong place.  You are on the right road now.

plasmahunt3r

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #18 on: December 28, 2016, 01:14:51 PM »
I was looking at my TL431 Trigger Circuit and I realized that the IRF9540N MOSFET I am using is rated for 100V and 23A.  Not bad for a 77 cent MOSFET.  Depending on your load and adequate heatsink, you can use the external circuit directly to run you load and omit the Solid State Relay.  The main reason for using the Solid State Relay is the High Amp MOSFET and the large Heatsink.

For example, if you set the Voltage to 11.8V, and you provide a large heatsink, you can use the Tl431 Trigger Circuit as a LVD circuit and run things such as Lights, VHF Radio, or whatever.  Just know your load requirements.

Be concerned with inductive loads.  For instance, Marine Refrigerators are very efficient and run in as little as 1.5A.  So on the surface, the IRF9540 would be more than adequate to run my refrigerator.  However, when Compressor Motors kick in, there is a large surge current and that surge could destroy your MOSFET. 

If I were to use the TL431 Trigger Circuit by itself for Refrigeration, I would add a large Resistor on the output side of the MOSFET (say 6 Ohm 25W) to limit current for the running refrigerator (up to 2A) and a large 2F Audio Capacitor to handle the surges.  Also I would add a large Heatsink for the MOSFET and make sure it had adequate ventilation.
« Last Edit: December 28, 2016, 05:53:58 PM by plasmahunt3r »

george65

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #19 on: December 30, 2016, 10:30:36 AM »
Hope this is not too off topic but can anyone tell me if the arduino boards can direct Drive one of the square white SSR's that come in 10 - 40?A Range?
the DC Side is 3-32V but not sure what amps are needed to switch the the high side of 24-380V

Want to drive one of them in PWM mode but I would like to know if I have to drive them with one of the smaller 2A SSR's First or if the Arduino is up to driving them direct?

madlabs

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #20 on: December 30, 2016, 10:42:16 AM »
George, the Arduino should run the SSR just fine. It sounds like you are switching AC with them and plan to PWM it so you should know that many SSR's are what they call "zero crossing" which means that when you turn them off or on they wait until the next time the AC voltage crosses zero before they turn on/off. You can still PWM them but you may not get the exact duty cycle you intended. Works fine for most applications. You can get "arbitrary" ones that will turn off when told if your application needs that.

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OperaHouse

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #22 on: December 30, 2016, 12:17:31 PM »
Depends on what you call PWM. A triac or SCr will trigger and stay on.  In the case of random firing SCR the standard PWM of 480 Hz really can not be used.  When ever two frequencies are mixed you will get sum and difference of the two frequencies.  Triggering that latches on makes that even more complicated.  In my younger years I used this property to make light shows.  If a long PWM time is used like one second, it will work quite well or if you trigger on the zero crossing with the UNO.  I did a test with standard uno PWM and a triac SSR, it was in no way proportional.

I am concerned with the FET modules.  Ones that I have looked at are slow.  It would be good if someone actually tested them.

DamonHD

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Re: Solid State Relays, Uses, and Trigger Circuits
« Reply #23 on: December 30, 2016, 01:22:15 PM »
In my application I used zero-crossing SSRs, and was aiming to drop out a small number of half cycles to reduce power delivery to a (dumb) appliance such as a kettle.  I didn't synchronise the AVR/Arduino to mains but it still worked well enough as I was generating approximately correct timings and the SSR was cleaning them up.

http://www.earth.org.uk/domestic-dynamic-demand-ideas.html#DD1

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