a diode is needed ? 1

[CarlosCheng]

Hi, I'm actually a computer enginneer and not an expert in electronics or electricity but I've done some applications involving relays before.

I used to work with 12V input relays(normal ones, not solid states), that usually required a diode in the circuit to prevent undesired voltage/current problems.

Currently I have a project in which I have to use 120V/220V input solid state relays, and I wonder if I still need to add a diode to the circuit in order to prevent electrical problems.

Thanks in advance.

 

[itsmoked]

Negative!

Solid state relays(SSR) are fully electronic.

Electromechanical relays have inductance that generates damaging kick-back when the coil is de-energized and so requires a diode often.

Just look at the SSR data sheet and hook it up that way. Pay particular attention to the de-rating curves!!

Remember too that most SSRs have leakage due to their output snubbers. This causes a lot of confusion in the field because they look like they are still on when checking them with a sensitive Digital Multi Meter(DMM). Often you need to use something like a lamp to shunt this leakage current to get "logical" readings.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Mike2468]

You said: 120V/220V input solid state relays

IF that input is coming from a triac (a triac output in a PLC or a triac output in a PID temperature controller), then you may need a burden resistor. This is to eliminate false turn ON due to triac leakage.

If that input is coming from a different location, such as a limit switch, then no burden resistor is usually required.

 

[nbucska]

tell us the type of the SSR, switched voltage and currect and control voltage.

Do you switch the control VTG with switch of with electronics ?

Plesae read FAQ240-1032
WEB: <

http://geocities.com/nbucska/>

 

[CarlosCheng]

Hi, thanks everybody for your help.

The problem is that I want to detect if an air conditioner is turned-on at a given time.

I've make a diagram of my problem here:

http://www.regalos-especiales.com/SSR_diagram.JPG

Mike2468 : does this circuit needs the resistor you were talking about? if so, where should the resistor be connected, and also where should it be connected?

 

[itsmoked]

CarlosCheng; Your picture is very helpful.

If you are using a PC to turn on the SSR you don't need a resistor.

I am having a problem with your power..

I would think that you have two hot wires and a safety ground running to the air conditioner. You would need to put the SSR in one of the "hot" wires NOT the safety ground.

I guess I am confused by the +,+,- expecting +,-,+.

Also you better make sure that is a hefty SSR because you have it starting a compressor (tough starting load). Remember that the SSR must get cooling!!! Or it will fail.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[nbucska]

If you want the computer to detect that the AC is on, the
simplest is to use an optocoupler OC ( LED & photo-transistor type)

Put a series R into the AC line which causes nominally
about 3V drop. rectify this with a four diode bridge.

The output diagonal of the bridge is connected to the
LED of the OC with a 200 ohm series resistor. Connect
a 5V zener parallel with this disgonal to reduce the start-up current surge.




Plesae read FAQ240-1032
WEB: <

http://geocities.com/nbucska/>

 

[waross]

The circuit you show would be used to control an air conditioner with the computer. That is with the input connected to the computer and the output connected in the Air Conditioning circuit. If the SSR is connected with the input in the air conditioner circuit, it will prevent the air conditioner from starting.
A small Current Transformer in the line to the air conditioner will give a voltage output when the A/C is running that you can use to control the SSR.
You can use nbucska's circuit on the output of the Current Transformer.
A series resistor should be rated with consideration for the Stalled Rotor Current of the Air Conditioner. (Locked Rotor Current times the time for the thermal overload to open)
With 10 amps and 600% Locked Rotor Current your 3 volts will become 18 volts under stalled rotor conditions. At 600% of 10 amps that's 60 amps times 18 volts for a few seconds.
That's a little over a Kilo Watt. You may not find a suitable resistor at Radio Shack.
respectfully

 

[imok2]

Wouldn't a simple currnt transormer in the common leg of the compressor and connected to the ssr all that's needed?

 

[machmech]

Hello Carlos

I would just like to clarify something, Your second post reads "The problem is that I want to detect if an air conditioner is turned-on at a given time." but your diagram appears like a control device. -----

I'm thinking if you just need to detect when it comes on maybe you could work off a lower voltage perhaps from a thermostat on the unit, is this what you are looking for?

Chuck

 

[machmech]

oops it's early in the morning, forgot to ask. How are you able to sense current with the P.C. what kind of signal are you looking for?

Also just out of curiosity what software are you using for you program?

 

[kboggs]

if it were me I would use a current sensing relay instead

Kenneth H Boggs

 

[nbucska]

Hi Carlos:
Since the AC represents an almost costant load, the simplest would be a current transformer -- diode bridge--
clamping zener and optional filter cap i.e. up to 7 components


Plesae read FAQ240-1032
WEB: <

http://geocities.com/nbucska/>

 

[OAHCG]

If you are going to control a 220vac appliance. You Should have a 2 pole SSR for both of the Hot Leads on the incoming AC.

 

[itsmoked]

Guys...It's been 1/2 a year!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com