How many AMPS do my SSR's really need to handle?

[BSD]

I have been slowly piecing together the info for my own controller build and it seems like some people are using solid state relays that will handle up to 25 AMPS but they are pretty expensive.

I have seen Sainsmart makes a 8 channel 10A relay board http://goo.gl/KCT5lW

They also have an 8 channel 2A solid state relay board
http://goo.gl/NtK46

I am planning on building a small tank so each plug wouldn't have anything more than a small pump, vortec, or a single LED bulb. Do I really need such a high amp SSR?

[asid61]

Not really, if it's only a few watts. If you have like 10w of pump, you shouldn't need a heatsink.
Figure around 5-10x the pump power for the relay.

[BSD]

do you think more AMPS or a less AMP solid state relay would be better?

[TimmyD16]

10A is fine... 2A is fine in most situations... 2A equates to 240W in power. Especially on a smaller tank you shouldn't ever go over that.

That being said the 10A will be much more reliable and last you longer and you should go with that if you can. You loose power in heat and noise, so there needs to be some wiggle room when figuring it out. IE if you have a 50W pump it would only be around .42 Watts, but all your circuits should be rated for at least 1W when taking into consideration power loss and power factor.

[mcgyvr]

There is no need to use SSR's.. Relays work just fine for these applications and cost fractions of the price of an SSR..

YOU need to determine how many amps your relays contacts need to handle based on what you are attaching to it.. But that 10A 8 ch relay board "should" be just fine for anything you plan on attaching to it..
Voltage x Current = Watts

[asid61]

I would stick with an SSR for pump control, as they last millions of on/off cycles. However, for just on/off once a day, then a regular relay will do.
And even though the SSR is rated for 2a, or about 240w, you can't run it so high without a heatsink. You should only run it at a little above 1/10 power. 1/5 if you have a fan.

[BSD]

Thanks for all of the info!!

[HippieSmell]

Originally Posted by asid61

I would stick with an SSR for pump control, as they last millions of on/off cycles. However, for just on/off once a day, then a regular relay will do.
And even though the SSR is rated for 2a, or about 240w, you can't run it so high without a heatsink. You should only run it at a little above 1/10 power. 1/5 if you have a fan.

Ssr's aren't the best for controlling pumps because the pump is inductive and should have some sort of counter electromotive force suppression across the load to protect the relay when the pump is turned off.

Or, just use it as is and have a reduced relay life. It might still last for years.

[uncleof6]

A high volatge diode across the relay contacts, is usually all it takes to protect a relay from voltage flyback when the inductor (motor) shuts down.

[d0ughb0y]

if you go with 8 channel relays, use 12v one if you can. It will only take 20ma-25ma per relay or around 200ma at 12v if all relays are on. Although the coil power consumption is the same for 5v relay, it will require about 500ma if you use 5v relay. The 12v relay needs a mod for the input lines to work though. Even though they advertise it as TTL level, they are actually designed for 12v input. The series resistor is 20k for 12v relay, but only 1k for 5v relay, so if you apply TTL level to 12v relay input, it barely runs enough current to enable the opto isolator led. I had to short out the led display on the input side. Using 12v relay also simplifies the circuit so all you need is a single 12v power source and don't need the extra 5v step down to power a 5v relay. Note that I have only tried the relay with blue board and not the red board, which seems to be a newer model, so you may not need to do any mod for the TTL input.

my diy controller

[uncleof6]

Originally Posted by uncleof6

A high volatge diode across the relay contacts, is usually all it takes to protect a relay from voltage flyback when the inductor (motor) shuts down.

EDIT: Change contacts: to coil + and -. I am very tired...

[d0ughb0y]

for the 8 channel relay boards, the coils already have diodes installed.

[HippieSmell]

There's a difference between placing a diode across the relay coil contacts and placing one across the load. Many relays come with built in diodes or other surge protection across the coil, but any inductive motor and really any DC powered device, should have something across the load.

For aquarium purposes, I would stick with mechanical relays because of all the pumps we use. SSR's are overkill unless you're using something like a PID loop for heater control and rapidly turning the heater on and off.

[d0ughb0y]

There is absolutely no need to put a diode across the 110v side of the relays with the 8 channel relay. It is just an SPDT switch that completes the 110v circuit. It is no different from plugging in your pump to the wall outlet.

[HippieSmell]

It's not different as far as the load is concerned, but the relay needs protecting. SSR's are electronic circuits, they're not very robust and back emf can damage them. That's why I'd stick with mechanical relays, because the back emf will just cause a little bit of arcing on the contacts, which isn't as big a deal as frying a transistor.

Another problem with inductive loads and SSR's is that the relay can get stuck turned on. I've had this happen many times with my magnetic ballast.

[sleepydoc]

Originally Posted by HippieSmell

That's why I'd stick with mechanical relays, because the back emf will just cause a little bit of arcing on the contacts, which isn't as big a deal as frying a transistor.

The problem is, the airing can cause failure of the relay contacts, which can end up being the same as frying a transistor.

[HippieSmell]

Yeah, but mechanical relays are cheaper and a single jolt can fry a SSR.

How often is the OP going to be turning the loads on and off? If it's once a day, then it probably doesn't matter either way.

Don't forget about the load not turning off, though. That's a bigger problem, IMO.

I'm not trying to sound like an expert, because I'm not. I've just read way too much about this issue over the last few months.

[uncleof6]

Originally Posted by sleepydoc

The problem is, the airing can cause failure of the relay contacts, which can end up being the same as frying a transistor.

Well Doc, this is another area where some know, and most don't. Motor controls are a speciality requiring considerably more education than basic electrical theory, and electronics.

In most cases with "normal" sized AC motors (a wide range) SSRs don't have the guts, nor do the mechanical relays most hobbyists would seek to use, due to high in-rush current levels alone, and really should not be tasked in this manner. Thus the concern over burning out relays and such. Motor controls use special relays designed and engineered for use as motor controls, they can handle the "environment," and in almost all cases utilize a latching circuit and seperate start and stop buttons. I was going to post an image of a motor control, however, it is too hot to dig around and find it. They are not cheap (why aquariums hobbyist aren't interested,) but they can handle the abuse with very good longevity, as long as the installer knows what they are doing.

[HippieSmell]

Originally Posted by uncleof6

Well Doc, this is another area where some know, and most don't. Motor controls are a speciality requiring considerably more education than basic electrical theory, and electronics.

I suppose this is a passive aggressive dig on me. Cool.

Originally Posted by uncleof6

In most cases with "normal" sized AC motors (a wide range) SSRs don't have the guts, nor do the mechanical relays most hobbyists would seek to use, due to high in-rush current levels alone, and really should not be tasked in this manner. Thus the concern over burning out relays and such. Motor controls use special relays designed and engineered for use as motor controls, they can handle the "environment," and in almost all cases utilize a latching circuit and seperate start and stop buttons. I was going to post an image of a motor control, however, it is too hot to dig around and find it. They are not cheap (why aquariums hobbyist aren't interested,) but they can handle the abuse with very good longevity, as long as the installer knows what they are doing.

I'll burden the OP with more of my educationless knowledge. If you want to use a 15 amp contactor for something that uses 3 amps, go ahead, I did. But, it's expensive and total overkill. Otherwise, I'd stick with simple, cheap, mechanical relays rated for at least 5 amps with built in suppression across the coils and be done with it. It might not last forever, but it'll work. Otherwise, do like I did and read for yourself, listen to random people on the internet, and get four different types of relays for all of your different equipment. Good luck.

[d0ughb0y]

Originally Posted by HippieSmell

There's a difference between placing a diode across the relay coil contacts and placing one across the load. Many relays come with built in diodes or other surge protection across the coil, but any inductive motor and really any DC powered device, should have something across the load.

For aquarium purposes, I would stick with mechanical relays because of all the pumps we use. SSR's are overkill unless you're using something like a PID loop for heater control and rapidly turning the heater on and off.

ok, I am actually running into this problem with my relay circuit.
whenever the relay switches, some noise causes my ethernet shield to drop off the network. If the arduino is connected to my laptop via usb cable, I can also see the USB drop out when the relay switches. Funny thing is, the program keeps going and does not reset. It does not happen when I unplug the powerhead connected to the relay load. BTW, I don't think you can connect a diode across an AC load, that will essentially short out the circuit on the half cycle of the AC like a half wave rectifier.