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Old October 28th, 2015, 02:42 AM   #1
rQx
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Use of solid state relays (SSR)

Hi!

Hopefully I'm allowed to ask this question here, if not the admin is free to delete it.

We have a standard cabinet we ship to our customers. Now there is a customer that want's to have SSR relays instead of regular mechanical relays. We tried talking him out of it with no succes. So now I've tried gather information how to use SSRs. I know it's basicly same functions as a mechanical relay and here iare my concerns.


It seems they are very sensitive on the input side, how sensitive is that? I will try to wire to seperate the main AC and the control circuit DC as mush as possible.

The SSR relay only has 1 "pole". I have relays that have 4 poles, my best guess here is that I use 4 SSRs in parallell?

I have mechanical contacts i series and in parallell, with SSR is it possible to connect them in series or parallell? I think you can but better safe then sorry.

I have attached a file with little schematics and text.

Thanks /Tim
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File Type: jpg SSR.jpg (27.9 KB, 234 views)
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Old October 28th, 2015, 03:25 AM   #2
kallileo
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This could be useful:

Using Solid State Relays in parallel and/or series
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Old October 28th, 2015, 05:11 AM   #3
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Are there any contactors in the panel? If so and your customer is happy with contactors then just replace the relays with contactor/relays:

http://www.eaton.eu/Europe/Electrica...DILA/index.htm
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Old October 28th, 2015, 05:39 AM   #4
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My first impression, is that your customer is a complete idiot.
Why does your customer want this?

SSR's are great as replacements for mercury relays for heaters, yes. Or other power switching duty.

For controls, they are completely stupid.
Poor isolation, high insertion loss, problems with analog or mixed voltage signals...
/shrug.
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Old October 28th, 2015, 07:59 AM   #5
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If he *wants SSRs, there are some electronic factors to consider. In your diagram of the contacts in parallel, SSR should be fine because they have a load (the coil). But if one were wiring four SSR contacts in parallel to a high-impedance input (for example, a PLC input), there could be enough leakage current to trigger an input. SSRs should have a load with enough impedance to pull down leakage.

Better safe THAN sorry - you don't really want to be both safe and then sorry . . .
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Old October 28th, 2015, 10:06 AM   #6
Phil Buchanan
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I can understand the end user asking for SSR's where possible as it will give him a more reliable control system with a higher MTBF if done correctly.

I use the units in the link below a lot for control as you can get them with built in leakage protection and the handle a fair load size.

http://www.ab.com/en/epub/catalogs/1...roduction.html

SSR's are no issue on digital signals if you use control rated SSR's and they are sized correctly for the impedance of the circuit and devices being controlled. The problem comes in when people think they can just throw an SSR where a mechanical relay was with not though and it will be all smiles and that's just not the case most often.

With that said though I try to avoid relays if at all possible. Some people in the industry get carried away with isolation and interfacing. i have seen panels that have a relay on every input when the input was a prox or limit switch which could have been directly connected.

Systems like this are more complex and harder for the average maintenance tech to troubleshoot and repair and have a lower MTBF.

PLC's were designed to simplify control systems and replace a lot of relay logic and some add a plc and ton's of relays where they are not need and it makes no sense.

I have ran across a lot of systems with a plc and so many interface relays that i could have used relay logic alone with not plc and had a much more simple and reliable circuit.

Not saying that's the situation the OP has but just something to keep in mind.

Last edited by Phil Buchanan; October 28th, 2015 at 10:08 AM.
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Old October 28th, 2015, 10:11 AM   #7
Phil Buchanan
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Here is another piece of literature that may be helpful to you. Beware it's a real page turner.

http://literature.rockwellautomation...t001_-en-e.pdf
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Old October 28th, 2015, 10:38 AM   #8
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The system is relay logic. I'm thinking the customer is asking for Solid State, not SSR.
Replace the entire system with a PLC.
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Old October 28th, 2015, 12:53 PM   #9
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They have only stated that the relays must be SSR but not the contactors. Which is wierd because they said they didn't want any mechanical switches, we must switch our level sensors to non mechanical and pressure sensors to analog etc etc.

Maybe we can use contactor relays instead, I'll have to ask the customer about that. Problem is that I haven't got direct contact with the end customer.

Our SSRs contacts will go to a PLC aswell, but then I can find SSRs designed to not leak current?

PBuchanan, I have read that and that's where all my questions come from

The system contains both some relays and a LOGO usually. But this customer doesen't want the LOGO aswell. So I have to run every input and output to terminals to their DCS. Making our cabinet a slave cabinet with only some manual functions.

This customer is like.... give me a break.
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Old October 28th, 2015, 01:22 PM   #10
just the cowboy
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Hard to find NC contacts

Also try searching for the SSR you will have a hard to find NC contacts.

Solid state for power contacts are fine but if they want control just go PLC.
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Old October 28th, 2015, 01:47 PM   #11
rQx
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Quote:
Originally Posted by just the cowboy View Post
Also try searching for the SSR you will have a hard to find NC contacts.

Solid state for power contacts are fine but if they want control just go PLC.
Yes I know NC is hard to find but I think that maybe I can redesign so I only have NO.

Problem is customer is specifik about they don't want PLC.
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Old October 28th, 2015, 02:02 PM   #12
MHennel
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I have used Crydom relays for many years with no problems

http://www.crydom.com/en/. They have a wide selection SSR of both AC and DC.
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Old October 28th, 2015, 07:33 PM   #13
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You are going to need to know if that control circuit is AC or DC, it makes a big difference on the type of SSRs you use. The drawing you clipped doesn't show that.
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Old October 29th, 2015, 02:12 AM   #14
rQx
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It is 24VDC, I decide what control circuit we are using since I am designing the cabinet
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Old October 29th, 2015, 01:36 PM   #15
rQx
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Ok, so if I got this straight.

The SSR have a leakage current when they are off and a voltage drop when they are on.

Am I right in assuming that if I want to have two SSR contacts in series, I want a low voltage drop so that I have enough voltage left for my load?

And it's always better to have a SSR with low leakage current? So that I don't trigger more sensitive inputs on a PLC for example? In other words it's not bad to have a low leakage current even if I don't need it to be?

I have found a relay from Crydom: http://www.crydom.com/en/products/catalog/e_d_dc.pdf

Load: 5A
Voltage drop: 0.3V
Leakage current: 20uA

I think this would suit me pretty good in all my areas.
For the most part my relays will go to indication lights (Allen Bradley 800F series LED), for controlling 24VDC contactors (with low consumptions coils and diode) and to a LOGO module from siemens.

Am I on the right track here?
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