Hello,

I've got a PLC with outputs, open collector or sourcing, that's limited to driving or sinking only 100mA out each output.  Of course nothing I ever need to drive needs only a measly 100mA unless its a relay.

All the solenoids in this job require about 125mA to 200mA.  So I need just a wee bit more.  Since the cycle time in Today's Machine Project is about 4 million cycles a year this project requires non-mechanical relays.   My problem is that the only solution I'm seeing is to use Solid State Relays.  The very cheapest ones I can find are $22 a piece.   I don't need more optical isolation.  The PLC has already provided that.  All the solenoids are being driven from the same supply.   All I need is to boost the lame, er... weak, PLC outputs up to about 250ma or so.

This seems like such a painfully obvious situation that someone must sell something that provides this for less than a forest of optos.. 16 channels of optos cost a minimum of $350.  More than the PLC!  Has anyone seen a solution for this?

Keith Cress
kcress - http://www.flaminsystems.com

Use a solid state relay rated for this application.

Allen Bradley
Bulletin 700-SK Slim Line Relay with Plug-in Quick Connect Terminals

or any other cross for this relay will work, Phenoix contact or square d or whoever has same.
 

You're telling me those SKs and the sockets they require are going to be less than $22 a point?

Keith Cress
kcress - http://www.flaminsystems.com

An hour or two with a CAD program and a few dollars for a doublesided board delivered within a week? Isn't that an option? Doesn't your customer accept home-brew?

Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.

If the solenoids are the heavy duty type (driving heavy machine tool valves) then isolation SS relay is your only option.  Then its a custom job like gunner explained as an option.  Most PLC cards  cant handle the current spikes and fail.

Gunnar; The problem is - I'm crazy enough to do that.  I have the board layout stuff and even the SW to convert the layout to g-code for my CNC router..


If I go that way I'll show you here.

 

Keith Cress
kcress - http://www.flaminsystems.com

I knew you would! I know you will. I have no objections  

Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.

panel mount SSR's from newark or automation direct start at $14 or so.. Just got to keep checking around.. SSR's aren't cheap but $22 is a bit high..
 

Thanks mcgyvr.   I didn't realize the pucks got down that low.  Too bad what they give you in better price they take away in needing acres of space..

Keith Cress
kcress - http://www.flaminsystems.com

Why dont you use a PLC with relay output?

Couldn't you use something like a 244 or 245 or even a Darlington array like: http://html.alldatasheet.com/html-pdf/12687/ONSEMI/ULN2803/367/2/ULN2803.html

If the only requirement is to sink gobs of current from a DC relay, then the Darlington array would seem to fit the bill.  The Vcesat's a trifle high, but I would think that you could gang upall 8 of them, to sink the relay.  Nonetheless, there are single Darlingtons or TTL-drive n-channel MOS drivers that could fit the bill.  This guy is gross overkill: http://www.irf.com/product-info/datasheets/data/irf7807d1.pdf but it should sell for under a buck.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

Get this... They handed me pneumatic valves that are made to be directly driven by PLCs.  They draw only 65mA (!)

Keith Cress
kcress - http://www.flaminsystems.com

Just FYI for future reference, those A-B 700SK slimline relays are $30 list each, so depending on how many you want, well under $22 net to an OEM panel builder would not be unreasonable to expect.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

It may not need mentioning, but when you drive solenoids a back swing diode should be put across the coil or you may damage solid state outputs. Some solenoid drivers already have this.

I have found that in addition to a diode a small ceramic cap (100 nF or 0.1 uF) in series with a 2 to 5 ohm resistor is very effective. This is sometimes called a Boucherot Cell or Zobel network. If you are going to make a small printed circuit board these are easily added.

- Stephen

Welcome stephenjvb
You seem to be new in here. I think that you are giving advice with good intention. But sometimes, as you say initially, they are not needed.

Itsmoked is one of the more experienced members of EngTips. And has been so for decades - well at least one decade. If you search the fora (use well-functioning search function) before you post, you will get an idea about what has been discussed and what is well known. Do that before you answer a question. It saves embarrassment.

It is interesting that you use the term "Zobel network". It is very rarely used. Last time I saw it was in a Philips book on audio power stages. The term normally used in control and automation is "snubber" or simply "RC unit".

Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.

Why not current amplification?