I'm working on an electronic product based on a microcontroller that must be able to control a series of actuators: 230V AC motors ON/OFF, 24V DC motors ON/OFF, 80V/20A heating element ON/OFF. The obvious way would be to use relays (in combination with optocouplers perhaps), but they have the downside of having a short life cycle. My product must have a +10 years lifetime, so what the best way to replace the relays in this design and still have galvanic isolation?
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\$\begingroup\$ How many on/off cycles do you have a day? What's the motors' current? \$\endgroup\$– stevenvhCommented Jul 15, 2011 at 8:30
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\$\begingroup\$ I'd say that no more than 720 on/off cycles per day. As for the motor's current: 2A \$\endgroup\$– m.AlinCommented Jul 15, 2011 at 8:56
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\$\begingroup\$ NO MORE(!) than 720. That's ON - 1 minute - OFF - 1 minute - etc! \$\endgroup\$– stevenvhCommented Jul 15, 2011 at 9:01
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\$\begingroup\$ That's right. But only the most fast switching actuator has that number of cycles. If the product operates 24h continuosly, the relay must switch on and off (for a few seconds) every 2 minutes. \$\endgroup\$– m.AlinCommented Jul 15, 2011 at 9:07
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2 Answers
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You don't need an optocoupler to drive a relay; the relay has insulation already.
But your switching requirements are indeed high for a typical relay; we're talking about 2.6 million operations over the 10 year lifetime. There are relays which can handle this, but they're high endurance industrial devices, which will cost a lot more than an electronic solution.
There are a number of solid-state solutions.
For the 230V AC motors you can use a solid state relay (SSR), either as a module, or built with discrete components around an opto-triac + power triac.
The DC motors can be switched with MOSFETs, again controlled by an optocoupler, or with a DC SSR.
For the heating I presume the power is also AC. Then you can use the SSR, like for the 230V motors. If it's DC I would use MOSFETs.
(the linked to datasheets are just examples to get you started)
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\$\begingroup\$ And what would be the downsides of using SSR / MOSFETs as opposed to relays? do they require any extra components to protect them from back EMF from the actuators? Also, could I use Darlington transistors instead of MOSFETs? \$\endgroup\$– m.AlinCommented Jul 15, 2011 at 8:50
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1\$\begingroup\$ You can use darlingtons, but they will dissipate much more power. The MOSFET I linked to has an \$R_{DS(ON)}\$ of about only 50m\$\Omega\$. The triac will need a snubber R+C to protect it. The MOSFETs need a flyback diode over the motor. \$\endgroup\$– stevenvhCommented Jul 15, 2011 at 8:55
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\$\begingroup\$ Just checked out the AC SSR you linked; they're very expensive compared to a relay: $35 on Digikey. But the opto-triac + power triac seems more cost effective. Great answer, BTW. Thanks! \$\endgroup\$– m.AlinCommented Jul 15, 2011 at 9:44
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1\$\begingroup\$ Check out PCB mounted SSRs, they're much more affordable, and your current isn't that high. I guess between $5 and $10. \$\endgroup\$– stevenvhCommented Jul 15, 2011 at 9:49
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\$\begingroup\$ If this is for a volume product then use of discrete power components rather than commercial SSRs will be offer lower cost. "Properly designed" [tm] the devices mentioned will allow many years of reliable operation. 10 years doable with proper care. Triacs fine for AC. MOSFETS the easiest and entirely acceptable choice for DC. Fully controlled AC switch can be made with a MOSFET in a full bridge rectifier but this is unlikely to be required. \$\endgroup\$– Russell McMahon ♦Commented Jul 15, 2011 at 10:28
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For field service, maintenability sake, you can choose 100% off-the shelf solution. DIN rail and few DIN components from miriad of industrial automation vendors. This way you will allow user to service it, replace opto-relays, wiring etc.
This way you will cover more customer's aspects, geographies, long-term investment thoughts, certifications, fire safety regulations etc.