What would you use relay, ssr, etc? - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: October 23, 2015, 04:05:59 pm
content: I am going to try to finish the GPSDO project I started years ago. I have a RFTG unit that I am going to take out the RFTG pcb and LPRO-101 rubidium module and mount them in a larger enclosure with a fan for cooling, control pcb, GPS pcb, etc. I have been using an AVR to convert the ublox GPS to the motorola oncore GPS expects and this is working fine, the RFTG will discipline itself. I just have a mess of cables and modules and want to finally put it all in its own box. I also don't want to keep a notebook plugged into it with the RFTG software, so I want to make the control board also provide a limited RFTG software interface where I can get the status and see the frequency adjustments on a small front mounted UI. I've got a nice 24x6 VFD that Noritake gave me as a sample that I think might look pretty cool.So - my question - The RFTG/LPRO-101 is 24 Volts and can generate a bit of heat. Once settled down it consumes about 800mA @ 24V which is 19.2 W. I will have a case fan that I can alter the speed of and I also plan to have a temperature sensor connected to the LPRO-101 base plate so I can attempt to keep its temperature more stable than just running the fan at a constant RPM.Often this will be left on for months, and it may be unattended, so if the fan stops or fails, I want to shut down the RFTG/LPRO-101 so it does not generate heat any longer. What is the best way to control this from a 5V microcontroller? A relay? a DC SSR? The LPRO-101 may draw 1.5A at startup, so probably something that could handle 2-3A @ 24V would do the job.

author: calzap
author_rank: Frequent Contributor
timestamp: October 23, 2015, 09:53:31 pm
content: How about a manual reset thermal switch? Simple and protects against all sorts of over-temp situations.Mike in California

author: promacjoe
author_rank: Regular Contributor
timestamp: October 24, 2015, 12:28:09 am
content: a relay is your best choice for anything less than 32V. Most SSR do not work correctly with low voltages. also an SSR can produce a lot of heat, Compared to a standard relay. In some cases, a MosFET can work very well. But again It will produce some heat. The only reason why I would choose a MosFET or a SSR over a relay is if I need a high-frequency on/off state. Anything less than about a second or two duration. remember if it's producing heat, it's wasting energy. you need to minimize the wasted energy.Also you may want to use a fan that has an RPM output. If an RPM signal is not detected by the microcontroller, the fan is not running, and the microcontroller can shut down the unit.

author: Monkeh
author_rank: Super Contributor
timestamp: October 24, 2015, 12:29:58 am
content: Quote from: promacjoe on October 24, 2015, 12:28:09 amYou mean like the 0.25-0.5W a standard relay coil burns all the time it's energised?I beg to differ, for ELV DC applications requiring relatively little current, a relay is yourworstchoice.

author: alank2
author_rank: Super Contributor
timestamp: October 24, 2015, 12:54:35 am
content: What does ELV mean?Switching will be very rare. It will start off when power is applied and then the AVR will turn it on within a few seconds of reset. It will stay on unless there is a fan failure, in which case it will turn off once and stay off.I agree about the fan RPM monitoring.So would a power mosfet that is almost never being switched use less power than a relay coil typically?

author: Monkeh
author_rank: Super Contributor
timestamp: October 24, 2015, 01:03:04 am
content: Quote from: alank2 on October 24, 2015, 12:54:35 amExtra Low Voltage. <=50VDC.QuoteA MOSFET would use however much energy is dissipated in leakage from the gate. So.. effectively zero when not switching. Not even worth trying to measure.It would, of course, waste energy as a resistor. Getting sub-10-millohm FETs is, however, absolutely trivial, which means for any current below 7A you immediately are wasting less energy than holding a typical relay closed. And that current goes up the lower your RDS(on), which can be cut down easily.Of course, you could get latching relays (expensive) or normally-closed relays (which have certain electrical limitations as you're not forcing the contacts closed as hard, and can't pull them apart as fast..).Also, small FETs use less board space, take no additional components to drive in many low-frequency switching applications (a gate current limiting resistor may be sensible, however), and cost less.

author: promacjoe
author_rank: Regular Contributor
timestamp: October 24, 2015, 01:35:31 am
content: Quote from: Monkeh on October 24, 2015, 12:29:58 amYour wattage estimate is a little high for the relay needed for that application. For relays up to 10A. 100-150 mW should be all that is needed. And that is turn on wattage. Hold wattage should be a lot less. and yes that is higher than what a SSR would need, But that's not where the heat is generated within an SSR. And the average SSR will not work with a 24V output, AC or DC. you could do it with a MOSFET but, Since it would be on all the time it might generate more heat than a standard relay. a latching relay might be a better choice. Apply current to it once it comes on, Remove the current and it stays on. You have to apply current to it again to turn it off. It can stay on for ever but not use any more current.

author: Monkeh
author_rank: Super Contributor
timestamp: October 24, 2015, 01:47:47 am
content: Quote from: promacjoe on October 24, 2015, 01:35:31 am*pulls relay out arse*http://www.digikey.com/product-detail/en/1461069-5/PB1321-ND/3318145400 ohms at 12V = 360mW. This is a pretty typical low cost relay.QuoteDoesn't matter, heat is heat, wasted energy is wasted energy.QuoteOnly at relatively high currents. In this proposed application (<1A!), there are few affordable relays on the market which will waste less energy and create less heat than a nearly jellybean MOSFET.

author: alank2
author_rank: Super Contributor
timestamp: October 24, 2015, 01:59:31 am
content: I like the sot223 sized ones - would something like this work:BUK98150-55A,135Does it make any difference if it is a low side or high side driver?

author: Monkeh
author_rank: Super Contributor
timestamp: October 24, 2015, 02:07:09 am
content: Quote from: alank2 on October 24, 2015, 01:59:31 amThat'd work alright in the low side, a little high RDS(on), but not bad.How about a (slightly large) PFET for high side:http://www.digikey.com/product-detail/en/ATP112-TL-H/ATP112-TL-HOSCT-ND/5213190Have to watch out for the Vgs though, that can complicate matters.If you switch the low side you need to be aware of a difference in 'ground' potential, and other return paths (chassis, signal wiring?). High side is likely better.