Sainsmart 8 channel relay with LM1117-5 - Page 1
Forum section: electronics / beginners

author: jerryk
author_rank: Regular Contributor
timestamp: December 21, 2017, 01:28:56 am
content: I'm trying to run the sainsmart 8 channel relay with an LM1117-5 (http://www.ti.com/lit/ds/symlink/lm1117.pdf) and it goes into thermal overload when the third relay coil is energized. The coil datasheet (http://www.lemicro.cn/rar/SRD%28T73%29.pdf)shows each relay coil nominal current and 71mA and power at 360mW per relay. The input voltage on the LM1117 -5 is 12V and is wired per the schematic below. I am observing approx. 852mW load rather than the 360 mW which input voltage x the nominal coil current. The datasheet for the LM1117 has the following formula for Pd - PD = (Vin-Vout)I load + VinIg. I'm having trouble applying this formula to my observations. The end goal is to know if this LM1117-5 can carry the load of the relay board and if so how figure the surface area of the heat sink needed.My guess is the LM1117 will not work. If this is the case I would take any ideas on another way to power this where I can have all relays triggered at the same time.Any help is appreciated.Jerry

author: danadak
author_rank: Super Contributor
timestamp: December 21, 2017, 02:35:00 am
content: Ref material -http://www.ti.com/lit/an/slva118a/slva118a.pdfhttp://rohmfs.rohm.com/en/products/databook/applinote/ic/power/linear_regulator/three_terminal_heatsink_appli-e.pdfYou add up all the loads on the 1117 output, then calc the total Pdiss the regulatorhas to deliver. Then per the ap notes back into the necessary Thermal R the heatsinkmust supply.If I take 8 x 71 mA x 7V thats ~ 4W nominal (not worst case) and thats a lot of power.Look at the package Tj-c value for the package you are considering, you can quicklyget a feel for what Tjmax will wind up being.Whatever you do worst case this design, max Vin to reg, min coil R, worst caseVout reg.........Regards, Dana.

author: Cliff Matthews
author_rank: Unknown
timestamp: December 21, 2017, 03:39:48 am
content: If in a momentary pickle (likely not the case here), you could always help-out LDO 1117 with 4-6 series-wired 1 amp rectifiers.

author: floobydust
author_rank: Super Contributor
timestamp: December 21, 2017, 05:09:26 am
content: I don't run relay coils off a voltage regulator. Use 12V relays instead, powered off the raw DC.Relays are quite tolerant of their coil voltage being imprecise, after all they are just a big resistor made of copper wire.If you must use 5V relays, you can do PWM to lower holding current, 50% PWM.

author: jerryk
author_rank: Regular Contributor
timestamp: December 22, 2017, 09:04:39 am
content: Will this work as a substitute for the LM1117.http://www.mouser.com/ds/2/687/tsr1-537631.pdf

author: mariush
author_rank: Super Contributor
timestamp: December 22, 2017, 12:13:01 pm
content: Also worth noting that 1117 is one of those linear regulators that needs a capacitor on the output with some minimum ESR. like 0.1 ohm or more... so don't use a ceramic capacitor on the output, or put a 0.1 - 1 ohm resistor in series with that capacitor to fake out a higher esr.The others have already mentioned the thermal issues.. it's simple (vin - vout ) x current = how much heat it makes.. for sot-223 you shouldn't go over 1w ... use to252 (dpaq) or some other larger chip and have some vias around it to dissipate heat to the bottom layer, if you want more heat dissipationyou can reduce the input voltage with some diodes in series in front of the regulator if you really have to.