Relay power dissipation calculation

ArakelTheDragon

Joined Nov 18, 2016
1,350
I want to use a solid state relay to switch a load of 24V, 25A. I have picked 2 relays that can be used:
https://www.mouser.com/datasheet/2/404/s60dc40-3426.pdf
https://www.sparkfun.com/products/13015

The first one has "8 degrees C per watt" dissipation of the built in heat sink. I was never good at calculating these heatsinks, can anyone give an idea is this enough of not and how to calculate it? Simpler if possible please.

It is possible that the load is not at 25A all the time, but drops to 12 half of the time, still I think the wors option should be considered.

Sensacell

Joined Jun 19, 2012
2,632
The first one is an AC only SSR, not usable.

The other DC unit has a 70 milliohm ON resistance: at 25 Amps, (25 * 0.07) = 1.75 volt drop.

24v - 1.75v = 22.25v at the load, is this a problem?

1.75 V * 25 A = 43.75 W loss, about 8% - not a very good "switch"

I would look for a better solution.

ArakelTheDragon

Joined Nov 18, 2016
1,350
You mean the first one is usable, and the second one is only an AC relay.

The battery is 120A/h. Yes its a lot. I am guessing a MOS with 4mOhms to 20mOhms would be better, howeve I still need to cool it and I do not want to put a fan.

(I know what case to ambitient is, junction to case, thermal paste, heat sink).

ArakelTheDragon

Joined Nov 18, 2016
1,350
I think this is better with 0.625V loss and about 2% wattage loss

http://www.crydom.com/en/Products/Catalog/1_dc.pdf.

Edit:
I have about 3A/h of the total capacity lost for the relay, lets say the rest of the PCB takes another 1A/h, total the consumption will be 4A/h.

The load is using the battery for 8 hours. About 3.5% of the battery are used.

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ArakelTheDragon

Joined Nov 18, 2016
1,350
So no ideas for cooling these 15W? Perhaps a general heat sink with thermal paste will do?

crutschow

Joined Mar 14, 2008
25,677
So no ideas for cooling these 15W?
You might want to consider a PC processor type heatsink with a built-in cooling fan.

ArakelTheDragon

Joined Nov 18, 2016
1,350
I prefer not to put a fan if possible. For MOS transistors I know that over 1W you need a heat sink, is there any general information about solid state relays?

AnalogKid

Joined Aug 1, 2013
8,705
Crydom has their own line of aluminum extrusion heatsinks for their products:

http://www.crydom.com/en/tech/brochures/crydom_hs2014.pdf

They probably are more expensive than a plain extruded fin heatsink from Aavid, Thermalloy, Wakefield, etc. Look for "free air" or "natural convection" specs.

8degC/W is not much of a heatsink at all. It allows the contact area of the device to rise 8 degrees C above ambient for each watt dissipated. A device dissipating 10 W would rise 80 degrees. If the ambient is 25C, that is a total of 105C. If the part is rated for 100C, failure. If it is rated for 150C, success, but it will get hot enough to blister skin on contact.

ak

ArakelTheDragon

Joined Nov 18, 2016
1,350
Thats bad. I can put a desktop fan at 5V, I think the relay is documented as 125 degrees C. My other option is the Finder 66 series, but I do not know the voltage drop over the coil at 15 or 25A, 28.8V fully charged battery pack.

EDIT: Also on the datasheet I see rated 30A, but a little lower I see "Breaking capacity DC1: 30/110/220 V A - 25/0.7/0.3".
That means the breaking capacity at 30VDC is 25ADC?

https://www.finder-relais.net/en/finder-relays-series-66.pdf

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AnalogKid

Joined Aug 1, 2013
8,705
The way I read it is that the max rated current is 30 A when making, and 25 A when breaking.

ak

ArakelTheDragon

Joined Nov 18, 2016
1,350
What exactly does this means "30 A when making, and 25 A when breaking"?

ericgibbs

Joined Jan 29, 2010
11,154
hi ATK,
Making is when the relay contacts Close, 30Amps max
Breaking is when the relay contacts Open, 25Amps max
When Opening there is a possibility that the breaking/opening contacts could create an 'arc' between the two contacts, that is why the Amp rating is lower,
E

ArakelTheDragon

Joined Nov 18, 2016
1,350
So if I pass 30A, there be an arc during the switching from NC to NO, meaning pin 12 to pin 14. Because I do intend to use a normally open relay and pass 25A through it, while when I close it, nothing will pass. Its just a defence to stop the battery from charging if it reaches 14.4V.