New relay boards - heat sinking the voltage regulator

[The_Gaz]

I noticed on the new relay boards that the regulator is standing up, and not heat-sunk. I presume then it's not necessary to do so, but am wondering how one can calculate whether or not a regulator should be heat sunk. Like, what if I wanna run 4 30mA relays? Would it need to be heat sunk then? Thanks.

[kagliostro]

An LM7805 can give 1-1.5A (depends on the brand)

with 4 x 30mA your relays circuit has a consumption of 120mA

I think you can use a very modest heat sink

K

[G._Hoffman]

An LM7805 can give 1-1.5A (depends on the brand)

with 4 x 30mA your relays circuit has a consumption of 120mA

I think you can use a very modest heat sink

K

What's the input voltage?  If the DC input voltage is 7V, than 120mA X 2V drop across the regulator is .24 Watts to dissipate as heat (approximately).  Bolt it to the chassis or use a small heat sink if you want, but you probably don't really need it.  If you are inputting 15V, though, then you have 120mA X 10V drop, for 1.2 and you'll need a heatsink of some sort - they should be marked with some sort of dissipation ratting, though it is very dependent on the temperature of the surrounding air and the volume of air flow.  So, yes, a 7805 can handle 1.5 amp, BUT the heat sink required is very dependent on the voltage drop across the regulator.  If you want to input 25V at 1.5 amps you are going to need a VERY big and rather heavily engineered piece of aluminum if you want to keep it from glowing red!  Not to mention a good heatsink compound.

I'm sure PRR will come along and correct me on the details, but it's enough to be getting going on.


Gabriel

[eleventeen]

What's the input voltage?

That's the key question. With a series resistor (cheap) you should be able to drop the input volts to say 9-10 volts and that's ideal for a 7805. Assume that the device wants to throw away a solid 3+ volts to operate properly. I've been down that road plenty of times. Here you have this beautiful, wonderful super-high reliability device (3 terminal regulator) that makes providing a very clean 5 volts (which is what 74LS--- chips required) as easy as pie, but you don't give it enough input volts to work properly and you end up hating life.

If the DC input voltage is 7V.....

Ehhhh, you want a little better than 7 volts on the 7805 input. You just do. I have not checked the specs but you get into a low line voltage situation and the regulator chip will drop out and produce very little on the output side. By the way, there are cheap little folded-sheet metal heatsinks for TO-220 packages (like the 7805) that you'll find in many junk PC power supplies. They just hold themselves on with friction. The ratings of a 7805 are for the chip by itself, standing up in free air, the little metal slab providing all the heatsink needed UNLESS you overfeed the input side as G. said.

[phsyconoodler]

G.Hoffman,
  I saw your feature on TV. Very impressive! It is so incredibly cool that you would rather build guitars than be a college professor.We may have some nice guitar wood for you in the future.We have some plans in New Brunswick Canada.
 
 To the OP: The chassis can be your heat sink if it's aluminum.Aluminum dissipates heat extremely well.

[The_Gaz]

Thanks for the replies, I was planning on running the boards from a 6.3V or 5V AC supply. I was just wondering at what point (how much dissipation) a 7805, for example, needs to be heat sunk. Doug clearly did not see the need on his boards, and while I trust his experience, there's no way for him to know how may relays are going to be run down stream.

[PRR]

A TO220 can shed about 1 Watt naked.

What are your relays?

Say one relay at 50mA, 0.050A.

Say raw 9V.

9V-5V= 4V dropped in reg.

4V*0.050= 0.2 Watts in reg.

You can run several relays.

No harm will happen. With too-many relays, you can energize one or two, but when you go three or four the "energized" ones will drop out. If you put a meter on your "5V" you will see less than 5V. If you put a finger on the reg you will say "ouch". When 7805 over-heats, it shuts down. Nearly impossible to kill with excess current or excess power. (>35V is fatal.)

[G._Hoffman]

G.Hoffman,
  I saw your feature on TV. Very impressive! It is so incredibly cool that you would rather build guitars than be a college professor.We may have some nice guitar wood for you in the future.We have some plans in New Brunswick Canada.
 
 To the OP: The chassis can be your heat sink if it's aluminum.Aluminum dissipates heat extremely well.

That was about my dad, actually.  No one wants to do something like that about me, sadly.



Gabriel

[The_Gaz]

Thanks, PRR, that was very clear. The 5V relays I'm using only draw 35mA or so. I actually only plan on running two at a time, but sometimes you have to pretend like you'll be doing something extreme to get a detailed replay on these amp message boards: "I wanna run 10 relays off one of these suckers." If I would have said I was running single relay some would have said "It'll be fine" without qualifying the statement :) Thanks again all for the advice.