Proper way to wire up the fogs w/out fog switch?
#11
02-16-2012, 01:26 AM
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A headlight switch that can safely handle the current costs much more than the relay, and you will need heavy gauge wire to the switch and back to the lights, which also costs more.
Ever wonder why most switches are rated for AC only? Because the DC rating at the same voltage is 1/30th to 1/60th the AC current rating, and it's embarassing to stamp that piddly current rating on the switch housing.
Drop to voltage to 1/10th (ie 12VDC on a 125VAC rated switch) and you might be able to go 1/10th the AC rated current (Amps), but the reality is there is no real rule; it's all guessing, since the way the switch is constructed affects that ability.
That's because with AC the arc across the switch terminals is self-extinguishing since there is zero voltage 60 times a second. The zero voltage point will kill the arc. That doesn't happen with DC ... its full current 100% of the time. There are many AC-rated switches that won't handle DC at all; the contacts fail rapidly.
So, to have the equivalent safe operating load of a $10 Bosch-type 30A DC relay, you might need a switch that would be capable of switching 300A house current.
YOu can get switches made specifically for large-ish DC loads, but you will pay 3x as much for it as a similar AC-rated switch. And they usually are huge things; they need more distance between the contacts to kill the DC arcing.
Just use the relay, already.
You only need heavy gauge wire from the battery to relay and then to the lights. You can use light gauge wire from your light-duty switch in the cab to the relay, because that side of the circuit is very low current.
I know, I know, you see nice fancy AC-only rated switches operating directly, no relay ... in automotive installations all the time. You're right, you do. But, try that in a Cessna or a homebuilt airplane. You'll be doing it over, guaranteed. The airplane guys are funny about fires.
A 50 watt lamp is going to draw about 4+ Amps; run two and that's 8A 12VDC. That's too much for any AC-rated toggle or rocker switch.
You can either get a real automotive headlight switch or use the relay. These relays will have about 150 mA DC on the low current side; you can use almost any switch you like for that current.
Assuming two 50 watt lamps, with a 20% safety margin = 10A, then 14Gauge wire is OK if the total length of the circuit is less than 8 feet (battery to relay to lamp to lamp to ground). 12GA is good for a 13 foot run. The wiring on the low current side of the relay (fuse box/headlight switch to fog lamp switch to relay to ground) can be almost anything; down to 20 GA.
If you were to run a switch with no relay, you need 10GA for everything if the total run is under 22 feet; 8 GA will cover you for 33 feet.
Put a fuse in the high current line.
Hot Tip: don't buy your Bosch 30/40A relays from auto parts stores. Electronics parts companies sell them cheap ... PartsExpress sells them for $2.09 each. Look on the sidebar of that page if you want genuine Bosch ... now Tyco ... for about $4.50.
http://www.parts-express.com/pe/show...number=330-073
Wiring Diagram (see page 14)
http://www.parts-express.com/pedocs/...tion-guide.pdf
Ever wonder why most switches are rated for AC only? Because the DC rating at the same voltage is 1/30th to 1/60th the AC current rating, and it's embarassing to stamp that piddly current rating on the switch housing.
Drop to voltage to 1/10th (ie 12VDC on a 125VAC rated switch) and you might be able to go 1/10th the AC rated current (Amps), but the reality is there is no real rule; it's all guessing, since the way the switch is constructed affects that ability.
That's because with AC the arc across the switch terminals is self-extinguishing since there is zero voltage 60 times a second. The zero voltage point will kill the arc. That doesn't happen with DC ... its full current 100% of the time. There are many AC-rated switches that won't handle DC at all; the contacts fail rapidly.
So, to have the equivalent safe operating load of a $10 Bosch-type 30A DC relay, you might need a switch that would be capable of switching 300A house current.
YOu can get switches made specifically for large-ish DC loads, but you will pay 3x as much for it as a similar AC-rated switch. And they usually are huge things; they need more distance between the contacts to kill the DC arcing.
Just use the relay, already.
You only need heavy gauge wire from the battery to relay and then to the lights. You can use light gauge wire from your light-duty switch in the cab to the relay, because that side of the circuit is very low current.
I know, I know, you see nice fancy AC-only rated switches operating directly, no relay ... in automotive installations all the time. You're right, you do. But, try that in a Cessna or a homebuilt airplane. You'll be doing it over, guaranteed. The airplane guys are funny about fires.
A 50 watt lamp is going to draw about 4+ Amps; run two and that's 8A 12VDC. That's too much for any AC-rated toggle or rocker switch.
You can either get a real automotive headlight switch or use the relay. These relays will have about 150 mA DC on the low current side; you can use almost any switch you like for that current.
Assuming two 50 watt lamps, with a 20% safety margin = 10A, then 14Gauge wire is OK if the total length of the circuit is less than 8 feet (battery to relay to lamp to lamp to ground). 12GA is good for a 13 foot run. The wiring on the low current side of the relay (fuse box/headlight switch to fog lamp switch to relay to ground) can be almost anything; down to 20 GA.
If you were to run a switch with no relay, you need 10GA for everything if the total run is under 22 feet; 8 GA will cover you for 33 feet.
Put a fuse in the high current line.
Hot Tip: don't buy your Bosch 30/40A relays from auto parts stores. Electronics parts companies sell them cheap ... PartsExpress sells them for $2.09 each. Look on the sidebar of that page if you want genuine Bosch ... now Tyco ... for about $4.50.
http://www.parts-express.com/pe/show...number=330-073
Wiring Diagram (see page 14)
http://www.parts-express.com/pedocs/...tion-guide.pdf
Last edited by Johnny2Bad; 02-16-2012 at 02:19 AM.