What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

question about switches/amp rating

D

Desert Rat

Well Known Member
So I feel like I should be able to figure this out, but I'm drawing a blank here. Can anybody tell me this;

If a switch is rated for 10A at 125 volts, and 15A at 28 volts, how many Amps can it handle in a 12v system (yeah, I know, it's actually around 14v)

Specifically, this is a Honeywell 2TL1-2, sold by Stein.
 
Last edited:
Amps are amps, doesn’t matter the voltage. However, you might see different ratings for AC and DC depending on who’s doing the math;) , and the contact material and type.
That switch is rated for 15A AC. I did t find a DC rating.
 
A switch rated at 15 amps at 28 volts should be capable of switching at least 15 amps or more at 14 volts.
As a general rule, any snap action switch should be capable of switching the same current at 12 VDC as it can at 120 VAC.
The fact that the DC voltage is one tenth that of the AC voltage automatically de-rates the power carried by the switch.
Be sure to install a diode in parallel with an inductive load with banded end of the diode connected to positive.
A diode will minimize arcing across switch contacts, thus prolonging switch life.
 
Bob Nuckolls says...

At the bottom of page 2 of Bob Nuckolls' Switch Ratings, What's it all Mean? he says "In a nutshell, 125 VAC ratings equate favorably and conservatively to 14 VDC ratings - as long as the switch has a healthy "snap" action… all toggle switches and most rocker switches do.”

In this document, Bob covers many technical details including:
  • Current ratings.
  • Load types Inductive, Resistive, and Lamp (incandescent) you see on switch data sheets. He also covers Motor and Capacitive loads.
  • Toggle switch action numbering... the 1-3, 2-3, etc switch labels we see on schematics.... and in what applications they are useful.
  • Operating Range... the current levels switches typically operate in based on contact materials. Low Level, Power Level, and Power Level or Low Level aka Dual Rated (Note a Dual Rated switch reverts to a Power Level switch once it's used in a Power Level application.).
.
 
Last edited:
I feel there is much more to this than being discussed here. I have seen many dual rated switch TDS' that show substantially lower Amp ratings for DC over the AC rating. Also many switches have different ratings based upon the DC load type. Resistive and inductive often have different Amp ratings. There is a third category of DC that I am not remembering.
 
Last edited:
lr172 said:
I feel there is much more to this than being discussed here. I have seen many dual rated switch TDS' that show substantially lower Amp ratings for DC over the AC rating. Also many switches have different ratings based upon the DC load type. Resistive and inductive often have different Amp ratings. There is a third category of DC that I am not remembering.
Click to expand...

The 3rd category is "Lamp". I don't believe this is talking about lighting as that would fall into the resistive category, so I don't exactly know what it means. Perhaps one of the electro guys will chime in?

Per the data sheet, at 28v the amp capacity for this switch is

inductive = 15

Resistive = 20

Lamp = 5
 
Last edited:
Get Bob’s Aeroelectric book. There’s a couple pages dedicated to this subject.
Lamps have a pretty large inrush current.
 
The Electronic Design article has factual errors. It confuses inductive loads with lamp loads.

Incandescent lamps have very high inrush current, which is hard on switches and connectors. In my aircraft, I add inrush current limiters to my lamp circuits, since I wig-wag them in flight, and the current pulses normally can exceed the switch lamp ratings. LED lamps are OK.

The old fashioned gas-tube strobe power supplies are the worst of all. They have a characteristic of increasing load current as input voltage drops, which can cause thermal runaway on switches and connectors, resulting in failures. I have experienced this and I have photos of fried switches somewhere in my piling system.

The flip side of current rating is the voltage rating. As others have pointed out, fast-acting switches reduce contact erosion. Most significant for DC circuits is voltage spikes caused by switching inductive loads. These are normally protected with 1N5400 type diodes (example contactors). The forgotten ones are electric primers, pumps, and flap motors. It's good practice to use protection diodes for these types of loads as well. The flap motor needs back to back Transzorb type diodes because it runs both ways. If you have a flaps controller, these will be built in.

Of course, with Honeywell discontinuing the TL switches, we have a bigger proble…

V
 
Last edited:
I should have read the article more thoughtfully before referencing it.

Be careful with LED.
LED’s can have the highest inrush of all lamp sources, basically a dead short. However, it happens really, really fast. This was a huge problem when LED’s started to become more prevalent in commercial lighting systems before driver technology started to incorporate circuitry to mitigate the inrush.
Still, review all specifications for your loads and be conservative.

It isn’t the LED, it’s the driver that regulates the power to the LED. An LED by itself will try to draw infinite power, but obviously blows itself up very quickly.
 
Another factual error is the statement:
Conversely, an inductive (L) load, such as a transformer or electric
motor, will initially draw a large amount of "inrush current" when first switched
on before settling back down after a few seconds to the load’s full running
current.
Click to expand...
It is true that motors have high inrush current. It is not true
for other inductors. Capacitors resist any change in voltage. Inductors resist
any change in current. When first energized, an inductor will resist the flow of
current. That is just the opposite of large inrush current. When an inductive
circuit is switched off, the inductor opposes any change and tries to keep
current flowing the SAME direction. That induced voltage combined with the
power supply voltage tries to arc across the opening contacts of the switch.
 
You must log in or register to reply here.
Back
Top