Switches VAC amp rating to VDC amp rating - Page 1
Forum section: electronics / beginners

author: geratheg
author_rank: Regular Contributor
timestamp: March 14, 2015, 10:32:13 pm
content: So I need to use a switch that can handle about 20A of current with 12V.I found this:http://www.ebay.com/itm/Heavy-Duty-20A-125V-DPDT-6-Terminal-ON-ON-2-POS-Toggle-Switch-w-Waterproof-Boot-/111415578187?pt=LH_DefaultDomain_0&hash=item19f0e2ce4bBut it's rated for 20A at 125VAC.Would it be able to handle 20A at 12VDC?Thanks!

author: ludzinc
author_rank: Unknown
timestamp: March 14, 2015, 11:21:27 pm
content: Probably not - most switches have lower current rating at DC than AC (I think it due to breaking capacity).Is there a reason to not use a relay?

author: Vgkid
author_rank: Super Contributor
timestamp: March 14, 2015, 11:28:30 pm
content: I wish they gave a part number.

author: electr_peter
author_rank: Unknown
timestamp: March 14, 2015, 11:29:47 pm
content: Search around on google about AC vs DC rating for switches.Bottom line is - typical switch will have much smaller voltage/current rating for DC vs AC. DC creates much bigger destructive non-self distinguishable spark ().Check switch datasheet to be 100% certain. DC current require much beefier contacts.

author: picandmix
author_rank: Frequent Contributor
timestamp: March 14, 2015, 11:47:43 pm
content: Search on 'car 12v 25amp switch' and you should find at lot.If you are switching a true full 20A then get a switch of at least 25A / 30A or higher rating otherwise it will not last long or burn out.

author: Psi
author_rank: Super Contributor
timestamp: March 15, 2015, 12:41:15 am
content: Yep, automotive switches would be your best bet.They are designed for switching high current DC with a long lifespan.Another option is a automotive relay controlled by a more generic switch.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2015, 03:57:17 am
content: Yes, it will be OK.DC over about 30V becomes a big concern with being able to open the switch. DC arcs do not self-extinguish easily, and this becomes a big problem beyond 30V.Tim

author: geratheg
author_rank: Regular Contributor
timestamp: March 15, 2015, 04:15:25 am
content: Thanks for all the responses!It won't be continuous at 20A, I just rated that as the max I would need it to handle.I looked into relays, but kind of wanted a toggle switch and didn't want it to be too bulky by adding a relay in addition to the toggle switch.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2015, 11:28:11 am
content: There are many good reasons you might still use a mechanical switch, but don't leave MOSFETs out of consideration either. They're an excellent choice for low voltage DC.Tim

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2015, 12:25:33 pm
content: Quote from: T3sl4co1l on March 15, 2015, 03:57:17 amIt also depends on the load. If it's inductive then the instantaneous voltage at the point of the switch opening will be much higher, possibly as much as several kV.

author: geratheg
author_rank: Regular Contributor
timestamp: March 15, 2015, 05:13:46 pm
content: Is a motor considered inductive or resistive?

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2015, 06:33:49 pm
content: A motor will be inductive so it's even worse.

author: wraper
author_rank: Unknown
timestamp: March 15, 2015, 06:51:45 pm
content: Usually switches/relays have the same DC current rating as AC but at lower voltage.

author: SeanB
author_rank: Super Contributor
timestamp: March 15, 2015, 06:56:05 pm
content: Not always. A switch might be rated at 16A resistive at 250VAC, 10A inductive at 250VAC and only 2A resistive or relay drive at 28VDC. They may also specify the relay to be used must incorporate either voltage clamps or a parallel resistor to limit the voltage across the switch during the opening time.

author: geratheg
author_rank: Regular Contributor
timestamp: March 15, 2015, 07:10:05 pm
content: Thanks for the responses. Looks like a relay would be the best way to go. Just wanted to avoid extra costs and power consumption as would be the case with a relay since it also requires a toggle switch, but if it isn't too much then it shouldn't be a problem.I decided I need a 3PDT switch. Are relays rates for VDC?Will this work?http://www.walmart.com/ip/MAGNECRAFT-389FXCXC1-12D-Relay-Power-11-Pin-3PDT-20A-12VDC/41243410Or this:http://www.digikey.com/product-detail/en/S32F/360-2086-ND/1007143

author: Paul Moir
author_rank: Frequent Contributor
timestamp: March 16, 2015, 12:37:22 am
content: I think the digikey one should be fine. If you check the datasheet, it says it's good for 20A DC.Note you may be able to find such a switch cheaper at a shop that deals with automotive, hydraulics or trailers.If you're worried about contact life, you should do something about arc suppression:https://www.idec.com/language/english/AppNotes/Relays/contact_circuit_protection.pdfIt can make a huge difference.Are you reversing the motor, such as FWD-STOP-REV on the switch?

author: geratheg
author_rank: Regular Contributor
timestamp: March 16, 2015, 01:00:03 am
content: Nope, I'm not reversing the motor.

author: Zero999
author_rank: Super Contributor
timestamp: March 16, 2015, 10:35:17 am
content: That relay only specifies the current at AC.The switch is your best bet as the current is specified at 30VDC.If you only need one pole then you can connect the three sets of contacts in parallel to spread the load between them and increase the life.

author: ConKbot
author_rank: Super Contributor
timestamp: March 16, 2015, 01:47:12 pm
content: Quote from: Hero999 on March 16, 2015, 10:35:17 amSeries switch contacts, not parallel. If you series them, then the current has to flow though all them, and as it arcs as all 3 are opening, your arc length increases rapidly and quenches faster. Parallel, if one opens even moments after the others, it will be the only one to draw an arc, so only one contact is extinguishing the arc. For non breaking current (I.e. just passing current though a closed switch) 20A is 20A, so if a switch can handle 125V @ 20A, it can handle 12, 24, whatever, VDC at 20A. Its transitioning from open to closed which is hard on the switch (much like mosfets and switching losses) Learned this from an ABB rep, trying to spec a contactor that would need to break 28V @ ~25A.That being said, most switches that are dual rated, for 125VAC usually have a 28-30 VDC rating of about the same current. So 12VDC @ 20 A on a switch rated for 125V @ 20A probably wouldn't be horrible to it. Would I spec one without the rating on a product to be manufactured? Nope. Would I used one for a one-off or a replacement/repair? Probably.

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2015, 02:24:14 pm
content: The switch from DigiKey is rated for 20 A at 30 V for DC.With specifications, one must be careful not to choose the numbers that one likes without checking that they correspond.

author: geratheg
author_rank: Regular Contributor
timestamp: March 16, 2015, 11:44:04 pm
content: Quote from: ConKbot on March 16, 2015, 01:47:12 pmHow would you connect them in series?

author: ConKbot
author_rank: Super Contributor
timestamp: March 17, 2015, 05:21:11 am
content: On a dpdt, if you're only switching a single line of and on then feed your power into one common terminal, jumper your two NO terminals and hook the second common terminal to the load. Or if your load is floating (not most stuff in automotive applications that is grounded to the chassis) you can switch both the power and return leads, which puts both switches in series with the power source and load too.