Switching a very big current with a very small current? - Page 1
Forum section: electronics / beginners

author: dog80
author_rank: Regular Contributor
timestamp: November 23, 2012, 01:35:16 pm
content: I have an output of 200mA DC and want to use it to switch 10A DC (or more). There are relays with coils that work with 200mA or even less, but the max current they can switch is a couple of amps.I thought of using a small relay to drive a bigger relay that can handle 10A, but it looks a bit inefficient. Are there other ways to switch big currents with just 200mA?

author: 8086
author_rank: Super Contributor
timestamp: November 23, 2012, 01:40:09 pm
content: You should be able to find a relay that will switch 10A with less than 200mA coil current...What is your coil voltage? Assuming it's 5V:http://uk.mouser.com/ProductDetail/Panasonic-Electric-Works/ALQ105/?qs=sGAEpiMZZMtSzCF3XBhmWxlIP1AFKBDB2vzHMCD%252beUk%3dOf course, you could always use a MOSFEThttp://uk.mouser.com/Semiconductors/Discrete-Semiconductors/MOSFET/_/N-96nid?P=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&Keyword=mosfet&FS=True

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2012, 11:25:11 pm
content: Is it important that the two circuits are electrically isolated from one another?An ordinary BJT may do.

author: c4757p
author_rank: Super Contributor
timestamp: November 24, 2012, 12:53:18 am
content: I don't think I'd want to use a BJT - most BJTs with that kind of current rating don't have the hFEto do 10A with 200mA at the base (50). Many of the ones that do are pricey. I think I'd be more disposed to a MOSFET.Edit: Obviously a Darlington pair could work.

author: Psi
author_rank: Super Contributor
timestamp: November 24, 2012, 01:55:07 am
content: I'd just use a mosfet, there are plenty that can switch 10A no probs (rated for like 60A).And if you get a logic level fet it will switch with 5V using much less current than 200mA.(From what you describe it seems you just want human speed on/off, so there's no issue with switching frequency and gate capacitance)

author: tom66
author_rank: Super Contributor
timestamp: November 24, 2012, 01:57:09 am
content: I'd use a MOSFET too, if isolation isn't crucial. Although a Darlington may work, it will have a Vce of about 1.5V, so dissipate upwards of 15W. Even a terrible fet with 50 mohm resistance would dissipate only 5W.

author: NEDM64
author_rank: Contributor
timestamp: November 24, 2012, 03:46:58 am
content: Can't beat a mosfet for the matter.... don't forget a protection diode.@Psi, those mosfet's are nice, but costly, and the OP must have already the relay...

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 24, 2012, 04:20:02 am
content: costly? a 50a 60v nmos is like $0.50...i can't imagine a relay to be cheaper?

author: johnboxall
author_rank: Unknown
timestamp: November 24, 2012, 04:32:39 am
content: Quote from: dog80 on November 23, 2012, 01:35:16 pmhttp://www.freetronics.com/collections/modules/products/n-mosfet-driver-output-module

author: NEDM64
author_rank: Contributor
timestamp: November 24, 2012, 05:09:35 am
content: Quote from: peter.mitchell on November 24, 2012, 04:20:02 amCostly compared to a BS170 or so...He already has the relay switch...

author: dog80
author_rank: Regular Contributor
timestamp: November 24, 2012, 02:01:23 pm
content: Ok, some background.The output is 200mA @ 12VDC. It is an Open Collector output on an alarm control panel that I want to use to switch some LED lights (also working at 12VDC) and the documentation says not to exceed 200mA.Someone at another forum suggested to use an automotive relay. I thought it would draw too much current but I actually tested one (rated for 30A) and it drew only 160mA.So I guess I will go with the automotive relay solution. Thanks for all the replies!

author: Neilm
author_rank: Super Contributor
timestamp: November 24, 2012, 04:38:19 pm
content: One thing to check is that the relay is rated to switch the 10A. Quite often I have found relays that will carry the 10A but not switch them. They fail after a few operations.YoursNeil

author: Zero999
author_rank: Super Contributor
timestamp: November 24, 2012, 06:51:45 pm
content: Quote from: c4757p on November 24, 2012, 12:53:18 amYes, you're right, a MOSFET is probably a better choice here.Quote from: Neilm on November 24, 2012, 04:38:19 pmAre you certain you weren't using an AC-rated relay to switch DC?A typical 10A AC relay won't reliably switch DC - you'll need to de-rate it. Switching AC is easier because there's a zero current point when the arc is easier to break. Switching a DC inductive load is most difficult because stopping the current results in a high voltage spike.

author: T4P
author_rank: Super Contributor
timestamp: November 24, 2012, 07:04:37 pm
content: Relays need to be derated about 4 times for DC if rated for 240VAC = 60VDC

author: Neilm
author_rank: Super Contributor
timestamp: November 24, 2012, 08:23:36 pm
content: Quote from: Hero999 on November 24, 2012, 06:51:45 pmIt was a DC relay, but there was a limit on the switched power (75W if my memory serves)Neil

author: ElectroElvis
author_rank: Regular Contributor
timestamp: November 24, 2012, 09:28:59 pm
content: Where I am i havent found relays cheaper then 2€ which is more that 2.40$, and most of them are from company named Omron.I would make circuit with usage of Mosfet and diode dont forget that one.

author: tom66
author_rank: Super Contributor
timestamp: November 25, 2012, 01:52:38 am
content: Yeah, or just use a P-FET.

author: SeanB
author_rank: Super Contributor
timestamp: November 25, 2012, 05:44:28 am
content: As it is an alarm open collector output I assume it is active low. I would use a relay rated at 30A automotive and with a low power draw, as the 200mA rating is really an absolute max rating, they typically want to see a 50mA draw on these outputs. As long as the coil resistance is above 200 ohms it will work fine, lower than 120 ohms is not recommended. There are a lot of automotive relays that will do, often available cheap at the scrapyard if you look for them. I have gotten a few there myself.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 25, 2012, 07:12:51 am
content: If you use a relay, remember to put a diode across the coil!

author: SeanB
author_rank: Super Contributor
timestamp: November 25, 2012, 08:00:37 am
content: Alarm outputs have the diode built in, along with a 14V transient suppressor diode as standard parts. They know it will be connected to long wires and exposed to strong transients.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 25, 2012, 08:30:35 am
content: Quote from: SeanB on November 25, 2012, 08:00:37 amMost alarm outputs do, but I know some manufacturers that don't. That said, automotive relays often have they built in, but it's good practice to get into putting the diode across them, as with everything, they time you don't do it is the time you should have, plus, what is a diode worth?