I have an output of 200mA DC and want to use it to switch 10A DC. 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?
This link was the first hit on Google.
Depending on what voltage the 10A is and the type of load (resistive or inductive) you might have to try one of the others on the page.
This is an AC relay. AC relays due to the natural impedance of the coil draw very little current.
These relays do not work with DC (at least for long). They draw too much current, overheat and burn out.
Have you checked automotive relays? Don’t know the signal current is, but headlight relays typically switch 10A or more.
We do this all the time with contactors, i.e. use a small relay to switch a large contactor.
What is the open circuit voltage for the load you’re trying to switch? In other words, when the relay contacts are open, what voltage will be across them?
When you switch the load, will you be switching when the load is on (and drawing up to 10 ADC)? Or will you be “dry switching,” i.e. only switching the load when the load is not drawing any current?
Another question just occurred to me: do you need isolation? If not, then a few MOSFETS connected in parallel (and to a heat sink) might do the trick. Though it should also be kept in mind that a MOSFET is not 100% “off” when it is off… there is a small bit of leakage current through the load when it is off.
Also, how quickly do you need to be able to do your switching? Are you talking something like the user flips a switch and the lights in the room come on, or do you need something that switches back and forth 60 times a second?
How about an SCR? 200 mA is too much for gate current, but you could easily limit it.
Just use two relays - a little sensitive one to control a much beefier one.
Use a power transistor (TO-220) to drive a suitable relay. Or search the Galco and Mouser catalogs for one that does the job. Pretty sure there are some very high capacity models with light coils.
It is an open collector output on an alarm control panel.
It says on the manual not to exceed 200mA and I want to use the OC to switch some lights that will draw around 8 amps.
But I found an automotive relay (as Tripolar suggested) rated for 30A that surprisingly draws only 160mA! For reference, a smaller 5A relay draws 120mA at the same voltage.
Just checked Galco and they have many relays with 10A+ contacts whose 12V relays work at well under the 2.4W you’re asking. They have a great categorical search tool, too, and ship small quantities.
Watch it with automotive relays - the contacts are not always suited to 120VAC switching.
What voltage are the lights operating at?
It is possible to run an AC relay coil on DC, as long as the voltage is set so that the rated current isn’t exceeded (in this case, 9.2 mA, knowing the DC resistance would help), since it is current, not voltage, that determines the magnetic field strength (amp-turns) and resistive losses in the coil.
Also, what is the voltage rating of the 200 mA output? In addition, since it is an open-collector output, using a transistor to drive the load would be a bit tricky, assuming it is normally high (open circuit); you’d want to use a pull-up resistor, then a small transistor (2N3904/2222 or equivalent), then a MOSFET (if the voltage is fairly low, you can easily find MOSFETs that can switch 10 amps without a heatsink; for example, this one (30 volts) has an on resistance of 3.4 milliohms, or 0.34 watts of power loss at 10 amps, about 2 watts max without a heatsink; I intentionally found a through-hole part but I often use SMT parts by soldering wires to them). Alternatively, you could use a PNP transistor (2N2907 or equivalent, 600 mA max) to drive a relay coil (connected between collector and ground, with a resistor between base and the OC output). In any case, don’t forget to put a diode across the relay coil.
Oh, almost forgot - if the DC voltage is fairly high, you could run into trouble with a relay since DC tends to arc when you break the circuit since current doesn’t go to zero like it does on AC (see this example, skip to 3:00; this is 2.5 amps at 80 volts, which causes the relay contacts to catch fire(!); this is also why switches and relays may be rated at 125-250 VAC but only 32 VDC or so).
The lights are 12V DC
In that case, which side of the line (+/-) is switched, or can you switch either of them? If you can switch the - line to ground, you can use the transistor circuit I described in my previous post, sort of like this circuit, except that Q2 is a MOSFET similar to the one I used as an example (to avoid the hassle of needing multiple MOSFETS and/or heatsinking). Note that D1 and R2 can be omitted if the output is open-collector and can handle 12 volts (might want to put a 12 volt zener on the MOSFET gate anyway) and this assumes the output is active low, which it probably is for an open-collector output. You could also use a relay (12 volts DC can be safely switched), but this circuit would be more reliable and cheaper (a similar version could be used to switch the + line, but you’d need a P-channel MOSFET, which are hard to find and expensive in low on-resistance versions).
While **Michael’s **approach would certainly work, a simpler solution would be to use this. It’s $30, but if time is money…
Found some cheaper ones. Here’s a 20 A SS relay for $12.95 and a 40 A SS relay for $16.95.
Ok, thanks, I’m done with the automotive relay for the time being, but these solid state relays look good. They do not mention the control current, but I guess it will be a few tenths of mA.
What supply voltage will be using with the open collector output and pull-up resistor? 12 VDC?
