I am trying to build a system where first i would charge the battery (3.7V,40A), and then discharge it through a high wattage Resistor. For this purpose i will be connecting a power supply (15V-50A) with my AC mains. It would be followed by a synchronous buck converter which would charge the battery (being controlled via PWM signal by the micro-controller). The purpose of using a synchronous buck converter is its bidirectional current nature, which mean it would be able to supply the current from the power supply to the battery (during charging) and from the battery to the Load (during discharging). I chose IR3553 from International Rectifier Synchronous Buck Converter to be used. The rough idea i have drawn, and it can be seen in the image below.

As can be seen in the image there's a switching device which would help me (by the help of my micro-controller) to select between the power supply (during charging operation) and Resistor (during discharging operation) drawn in red. So my questions are :

1) What device would this be ? Is it a relay ?

2) I was going through relays online and they are used for these purposes. But what i also found was that there are different kind of relays mainly Electro mechanical relays and solid state relays. I also found in one article online that electro mechanical relays are used when current is upto 15A. And solid state relays have higher currents upto 100A. So if my power supply is providing a max of 50A current, does that mean that i will have to use Solid state relays ?

3) Also I found out that relays are present with different no. of poles for eg 2,3,4,6,8 pole relays are there. In my case i think a single pole relay would do the job, am i right ?

4) What other things should one look for in relays data sheets ? for eg in my case (15V-50A) what things i should be concerned about when i am selecting a relay ?

Thank you. enter image description here

  • \$\begingroup\$ On your previous question I note that you neither upvoted any answers nor "accepted" the answer most useful to you: electronics.stackexchange.com/questions/173593/… \$\endgroup\$
    – Andy aka
    Jun 16, 2015 at 13:41
  • \$\begingroup\$ i am sorry, i am new to this community, like i joined it a couple of weeks back, i didn't know that one has to do so, but now i do. \$\endgroup\$
    – yiipmann
    Jun 16, 2015 at 13:43
  • \$\begingroup\$ That's no problem. BTW did you get the device to work in reverse as suspected that it would? \$\endgroup\$
    – Andy aka
    Jun 16, 2015 at 13:51
  • \$\begingroup\$ Be mindful of the fact that if you get the IR-chip to work in reverse, you may have to keep it on, since it's minimum VCC level of 4.5V is above your battery voltage. Once you let it turn off, you may not be able to get it to turn back on again. Just an implementation detail you should be mindful of once you get it to work as you hope. \$\endgroup\$
    – Asmyldof
    Jun 16, 2015 at 14:47
  • \$\begingroup\$ @Andyaka sorry for such a late reply, i didnt order that synchronous buck converter IC IR3550 about which i also posted a question . And the reason is that this IC is ONLY available in PQFN package and unfortunately i dont have the facility for its soldering. So now i will be building up a synchrounous buck converter my self using NCP5351 driver IC and IRlb8748pbf NMOS power transistors. \$\endgroup\$
    – yiipmann
    Aug 13, 2015 at 13:32

2 Answers 2


You can certainly use an electromechanical switch, but at these current levels it's called a contactor https://en.wikipedia.org/wiki/Contactor. You'll need a fairly hefty circuit to drive its coil, too, so you won't be able to just connect it to your MCU.

In your case, you only need the simplest form of contact arrangements, single pole, single throw (SPST).

The obvious alternative to a contactor is a solid state relay (SSR), and some good information about using them is here http://www.sos.sk/a_info/resource/Crydom/Selecting%20a%20solid%20state%20relay.pdf. You can get current ratings in excess of 100 amps, so that's not a problem. If you do decide to go this route, be aware that you MUST use a DC-rated SSR, NOT an AC-rated unit. If you mistakenly use an AC SSR you will not be able to turn it off.

Rolling your own is possible, using commercially-available MOSFETs. Just be aware that dealing with 50 amps has its own peculiar set of problems. Things like getting enough copper trace on a pc board to keep from burning your traces. Things like making contact with the load-carrying wires mechanically strong enough (the wires are thick and unwieldy) to be reliable. And being absolutely certain that you get enough gate drive to turn the MOSFETs fully on. It's certainly doable, but not necessarily on the first try. It's a classic case of doing it yourself or paying someone else for their experience in how to do it right.

  • \$\begingroup\$ You said that i will be needing a single pole single throw relay. But after going through it i came to the conclusion that may be i will be needing a single pole double throw relay. As i want to SWITCH BETWEEN my power supply and the load, rather than just connecting or disconnecting some single device. And after going through SPDT relays i think they would be my choice. What do you say about it ? Am i right ? Also shown in this image \$\endgroup\$
    – yiipmann
    Jun 18, 2015 at 12:14
  • \$\begingroup\$ Oops. Sorry. You are correct, and I forgot aaaall about the discharge requirement. SPDT it is. \$\endgroup\$ Jun 18, 2015 at 14:59
  • \$\begingroup\$ So does that mean i should look for a DC SPDT solid state relay with vdc >= 12VDC and Load current >= 50A ? Because i have been looking for quite some time and still i haven't found any device with such current rating. Most of them i am coming across are rated at load current <= 2A. \$\endgroup\$
    – yiipmann
    Jun 18, 2015 at 16:18
  • \$\begingroup\$ Also SSR SPDT relays are very rarely available online (also with a very low load current rating). Mostly SSR are available in SPST. So does that mean i will have to look for contactors ? Or i can find SSR SPDT rated at my requirement ? As you said above and i also found out online that one has to design driver circuitry for contactor/electromechanical relays whaen driven by MCU, so i think my preference would be to find a suitable SPDT SSR. Can you help me finding one please ? \$\endgroup\$
    – yiipmann
    Jun 18, 2015 at 16:45
  • \$\begingroup\$ You're looking in the wrong places. Try Digikey, Mouser, Allied Electronics, Arrow, Future, etc. If you absolutely must, you can try eBay, searching for "solid state relay DC output 40A" - but you'll get Chinese stuff of unknown parentage, buyer beware. For instance, ebay.com/itm/… will do you, as long as you trust it. Also, be aware that you'll need a decent heat sink - cheap SSRs can have a 1 volt drop, so you might get 40-50 watts. \$\endgroup\$ Jun 18, 2015 at 18:40

You can certainly use a relay but I'd be tempted to use MOSFETs also. The top wire of the load could be connected to the sync buck regulator input all the time with a beafy N channel device grounding the lower end of the load. This makes it fairly easy to control.

As for disconnecting the "commercial voltage source", given that in boost mode the voltage from the buck's input can be significantly higher that the voltage from the "commercial voltage source", you could use a diode in the voltage source's output that would normally feed current when charging but then blocks in boost mode.

This basically means an N channel fet and a diode: -

enter image description here

  • \$\begingroup\$ Your idea seems very helpful. But i think as WhatRoughBeast said in the above post that i will have to take care of other things too while implementing this technique(PCB tracing for e.g), i think i should not opt it as i have never ever done pcb designing before and it would be my first time. So may be i should go for the Relays. \$\endgroup\$
    – yiipmann
    Jun 18, 2015 at 12:19
  • \$\begingroup\$ If you are drawing 50A from the "boosted" output then the current taken from the battery is going to be more. If you haven't designed the buck converter correctly (to cope with +50A in reverse boost mode) then the whole implementation is going to fail. I have presumed that your buck converter is designed by you (hence your previous question about the IR device) and that because you are designing the buck, implementing a power rail switch is relatively more simple. Are you not designing the buck/boost circuit? Basically, any MOSFET used as the synchro FET will have to cope with currents >50A. \$\endgroup\$
    – Andy aka
    Jun 18, 2015 at 12:48
  • \$\begingroup\$ So if i use a N channel Power Transistor like This One it would mean that i would be dissipating (@40A) around 1600x4.3mOhm = 7 Watts of power in the transistor right ? Dissipation is okay with me as my main aim IS to dissipate power. But what about heating of the transistor ? would i need to use a heat sink for that ? or shall i drive my power transistor in PWM mode ? \$\endgroup\$
    – yiipmann
    Jul 6, 2015 at 12:20
  • \$\begingroup\$ I'd reckon on maybe 8 to 10 watts and a heat sink will be required if discharging for a more than a few seconds. Driving the FET is PWM mode is irrelevant unless you want to apply a pulsing load. \$\endgroup\$
    – Andy aka
    Jul 6, 2015 at 12:37

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