# Creating DPDT Solid State Relay with FETs

I feel like this should be a very simple question but so far I have not found a solution. My goal is to make the following DPDT relay circuit using MOSFETs:

The goal is to connect INX to OUTX or OUTY and INY to OUTY or OUTX depending on the state of an input signal (I have two data lines available for the switch control). Normally I would use a mechanical relay but due to the nature of my project this really won't work, I need a solid-state option. DPDT solid-state relays are very difficult to come by, and any I have found cannot be connected to work in the way shown in the image.

I was looking at using two enhancement mode and two depletion mode N-channel MOSFETs, but since a FET needs a G-S voltage to switch on I would not be able to simply pass the data signals straight through the FETs as they need to be isolated from ground.

So how might one construct a DPDT solid state relay entirely out of MOSFETs that would allow them to swap the two outputs (connecting INX to OUTY and INY to OUTX, or vice versa)?

EDIT: I forgot to mention, I had been looking at using bilateral switches but most of them can only handle about 30mA. These particular data lines can carry up to 200mA, so I'm looking for a ~500mA solution (hence using discrete FETs).

My thought was to connect them as follows (please pardon the crudity of the drawing, it had to be done in MS Paint, and I didn't want to put detail of the FETs and all in it. Just know that each pair of wires on the output are effectively connected to a switch:

• Is that not basically an H-Bridge?
– JRE
Jun 11, 2015 at 13:34
• Not quite. An H-bridge simply switches direction of current through the load. I want to be able to swap data lines based on the "position" of the "switch" Jun 11, 2015 at 13:41
• ^this. You can search for ready made H bridges, they usually include a drive circuit for the high side mos. If you just wanna switch some signals what you are searching for is passgates. Jun 11, 2015 at 13:41
• So, connect your one data line to the top and one data line to the bottom of the H-Bridge. Your outputs are where the motor would go.
– JRE
Jun 11, 2015 at 13:43
• I would still need the G-S voltage to switch the FETs, which means the return of my control signal would have to be connected to the data lines, no? Jun 11, 2015 at 13:44

You could combine multiple SSRs to do this. Two form B and two form A will give you a DPDT SSR. Cost would be fairly high (about $20 in singles) but this is really a weird set of requirements. Switching will be slow. For example, two LCB710 and two CLA230 (IXYS) would handle 700mA. You could also buy PV optoisolators and use them to drive back-to-back MOSFETs for each switch. Since the former come in pairs you'd end up with 8 parts (plus maybe 4 resistors). Again, slow switching. Edit: Since you imply in the comment below that there is no requirement for power-off normally closed, the simplest method is probably to use two dual form A SSRs such as TLP222A-2. Again you could use dual MOSFETs and PV drivers, but I don't see much point in that since the current requirements (and presumably your voltage requirements) are met by common parts. • I agree, this is a very weird set of requirements, and I appreciate your help. I was hoping to keep the part count as low as possible to avoid significant switching time differences between chips. Otherwise I would use 4 SPST SSRs which are very cheap and plentiful. If I must use more than one or two devices, then I must. I was just hoping there would be a way to do this with, say, 4 discrete transistors Jun 11, 2015 at 14:07 • Okay, you mentioned depletion mode which I took to likely be a requirement that it conduct with no power. Pls. see edit. Jun 11, 2015 at 14:13 • I mentioned depletion mode in case I could use my two control lines in a bipolar way (switch "direction" of the current to switch on a different relay "contact"). It's funny you should show the opto-relay above, I was just looking at this: download.siliconexpert.com/pdfs/2014/7/28/1/10/31/351/aro_/… Jun 11, 2015 at 14:16 • I have made an edit to my initial post. What I added allows the use of only two devices, and though it's still not ideal it may still work. Jun 11, 2015 at 14:37 As the OP said, it is very difficult to find DPDT SSR (solid state relays). But there are a some ways around this; use two SPDT relays, like this one, or use one DPST-NO and one DPST-NC as proposed in another answer, or finally, use two DPST, with an inverter in front of the control for the second relay. For this particular application, the need to handle relatively high currents (at least 500 mA) with a low on-resistance weere the critical specifications. I found this DPST C347S SSR which is rated for 1A. 150 mΩ. So you would need two, and use an inverter in front of the second IC's control lead.$8.09 @ in singles, \$4.72 in thousands.

• I think you and I are on the same track. This sounds very much like what I've done in the latest edit of my original post =) Jun 11, 2015 at 17:55

I think I found the simplest option: Instead of a "Solid-State Relay" like I kept asking for, what I think I was actually looking for was a DPDT analog switch like the NLAS44599 (Datasheet: http://www.onsemi.com/pub_link/Collateral/NLAS44599-D.PDF). The functional diagram looks like this:

I hope this might help someone in the future who's looking for the same thing I was. Sure, it's a 16-pin device, but it does exactly what I need it to.

this is the digital equiv of an h circuit, fwd rev motor drivers. try the tlp222 optical isolated fet driver, and if the current demands are really high follow the design and use opto isolators to keep one side that is on from inverting the side that is off and reverse biasing the fets. some diodes might come in handy during development

• I suggest you read some of the comments, this is not quite the same as an H-bridge. Jun 11, 2015 at 14:22
• sorry h bridge. out of pete's French roast. It is not too much different. what the objective here, is to make a bidirectional on off switch. right? Jun 11, 2015 at 14:41
• Nope, not at all. Please read my initial post and the comments. Jun 11, 2015 at 14:47
• well good luck, use gloves and goggles, the one I have on my boat works at 40 amps 36 volts. Jun 11, 2015 at 18:35

I would approach this as follows:

Use a pair of photovoltaic optos to generate an isolated voltage source.

Connect enough N-channel MOSFETs (back-to-back in pairs) as you need. That would be 4 pairs.

Finally, use opto-isolators to drive the gates of the MOSFETs. You want to use the optos in a tptem-pole configuration so as to minimize the current from the photovoltaic source.

I'll draw up a quick schematic when I get in front of a computer.

This gives you the ability to switch very quickly and handle as much current as you need.

I've done it for a bms using discrete FETs

I've looked at analog switches as well, but the issues I seem to run into is the difficultly in managing the supply and common mode voltages