Solid state relay/switch that needs low supply current (uAmps)? - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: March 28, 2020, 08:58:39 pm
content: Hi I'm looking for a solid state SPST Form B relay that can be activated with current in the order ofmicroamps. Even a simple digital switch that can work with a supply as low as 1.5-2V and microamp would do the job.All the relays I found need at least 1mA for activation.Thank you

author: David Hess
author_rank: Super Contributor
timestamp: March 28, 2020, 10:23:16 pm
content: If it does not need to be isolated, then any analog switch can meet that requirement.There are bistable latching relays which do not require any power to maintain state.If I wanted a minimum power all solid station solution with isolation, then I would pump charge through a pulse transformer to control one or more power MOSFETs. A forward polarity pulse through the transformer charges a capacitor which holds the MOSFET gates on. A reverse polarity pulse discharges the capacitor shutting the MOSFETs off.

author: Zero999
author_rank: Super Contributor
timestamp: March 29, 2020, 10:13:24 am
content: Quote from: raff5184 on March 28, 2020, 08:58:39 pmForm B means normally closed. David's idea is a good one, but will need depletion mode MOSFETS and the transformer set up to give negative gate voltages, to turn them off.Another option is to AC couple the signal via small capacitors, assuming the capacitance bridging the two circuits is acceptable and a slow off time isn't a problem. The resistors will need to be high value >1M for low current draw and a dual gate IC such as the 74HC2G14 will also help.If safety/mains isolation is required, use Y1-rated capacitors for C1 and C2 and adhere to the minimum creepage distances on the PCB.

author: raff5184
author_rank: Regular Contributor
timestamp: March 31, 2020, 12:19:19 am
content: thank you for your replies. Timing is not an issue. But what do you mean with isolation?

author: wizard69
author_rank: Super Contributor
timestamp: March 31, 2020, 12:57:07 am
content: I'm not convinced that detecting current is the easy way to go here. When this device is activated is there a signal you can tap into to indicate operation?Quote from: raff5184 on March 28, 2020, 08:58:39 pm

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2020, 03:16:08 am
content: Quote from: Zero999 on March 29, 2020, 10:13:24 amIt is commonly done with enhancement mode MOSFETs but either could be used. A similar circuit is used for transformer isolated gate drivers where amplitude does not vary with duty cycle.

author: Jwillis
author_rank: Super Contributor
timestamp: March 31, 2020, 04:52:39 am
content: Quote from: raff5184 on March 31, 2020, 12:19:19 amRelays are used to isolate between a low current low voltage control over high voltage high current. If that is your intention then simply using a transistor to boost the very low uA current signal to trigger a latching relay would work.

author: raff5184
author_rank: Regular Contributor
timestamp: March 31, 2020, 02:09:45 pm
content: I'm not sure if my question and problem were clear, so I'll try to clarify them, even though you have probably already answered. I need something very basic like Figure 2:https://www.allaboutcircuits.com/technical-articles/basics-of-ssr-solid-state-relay-the-switching-device/my circuit is DC and I have no timing constraints.I need avoltage controlled switch, something like these:https://www3.panasonic.biz/ac/e/search_num/index.jsp?c=detail&part_no=AQY412Shttps://www.mouser.com/datasheet/2/240/lcc110-1546141.pdfThe problem with those is that, if I understood correctly, the minimum LED current to make the switch toggle is typically of at least 1mA. Therefore, for all the duration that I need the switch to stay "active" I need to power the input LED with 1mA but that is too much. So I'm looking for an option that absorbs less current

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2020, 03:03:54 pm
content: Quote from: David Hess on March 31, 2020, 03:16:08 amIn this case they must be depletion mode because the original posted asked for a form B relay, which is normally closed, so the transistors will have to be normally on.Quote from: raff5184 on March 28, 2020, 08:58:39 pmhttps://forum.digikey.com/t/understanding-form-a-form-b-form-c-contact-configuration/811Quote from: raff5184 on March 31, 2020, 02:09:45 pmIn your first post you said you wanted a form B relay, which is normally closed.Can we see the bigger picture? What exactly are you trying to do? Please post a schematic of the circuit where this SSR is going to go.

author: raff5184
author_rank: Regular Contributor
timestamp: March 31, 2020, 03:11:19 pm
content: Quote from: Zero999 on March 31, 2020, 03:03:54 pmYes and I still need a N.C. type of relay/switch, but low current.Sure, I'll post a schematic

author: raff5184
author_rank: Regular Contributor
timestamp: March 31, 2020, 03:27:25 pm
content: It is the design of my other post here:https://www.eevblog.com/forum/projects/power-consumption-of-circuit/msg2985292/#msg2985292This is the schematic:https://www.eevblog.com/forum/projects/power-consumption-of-circuit/?action=dlattach;attach=957974I want a N.C. switch on the right to replace the CPC1114N

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2020, 04:40:54 pm
content: Quote from: raff5184 on March 31, 2020, 03:27:25 pmWhy create a new thread? If you'd stuck to the other one, you could have saved both yourself and everyone else a lot of time.What does the CPC1114N switch?The logic of the comparator circuit can be reversed an a normal open relay can be used.

author: raff5184
author_rank: Regular Contributor
timestamp: March 31, 2020, 05:55:15 pm
content: Quote from: Zero999 on March 31, 2020, 04:40:54 pmIt closes the power on a PCB that has FPGA, microcontroller, ADCs...The comparator as you can see has hysteresis. Its output is high (switch is OPEN) when Vin goes from 0 to 4.4V. At 4.4V output goes LOW (switch goes to NC position). Then, output of comparator goes high again only when Vin goes below 2.2V. Notice that I changed the values of the resistors according to the suggestion in the other thread.Yes, I tried to reverse the logic but probably did something wrong and it wasn't working

author: Zero999
author_rank: Super Contributor
timestamp: April 01, 2020, 08:06:21 am
content: Quote from: raff5184 on March 31, 2020, 05:55:15 pmThen power the comparator off the same power supply as the FPGA/MCU PCB and use it to switch a MOSFET.

author: raff5184
author_rank: Regular Contributor
timestamp: April 01, 2020, 10:52:01 am
content: Ok I'll try with a MOSFET. Buy why do I need to power from the other source?

author: Zero999
author_rank: Super Contributor
timestamp: April 01, 2020, 02:37:59 pm
content: Quote from: raff5184 on April 01, 2020, 10:52:01 amWhy not?Obviously if the comparator isn't powered from the super capacitor, then it won't discharge it.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: April 01, 2020, 11:22:18 pm
content: You can get photo voltaic opto-couplers to supply enough power to switch mosfets.

author: Zero999
author_rank: Super Contributor
timestamp: April 02, 2020, 10:44:47 am
content: Quote from: nigelwright7557 on April 01, 2020, 11:22:18 pmThe trouble is they require milliamps for activate but the original poster wants something which will work with a few µA.

author: Ian.M
author_rank: Super Contributor
timestamp: April 02, 2020, 12:02:28 pm
content: Hmmm, (ignoring the O.P's requirement clarification and considering the more general case he originally requested), if gate leakage current of the MOSFETs is low enough, a pair of back to back MOSFETs driven by a 4000 series CMOS flipflop powered by a small 12V alkaline battery (e.g A23) could be used to implement a latching SSR, good for close to the shelf life of the battery. Use pulse transformers to drive S and R as optocouplers could cause excessive battery drain. Although it would need significant input current to switch, that would only be for a few tens of us S or R pulse, so the average input current could be sub 1uA.

author: raff5184
author_rank: Regular Contributor
timestamp: April 04, 2020, 09:05:10 pm
content: Quote from: Zero999 on April 01, 2020, 08:06:21 amYou mean a p-channel type? I'll need to order some. I only have n-ch type, meanwhile can I use it so it works in the same way?

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2020, 09:41:36 pm
content: Quote from: raff5184 on April 04, 2020, 09:05:10 pmIf switching the 0V side is acceptable, then you could use a N-channel MOSFET.Looking at the other thread, you've got the TLC3702 running from the LM285LPE, givign it only 2.5V. Not only does this waste power, but the TLC3702 is only specified to work down to 3V, so it might not even work. Run it from an unregulated voltage between 3V and 16V. The unused section can also be used as a logic inverter, if you want the opposite output logic, without having to change any of the resistors and inputs round.

author: raff5184
author_rank: Regular Contributor
timestamp: April 04, 2020, 10:37:38 pm
content: The ideal would be that the MOSFET allows current only when gate voltage (output of comparator) is LOW. That's why I thought of a p-ch MOSFET.I actually removed the LM285LPE driving the TLC so I don't waste power. I'll use an unregulated voltageIf I use the other section of the TLC3702 (that I now disabled connecting the input terminals to V+ and GND using resistors) doesn't it require twice the power? But regardless, would reversing Vref and Vin on the input pins of the second comparator work, I mean Vref to IN- and Vin to IN+ ?

author: Zero999
author_rank: Super Contributor
timestamp: April 05, 2020, 11:44:01 am
content: It makes no difference whether the other comparator is doing anything useful or not, since it's on the same IC and will therefore be drawing current irrespective. It's much easier just to use it to invert the signal and drive an N-channel MOSFET, than modifying the circuit to invert the logic. Some of the resistor values could also be increased to reduce the current further and a decoupling capacitor is recommended, as close to the comparator IC, as possible.

author: raff5184
author_rank: Regular Contributor
timestamp: April 05, 2020, 12:17:34 pm
content: Fantastic! Very clear. Thank you so much, can't wait to test it