Solid state relay for switching a negtive rail - Page 1
Forum section: electronics / projects

author: Sariel
author_rank: Regular Contributor
timestamp: November 02, 2024, 05:54:26 pm
content: Hello,I am planning to use the CPC1706Y to switch between different sense resistors for measuring input current across various ranges (A, mA, μA).I need to measure input current on both positive rails and a negative rail (~24V).Here are my questions:1. How should I wire the relay for the negative rail connection? Can the FET inside the SSR handle a negative voltage? Should I reverse the connections of pins 3 and 4?2. Is there a minimum current that these relays can conduct effectively? Can they handle very low currents, such as in the microampere (μA) or even nanoampere (nA) range, while still functioning correctly?Thanks!

author: inse
author_rank: Super Contributor
timestamp: November 02, 2024, 07:23:35 pm
content: The MOSFET inside the relay works as every other MOSFET, it’s a switch in one direction and a diode in the other - see the polarity indication.You should check also the second page of the datasheet whether on-resistance and off-state leakage fit to your application.

author: ArdWar
author_rank: Frequent Contributor
timestamp: November 03, 2024, 02:59:52 am
content: Yes, you have to "reverse" the SSR for the "reverse" rail switching with this particular SSR since this one doesn't use back-to-back connected FET like most of this kind do.There are no minimum load current (or voltage for that matter) for FET-based SSR (unlike SCR-based one). However if you're switching at uA level you have to look at its off-state leakage current. There are generally compromises between parts performance. Usually devices designed with low ON resistance (i.e. high current, like this one) will have comparatively larger leakage current and output capacitance.

author: Sariel
author_rank: Regular Contributor
timestamp: November 03, 2024, 07:56:10 am
content: Thank you for your response.Could you share a part number that illustrates how an SSR with internal back-to-back FETs is constructed?Additionally, would an SSR designed for AC or both AC and DC perform better when connected to a negative voltage?I am considering connecting the SSR according to the attached diagram. Will it function correctly? In this setup, both the drain and source will be at 15V. Will there be sufficient VGS to turn on the FET, given that the source is not at 0V potential?Am I using the SSR properly in this configuration?Thanks again!

author: voltsandjolts
author_rank: Unknown
timestamp: November 03, 2024, 11:11:27 am
content: Other than (probably erroneous) polarity symbols on the SSR output, I don't see anything suggesting this device has a 'polarised' output. I expect it's just a standard SSR output which doesn't care about polarity. Just use it as a switch, within the 4A current limit.Also note the SSR on resistance is almost the same as your 100mOhm sense resistor, which will cause a large reading error. You could calibrate this out but it would likely have poor temperature stability.

author: fchk
author_rank: Frequent Contributor
timestamp: November 03, 2024, 11:32:39 am
content: Suggestion: TLP3545A(F)https://toshiba.semicon-storage.com/info/datasheet_en_20230526.pdf?did=60318This part has the same switching capabilities (60V, 4A), but has two FETs in back-to-back configuration. You can switch AC and DC and don't need to care about polarity or current backflow.

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2024, 11:39:53 am
content: Why not use a mechanical relay?The on resistance of your SSR can be as high as 0.09Ω, which is 90% of the value of the lowest sense resistor of 0.1Ω.

author: Sariel
author_rank: Regular Contributor
timestamp: November 03, 2024, 02:33:55 pm
content: Quote from: Zero999 on November 03, 2024, 11:39:53 amMechanical relays are also an option.But:1. They are much bigger and more expensive.2. They have about 100mohm, on resistance- very similar to high current SSR3. Require a lot of coil current for switchingI will need to calibrate out the SSR ON resistance. The card will work mostly in 25C-30C in lab environment.

author: Sariel
author_rank: Regular Contributor
timestamp: November 03, 2024, 02:34:42 pm
content: Quote from: fchk on November 03, 2024, 11:32:39 amThank you.So I should use the "A connection", right?

author: themadhippy
author_rank: Super Contributor
timestamp: November 03, 2024, 02:41:13 pm
content: reed relays aint much bigger than a resistor and the coils only need a few milliamps to fire

author: inse
author_rank: Super Contributor
timestamp: November 03, 2024, 03:45:29 pm
content: Quote from: voltsandjolts on November 03, 2024, 11:11:27 amThe datasheet distinctively says DC and the outputs are marked + and -Quote from: voltsandjolts on November 03, 2024, 11:11:27 amThat’s definitely an issue

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2024, 05:13:28 pm
content: Quote from: Sariel on November 03, 2024, 02:33:55 pmThat not true for all mechanical relays.As mentioned above, reed relays are tiny and require a similar current to a solid state relay. #2 is false. The contact resistance of most mechanical relays is lower than to 90mΩ of the SSR you specified. It's true, a higher contact resistance often is given on data sheets, but it's only to account for low currents and voltages which are too low to fully wet the currents.You can calibrate out the on resistance to some extent, but stability will be an issue. Ideally you should measure the voltage across just the sense resistor and not the relay,.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 03, 2024, 09:11:24 pm
content: Quote from: Sariel on November 02, 2024, 05:54:26 pmIf you have one, measure for a reverse diode.Otherwise, they clearly tag the pins + and -, so ensure + really is most positive.Quote from: Sariel on November 03, 2024, 07:56:10 amNot really, as mentioned above, the On resistance is massive relative to your circuit.If youmusthave isolated parts, look at photovoltaic optocouplers. (eg TLP3906)These are strings of photocells, that can drive a FET gate directly, and are cheaper than the CPC1706Y.You then pick a FET for each task that has the ratings you need.For your 100mΩ, you might pick a sub 10mΩ or sub 5mΩ FET, with low capacitance.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 03, 2024, 09:21:18 pm
content: Quote from: Zero999 on November 03, 2024, 05:13:28 pmGood advice.In this circuit, the OP can trade off a little gain for more freedom of device choice.If you feed from each resistor with equal resistances, you have a gain of 1/3, but in each case you measure only the sense resistor, the FET is no longer part of the measurement.You would still use photovoltaic optocouplers, but can pick a higher resistance FET, based on current rather than lowest on resistance.

author: tooki
author_rank: Super Contributor
timestamp: November 03, 2024, 09:32:32 pm
content: Quote from: voltsandjolts on November 03, 2024, 11:11:27 amHow do you arrive at this conclusion after seeing the datasheet, which clearly shows the internal schematic consisting of one MOSFET as the output stage?

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2024, 10:14:12 pm
content: The data sheet clearly shows a MOSFET, with the polarity clearly marked. The only time it'll work when reverse biased, is if the voltage is tiny <0.5V and even then, the leakage current will be high.https://www.mouser.co.uk/datasheet/2/240/media-3320585.pdf

author: voltsandjolts
author_rank: Unknown
timestamp: November 04, 2024, 09:35:48 am
content: Yeh, I see now, I was talking crap. We all do that sometimes don't we?!

author: ArdWar
author_rank: Frequent Contributor
timestamp: November 04, 2024, 09:48:40 am
content: Quote from: Sariel on November 03, 2024, 02:33:55 pmCalibrating for RDS(ON)will be a bit tricky. The FET will self heat die to current flow and TJcan get quite significantly hotter than ambient, or TCASEfor that matter.

author: r6502
author_rank: Regular Contributor
timestamp: November 05, 2024, 12:25:48 pm
content: hello Sariel,I would go a different way. I would not switch the current sense resistor, I would just use 0.1Ohm resistor and switch the range of the connected meter or, if you are using your own electronic, use a pga (programmable gain amplifier) to switch between the different ranges:Remark: GND Connection for both rails are not shownGuido