Reed relay -- exceptionally high internal resistance - Page 1
Forum section: electronics / projects

author: blueskull
author_rank: Unknown
timestamp: April 01, 2018, 01:52:57 am
content: I was designing a HiFi DAC, and I need a linear (unlike semiconductor which has Cds/Vds and Rds/Ids dependency) switch for gain control.I chose Standex Meder CRR05-1A, and I finally was able to get a few samples on my hand.The datasheet states maximum 200mR internal resistance at 40% over drive (5V relay, driven at 7V), but I wonder why they rate it 5V if it can't achieve its stated performance at 5V.I don't have a second power supply, so I can only test the relay with my only power supply connected as a current source and a USB plug as 5V relay coil supply.I first powered the coil with 5V, then set contact current to 1A (which is stated maximum carry current), this 1A has been calibrated with a DMM.I then did a Kelvin measurement on contact pads right on the relay package, and I read 248mV (at 0.9994A), that means I get 25% more contact resistance than rated value.Keep in mind that this is a fresh, new relay from cut tape, and the tape came from a Standex Meder listed distributor. The relay is definitely new with 1709 data code.I'm pretty sure that the lack of over driving is not the issue here. I tried to drive the relay with 3.3V coil voltage, and the resistance did increase, but not by that much.I got 264mV at 0.9994A at 3.3V coil. I don't think by cranking up the coil voltage to 7V the resistance will magically drop down to <200mR.The meter used to calibrate PSU current and monitor Kelvin voltage is a Fluke 289, recently repaired and calibrated by Fluke themselves.So, did I miss something when testing this thing, or it's just the relay that sucks?Also, is there a good recommendation on an ideal linear switch that I can use to attenuate audio signal with less than -130dB added THD?

author: C
author_rank: Super Contributor
timestamp: April 01, 2018, 02:36:00 am
content: Think you missed a detail.Sheet says you can switch 0.5A not 1AAfter connect is made then up to 1A.Think you have damaged the contacts.

author: C
author_rank: Super Contributor
timestamp: April 01, 2018, 03:00:42 am
content: Contacts are a problem, Too low or too changes how they act.Have see very long long life switches fail when switching to low a current. They need that little extra to clean the contact.

author: tautech
author_rank: Super Contributor
timestamp: April 01, 2018, 03:47:45 am
content: Has the datasheet mislead you with a Max contact resistance whereas I take it to mean Min contact resistance ?Will better than 300 mohms suit your needs ?

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 01, 2018, 04:00:06 am
content: The datasheet linked by mouser not only mentions the 40% overdrive, but measured @0.5V and 10mA.So 25% more at 1A is veeery good

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 01, 2018, 04:49:00 am
content: Even with Kelvin contacts you are measuring the sum of three things - contact resistance at pad 1, relay contact resistance, and contact resistance at pad 2. Seems unlikely that your contact resistances are under 22 milliohms each. So the relay probably meets its spec of <200 milliohms.

author: Monkeh
author_rank: Super Contributor
timestamp: April 01, 2018, 04:55:32 am
content: Have you at all considered actually testing the relay under the conditions specified in the datasheet?I know, it's an out there idea.

author: Monkeh
author_rank: Super Contributor
timestamp: April 01, 2018, 05:02:07 am
content: Quote from: blueskull on April 01, 2018, 04:59:50 amThen why even bother reading and comparing to them?Test a few samplesunder the specified conditionsor just stop reading datasheets entirely.

author: SeanB
author_rank: Super Contributor
timestamp: April 01, 2018, 05:11:17 am
content: Try cycling the relay 1000 times and see if the contact area increases as the 2 contacts conform to each other. Sadly for the lowest on resistance you do need mercury wetted relays.

author: BU508A
author_rank: Super Contributor
timestamp: April 01, 2018, 07:50:03 am
content: I had a look at the datasheet from the Standex/Meder website (attached it below) and found this:Website:https://standexelectronics.com/products/crr-series-reed-relay/Found also another datasheet here (German electronic distributor):Website:https://www.reichelt.de/Reedrelays/CRR-05-1A/3/index.html?ACTION=3&GROUPID=7617&ARTICLE=85096&SEARCH=meder&START=0&OFFSET=100&This datasheet states a contact resistance (initial) of 250mOhm.It seems, that there were some changes over time, so I would go after the datasheet from their websiteand I would also probably asking them some questions.

author: Marco
author_rank: Super Contributor
timestamp: April 01, 2018, 09:33:52 am
content: Quote from: blueskull on April 01, 2018, 01:52:57 amWhy don't we have cheap electrowetting liquid metal switches yet? Patents? Probably patents ... sigh.If it's one off and the fixed vertical orientation don't matter, why not buy some mercury wetted relays?

author: Marco
author_rank: Super Contributor
timestamp: April 01, 2018, 10:08:44 am
content: Quote from: blueskull on April 01, 2018, 09:39:08 amAgilent hasa patent. It's most interesting for RF because of the trivial impedance matching.On second thought though, it's probably because gallium wets and dissolves damn near everything and mercury hysteria would make new mercury based switches hard to sell.

author: Gyro
author_rank: Super Contributor
timestamp: April 01, 2018, 11:47:35 am
content: As a bit of a Mercury wetted relay 'anorak' (I use them for my personal dry circuit and audio applications) I can tell you that C P Clare introduced Ruthenium sputtered contacts as a kindred series with their Mercury wetted relays. That might be a contact material you want to look into. Datasheet attached.

author: Marco
author_rank: Super Contributor
timestamp: April 01, 2018, 01:51:56 pm
content: Quote from: blueskull on April 01, 2018, 01:52:57 amWhat's wrong with MOSFETs BTW? I don't see how channel length modulation or off-state isolation going to be an issue given the impedances involved and with unprotected gate MOSFETs gate leakage current isn't an issue either.

author: dmills
author_rank: Super Contributor
timestamp: April 01, 2018, 02:27:42 pm
content: Cadac (A VERY high end manufacturer of mixing desks) used to employ photomos relays for gain switching in the M16 mic to MADI box, if it worked for them on the ingest side I see no reason it would not be more then good enough for simple line level playout.Regards, Dan.

author: BrianHG
author_rank: Super Contributor
timestamp: April 01, 2018, 04:23:17 pm
content: When you say gain, are you talking about a full relay switched volume control, IE, 7 bits, -1db per step?Or, just 2 or 4 output gain settings?I've used Telecom latched relays in a 6bit constant impedance attenuator balanced mono, or stereo preamp which had a remote and could run on a few batteries for years as the coils only need a 10ms set/reset, then the system is near 0 power consumption. Also, no MCU noise on the coils with everything being off.Anyways, the telecom relays, a bit larger than your reed relays were DPDT, with <50mOhm on resistance. The only issue is they do not specify the off resistance, though my system was a 10k and 600ohm impedance, I could not find any leakage on the off relays.The mechanics of these relays is like a tiny slider switch, not a long flexible wire to a lever with contact which goes up and down, so you get nearly no inductance and near 0 wire length.Note they are hermetically sealed as well. No dust or contact noise over years of operation.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 01, 2018, 05:17:09 pm
content: Quote from: blueskull on April 01, 2018, 04:56:20 amIf soldered in the contact impedance between the relay pin is likely under 22 milliohms. But unless you soldered your Kelvin probe to your test relay you have contact resistance that won't be there in your application circuit.

author: BrianHG
author_rank: Super Contributor
timestamp: April 01, 2018, 09:52:27 pm
content: @blueskull, if you are using a reed relay within a gain circuit of hifi audio, why did you not choose a reed-relay where the reed tube in encased inside a coaxial EMI & Electrostatic shield, or, RF grade?RF with reed switch embedded in shield relay:https://www.digikey.com/product-detail/en/coto-technology/9202-05-20/306-1070-ND/301704

author: BrianHG
author_rank: Super Contributor
timestamp: April 01, 2018, 11:28:09 pm
content: Quote from: blueskull on April 01, 2018, 10:05:18 pmNow that's a nice part. Especially the 6 pole version. Good luck finding stock.Too bad about your open capacitance & resistance requirements, Toshiba now has an extensive range of photorelays with 100mOhm on resistance & off currents of 0.01ua up at 100v, dirt cheap. 60mOhm for the 60v version

author: floobydust
author_rank: Super Contributor
timestamp: April 02, 2018, 01:29:00 am
content: All small reed relays and switches I've looked at have 0.1-0.2ohm contact resistance, on the Bell curve. Example graph shows test limits.Why is 0.2ohm significant for controlling gain for audio?

author: BrianHG
author_rank: Super Contributor
timestamp: April 02, 2018, 10:58:00 am
content: Quote from: blueskull on April 02, 2018, 02:29:15 amWithout knowing your design, and such tight quarters on control, I would recommend using all 4 reed relays in 1 quad package in parallel. You never know every time a reed switches on exactly how good the contact being made is with age and everything else. You don't want your left and right channels to become unbalanced when the reeds are on. Personally, I would explore all other possibilities of doing this electronically.I just recently saw a website posted by Paulca called :http://reference-audio-analyzer.pro/en/report.phpThey evaluate by measurement, headphones, dacs, preamps, amps, ect. If you were to get your DAC evaluated by them, you don't want this problem which popped up when Paulca listed his headphones, and I out of curiosity went to compare mine. Here is Paulca's Audio-Technica ATH-M50x VS my Grado SR 325i L-Cush. This is one of the measured graphs further down the page:Now, you can see, the Audio-Technica looks flatter than mine, but, I had the chart set to mono 'CENTER', take a look at what happens when I switched the charts to stereo 'LEFT RIGHT'.Audio-Technica's left and right channel aren't even a close match. You need the left and right to match if you want that proper mono and stereo imagery. The last thing you need is having your DAC being evaluated and measured for that super strict db level you are targeting and the left and right come out un-matched.Example DAC technical evaluation:http://reference-audio-analyzer.pro/en/report/dac/lynxaudio-lynx-d48.phpScroll down and look at THD in the higher db settings with left/right, and you will see the 2 channels don't match, which is kind of normal at such high degree of measurements.

author: Marco
author_rank: Super Contributor
timestamp: April 02, 2018, 12:50:44 pm
content: Why are you putting a signal into a 1K impedance?

author: dmills
author_rank: Super Contributor
timestamp: April 02, 2018, 01:03:52 pm
content: Multiplying DAC for gain control? You are aware that only SOME of the DAC noise and distortion sources scale with reference voltage?You could I suppose use two DACs and subtract one output from the other then vary the reference to one of them to control the gain, has the advantage that the reference voltage is maximum for minimum gain, but it still feels clunky, and matching to get good attenuation will be a pain.In particular if you do this after the reconstruction filters then precision gain matching in them becomes a problem, possibly better to use differential current outputs and just directly subtract the currents before hitting the I/V and reconstruction filters.One of the 'ick' things about this is that the control voltage will be linear, not usually what you really want for audio gain.For -130dB THD (Or are you trying for THD+N?) resistor linearity is going to be a big issue, thin film is the way to go, but not all thin film parts are created equal, you will need to do some experimentation here. Last precision audio I did we ended up using susmu but I cannot remember the series.MELF while better in some ways is basically a pain in the arse, go bigger in a standard package your assembly house would rather place 1206 then MELF.Regards, Dan.

author: BrianHG
author_rank: Super Contributor
timestamp: April 02, 2018, 02:38:07 pm
content: Sounds like if this DAC becomes a consumer device, one of us will have to purchase one to do an eval and tear-down.

author: BrianHG
author_rank: Super Contributor
timestamp: April 02, 2018, 04:13:47 pm
content: Quote from: Marco on April 02, 2018, 12:50:44 pmI have a feeling it's part of his secret sauce and we wont be told... Even my latched relay switched passive volume control I posted on the previous page operated at a 600 ohm impedance and the -1.5db and -3db relay switch had resistors in the really low ohm range and the relays had to be precise to make that perfect -1.5db / step attenuation.

author: Marco
author_rank: Super Contributor
timestamp: April 02, 2018, 04:37:20 pm
content: On second thought it's probably because his fully differential buffer has an annoyingly low input impedance.

author: dmills
author_rank: Super Contributor
timestamp: April 02, 2018, 04:41:19 pm
content: If the DAC has a mute control, and is voltage output you can of course just use an R/2R ladder attenuator, you mute the dac before changing the attenuation to avoid bloody great thumps as the contacts move, but it is not rocket science.If you were doing current output with a resistor I/V converter (kind of stupid, but fashionable in 'phile circles), then just switching in a low value shunt would get you your 40dB gain reduction, this makes contact resistance matter, if the full scale shunt was say 1k then for -40db you would be looking at switching in 10 ohms.....Most of audiophile product design is about fashion not engineering, it is really far more of a lifestyle marketing exercise then it is an engineering one.Regards, Dan.

author: BrianHG
author_rank: Super Contributor
timestamp: April 02, 2018, 06:08:52 pm
content: Quote from: dmills on April 02, 2018, 04:41:19 pmWhere within this entire thread does blueskull say that he is making an audiophile dac. He just says it's HiFi in the opening post. It just sounds like he is using the dac at full unscaled digital code and has come up with a -0db to -80db volume which doesn't loose any of that original code. I know that's perfect to feed directly my 500watt/channel audio amp without a linear preamp. And yes, my amp and speakers are good enough for low volume listening at around -60db with as much refined detail as the full volume -0db, as well as a an undistorted spare +6db headroom. Design innovations + refinements can make differences, especially when you try to push extremes. I just hope he has an option to pump out +6db as I like to push my equipment and a lot of my HD grade audio sources rarely exceed even -10db except during the loudest events.It's going to come down to how he can market and sell that feature for big $$$. Get the right reviews, the right looking case, the right marketing, and for a change, be truthfull no BS, and he need not sell many to make quite a bit from his first consumer product. By the second year, if he can manage 100 units for that year, keep it up and he could achieve 1k units for year 4 or 5. Then he needs to be careful, since like PassLabs and their XA200 amp, if he can't keep up with orders due to availability of parts, he will loose his opportunity to maintain sales, otherwise, a different product will take it's place. (I'm not saying this will be easy. He needs money for trade shows, manufacturing, testing, demo units, advertising... But if he has financial backing anything is possible.)

author: calexanian
author_rank: Super Contributor
timestamp: April 02, 2018, 08:21:06 pm
content: +1 for bad application for a reed switch here. Use it to switch a intermediate relay or something that can handle the full load.

author: BrianHG
author_rank: Super Contributor
timestamp: April 03, 2018, 02:41:13 am
content: I'm surprised you didn't user multiple 12Ghz serial outputs from a FPGA as a 12Ghz clocked 1 bit pwm driving a full-bridge EPC eGaN as a home made 1 bit dac with a varying VDD as volume. Having a 50v or 100v 1 bit dac, resistor divided down to line level would be really clean and free of noise if done right. Get rid of any stinking opamp all together. (Something which can be marketed. 0 opamp, 0 linear transistor, DAC straight thought 1 resistor divider to output DAC) Or, make it high current direct drive to speaker as an all digital path 0 feedback noise free power amp.TI's all in 1 package H-bridge might just do it for a max 80v design.

author: BrianHG
author_rank: Super Contributor
timestamp: April 03, 2018, 04:04:45 am
content: Quote from: blueskull on April 03, 2018, 02:56:47 amYou misunderstood. The loop frequency would only be a small multiple of 192Khz, eg x8, 1.536Mhz, the 12.6Ghz is the sub-clocking at 8192 x 1.536Mhz giving you 8192 possible clock events of high & low for each 1.535Mhz cycle, or, you can call this 0.08ns resolution. I know you wont get the eGaN switching in the 12Ghz speed, but, you can set the switch transition position with 0.08ns positioning feeding the FPGA serdes in manual mode all 0s, then all 1s at the right lengths. Many audio 1 bit DACs, or sometimes 2 bit DAC really depend on inserting a lot of random noise to construct the output since they have far fewer clocked steps as their PWM may run at 768khz, or 1.356Mhz, but they haven't gone to a 12.6Ghz internal counter for their PWM.

author: BrianHG
author_rank: Super Contributor
timestamp: April 03, 2018, 04:21:34 am
content: I guess I'm still waiting for that perfect near 0 ohm on switch, ultra low pf gate before I attempt such a medium-high voltage DAC.

author: BrianHG
author_rank: Super Contributor
timestamp: April 03, 2018, 05:54:35 am
content: You can always double DAC if that part costs just as much, though you might need to rethink your gain section.

author: Marco
author_rank: Super Contributor
timestamp: April 03, 2018, 12:06:31 pm
content: Quote from: blueskull on April 03, 2018, 12:48:42 amDoes a FDA really offer much improvement over what you can do with two buffers?

author: Marco
author_rank: Super Contributor
timestamp: April 03, 2018, 01:03:32 pm
content: I thought the rest of your chain was also differential, not that you were using the FDA for differential->single ended.

author: Zucca
author_rank: Unknown
timestamp: April 03, 2018, 08:38:02 pm
content: Killing myself...I admit I am totally lost here.. too many ABC, DEF and so on.If you have 1 minute can you provide what thoseQuotemeans?I got the following (I hope):Quoteso sorry for slowing down the discussion...