Automated Relay testing. Need help! - Page 1
Forum section: electronics / projects

author: spikeyeddy
author_rank: Newbie
timestamp: October 16, 2012, 01:43:32 pm
content: Hey all,A little info to my project.Its basically relay testing. i have to design a circuit board first that fits all the different types of relays i have to test.I have an SMU that I ll use to source and measure the relays.The whole process has to completely automated and operated from a GUI. So i have to write an interface in C and write measruement programs for the SMU.I am designing the circuit board right now and have a very basic question. As I just have to do measurements, I do not have any loads.What appropriate type of load I need to use in the contact circuit of the relay?? the test voltages on the contact pins of the relay range from 1V to 50V (maybe less).Any suggestions? Let me know if you need more details...Peace

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 16, 2012, 01:56:37 pm
content: What are you trying to measure?Activation and holding current for the relay?Resistance of contacts on load side?Speed of activation?If you don't have to measure anything on the load side, then I don't see why you even need a load. You might just be using the contact closure to verify that it activated and if needed, the speed of activation.If you are testing the load side, then you need a load that will limit the current of the relay to under the rating, with the voltage you are testing. If you are varying the voltage and need to vary the load, then you might be looking at something like the constant current load that has been discussed many times in here.With more details of exactly what you need to measure, I'm sure we could be of more help.

author: robrenz
author_rank: Super Contributor
timestamp: October 16, 2012, 01:57:25 pm
content: What specification of the relays are you testing for? Contact related or coil related or both, are any relays considered dry contact, etc.?edit: sorry sacherjj I was too slow

author: spikeyeddy
author_rank: Newbie
timestamp: October 16, 2012, 02:07:25 pm
content: @sacharjj and robrenzI am doing tests on both coil and contact sides.the test/measurements are:coil resistance measurementpull in /drop-out Voltage testContact resistanceand the main test: Reliability of the relay =>I ll switch the relay on/off, say, 1000 times. While doing this I want to measure/test- operate/release times in each cycle- bounce timesThere are various relays that I have to test, but all relays have to go through the above-stated tests.For example, this is the relay Im working with right now.http://pdf1.alldatasheet.com/datasheet-pdf/view/317453/MEDER/SIL12-1A31-71L.htmlThanks

author: spikeyeddy
author_rank: Newbie
timestamp: October 16, 2012, 02:10:50 pm
content: @sacherjjConstant current load?Yes the voltage on the contact pins would be varied to check the reliability.Thanks a lot =)

author: robrenz
author_rank: Super Contributor
timestamp: October 16, 2012, 02:16:27 pm
content: Depending on the level of accuracy necessary you will need a 4 wire kelvin connection at least to the socket pin pads. you may need custom sockets with separate source and sense contacts for each pin that goes to the relay contacts. If you havent read it yet you need to thouroghly digest the Keithly low level measurements handbook 6th ed. its a free book form Keithly both hardcopy or download

author: spikeyeddy
author_rank: Newbie
timestamp: October 16, 2012, 02:20:32 pm
content: @robrenz,I am using a keithley 2600 series as SMU and the measurements will be 4-wire (sense option).I have read the manual completely but of course it wasnt thoroughly.Thanks,

author: robrenz
author_rank: Super Contributor
timestamp: October 16, 2012, 02:30:22 pm
content: Do you understand what the first two sentences of my last post mean? And what the difference between the two measurements would be? do you understand contact resistance itself? If not you need to study the applicable sections of that book. I am not trying to be a wise guy I am trying to help

author: spikeyeddy
author_rank: Newbie
timestamp: October 16, 2012, 02:35:57 pm
content: @robrenzDont worry, I appreciate it!I know what you mean. I know about the custom sockets and I am working on that too.I know the contact resistance could be measured without any load but that would be in presence of a very small voltage.What I was actually wonderingis, if i could measure the contact resistance somehow WHILE I am doing the switching test (at various voltages). Is that possible?Thanks =)

author: robrenz
author_rank: Super Contributor
timestamp: October 16, 2012, 03:02:34 pm
content: You can always compute the contact resistance from what you are sourceing and sensing regardless of level. But you will have source minimums that you can go to depending on the resistance resolution you need.

author: free_electron
author_rank: Super Contributor
timestamp: October 16, 2012, 06:53:00 pm
content: you have to be careful when measuring contact resistance...you need to send some current through the contact. using a fe wmicroamps may yield incorrect results for certain relays. for others : do NOT switch the contact when current is flowing.the relay manufacturer will have specified how they measure the contact resistance. you need to use the same method.

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 16, 2012, 07:03:45 pm
content: Quote from: free_electron on October 16, 2012, 06:53:00 pmWhy wouldn't you switch the contact when you are powered? If you are emulating the expected switching use of the relay, then using a current and voltage drop measurement of the load in operation seems like the best way to characterize that actually contact resistance.I guess it depends on validating the relay to spec or to actual intended use?

author: poptones
author_rank: Frequent Contributor
timestamp: October 16, 2012, 09:39:49 pm
content: Because dry current and wet current vary. If you are trying to measure the dry current breakdown, actuating the relay will corrupt that because the contacts will be wiping. You need to have the contacts actuated then test for the dry current while they are stationary.

author: robrenz
author_rank: Super Contributor
timestamp: October 16, 2012, 10:35:54 pm
content: One thing that had confused me for a while is that when relay and switch specs say contact resistance they really mean the bulk resistance from the external physical connections of the unit to the actual contacts plus the true contact resistance of the contacts. I designed Kelvin banana jacks that eliminate all bulk resistance from the connection and read only the contact resistance. thread and video herehttps://www.eevblog.com/forum/projects-designs-and-technical-stuff/video-of-design-and-build-of-precision-kelvin-banana-jacks-and-header-pins/msg148204/#msg148204

author: free_electron
author_rank: Super Contributor
timestamp: October 16, 2012, 11:19:26 pm
content: Quote from: poptones on October 16, 2012, 09:39:49 pmbingo!if you have the contacts powered you will spark the contact clean. any surface oxidation will be cleaned off and you get a 'false good' while the contacts are in reality corroded.second, certain relays cannot be powered when switching. teledyne signal relays for example , or reed relays. current will keep the contacts to stick

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 17, 2012, 12:52:07 am
content: Quote from: free_electron on October 16, 2012, 11:19:26 pmMakes total sense now. Good info.So it sounds like a MOSFET style current load setup would be good for this. It would allow you to problematically stop current flow, then ramp it up to accomplish that.

author: poptones
author_rank: Frequent Contributor
timestamp: October 17, 2012, 02:30:12 am
content: It's cool to see this topic. One thing I learned a LOT about was testing wiring harnesses. I designed lots of testers for the auto industry. When everyone was having trouble with dry current breakdown on auto harnesses we were delivering flawless testing. Why? Because we didn't use muxes - we delivered a short of 40mA across the node and measured the voltage drop. BANG! Patent city!

author: spikeyeddy
author_rank: Newbie
timestamp: October 18, 2012, 12:09:31 pm
content: Hey guys,Thanks a lot. I read the Keithley book and now I know how I am going to measure the contact resistance.Now I have one more question. As I have already described, I have to do a reliability test also in which I would switch the relay for a couple of 1000 cycles.The voltage in the CONTACT circuit would vary in these cycles. For example: 5000 cycles with 1V, then 2V, then 5V, then 10 V and then 50V.What appropriate load would I use in my Contact circuit for this? Dummy Loads?Thanks,

author: robrenz
author_rank: Super Contributor
timestamp: October 18, 2012, 12:27:36 pm
content: If you only have to do one test the programmable load would be great. But if you need to do a lot of these simultaneously you may want to have relay controled load resistances to suit each voltage.

author: spikeyeddy
author_rank: Newbie
timestamp: October 18, 2012, 01:34:42 pm
content: what if i use the compliance function from my SMU and limit the current to, say, 10mA for each voltage; would I still need a load?

author: robrenz
author_rank: Super Contributor
timestamp: October 18, 2012, 01:49:30 pm
content: I got ready to post that same thing, just use current limit, but then I realized I was asking the power supply to defy Ohms law. It will limit the current but it has to do that by reducing the voltage.

author: spikeyeddy
author_rank: Newbie
timestamp: October 18, 2012, 02:04:50 pm
content: Oh.. yeah I realize that too right now. But I thought that there could be an adjustable circuit in the SMU that would somehow direct the limited amount of current through the DUT and still have the same voltage across it.the relay controlled Load is not a bad idea. As i have 10 different kinds of relays with different contact circuit voltages, that might be just the thing Im looking for.Even if i use that, what should the load look like? Simple resistors? because i read something like, if the load is inductive/capactive, then there could be spike currents in the circuit as the relay goes OFF and then I would require a safety circuit like a snubber or something.Thanks again for ur patience and answers. i really appreciate it.

author: poptones
author_rank: Frequent Contributor
timestamp: October 18, 2012, 02:09:53 pm
content: If you use a programmable current you won't have to worry about that. You can ramp down the current before opening the relay contacts.

author: robrenz
author_rank: Super Contributor
timestamp: October 18, 2012, 02:40:53 pm
content: Quote from: spikeyeddy on October 18, 2012, 02:04:50 pm

author: robrenz
author_rank: Super Contributor
timestamp: October 18, 2012, 02:54:35 pm
content: You could also build your own variable resistive loads instead of the relays/resistors, but the loads response to a step input may mess with what you are trying to measure like contact bounce time. This may also make an Eload unsuitable.

author: qno
author_rank: Frequent Contributor
timestamp: October 18, 2012, 05:59:30 pm
content: Hi DC voltages are most damaging to the relays contacts.What load is it going to be switching in the real world?The manufacturer claims 1000 V. Hm Interesting.I have tested relays once 110 V AC 200 mA, 3.7 million times minimum.

author: robrenz
author_rank: Super Contributor
timestamp: October 18, 2012, 06:26:49 pm
content: Quote from: spikeyeddy on October 18, 2012, 02:04:50 pmA 50W (relay max contact wattage) minimum power resistor is probably going to be a wire wound type and will have inductance inherently.

author: free_electron
author_rank: Super Contributor
timestamp: October 18, 2012, 11:43:05 pm
content: just make a load from cerpack resistors i typically use either ohmite alox substrate( they are 5 watt . simply clamp them on a heatsink with some thermal past ) or use resistors in to220 package. those are inductionless. thick film is the way to go. wirewound resistors is asking for misery.