Voltage leak in cheap relay? - Page 1
Forum section: electronics / beginners

author: wblackledg
author_rank: Regular Contributor
timestamp: May 16, 2015, 04:19:35 am
content: I've never worked with relays before and new to the hobby.I bought a $4 circuit from China via ebay. (see link below).http://www.ebay.com/itm/Liquid-Level-Controller-Sensor-Module-Water-Level-Detection-Sensor-Low-pressure-/311336921213?pt=LH_DefaultDomain_0&hash=item487d20687dThe Chinglish instructions in the one I bought were lacking..Once I hooked it up to my multimeter I noticed in the "off" setting I was getting about 400mv to 200mv leakage it would fluctuate all the way up to 1 Volt. I bought two, and the second one acted the same. In the On position it worked fine at 12V.Is this because its a cheap relay/circuit or is that normal for relays?Better question will the leakage damage my bilge pump I'm hooking up to it over time?thanks in advance.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 16, 2015, 07:16:00 am
content: QuoteHow did you measure that?If there is indeed a leakage, then leakage current.Leakage voltageis not quite a technical termQuoteCertainly not, but I guess you have measure something wrong there.QuoteAC or DC?

author: richard.cs
author_rank: Super Contributor
timestamp: May 16, 2015, 08:21:16 am
content: It's probably a few microamps tracking across the underside of the pcb in which case it'll probably go away if you clean it with alcohol. For a pump that kind of leakage isn't a problem anyway, it'll collapse to no volts at all when you connect the pump.

author: TimFox
author_rank: Super Contributor
timestamp: May 16, 2015, 01:37:42 pm
content: If the switched circuit is AC, then there may be current through a contact-protecting capacitor when the contact is off.

author: wblackledg
author_rank: Regular Contributor
timestamp: May 16, 2015, 02:41:05 pm
content: Quote from: PSR B1257 on May 16, 2015, 07:16:00 amI used a multimeter hooked up where the pump would be at the end of the circuit (hope end of circuit is a technical term.) Its DC only.

author: wblackledg
author_rank: Regular Contributor
timestamp: May 16, 2015, 02:43:45 pm
content: Quote from: richard.cs on May 16, 2015, 08:21:16 ammaybe thats me? I had it laying on top of a latex painted wooden table (not an Electrostatic mat) when I tested it.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 16, 2015, 03:28:05 pm
content: Some DMM show a reading on AC or DC voltage when their input terminals are floating (due to static charge in combination with the high input impedance).If you connect this meter to a relay, which is also an open circuit, it show's the same behavior.What does your DMM read with it's leads in free air?

author: wblackledg
author_rank: Regular Contributor
timestamp: May 16, 2015, 03:37:03 pm
content: Quote from: PSR B1257 on May 16, 2015, 03:28:05 pmIt a high quality "Sinometer" brandIt reads between negative 20mv and negative 40mv.However I have a used HP 3478A that gave about the same fluctuating readings when I had it hooked up to the circuit.

author: dom0
author_rank: Super Contributor
timestamp: May 16, 2015, 06:04:06 pm
content: Which really isn't surprising with the 10 M? input resistance in the higher ranges and >10G? input resistance in lower ranges.

author: wblackledg
author_rank: Regular Contributor
timestamp: May 17, 2015, 01:01:29 am
content: After cleaning it with 90% alcohol the voltage has jumped to 2.5 volts and fluctuates down to 1.2 volts DC . I'm confused. when triggered its stays on a steady 12VDC

author: Monkeh
author_rank: Super Contributor
timestamp: May 17, 2015, 01:14:34 am
content: Which means rather than cleaning it you've just given the contamination a better distribution

author: wblackledg
author_rank: Regular Contributor
timestamp: May 17, 2015, 01:47:54 am
content: Quote from: Monkeh on May 17, 2015, 01:14:34 amGreat...Its not like I spent a lot for it. Any suggestions?

author: Monkeh
author_rank: Super Contributor
timestamp: May 17, 2015, 01:51:00 am
content: Clean it better or don't worry about a bit of leakage?

author: wblackledg
author_rank: Regular Contributor
timestamp: May 17, 2015, 01:53:51 am
content: Quote from: Monkeh on May 17, 2015, 01:51:00 amjust don't want a cheezy $4 part to burn up a $75 pump..So its not possible that one or more of the components on this board are just crap and not working to specs, or is it the end user?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 17, 2015, 06:06:45 am
content: QuoteSure it does. In this case the relay act's just like it should. ItQuoteAs already sayed, it's the meter.And as also already sayed, there is no such thing likeleakage voltage. If you measure the current when the relay is OFF, the meter has to read zero.And if you hook up a load (i.e. your pump) the phantom voltage will disappear also.However, you will not kill the pump, if you connect it to the relay borad.

author: Graphite
author_rank: Newbie
timestamp: May 17, 2015, 07:01:43 pm
content: Measuring an open relay will be like measuring the voltage of your two multimeter probes as they are suspended in air. It is meaningless.A better way to test your relay is to disconnect the output from power, then measure the resistance through the relay with the relay open, then closed. When the relay is open, you should see a very high resistance. With the relay closed, it should read near zero.Another way to determine if the relay is good is to measure the current through the output side of the relay. It should read the expected current when the relay is closed, and it should read no current when the relay is open.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 18, 2015, 09:31:08 pm
content: Put a 1k resistor on the relay where the motor will go.Then test again.You will probably then see that there is 0v when off and 12v when on.If that is the case, then all is fine, you can remove the meter.My guess is that your meters high impedance isn't loading the system enough. And that is allowing the smallest current tracking across the board to be picked up by your meter.The motors impedance will do the same thing as the 1k resistor. It will load down the circuit, and you should not have a problem.High impedance comes in handy sometimes when I test mains. If I just want a quick confirmation that the circuit is live/dead, I hold one probe in my hand (touching the metal of the probe.), and touch the other probe on the terminals. The Live terminal normally gives me a reading of 100v or above (and this is through safety shoes), and the Netural terminal (or a dead terminal) gives me 0v...

author: wblackledg
author_rank: Regular Contributor
timestamp: May 18, 2015, 10:25:09 pm
content: Quote from: kolbep on May 18, 2015, 09:31:08 pmthanks. I used my 3rd and final Multimeter on it (the AMPRO that Dave recommended in his shootout). It showed about 200mv before I switch the power on, then 0mv once the power was on, and them 11.96 volts when the relay kicked in.thanks to all!!PS, does mean I should "calibrate" those meters or is that even possible?thanks.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: May 20, 2015, 04:20:23 am
content: QuoteSeriously? Youmeasurethe open circuit voltage of the meter itself (which is as already established meaningles) and want to calibrate it against another - go for it O0