[Solved] Optomos Relay Neg Voltage. - Page 1
Forum section: electronics / beginners

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 07, 2017, 11:35:49 am
content: I wanted to test an optomos relay that arrived this week (Clare LCA717) so I made this simple circuit to check how to wire it. It works as it should but I get a negative voltage from -1.8V to -0.7V where I expected 0V when the SPST switch is not pressed or even when the circuit is not powered at all. This reading was done with a new BM235 that I bought to replace my first DMM (Mastech 830). Last one reads 0V as expected.What is this voltage?Thanks in advance!

author: rs20
author_rank: Super Contributor
timestamp: April 07, 2017, 11:42:45 am
content: So when you say "the circuit is not powered at all", have you removed all 5V supplies, from both sides of the circuit? Have you shorted out the supply, left it floating, or left it connected to a dodgy power supply displaying "0V"? Let's focus on the "not powered at all" case and focus on very clearly stating how the circuit is laid out and what we're observing.Have you tried removing the OptoMOS Relay from the circuit entirely, i.e., basically just measuring the voltage across the LED (the 470 ohm resistor will be irrelevant) on its own? Perhaps your LED is acting as a solar cell. I can get over 1.0 V out of a random green LED I had lying around here.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 07, 2017, 12:02:43 pm
content: Quote from: rs20 on April 07, 2017, 11:42:45 ama) When I fully disconnect the circuit from the power supply I get 0.008Vb) When the power supply is fully off BUT it's still connected to the circuit I get the same voltage about -0.7VQuote from: rs20 on April 07, 2017, 11:42:45 amc) When the power supply is disconnected from circuit and optomos is out of the board I get 0.008VAlso checked BM235 user's manual to verify if I'm not using it correctly.Pic Negative2 is answer b)

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 07, 2017, 12:16:03 pm
content: Quote from: rs20 on April 07, 2017, 11:42:45 amI removed the green led from the board and put a lamp above it and I get 0.4V to 0.1V depending on how I move the lamp. I get negative voltage depending on how I touch the test leads.

author: tatus1969
author_rank: Super Contributor
timestamp: April 07, 2017, 12:19:35 pm
content: is the circuit connected to something else, like a computer via USB?My guess would be that you receive mains AC via the power supplies' EMI capacitors. Try having it connected and switched off, and disconnect the mains plug then.

author: rs20
author_rank: Super Contributor
timestamp: April 07, 2017, 12:31:43 pm
content: Quote from: HoracioDos on April 07, 2017, 12:02:43 pmRight, so power supply disconnected, you see a tiny insignificant voltage that is basically noise or whatever. When you connect the power supply, you see weird negative voltages. So the weirdness is entirely inside your power supply; don't think too hard about it.

author: danadak
author_rank: Super Contributor
timestamp: April 07, 2017, 12:34:21 pm
content: Might be possible that LCA has a charge pump inside to be ableto drive the MOSFETs hard on via some form of bootstrap circuit.But then you said you had one multimeter that read 0V, not negative.So I suspect either DMM or above boostrap circuit possibility.Conflicting measurements......Regards, Dana.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 07, 2017, 12:49:52 pm
content: Quote from: rs20 on April 07, 2017, 12:31:43 pmOk thanks! the green led works definitely as a photodiode. I put everything back in the board and I put a still lamp above the circuit and I get a very stable 0.032V. When I turn off the light I get 0V. This never happened before or at least I never realized about it. I don't understand why I get negative voltage depending on how I touch the test leads.I must say that I don't have a proper earth in my house. I'm looking for alternatives to make it without redoing all the electrical circuits.Thanks again

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 07, 2017, 01:03:02 pm
content: Quote from: danadak on April 07, 2017, 12:34:21 pmThis could be a good reason to get a third DMM.Thanks for your reply.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 07, 2017, 01:51:51 pm
content: Quote from: HoracioDos on April 07, 2017, 01:03:02 pmI remembered that I already have a third DMM. I get constant -0.6V with a Brymen BM079 clamp meter. So the Mastech must be wrong.