measurement error or bad part? - Page 1
Forum section: electronics / projects

author: electrolust
author_rank: Unknown
timestamp: July 15, 2020, 12:28:41 am
content: i'm using a Coto Tech C347S SSR. Datasheet attached.just testing it for on resistance, i measure >50M when the LED (switch) is off, and 0.2 ohm when the switch is on. just like the datasheet says. great for DC.my test AC signal is a 1kHz sine wave, 100mVpp. with the switch on, i measure 100mVpp at the output. with the switch off, i measure 60mVpp. this also scales to 1V, where i measure 600mVpp. so, 60% of the signal is getting through.i was initially sharing the ground with power and signal. i changed that to have a different signal ground,even though in my actual circuit they will be shared. then i went further and used a differential probe on the output signal. so only the signal and input probe share a ground. same results!The C347s looks like it is a standard back-to-back MOSFET so it should be ok for AC signals, shouldn't it? I have the LED pins floating, no pulldown resistor, but this should be ok I think.Even though I measured DC off resistance at >50M, I do notice that an RC filter with 1M / 190pF (the output capacitance of the C347S) has -3dB frequency of 840Hz. So I played with the frequency and I see that by 5kHz the SSR is passing the full signal even in the off state, and at 20Hz it's down to 20% of the signal. I guess I could do a bode plot later, but anyway it appears to be acting as a high pass filter in the off state.this doesn't seem right, but at the same time, i can't imagine what's wrong with my measurement setup.

author: wraper
author_rank: Unknown
timestamp: July 15, 2020, 02:24:41 am
content: You did not say a word about load resistance. As if you are simply measuring in series with multimeter/oscilloscope in series and no load at all.

author: wraper
author_rank: Unknown
timestamp: July 15, 2020, 02:29:56 am
content: In the datasheet you can find output capacitance plot and at low voltage it's about 200pF.QuoteMy gut feeling is that you have no load at all except multimeter/oscilloscope input resistance. And that certainly isn't right.

author: electrolust
author_rank: Unknown
timestamp: July 15, 2020, 04:40:04 am
content: it's correct, i have no load at all. however, on the far side of the switch, with the switch on I see 1:1 signal and with the switch off I should see 0 signal -- regardless of load. i mean, isn't the scope a high impedance load anyway? I also tried with 50 ohm and get the same results.i realized something. when i tested this with CD4066 it worked as expected. now that i test with SSR i get this signal passing through. so i havesomeconfidence in my measurement setup.i notice in the data sheet that the block diagram doesn't show a diode in the mosfets. i just assume it's a simplification. it does show the 2 leads are connected to the 2 drains. the data sheet does specifically say, 80V peak AC voltage. It shouldn't say that if it doesn't work with AC.

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 15, 2020, 10:35:05 am
content: Quote from: electrolust on July 15, 2020, 04:40:04 amIt actually does, visible with >500% magnification.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2020, 12:31:18 pm
content: Quote from: electrolust on July 15, 2020, 04:40:04 amThere is a fixed input to output capacitance which when combined with a high impedance load like a multimeter or oscilloscope, allows enough AC leakage to be misleading. Real loads for power switches have much lower impedances so this leakage would usually be insignificant.If lower leakage is a requirement, then a relay or a more sophisticated SSR is required.QuoteA 4066 has much lower input to output capacitance, only a couple picofarads.

author: electrolust
author_rank: Unknown
timestamp: July 15, 2020, 08:10:43 pm
content: ah. good to know that my measurement is ok. i will either source a different part (like one from this threadhttps://www.eevblog.com/forum/projects/low-leakage-triac-or-solid-state-relay/), or change the design.