Timeguard ZV900 questions about circuit - Page 1
Forum section: electronics / repair

author: perdrix
author_rank: Frequent Contributor
timestamp: January 03, 2022, 02:40:17 pm
content: This is a load controller installed between the output of a "two-wire" time switch and a load of LED or CFL lamps to stop them from flickering when the two-wire switch is nominally off.This unit is for a 230V/50Hz mains supply system.It activates a internal 48V relay not long after power is applied as the switch is turned on. Leakage power from the "two-wire" electronic switch in the "off" position doesn't turn on the relay.The one in my house failed, and after ripping off the plastic housing and loads of potting compound I attempted to reverse engineer it.Which seems to make some sort of sense except that the relay is a 48V relay and I can't see how the voltage across the relay coil can exceed about 31V or so (relay resistance is 5760 ohms nominal), and the pick-up voltage is specified as 33.6V!However read datasheets with a pinch of salt - measuring the relay on the C/T, it seems to turn on at about 25VI couldn't read the markings on the zener so I had to remove it and stick it on the curve tracer to determine its voltage (47V)I think R1 is 120 ohms - it was O/C and the colour coding was very hard to read. I'm not clear what function is serves in the circuit and why it needs to be that value and a 2 or 3W part. As far as I can tell it should only have about 12Vppacross it. Could someone elucidate what its purpose is and why it is so beefy!And for that matter what does R2 achieve, because AFAICT most of the current charge C2 goes through C1?

author: perdrix
author_rank: Frequent Contributor
timestamp: January 06, 2022, 04:32:51 pm
content: I guess this got missed in the New Year celebrations - I have it working now, but my questions still apply ...ThanksDavid

author: SeanB
author_rank: Super Contributor
timestamp: January 06, 2022, 06:42:15 pm
content: R1 serves to limit inrush current into the circuit, which occurs with either mains transients, or with the circuit being turned on near a mains peak. Saves cooking C1 and Zd1 with large current pulses. The circuit simply is a capacitive dropper, with the current being roughly 33mA into the relay coil, somewhat smoothed by C2, and with peak voltage limiting being provided by Zd1, which will probably barely pass any current. R3 is across C2, not in series with the coil, just so there is a discharge path, if the coil is open circuit, or it is 330R, to trim current flow in the coil somewhat.This simply provides a delayed switch on of the relay, around 2 to 10 mains cycles, and then, when power is switched off, the relay drops out, disconnecting the load, so there is no flashing, as the LED power supplies charge up from small capacitive coupled currents.Not right to switch the neutral, should switch line only, or both, and D2 is totally unneeded, along with R3, but what do you expect with cookie cutter designers, who slavishly follow some arbitrary rules without thinking of why they are there. At least they did put in R1, and C3 could be replaced with a 275 V MOV instead, after R1, which would actually work well to protect the unit.

author: perdrix
author_rank: Frequent Contributor
timestamp: January 06, 2022, 08:39:40 pm
content: R3 (3K3) is DEFINITELY in series with the coil of the relay - I double checked that. R2 (definitely 330k) is across C2. Definitely agree that switching Neutral is a no-no, but sadly ...Thanks for explanation of R1 as inrush limiter.Relay triggers after 350-400ms (approx.) so 15-20 cycles.At least they put creepage cuts in the PCB at the correct locations!David

author: TheMG
author_rank: Frequent Contributor
timestamp: January 07, 2022, 06:54:14 pm
content: Seems like an awful lot of effort went into this when they could have achieved the same thing using nothing more than a relay with a 230VAC coil. I'm guessing using a 48VDC relay with a capacitive dropper power supply was likely cheaper than trying to source a small 230VAC relay which are probably more expensive and less common.The slight delay I think is just a result of the circuit they used to energize the relay, not actually intentional as I don't see any reason there needs to be any kind of delay for this circuit to achieve its intended purpose.