How is this suppose to work? - Page 1
Forum section: electronics / beginners

author: u666sa
author_rank: Frequent Contributor
timestamp: September 16, 2023, 03:17:27 pm
content: I'm fixing a timer relay -- TDM ТРЭ-01It used to work, but then it broke when I had it hooked up into outlet with broken faze, outlet was giving really low voltage. Thus far I replaced Z1 stabilizer, because it was shot. I also checked all diodes, and transistor, all resistors, replaced battery. Checked relay. Relay works. It takes about 30 volts to activate it. Presently the problem is that 220V does not come into the rectifier.220V comes in from mains, red wire is hot, where my finger is the black wire, neutral.Here, neutral is going through 1M R2 and 33 uF condenser into rectifier bridgeI'm holding my finger where hot (red) goes through 150 ohm resistor into rectifier bridgeNow I got some schematics attached. But don't get your hopes high. It is not from this particular timer.Now, on schematics we have R2 970k. But on my board, I have R2 in sot25 with 105 marking. Which is 1 meg resistor. And also on board there is 1M marking.How is voltage suppose to come into rectifier bridge?I mean, there is a 0.33 uF condenser in parallel with R2, but I checked that thing, it checks out okey.

author: ataradov
author_rank: Super Contributor
timestamp: September 16, 2023, 03:33:39 pm
content: It is a very typical capacitive voltage dropper -https://en.wikipedia.org/wiki/Capacitive_power_supply

author: wasedadoc
author_rank: Super Contributor
timestamp: September 16, 2023, 03:39:26 pm
content: And typically the dropper capacitor loses a lot of its capacitance over years of being plugged in.

author: dietert1
author_rank: Super Contributor
timestamp: September 16, 2023, 03:47:43 pm
content: Did you check R1? It is a safety part that limits inrush current on power-on.The circuit works well with sine wave as mains input. It fails when used with a 220 Vac battery converter that outputs rectangular wave.Regards, Dieter

author: u666sa
author_rank: Frequent Contributor
timestamp: September 16, 2023, 03:52:24 pm
content: Yea. R1 is 150 ohm as it should be. It's a regular through hole resistor.Interesting, cap checks out okey. Anyway, tomorrow gonna run to the store get a new one.Just to be sure, how much volts should I get on rectifier input?

author: dietert1
author_rank: Super Contributor
timestamp: September 16, 2023, 04:55:58 pm
content: With a low frequency sine wave as input the capacitor acts as a high impedance source so the zener determines voltage.

author: soldar
author_rank: Super Contributor
timestamp: September 16, 2023, 08:08:32 pm
content: Quote from: wasedadoc on September 16, 2023, 03:39:26 pm+1I had a similar problemhttps://www.eevblog.com/forum/repair/repairing-panel-volt-amp-meter/msg2110735/which I resolved withhttps://www.markhennessy.co.uk/mdu/

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2023, 09:27:21 pm
content: This loss of capacitance can occur with non-C0G ceramic dielectrics, which are non-linear devices.Unfortunately, there isn't enough space (probably) in your housing for a good TH film capacitor: can you substitute a PPS or other film SMD capacitor (they tend to be in a larger package than ceramic devices)?

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 16, 2023, 10:15:06 pm
content: We're getting close to audiophoolery levels with so many C0G here and there! This circuit is stupidly simple, need for such expensive caps.They die after 10-15 years, probably took some nasty spikes from mains, or moisture got inside.Check the capacitance, if bad then just put a new x2 cap rated for >250V~.Same for the electrolytic and the zener, they're the usual failure points.

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2023, 10:41:56 pm
content: Even in non-audio applications, I don't like running substantial AC current through Class II ceramic capacitors if not necessary.However, I don't know what that capacitor actually is in his unit, since the diagram he posted is for another device.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 16, 2023, 11:10:48 pm
content: These capacitor droppers normally use X2 polyester or polypropylene caps. Never seen ceramics doing that work.

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2023, 11:11:46 pm
content: I agree that PE and PP are better dielectrics for AC current.I'm not sure if the OP's unit had a SMT capacitor or TH capacitor.

author: u666sa
author_rank: Frequent Contributor
timestamp: September 17, 2023, 05:09:29 am
content: Fixed!Turns out I put two diodes on rectifier bridge backwards, you can see on silk screen.I began to think how it happened, and turns out I did attempt to repair it right after it broke, in the winter, back then I didn't have microscope. So I ended up putting these two backwards.This time around I didn't question these, just ended up blowing them off and checking and then putting back, the way they were on there, not paying attention to the silk screen. Anyway, put everything back then way it should be and it works now, relay is clicking.Interesting enough, I'm getting 33V on relay. Zener was replaced with 1N4756A, supposedly. I don't know what they gave me at the store. So I might try to source proper zener, or leave this one in, since 33V is good enough for replay to click.