Master/Slave Power Strip Repair - Page 1
Forum section: electronics / repair

author: WannabeGeek
author_rank: Contributor
timestamp: February 07, 2016, 04:58:05 pm
content: The current sensing, surge protection power strip (that I was using to switch on my computer and all the peripherals together) died recently, so I decided to take it apart. The board didn’t look too complicated and there are no SMD parts. It seems to consist of a surge protection circuit and a current sensor circuit which uses a relay to switch on the slave sockets. The sensor circuit is powered by a capacitor dropper circuit. When I opened the case, I noticed that the original 33k resistor on the far left between the Load wire and the red LED was browned. It was reading around 34k. I didn’t think this would result in a total failure of the unit, but I replaced it with a new one anyway. The first thing I did was to check the thermal fuse between the two MOVs which is partially obscured by the brown LMS wire. The continuity check was good. Next, I checked the two 47uF/50V caps with my ESR and capacitance meters and they checked out. I checked the little ceramic disk caps with my L/C meter and they’re all good. I checked all the diodes with my multi-meter and they all passed. I checked the rest of the resistors and they were all good as far as I could test them on the board. No shorts, no unusually high or low readings. Even the 470K above the green LED, with the yellow stripe scraped off, checks out good. The LEDs are both good although they don’t light when power is applied to the strip. I actually desoldered the LM358 Op Amp and the transistor and they both passed basic tests out of circuit. While the op amp was out, I decided to solder in a socket so I could experiment with other op amps. The Sensor (which I’m assuming is a coil of some sort) has a resistance of around 90 Ohms. That seems rather high to me, since inductors of that general size usually have a resistance of less than 5 ohms. I have a flyback transformer tester which gives me no rings across the leads but also no short. My L/C meter just reads it as “over range.” The white round thing to the left of the op amp is a 25k trim pot that is used for setting the threshold to trigger the slave sockets depending on the power that the master is drawing. It also checks good. The resistance of the relay coil is within the tolerance on the datasheet. That’s all I could do without power, so I decided to live dangerously and check it out while it's plugged in. The input voltages are 230v AC around all the power traces as expected. The electrolytic cap near the op amp is reading 12v DC and the electrolytic cap near the transistor is reading 24v DC. I’m assuming that means the power supply portion of the circuit is good, so that leaves the triggering circuit. I’m getting 12v DC across the power supply pins of the LM358. The rest of the readings with regard to ground (with the LM358 installed) are as follows:Pin 1: (out 1) 9.6mVPin 2: (in 1 -) 3.33VPin 3: (in 1 +) 45mVPin 4: (gnd) 0Pin 5: (in 2 +) 2.8mVPin 6: (in 2 -) 3.3mVPin 7: (out 2) 34.2mVPin 8: (VCC) 12.02VThe reason I bothered to list the millivolt readings is that they are stable as opposed to just transients jumping around. I’m also assuming the input signal from the sensor will be pretty low. I accidentally shorted pins 2 and 3 (or 2 and 1 – I’m not really sure) and the relay clicked on with the green LED glowing. I only did that once, but that would suggest to me that the 24V relay and the transistor are functioning properly. I don’t have a replacement LM358 on hand, but I did install a 4558 in the socket to see what would happen. The only difference was that the green LED was glowing slightly. Measuring across the 2 sensor leads, I get .02v AC with or without a device being powered by the master and 0v DC either way. I tried to measure the Hz with my multi-meter but the readings across the sensor are all over the place. Whether or not there’s a load in the master, it still hovers somewhere around 50 Hz. I was expecting some noticeable change across the sensor but I’m unable to detect any. Does that mean the sensor is faulty? I was assuming that those things are pretty much indestructible. I’m not even sure how to test it or what exactly is supposed to happen when amperage is drawn through the hole. I can’t find any datasheets or comparable circuits using a 2 pin sensor. Does anybody have any experience with this type of (Hall?) sensor or a master/slave power strip?

author: mij59
author_rank: Frequent Contributor
timestamp: February 07, 2016, 06:24:33 pm
content: Your current sensor is a current transformer it's probably something like 1000:1, so 1A through the blue wire will generate an output current of 1mA.The output current from the current transformer is converted to a voltage by a resistor just below the LM358 , I think it's 10 Ohm.

author: WannabeGeek
author_rank: Contributor
timestamp: February 07, 2016, 10:18:51 pm
content: Thanks for the info. If it is a 1000:1 transformer I would expect it to act like one. For me, that would mean some sort of inductance reading or a low resistance. I'm not sure which resistor you are referring to but if I follow the trace from the right pin of the sensor, past the x-cap and the diodes, it ends at the junction of what looks like a voltage divider formed by a 1K resistor (to ground) and a 47K resistor. That 47K resistor is connected to pin 5 (non inverting input 2) of the LM358. Powering up again, there's no change in the voltage between ground and pin 5 when I switch on my fume extractor (which I have plugged into the master). It still reads exactly 2.8mV on or off. If I measure between ground and the junction of the 2 resistors, I get 0v. I am getting a 2v AC reading (sometimes) between ground and pin 5 that doesn't change when I turn on the fume extractor. I don't want to risk blowing up my oscilloscope and I'm a bit afraid of trying to measure milliamps around that circuit for fear of blowing the fuse in my multi-meter. I'm still fishing around in the dark with no real clue as to why the unit isn't working

author: mij59
author_rank: Frequent Contributor
timestamp: February 08, 2016, 04:53:22 am
content: I measured a 100A current transformer from a panel meter, the resistance is about 8 Ohm an the inductance is 94 mH, so the 90 Ohm resistance of your transformer is not that odd.What's the current rating of the fume extractor ?Try the AC measuring directly across the 1k resistor, maybe something is floating.You could replace the current transformer with a small toroidal inductor , or use two 470k resistors in series to connect between the line voltage and the junction 1k/47k resistors to test the circuit.

author: WannabeGeek
author_rank: Contributor
timestamp: February 08, 2016, 10:22:06 pm
content: I’m not exactly sure what the purpose of your experiment would be. I’m not an electronics engineer. Perhaps you could clarify a bit. My fume extractor consists of a small step-down transformer, a rectifier, a 7812 volt regulator, a few smoothing caps and a 12v computer fan with a filter. I figure it’s pulling less than 1 amp. I also had my desk lamp (which draws 1.6 amps) in the master socket with no noticeable change. When it was working correctly, I could get the relay to turn on the slave sockets with just about anything. If there was something that was drawing too little current, the red LED would light up telling me to adjust the threshold trimmer, which would then turn on the slave sockets. Even if my fume extractor is pulling too few amps to trip the circuit, I would still expect some sort of reaction from the sensor. I’m actually a little surprised that there are no LEDs lighting up regardless of the condition of the sensor. Aside from the large Ohm reading, I’m not really too suspicious of the sensor. Since I know the least about that part, and everything else is testing OK, I was naturally curious to find out how I could test it. Even though the LM358 is testing good with my simple op amp tester (see attachment) I suppose it could be failing in other important areas. Other than that, I can’t seem to find any suspicious components. I suppose I could start pulling out all the diodes and testing them one by one out of circuit.I’m a bit of a beginner regarding transformers. I have a box full of small transformers that I pulled out of switch mode power supplies. I also have a lot of inductors and chokes. My (home made) LC meter won’t go up that high in the millihenry range. The biggest transformer I have gives me a 34 millihenry reading with .9 Ohms. Of course, at such low Ohms, I might have to subtract the impedance of the test leads. Most of my other transformers and inductors are in the microhenry range, but I haven’t found one yet that tests above 1 Ohm with my multimeter. I’m assuming Ohms and Henrys don’t have a linear relationship. I was hoping I could test the sensor out of circuit because I don’t really have confidence that the rest is working properly. Anyway, back to your idea…Measuring across the 1K Ohm resistor with the power strip on (no load in the master) I get 0v AC. With my desk lamp as a load in the master, I get .102v AC. Just for fun, I did the same thing with the fume extractor and I got .027v AC, so I guess there is a big difference. The fume extractor is grounded and the lamp is not, so that may be a factor. Needless to say, the relay didn’t switch on in either case. At least I got some sort of reading for the first time, so that’s good news.I have some toroids here but I doubt that I could get 1000 turns on any of them. Even If I could, there would be no more room for a mains wire to go through the middle. The other toroids I salvaged are wound with relatively thick wire. I’d say the one with the most wire has about 100 turns on it. It gives me a reading of 380uH with a resistance of .4 Ohms.I’m not sure what you mean with the series resistors connected to the 1K/47K junction. If I put two 470k resistors in series (940K) with the (230vAC) line, that should inject .24 microamps into that junction. Is that what I would usually expect from a Hall sensor? What should the result be if everything except the sensor is working? I find it interesting that there are already two 470k resistors in series (just to the right of the green LED) between neutral and the rectifier diodes. By the way, I shorted the negative of the right (24v) electrolytic cap to the collector of the c9013 NPN transistor thereby bypassing the sensor circuitry. As expected the relay switched on, the green LED lit up, and the slave sockets received 230v AC. That rules out the relay at least.

author: mij59
author_rank: Frequent Contributor
timestamp: February 09, 2016, 06:28:35 am
content: The purpose was to test weather the current transformer is faulty, and no its not a Hall sensor, a Hall sensor would have at lease 3 terminals.The behaviour of the red led makes no sense there is no connection with the current sense circuit, it's used to indicate the failing of the protection circuit.Your AC measurement proves the current transformer is working, next step is to check the output 2 of the opamp for a AC signal, check if adjusting the pot makes any difference.

author: WannabeGeek
author_rank: Contributor
timestamp: February 09, 2016, 11:39:57 pm
content: Perhaps I was mistaken as to whether the red LED ever lit up. It could have been the green LED blinking along with the relay when the unit in the master socket was drawing too little current. Measuring between ground (pin 4) and output 2 (pin 7) I'm getting around .1V AC with the unit powered up (no load) and 1.4V AC with my desk lamp in the master socket (turned on). This is with the trim pot in the middle position. Strangely, if I adjust the trim pot either high or low, I get around .8v AC (with master load) in either position. Measuring DC volts, I get 34mV with no master load and 1.25V with load and the trim pot in the center position. With the trim pot either high or low, I get only .7V DC with load. Now I'm even more confused. First of all, the sensor is obviously sending something, but the op amp output is producing only negligible voltage; either AC with a huge offset, or DC with a huge ripple. Also the trimmer causes the highest voltages when the resistance is around 12.5K. Could the op amp be malfunctioning or is there is a load somewhere that is drawing all the power? I hear those capacitor dropper circuits are susceptible to that sort of thing, because they're designed to reduce the current to within a small tolerance of what the circuit needs to operate. Does any of this make sense to you?

author: mij59
author_rank: Frequent Contributor
timestamp: February 10, 2016, 05:18:37 am
content: The difference in AC signal on pin 7 of the opamp is important, seems fine to me.You have already checked the 12V and 24V supply voltages, and the esr of the capacitors, you could measure the ripple voltage on both capacitors.Check the output of the other opamp with and without master load.Please trace out the circuit and post the schematic

author: WannabeGeek
author_rank: Contributor
timestamp: February 10, 2016, 06:31:17 pm
content: The output 1 of the LM358 (measured between pin 1 and pin 4 gnd.) is .001 VAC (no load) and .002 VAC (with load). The output 1 DC is 9.2 millivolts (no load) and 10.2 millivolts (with load). I can’t really measure the capacitor ripple because I can’t isolate my oscilloscope from the mains. I’m getting .04 VAC and 12.01 VDC from C1 (with load). I’m now getting 25.6 VDC from C2, but the AC is either too small or too unstable. My multimeter is just giving arbitrary AC readings that jump all around. I’ve included a schematic as best I could figure it out. I hope there are no mistakes. I ignored the surge protection circuit.

author: mij59
author_rank: Frequent Contributor
timestamp: February 11, 2016, 05:40:56 am
content: Quote from: WannabeGeek on February 10, 2016, 06:31:17 pmChecking the ripple voltage with a DMM is sufficient.Thanks, the schematic looks good, I’ll check it.I noticed that the zener DZ2 is not powered.

author: WannabeGeek
author_rank: Contributor
timestamp: February 11, 2016, 10:38:30 am
content: Oops! I forgot to connect DZ2. Thanks. I've posted the revised schematic. It probably isn't the only mistake.

author: mij59
author_rank: Frequent Contributor
timestamp: February 11, 2016, 01:05:39 pm
content: Thanks for the update of the schematic, I'll give a short explanation of the circuit.Opamp1 = pins 5,6,7 of the LM358Opamp2 = pins 1,2,3 of the LM358The current transformer is loaded by R5, for accurate measuring the value of 1k is way to high, but for current sensing this will yield a higher input voltage for the circuit.The voltage across R1 is fed into the non inverting amplifier, opamp1, with VR1 the gain can be set between about 20 and 471.Diode D5 and D6 clamp the voltage on the non inverting input, R6 and C3 form a low pass filter for input noise suppression.The output of opamp1 is fed in to a low pass filter R12, C4, when a 50 Hz signal from the current transformer is present on the output of opamp1 the average voltage on C4 will be higher than 3.25 V and the output of opamp 2 will go high, turning on the transistor Q1, which turns on the relay RL1.Please check the DC voltage on pin 7 and pin 3 of the LM358 when the master load is on.Given the low AC voltage on the output of opamp1 when the master load is on I think there is a problem with the current transformer.

author: WannabeGeek
author_rank: Contributor
timestamp: February 12, 2016, 01:11:12 am
content: Measured between pin 4 (gnd.) and pin 7 of the LM358, I get 1.15VDC with load and 37mV without. On pin 3, I’m getting .99VDC with load and 48mV without. I’m still getting a steady 12V across the power pins.Thanks for taking the time to analyze the circuit ;-) When you wrote “The voltage across R1 is fed into the non inverting amplifier, opamp1” was that a typo? I don’t see any direct path from R1 (which connects only to the bridge rectifier) to the non inverting input of U1. The rest makes perfect sense to me.That’s bad news about the current transformer since it’s just about the only part for which I am unable to find a replacement. How would a current transformer go bad anyway? There are no moving parts and it barely gets an electrical load. I would expect almost any other part of the circuit to go bad before the transformer. Do you think I could get away with replacing it with this eBay offer (DL-CT1005a) if I tweaked a few resistor values?http://www.ebay.com/itm/DL-CT1005A-Wear-Core-Type-AC-Miniature-Sensor-Current-Transformer-50A-10A-5mA-/281752318805?hash=item4199bf3355:g:3SUAAOSwBLlVdkplOf course, it won’t fit into the unit if I mount it vertically, but I might be able to find another way.The ZCT505 on this page looks physically pretty close, but nobody is selling it anywhere:http://www.ningbo-electric.com/zero-phase-current-transformer.html

author: mij59
author_rank: Frequent Contributor
timestamp: February 12, 2016, 05:36:49 am
content: Quote from: WannabeGeek on February 12, 2016, 01:11:12 amThanks for noticing, yes it's resistor R5.There's probably a short in a part of the transformer winding.A Current transformer acts as a current source, with a high value of the load resistor, ( or no load) the output voltage will be very high, this could lead to breakdown of the wire insulation.Reducing the value of R5 will decrease the sensitivity of the circuit, you'll have to try what works for you, or you could place two diodes ( e.g. 1N4148 ) anti parallel across the current transformer terminals.The current transformer from eBay should work, there are also 5A current transformers available on eBay.

author: WannabeGeek
author_rank: Contributor
timestamp: February 12, 2016, 05:56:31 pm
content: Since this is supposed to be a learning experience and based on the information you’ve given me, I’m thinking I could devise a simple and definitive test. If the transformer is working and I were to remove it from the board (leaving the mains wire running through it) and bridge the two leads with say a 250 Ohm resistor, would I get a significant AC Voltage across the terminals if I put a load in the master socket? Also, if the transformer is bad or partially working, what would the voltage difference typically look like? Do you have a better idea for an out of circuit test? If I remove the transformer and inject a small AC voltage across R5, should I expect the relay to turn on if the rest of the circuit is working?It looks like that model current transformer from eBay is readily available. I don’t see any others that might fit on my board. As it is, I’ll have to lay it flat and solder some wires to it. Since I still can’t find an official datasheet for that model either, I’m a little confused as to why there are 4 terminals instead of 2. From the eBay description, a resistor is shown between terminals 1 and 2. Do you think that is a theoretical resistor or is there one built in? Would terminals 1 and 4 be connected together and also 2 and 3, thereby giving the option of connecting the device with or without the resistor, or are there separate windings? A maximum load of 250 Ohms is mentioned. Does that mean I should not use it in my circuit without lowering the value of R5 to a maximum of 250 Ohms? I’m guessing that would mean I would also need to change the values of R6, VR1 and R14 or at least one or two of them to make the circuit work properly. What exactly would the 2 diodes do? Would they be acting as a rectifier?

author: mij59
author_rank: Frequent Contributor
timestamp: February 12, 2016, 06:37:32 pm
content: You can test the transformer with the 1k resistor, just measure the AC voltage across R5, because the relay is no turning on the voltage will be quite low.To inject an AC voltage use two 470k resistors in series to connect between R5 and the neutral wire (N) in you schematic.Only the terminals 1 and 2 are connected, the show resistor is the load resistor.Use a 220 Ohm resistor, or the the two anti parallel diodes together with R5 being 1k.The diodes prevent the output voltage of the current transformer exceeding about 0.6V.

author: WannabeGeek
author_rank: Contributor
timestamp: February 12, 2016, 07:58:59 pm
content: I found another current transformer for sale (also with no datasheet).http://www.aliexpress.com/item/ZMCT103C-5A-5mA-Precision-Micro-Current-Transformer-Sensor-Module/1846875034.htmlThis one has the same pin spacing as mine and only two terminals. According to my calculations, I can just get it in the enclosure standing up. I'm wondering about the amp ratings. If the rated input current is 5 amps, does that mean the device in the master can only draw 5 amps before the transformer overheats? Max 550W is written on the master socket, which works out to 4.2 amps so I guess 5 amps should be enough. Perhaps using it to power a desktop computer was a bad idea since it's power supply is rated at 430W. On the description for the 1000:1 current transformer, it mentions a 100 Ohm "phase." Does this have anything to do with the value of the load resistor I can use? This transformer seems to be a better choice, do you see any reason why it wouldn't work?I'll try the 470K series resistors and let you know what happens. Sounds dangerous ;-)

author: WannabeGeek
author_rank: Contributor
timestamp: February 12, 2016, 08:55:34 pm
content: Just one more thing. When you suggested connecting the series resistors to the 1K resistor, did you mean to the ground or the junction of the 47K? In a previous post, you suggested, "use two 470k resistors in series to connect between the line voltage and the junction 1k/47k resistors to test the circuit." Just to be sure that I don't blow anything up, where exactly should I connect the series resistors to R5?

author: mij59
author_rank: Frequent Contributor
timestamp: February 13, 2016, 05:43:04 am
content: Quote from: WannabeGeek on February 12, 2016, 07:58:59 pmThe accuracy of the current transformation will less when exceeding the specified limits, this doesn’t mean it will overheat.I have no idea what the "phase" means.Which current transformer you choose is not that important, get what fits the best.To be save you could breadboard the circuit and use a (isolated) 12V power supply.

author: mij59
author_rank: Frequent Contributor
timestamp: February 13, 2016, 05:48:37 am
content: Quote from: WannabeGeek on February 12, 2016, 08:55:34 pmThe junction of R5 and R6 is the input of the circuit.To be save breadboard the circuit and power it from a isolated 12V power supply.

author: WannabeGeek
author_rank: Contributor
timestamp: February 13, 2016, 03:42:47 pm
content: I soldered the two 470K series resistors into the circuit between neutral (blue) and the junction of R5/R6 (see photo) and there was absolutely no effect. No green LED, no relay, no mains voltage to the slave sockets. I measured 223VAC and 6.8mVDC across the series resistors. I'm getting .223VAC and 1mVDC across R5. I have no load in the master. Would it make a difference if I removed the current transformer from the circuit?

author: mij59
author_rank: Frequent Contributor
timestamp: February 13, 2016, 06:10:44 pm
content: Quote from: WannabeGeek on February 13, 2016, 03:42:47 pmThe internal resistance of the current transformer is 90 Ohm, this loads the input voltage down, you'll have to remove it.Check the AC output voltage of opamp1 (as function of the position of the potentiometer ).

author: WannabeGeek
author_rank: Contributor
timestamp: February 14, 2016, 03:16:10 pm
content: I removed the current transformer from the board and soldered the series resistors directly to the R5/R6 junction. Powered up (with no load) there is still no reaction from the relay or the LED. The slave sockets are now reading 6vAC. I think they were around 15VAC before with the relay off. I'm assuming that's some sort of residual voltage but not enough to power anything. The only interesting thing is that it's less than half without the transformer. Across the series resistors, I'm still getting 223VAC but I'm now getting 49mVDC. Across R5, I'm now getting .234VAC and 13.6mVDC. The output of U1 (pin7) with the trim pot at the lowest resistance (about 30 Ohms) is 5.7VAC. With the trim pot at 25K Ohms the output of U1 is 2VAC. I'm still not convinced that the current transformer is faulty, but now that I have it off of the board, is there a definitive test to determine If it is partially shorted or not? As I mentioned before, my flyback transformer tester says it's not shorted but it also reacts like there's a problem. Of course, I don't think it was designed to measure current transformers specifically. If the voltage that I injected to the R5/R6 junction should theoretically be enough to power the circuit, this would suggest that there is another fault regardless of whether the current transformer is working or not. Would you say that sounds logical?

author: mij59
author_rank: Frequent Contributor
timestamp: February 14, 2016, 04:03:53 pm
content: Quote from: WannabeGeek on February 14, 2016, 03:16:10 pmMaybe a ring tester would workhttps://www.youtube.com/watch?v=CFyRO42JMbE] https://www.youtube.com/watch?v=CFyRO42JMbE, what kind of tester you got ?Make sure your measuring the voltages with respect to the circuit "grond" e..g. the minus lead of C1.The voltage across R5 should be enough to switch the relay on, what's the AC voltage at pin 7, and the DC voltage on pin 3 ?

author: WannabeGeek
author_rank: Contributor
timestamp: February 14, 2016, 10:28:41 pm
content: I'm using the Dick Smith flyback tester (which is a ring tester).http://members.ozemail.com.au/~bobpar/fbt.htmIt has 8 LEDs which light up according to how many rings are measured. With a normal good transformer, I usually get 7 or 8 rings. When I attach it to the current transformer, I get only one ring, which usually indicates a bad transformer winding. I'm just not sure that this type of transformer is built like the other transformers I've tested. Some good chokes that I have, test just as bad using the same meter.I'm usually measuring voltages using the negative of C1 but sometimes I use pin4 of the LM358 if it's closer. They should be connected directly. The AC voltage from ground to pin 7 is between 2 and 5.7V depending on where the trim pot is set. The voltage of U2 pin 1 is between 11mV and 10.2VDC depending on the trim pot. There are also 10.2VDC going into the base of the transistor. I've tested that transistor twice already (out of circuit) using a simple LED and a signal to the base. It's a very simple go/no go test, but that's all I can do. In circuit, it's giving me a reading across the emitter and collector that shouldn't be there, but I was attributing that to the green LED. I also figured that too much conducting between emitter and collector is the opposite of what my problem should be. I actually replaced that transistor with a similar one as a test and the result was the same. Of course, that was before I bypassed the transformer with the series resistors.

author: WannabeGeek
author_rank: Contributor
timestamp: February 14, 2016, 10:34:38 pm
content: Update: I decided to replace the C9013 with a BC337 again after all this. When I plugged it in, the relay clicked, the LED came on and the slave sockets got 230VAC! Could it be that it was the transistor all along? I removed the series resistors and replaced the current transformer. With a load in the master, it now switches on the slave sockets like it used to. So why wasn’t I getting the correct voltages in the circuit while the transformer was installed before? If I had been getting DC Voltage to the transistor base when I replaced it the first time, why didn’t it switch on the relay? While I was messing around with the current transformer, I noticed that the trim pot was a little loose. I thought it was because there was a white plastic extension on it, but when I examined it closely with a magnifying glass, I noticed a loose solder connection. I immediately fixed this, but since the faulty transistor was still installed, it didn’t make a difference. I think the trim pot was messing up the threshold current, which wasn’t allowing enough voltage to get to the new transistor, which tricked me into looking elsewhere for the fault. Unfortunately, I put the original transistor back in the circuit because there didn’t seem to be a difference with the new one.Doh!When I plugged the C9013 back into my transistor test circuit, it passed again. Testing it again with the mulitmeter set to diode mode, I get no voltage either direction across C and E. The voltage between B and E is .602 but the voltage between B and C is .972. The collector is, of course, where the load is. I didn't think that voltage difference would be so important. Anyway, I learned something about testing transistors. Even if they pass basic functionality tests, you can't call them good until the diode test voltages are very close to each other. Thanks again for all the help. I'm sorry I wasted a lot of your time, but at least I learned a lot too and that’s what Dave is always preaching ;-)

author: mij59
author_rank: Frequent Contributor
timestamp: February 15, 2016, 05:04:33 am
content: Quote from: WannabeGeek on February 14, 2016, 10:28:41 pmThat's not possible, either the transistor Q1 is faulty or the emitter of Q1 is not connected to "ground" .

author: WannabeGeek
author_rank: Contributor
timestamp: February 15, 2016, 05:32:57 am
content: Perhaps my last post was a bit confusing, but you're right. The transistor was faulty. I replaced it and the master/slave power strip is working again. I'm using it to power my computer right now. Thanks for all the help!

author: wootwoot
author_rank: Contributor
timestamp: July 29, 2025, 02:07:07 am
content: Thought I'd add my information here rather than starting a new thread.I recently repaired a similar device, that was momentarily switching on and then off. It is a Jackson PT9778I would have bought a new one, but these are actually pretty hard to find now.Took me quite a while to figure out that there is a section that acts as a very simple 24v power supply. The big polypropylene film capacitor had started failing, and it had reduced its capacitance from 680nF to about 107nF which limited the available supply current. When the relay closed, it pulled the power supply voltage down so far that the whole circuit failed.I traced the whole circuit before I was able to see which section was the power supply. Once I understood that, I could run it on a DC power supply and found the current sensing and switching part of the device actually worked fine. Plugged into the mains, and (carefully) measuring the power supply output voltage confirmed that the supply side was the issue. The voltage dropped down to 4V. Originally I assumed it must be a bad diode or something. Discovering the capacitor was bad took testing components one by one, and I actually resorted to disordering them.I traced the circuit directly into LT spice, so I was able to simulate it. I got the sim working and the simulated current draw matched what I was getting through the DC power supply. At first I couldn't understand how that low capacitance could limit the supply current, but changing the value in the simulation showed exactly what happens. I only had a 1000nF capacitor on hand, and I was also able to simulate what happened if I put it in. Which was simply 15mA wasted current at 24V.(bad on the left, new on the right)So yeah, spent way to long doing this. But I learned a surprising amount and have a functioning unit in the end.I use it to switch a set top box and an old HiFi amplifier on and off for an old TV.