Existing old type  LM324 based automatic voltage stabilizer circuit  evaluation  - Page 1
Forum section: electronics / projects

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 11, 2025, 07:52:36 am
content: Hi there,I wanted to build up a voltage stabilizer to study its performance.I got a simple circuit diagram from online, perhaps someone design it without understanding the LM324 op-amp action.This has made me curious because I wanted to know it works or not.Take a look below.The most obscure part is transformer TR1.The main winding (taps A … E) is most likely a specific, commercial part; but without knowing the expected voltages (or turns ratios) of that winding and taps AND the smaller, isolated one (possibly another 18V, like TR2?), it simply cannot be fully functional.The three relay drivers on the left side almost beg to be modelled as identical subsections, easily done with the information shown. Same with the High/Low cutout circuit on the right side.I was trying to simulate this circuit in LTspice, here is the image. Kindly help me to match with the upper one.Lets calculate the op-amp gain, and fault.I will be waiting for your response and guidance and comment. Your similar suggestions are appreciable.NB: DONT comment something that you are not sure about it, I might be silly for you.

author: synccond
author_rank: Contributor
timestamp: August 11, 2025, 11:49:48 am
content: Why did you move the 'ground' (better called circuit common) on BR1 - it was in the right spot. It is simply taking the AC voltage, rectifying it so the circuit can use it as DC, and referencing that DC to circuit common.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 11, 2025, 01:19:06 pm
content: QuoteThank you to take part here. If you look the original diagram, the deep green line (trace) goes all of the negative side of the component, perhaps the BR1 diode direction is wrong in Ltspice drawing. You are right that rectification helps to detect the DC. There is no 'gain' as such in the opamps, they are wired as comparators - it's a basic window comparator. The opamps have no functional part in the circuit as such, that is all done crudely by Q1, Q2 and Q3.can you calculate the voltage difference in inverting and non-inverting input ? From simulation VCC looks 8.2VDC. When do you think Q1,Q2,Q3 turned ON ? They are not depending on input voltage or rated current? Helping relays ?Do you believe this circuit will work ?What is your expected output looks like? My simulation result shows a sinewave in output 320VAC.

author: synccond
author_rank: Contributor
timestamp: August 11, 2025, 07:49:12 pm
content: Spice D1 is backwards.Yes, if the circuit is working properly, it will be completely sine wave output. It just selects how much autotransformer action to implement.Can look into more details of how the circuit works, but fix D1 for starters.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 12, 2025, 10:33:42 am
content: QuoteYes, I did change the D1 direction and now the output looks different, previously it was sine wave.Take a lookI am guessing that U4 and U5 (together with their associated components) are a "window" detector, checking to see if the rectified output level is within a window, neither too low nor too high. If so, then one of the pots (U8 or U9) sets the lower trip point (the lower edge of the window) and the other sets the upper trip point.To see the correct operation of switch S1 in the simulation, there should be a load connected to the OUT net. The LTspice switch is not an ideal switch, and even when it is "open" it is a 10G resistance. With no load connected to the OUT net, all of the voltage passes through the 10G resistor, as if the switch (relay) was closed even when it is open.

author: synccond
author_rank: Contributor
timestamp: August 12, 2025, 01:05:05 pm
content: Window detector makes sense, the circuit has to either buck or boost the input AC by selecting which portion of the autotransformer to select.I would start at the beginning, and look at each critical point along the way; what do things look like at the R16/R18 node (should be DC) with 120Hz ripple? It gets smoothed at C13. Do your VCC rails look correct?You seem to be jumping immediately to the output and getting confused. You need to understand the circuit all along the way. If you started with this approach, you would have identified D1 as backwards right away.When you are ready to start looking at the output, extend the simulation time to get yourself a few cycles (40 msec)

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 12, 2025, 01:40:01 pm
content: QuoteVCC was 8.2V when the D1 was in wrong direction.Now it drops to 1.2V. LM324 should not work now !All it needs to do is shunt the signal to ground when S1 is "off" but S1 is not an infinite resistance.(See main diagram)If I put a load of 1k, the output looks more strange, like a step output start at 3.4ms.

author: synccond
author_rank: Contributor
timestamp: August 12, 2025, 08:33:21 pm
content: Show your latest spice drawing without any mention of the ground moving. You have something wrong with the diode bridge.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 13, 2025, 09:15:36 am
content: QuoteNow I keep the GND back in original position, and the simulation improves may be. But dont you think both position should be grounded ? Take a look.

author: synccond
author_rank: Contributor
timestamp: August 13, 2025, 01:33:14 pm
content: Cannot read your schematic.The bridge MUST be configured as attached, which agrees with the original design.If you run the bridge as attached and it doesn't work, your spice file is wrong. Don't question the circuit, question your spice.This is a pretty standard and common design for basic non-precision measurement of an AC signal. It gets rectified to DC so the opamp circuit can act on it.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 14, 2025, 10:32:20 am
content: QuoteStill you believe this circuit is not working ? Do you expect a glitch at 3.5ms when the GND position unaltered ?I wonder if the glitch that happens near 3.5 ms in my simulation, happened because the output relay driver is close to the threshold of switching ON the relay. And in my simulation, it switched ON at about 3.5 ms.If that is what happened, then I think it is normal. This circuit probably powers up with the relay OFF at first, so OUT is disconnected, until the voltages come up and it decides the output voltage is within range. Then the relay closes and connects the voltage to the OUT net.But as I said, if it was very close to switching it ON, a small difference in the simulation might cause it to happen.Technically, that part is correct. The circuitry around the LM324 can be floating (no connection to ground), and that is exactly what they did. The green wire was not ground, in the published schematic. That is OK because that part of the circuit needs no actual connection to ground.But to simulate it in SPICE, it must have a Ground in that part of the circuit. SPICE can have no floating circuits. Even battery-powered circuits must have a connection to Ground to simulate them in SPICE.

author: synccond
author_rank: Contributor
timestamp: August 14, 2025, 01:16:56 pm
content: I was only trying to get confirmation on the incorrect polarity of D1, and working forward through the circuit block by block to make sure everything looked right. The issue is not one of "ground", it is whether you changed circuit common in your first post. Clearly communication is not working. I still have no certainty of D1 because you continue to only pay attention to AC output.I think you are asking for issues by always focusing on the AC output, and not paying attention to the intermediate sections of the circuit. Simply concluding 'it works' or 'it doesn't work' is not a proper methodology, especially when trying to use spice.But you do you; if you are convinced it's working go ahead and build it.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 16, 2025, 01:15:23 pm
content: QuoteIt was the same configuration, our eyes are just confusing, take a look.[ Specified attachment is not available ]

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: October 04, 2025, 02:53:09 pm
content: @synccondTake a look 2 similar circuit here and make some comment which one is comparative good that we were talking about