150W 24V SMPS mod to 5V (clicking noise) - Page 1
Forum section: electronics / beginners

author: dominicM
author_rank: Regular Contributor
timestamp: March 28, 2018, 01:02:27 pm
content: I have a few of cheap chinese 150w or 240w 24V or 48V power supplies. I would like to modify these to 5V or 12V.(24V PSU) I tried replacing R30 to 4.7k or R32 to 47k and got 4.4-4.7V output which is too variable. I also hear a fairly loud clicking similar to when TL431 had failed (replaced with genuine part now). Exactly why does this noise happen? Seems quite loud for such a tiny part with no moving parts. Does this harm anything in the circuit? More importantly what can be done to remedy the variable output voltage? LED glows quite dimply which I assume is because R31 connected to the power indicator LED is now the wrong value. I wouldn't think that could cause issue in itself though.They all have almost identical design that uses TL431. R30 connects to Vout. VR1 and R32 are in series and connect to GND.VR1 = ~430 (about middle POT setting)R30 = 47kR31 = 4.4kR32 = 4.7kR27 = 4.7k

author: wraper
author_rank: Unknown
timestamp: March 28, 2018, 01:09:31 pm
content: This happens because PSU needs skip hell a lot of pulses to keep the voltage low enough. It basically shuts down and restarts after a while. Sound comes from transformer.

author: mariush
author_rank: Super Contributor
timestamp: March 28, 2018, 01:10:41 pm
content: Yeah, you can't just change the output voltage so aggressively without changing other things... you changed the "duty cycle" too much.Here's a video that shows how changing the output can affect other things in a power supply:

author: madires
author_rank: Super Contributor
timestamp: March 28, 2018, 01:25:49 pm
content: I agree, for such a large voltage change you would have to change the flyback transformer (less secondary turns & larger wire diameter). A better option might be to add an additional DC-DC converter for 5V.

author: dominicM
author_rank: Regular Contributor
timestamp: March 28, 2018, 01:37:19 pm
content: Ok, so basically it sounds like it's not practical to mod it to 5V. Transformer change is certainly not an option for me. How would one try to estimate lowest/highest voltage mod that would work well? I also have 48V PSU and 12Vout would be useful to me as well.What specifically causes the variable voltage? Is it just inadequate filtering due to change in duty cycle?

author: wraper
author_rank: Unknown
timestamp: March 28, 2018, 01:40:28 pm
content: Quote from: dominicM on March 28, 2018, 01:37:19 pmIt's not change in duty cycle that cause clicking. It's that duty cycle cannot be kept low enough that PSU needs to skip cycles/shut down for some time. When it restarts, you hear click.

author: dominicM
author_rank: Regular Contributor
timestamp: March 28, 2018, 01:44:54 pm
content: That makes sense. I could swear that the noise is coming from the transistor not the transformer but I'll take your word. In either case how does such a distinct sound gets generated from a part with no moving parts?

author: wraper
author_rank: Unknown
timestamp: March 28, 2018, 01:44:59 pm
content: 24V to 12V mod might work fine. Although you may need to increase dummy load (decrease resistance) on the secondary side to keep it working fine without external load attached.

author: wraper
author_rank: Unknown
timestamp: March 28, 2018, 01:45:48 pm
content: Quote from: dominicM on March 28, 2018, 01:44:54 pmTL431 is not a transistor and it cannot click.

author: dominicM
author_rank: Regular Contributor
timestamp: March 28, 2018, 01:57:42 pm
content: Would you know the mechanics of how transformer generates clicking noise?

author: mariush
author_rank: Super Contributor
timestamp: March 28, 2018, 02:26:59 pm
content: Quote from: dominicM on March 28, 2018, 01:44:54 pmTL431 is just a voltage reference, it's used to send through the optocoupler "messages" to the controller like "dude, you made enough cycles through the transformer that the output voltage reached 24v" and then controller pauses for a few microseconds and the voltage on output drops and then the voltage references says through the optocoupler "dude, the voltage dropped too low, increase cycles through transformer" ... it's not literally saying messages, just on or off (you hit the voltage, stop or keep going) or some degree of "intensity" of signal through the optocoupler.The noise can come from the transformer, you have coil of wire wound around a ferrite core, you have magnetic field generated inside the transformer. When there's pulses of current going through the wires inside the transformer, the wires can heat up or cool down and can shrink slightly or expand a bit, so these physical changes can cause coil whine, friction of the coil against the ferrite core. The magnetic field being created and breaking down can also cause noise I guess.

author: wraper
author_rank: Unknown
timestamp: March 28, 2018, 02:49:59 pm
content: there is always some noise from the transformer, however normally frequency is above of what human can hear. Skipping pulses brings frequency down to audible range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2018, 02:58:51 pm
content: A change of more than +/-40% is unlikely to succeed. The problem is the auxiliary winding on the primary side, which supplies power to the controller.Second priority, switching and control functions are worsened. Too low output voltage (too high transformer ratio) forces a low duty cycle and continuous current flow, causing higher peak currents, and chaotic switching. Too high output voltage (too low transformer ratio) and the transistor and rectifier (and snubber, if TVS based) voltage limits are threatened.Tim

author: wraper
author_rank: Unknown
timestamp: March 28, 2018, 03:23:29 pm
content: Quote from: T3sl4co1l on March 28, 2018, 02:58:51 pmCompletely forgot about it. However this it not necessarily true as some PSUs don't have aux winding.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2018, 03:39:04 pm
content: Right. Very small ones are often powered just from a bias resistor off the DC bus. I think these become uneconomical over 5 or 10W.I think most designs in the 150W class will be of the described type (aux winding, UVLO causing 'hiccup' operation). The minority, then, may be secondary powered, in which case the same still applies, but depends on the output voltage directly, rather than an aux winding. I'd think this would be unlikely outside of 9-15V supplies, where the output is immediately useful. (For other voltages, there would probably be a secondary side aux winding, which ends up with the same problem as the primary side case.)The last case, is when there's an independent standby supply. These are fairly uncommon in single or few-output supplies, as far as I know (but they are quite common in certain applications -- like computer PSUs). If you can identify one of these, you will have a much better chance of success, setting it to a very different output voltage. You may even be able to make a variable bench supply with one!Tim

author: dominicM
author_rank: Regular Contributor
timestamp: March 28, 2018, 03:52:07 pm
content: Thanks for all the info, it satisfied my curiosity! Once the mod is made how would I determine that it was really successful? Is it just as simple as no noise + no smoke + stable Vout = success or are there other thing to check before use?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2018, 06:44:07 pm
content: Guh... I can think of many things thatI wouldcheck. But a minimal test? Uhh, maybe...1. Don't try to increase the voltage by more than, oh, 20%. If there is a crowbar circuit, adjust its threshold, or disable it.2. Don't try to decrease the voltage by more than, oh, 30%, if it's a self-powered circuit. If standby powered, any ratio (down to zero) may be possible.3. Test that it operates, at (the new) nominal voltage, over the full load range (use a selection of resistors to figure this out), without shutting down, drooping, or over/undershooting.4. Check output ripple over the same range, and any sounds it's making. If it's hissing or whining, it might still be okay (given that you'll have more output ripple), but it's not ideal.5. Run at full load (either the nameplate current, or no more than 90% of the actual dropout current, whichever is lower), for an hour, and make sure nothing is getting melty and burny inside. (Preferably, get a thermistor inside the enclosure to measure key hot spots, usually heatsinks, transformer or capacitors.)To measure the voltage stuff, note that a DMM reads DCV as just the average (no AC), and ACV as just the alternating part (no DC). So you can measure ripple (and under/overshoot if you toggle a load on and off rapidly) with the AC range. Or at least, that's how it should be; if it's obviously reading "AC" as everything (i.e., a 12V supply is measuring as ~12V "AC"), then your meter is dumb, and I guess, take that measurement with a series (coupling) capacitor instead.Note also that DMMs don't read high frequencies. The ripple will most likely be four things:1. Hiccup mode: output (probably as seen on both DC and AC) swinging wildly.2. Mains ripple: maybe output (AC) ripple rising with load resistance?3. Control loop stability: audio frequency, accompanied by a whining sound (from the transformer).4. Control or switch instability, chaos: audio frequency, usually a hissing sound; output ripple impact may be small, but noticeable.Expect reduced reliability. You may get lucky and end up with something not far off from design ratings, but don't expect that to be the normal outcome. When you can't afford down time, get the real deal.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: March 29, 2018, 04:27:21 am
content: Can you make out the name of the primary side controller IC?A clicking sound generally indicates a protection causing the controller to attempt a restart periodically. Magnetostriction is probably what's causing the sound. It's probably not worth speculating what's happening without at least knowing the topology/controller first.Have you tried applying a load at all? Whether the issues persists during loaded operation (or not) could shed light onto what's happening.Again, as others have suggested, you're probably not going to get great results out of this, but could be a good learning experience.

author: dominicM
author_rank: Regular Contributor
timestamp: March 30, 2018, 11:39:29 am
content: Quote from: TimNJ on March 29, 2018, 04:27:21 amTop of IC is filed off so no idea what it is. I forgot to try applying a load before I put it back together. It's not really worth it anyways since even 12V did not work properly and I really need 5V in any case.I didn't really find much as far as cheap variable power supplies go apart from bench supplies. I think I will try to duplicate the following PSU mod:http://imajeenyus.com/electronics/20151028_smps_variable_voltage/index.shtml4.8V to 15V seems like a very decent range.

author: Seekonk
author_rank: Super Contributor
timestamp: March 30, 2018, 01:15:52 pm
content: I buy 12V @1.2A in an aluminum case for a little more than a buck shipped. There ought to be 5V versions cheap. If your supply is higher power, It msy use a voltage doubler on the input. Just rewire it so it doesn't double. That should match 5V a little better. I use these 120-240V supplies on 60V DC which is quite a mismatch. I just reduce the value of the high ohm start resistor. Even adding a zener would waste less power than putting a load on the output.