Not seeing quite this. With the 9V battery disconnected, Drain-to-ground resistance reads Overload (OL) at all times as I turn the potentiometer and 0V across the 1 ohm resistor. Upon connecting the 9V battery, I get overload initially, but upon a slight turn of potentiometer drops to 2 ohms at all times as I turn the potentiometer the rest of the way up. On the second meter, getting 1mV across the 1 ohm resistor at all times as I turn the potentiometer with the 9V battery connected.

I think that's working OK. Your meter seems to deliver a constant 1mA on the ohms range, so you get a very on/off experience rather than the gradual change I'd hoped to see.

I think it's safe to try the USB supply.

 

Start with pot at fully clockwise (0) position. Put one multimeter across Drain/Tab and 0v - that's input volts and the other across 1ohm resistor as before. To start input volts will be highest, say 5.2v on a USB, and the other will read 0mV = 0mA. As you slowly turn pot anticlockwise the current will rise and the input volts will start to fall. at a current of 0.5A (0.5v across resistor) the input volts should not be below 4.95v if your USB output is good. At that point the resistor is dissipating 025W so should be warm but touchable (just) and the MOSFET 0.5 * 4.5 = 2.25W. Not sure of the spec of the heatsink but i'd guess the heatsink temperature will be around 70C in that test - as will the drain pin, so watch for melting of the breadboard if you go much higher...

Ok, that seemed to work, though not exactly as you predicted. Initially with POT fully clockwise, I got 5.06V on the power supply and 1mV across the 1ohm resistor. As I turned counterclockwise, indeed input voltage decreased as resistor voltage increased, until power supply voltage reached 4.75V and around 0.590mV across the resistor, at which point the power supply voltage dropped rapidly to about 1.70V and resistor voltage stayed around 0.599mV the rest of the turn. I noticed the MOSFET get a little warm but not hot, though of course it is on ice and I'm not leaving the POT at full for very long. As 1mV = 1mA, (V=IR) this behavior makes sense, as good power supply is rated at 550mA and probably is shutting down from overcurrent by the time we try to pull more than 590mA.

So, the point of this exercise, of course, is to connect the broken power supply to this contraption and figure out at what point it slips out of green mode, and at what point it just shuts down. So, I repeated this procedure with the 30V power supply now.

Initially, input voltage was at around 29.5V in green mode and 1mV across the 1 ohm resistor. As I slowly turn up the POT, input voltage slowly dropped as mV rose, until about 25.5V when there was 15mV across resistor. At that point, input voltage shot up to 30.7V and pretty much remained steady around 30.1V as I continued to turn up the POT. I kept my finger pressing the MOSFET against the ice to gauge its temperature and keep it cool, and certainly it did get hot pretty quickly at the higher POT settings and I quickly dialed back down to let it cool off (and avoid getting myself burned). Power supply is able to maintain output even as high at 650mV across the 1 ohm resistor without shutting down.

Eventually, however, if I ran it up to somewhere around 650mV - 750mV, the power supply shuts down and does not return to supplying any power even after I turn down the POT, until it rests unplugged for some time (like 10-15 minutes). This is the same symptom caused by the printer's various surges above the power supply's rating of 500mA, such as when it powers on and proceeds to advance the printhead.

There is one other oddity that I also noticed. Apparently, if I turn up the POT quickly from 0, the power supply doesn't seem to leave green mode at all, and voltage just keeps on dropping until, at max POT, input voltage is 1.5V and voltage across the 1 ohm resistor is about 295mV. In this scenario, however, turning down the POT reverses the process, and once at or near 0 on the POT, input voltage has returned to 29.5V and the power supply is functioning normally again.

 

this behavior makes sense, as good power supply is rated at 550mA and probably is shutting down from overcurrent by the time we try to pull more than 590mA.

Exactly so... And proves the e-load is working exactly as expected.

Initially, input voltage was at around 29.5V in green mode and 1mV across the 1 ohm resistor. As I slowly turn up the POT, input voltage slowly dropped as mV rose, until about 25.5V when there was 15mV across resistor. At that point, input voltage shot up to 30.7V and pretty much remained steady around 30.1V as I continued to turn up the POT.

This is much as I'd expect. It's in green mode & stays there until demand reaches some limit then PSU goes into regulation. It will be instructive to measure the voltage at VDD and with your diode/capacitor jig at R7 as you do this.

Power supply is able to maintain output even as high at 650mV across the 1 ohm resistor without shutting down.

Eventually, however, if I ran it up to somewhere around 650mV - 750mV, the power supply shuts down and does not return to supplying any power even after I turn down the POT, until it rests unplugged for some time (like 10-15 minutes). This is the same symptom caused by the printer's various surges above the power supply's rating of 500mA, such as when it powers on and proceeds to advance the printhead.

And this is again what I 1/2 suspected - and its something I suspect Q3/Q4 have something to do with... and now we have something we can use to investigate

There is one other oddity that I also noticed. Apparently, if I turn up the POT quickly from 0, the power supply doesn't seem to leave green mode at all, and voltage just keeps on dropping until, at max POT, input voltage is 1.5V and voltage across the 1 ohm resistor is about 295mV. In this scenario, however, turning down the POT reverses the process, and once at or near 0 on the POT, input voltage has returned to 29.5V and the power supply is functioning normally again.

The scenario you had when plugging the printer in was it dropped to 6v and recovers when you disconnect- can you replicate that?

 

The scenario you had when plugging the printer in was it dropped to 6v and recovers when you disconnect- can you replicate that?

The scenario that I have with the printer is what happens when I cause an overcurrent with the contraption. The power supply shuts down and will not recover until being left unplugged for 10-15 minutes. The various low voltage of 2V-6V that appeared afterwards must just be from capacitors or something while shut down; I don't think it is coming from anywhere, nor will it hold under any load (but that hasn't been tested yet). The constant current device always applies some load, even if 1mA, amd I did not try disconnecting the power supply from the apparatus after it shut down just to verify the stray voltage is still there, but I believe it would be.

You mentioned that I should check VDD and R7 as I apply load, and also check Q3/Q4 as well, so what do you have in mind?

 

Not quite sure what to check next, or are you thinking about it?

 

Sorry, had a busy couple of days. There's still something missing/wrong about the Q3/Q4 area on the circuit diagram. There has to be a connection between VDD and ZD1 somehow else it doesn't make any sense.

Logically Q3 & Q4 are arranged as a latch (though the drawing isn't right). so when Q3 gets turned on Q4 is turned on, which holds Q3 on and charges C9 up. C9 provides some timing lockout, which holds Q4 on across power cycling. When Q4 is on, this seems to force green mode/power supply shutdown. My belief is that this circuit, if working correctly, monitors over voltage and/or over current and locks out the supply for a time after such an event. That sort of ties in with what you're seeing, except as drawn it doesn't do anything useful...

 

Sorry, had a busy couple of days. There's still something missing/wrong about the Q3/Q4 area on the circuit diagram. There has to be a connection between VDD and ZD1 somehow else it doesn't make any sense.

View attachment 268990

ZD1, R15, and R17 are centrally connected to VDD (after R2b), but that connection is just missing on your diagram. The route goes under the center of J2. Adding this might resolve the confusion.

If C9 is central to lockout timing and keeping the transistors running during shutdown, could that be the culprit? If C9 is failed, Q5 would not be running and can't cancel the lockout?

 

ZD1, R15, and R17 are centrally connected to VDD (after R2b), but that connection is just missing on your diagram. The route goes under the center of J2. Adding this might resolve the confusion.

If C9 is central to lockout timing and keeping the transistors running during shutdown, could that be the culprit? If C9 is failed, Q5 would not be running and can't cancel the lockout?

Sorry not got back to you, been a busy few days

Will respond later today!

 

Sorry not got back to you, been a busy few days

Will respond later today!

It's fine. BTW - been curious if my IC tester should work on Q3 and Q4 while they remain in circuit? After all, we are essentially hunting for a defective component, and these guys are becoming the focus of our attention.

 

I doubt it, though one reason I think there is confusion is that I've drawn Q3 in the schematic as per its datasheet, but it makes much more sense - and works in simulation - if you swap base and emitter pins so using E-B-C not B-E-C!

Redrawn it looks like this:


So, we need to plot the voltages at various points as we increase the load current to see how this behaves...

The ones initially of interest are VDD, the junction of zd1 and R13, and the end of R18 connected to Q3, though the latter may disturb operation because there are a lot of high impedances around Q3 and Q4...

 

So, we need to plot the voltages at various points as we increase the load current to see how this behaves...

The ones initially of interest are VDD, the junction of zd1 and R13, and the end of R18 connected to Q3, though the latter may disturb operation because there are a lot of high impedances around Q3 and Q4...

Hi, sorry for the delay, I've had some other issues to attend to as well, and was not able to do this latest experiment until last night.

The task involved here required me to use 3 meters and watch them while adjusting the POT and also while monitoring and keeping the MOSFET cool, so for the third test here I had to solder a wire to the Q3-R18 junction at the point below so it could stay connected to a meter by itself:




At any rate, some interesting results to report on these tests.

VDD at ZD1:
Initially with no current drawn, VDD oscillates between 15V and 15.5V. If I turn up the POT quickly, triggering an immediately but temporary, shutdown, VDD rises to 16.5V during the shutdown. But, the more common case of gradually increasing current from 0A so that the power supply properly leaves green mode, I found that VDD gradually increased up to about 19.50V when shutdown occurs. Upon shutdown, VDD drops to around 4V and output to 0V. Shutdown cannot be disabled without unplugging and waiting. Once unplugged, I see VDD slowly drop to about 1.75V, and then charges up to 13.3V (not instantaneously) when the shutdown is over (unit still unplugged). So at least I have an indicator now of when I can plug it back in! Time to recover certainly varies, and if it recently recovered, it will shutdown again quickly without enduring as much load as before. Shorting VDD to ground can't speed up the process, after I did that, I saw it maintain 0.75V at VDD and then continue its slow fall to 0V.

ZD1 at R13:
Initially at startup, this is 0V. During normal, working operation as I turn up the current, this slowly rises in mV somewhat analogously to my mV meter on the 1ohm resister measuring current. So it seems like a similar current measurement portion of the circuit, and this also seems to be the area that triggers the latch as you called it. When overcurrent shutdown occurs, the voltage here initially jumps to something like 7V but then quickly drop and settles at 3.50V. When unplugging, this voltage will slowly drop, but if I plug it back in again too soon it just recovers back to 3.50V. When this voltage finally falls to about 1.25V, the shutdown is released, this voltage drops to 0V, and output voltage shoots up from 0V to 30V! Furthermore, I found that I could short this to ground (while unplugged) and immediately reset the shutdown without waiting. Definitely helpful, as these tests have been time consuming waiting for the random delay for a reset previously, plus I think this is a big clue that the transformer sensor is not really the cause of the shutdowns.

R18 at Q3:
As you suspected, just checking voltage with a meter here disturbs operation. Frankly, with the meter connected, power supply immediately shuts down even with no output current, and I think I saw 2.75V here during such shutdowns. Only with the meter disconnected after a reset could I apply any current. So, not much that I can discuss here.


I did not do any tests at R7 with the diode-capacitor jig since you didn't mention it again, and to do that I would have to do another difficult solder to connect a wire with the meter in order to check that.

So these new results seem encouraging to me, like maybe the ZD1 zener diode is bad or something?

 

Hi Irving -

Wondering if we're at a sticking point or what you suggest doing next to try resolving the issue with this power supply?

 

Hi there, sorry been busy...

Hard to know what to suggest next. Without a 'scope to look at waveforms its difficult to understand the failure mechanism - it seems the over-current latch is kicking in earlier but whether that's a fault in the PSU or in the printer is hard to say. If it is something in the Q3/Q4 block we need to understand how that relates to current limiting. My instinct would be to remove D9 to disconnect the Q3/Q4 block from the rest of the circuit and see how the PSU behaves then... but that involves some careful micro-soldering.

 

Hard to know what to suggest next. Without a 'scope to look at waveforms its difficult to understand the failure mechanism - it seems the over-current latch is kicking in earlier but whether that's a fault in the PSU or in the printer is hard to say. If it is something in the Q3/Q4 block we need to understand how that relates to current limiting. My instinct would be to remove D9 to disconnect the Q3/Q4 block from the rest of the circuit and see how the PSU behaves then... but that involves some careful micro-soldering.

Hi Irving - nice to hear from you again.

The over-current latch isn't always kicking in early is what I am seeing. It kicks in when the load is increased suddenly from 0. Otherwise, it kicks in properly when the load exceeds the 500mA rated maximum. The main problem really seems to be that the latch does not properly reset on its own once the load is reduced. Rather, it stays shut down until the capacitors lose their charge (but only in the true over-current situation).

Removing D9 would be tricky, though really only one side needs to be disconnected. I really don't want to do that unless there is no other way to get a clue and it will be helpful. With D9 disconnected, there would be no more shutdowns but also no overcurrent protection? So what would we be doing and looking for after that?

 

I also don't think there is anything wrong with the printer. We are not even using the printer to test the power supply now. Printer is working correctly with new power supply of the same design as well.

 

Irving - really could use some more advice on what to look for next. I had several questions above gone unanswered. If it helps, I am planning to get a pocket oscilloscope soon for another project of mine to test if a pure sine inverter is really generating and maintaining pure sine output.

 

Irving - not sure how I lost you here!

FYI - I just got a small digital handheld Oscilloscope 5Mhz Bandwidth 20MS/s Sampling Rate with a 1x/10x probe. Still learning how to use this thing and what all the modes are about.

So, since I never heard back from you, I'm not sure what to do with this on the power supply board. I assume you wanted to see the waveform at R18 at Q3 as I raise the load, is that correct?

 

Hi, sorry, been suffering from a heatwave here and other issues (including my 30v 40A power supply dying!).

Be careful attaching the scope - run it on batteries only. Make sure your connections are sound before you switch the PSU on and never try and look at the primary side wave forms. And do not connect to a PC or laptop unless that's running on batteries too.

As a first test, lets look at the waveform across pins 5 (signal) and 3 (ground) of the transformer when idling unloaded, when operating normally at a small load,and when overloaded...

 

Shutting down power supply could be symptoms of short circuit in the load, try to plug it to other load.

 

Shutting down power supply could be symptoms of short circuit in the load, try to plug it to other load.

Hi Adel,

We've done a lot of checks and tests already as documented in this thread. The power supply has been tested with a variable resistance OP-AMP current limiter contraption that Irving cooked up. Results show that the power supply's safety shutdown latch engages prematurely when the power supply experiences certain surges, as well as when overloaded. In the latter case, however, the latch also does not properly disengage when the overload is removed. So trying to figure out primarily what is wrong to cause this, and secondarily if there is any way to fix it.