HY5003 PSU. Constant Current mode broken.

[Fidelis45]

First, much thanks Dave for his videos and for hosting this forum. And much thanks to many of you who provide great contents here.

I've a MPJA 14602 power supply, its MPJA's rebrand of the HY5003 PSU (50V3A, single output) model. The problem is that Constant Current mode is broken.  The PSU works fine so long as current draw is lower than the set value.

The schematic is similar to one posted here a while back: https://www.eevblog.com/forum/beginners/fixing-my-mastech-hy3005d-3/?action=dlattach;attach=48903

This guy has the very same unit:http://magictale.com/1356/mastech-hy5003-bench-power-supply-110v-ac-rewiring-to-220v-ac/


My guess as to what caused it:

I don't use CC mode too often so I can't be sure, but about a month  ago I picked up an Electronic Load (Array 150W model), and was having too much fun I guess. I don't recall pulling more than 3 amps though. I think the PSU quit at about 2.7 amps or so.

Some symptoms:

1. When I short the output to set the current limit, the PSU simply quits.
2. When the current limit is fine tuned, the relays continuously click on and off, never settling on CC mode.
 (Works with pure resistive load)
3. Once it shuts down (No output voltage), the only way to restore the voltage is to remove the load.
( Again with resistive load it works).

Trouble shooting so far:
1. Visual inspection of the PCB's all look good, no heat stressed components or PCB area
2. Checked the electrolytic caps (in circuit), all acceptable values. (uncertain of ESR)
3. Checked larger resistors, all acceptable values.
4. Checked diodes, all good. Few that I couldn't get conclusive readings, I desoldered and tested, all good.(didn't test zener values)
5. Bunch of NPN transistors and one PNP, all good.
6. Voltage regulators and one ref (12V, 5V, 2.5V ref) all good.
7. Current Potentiometer resistance range. Good.

At this point I'm left with opamps. The circuit uses one LM324N and two HA17741.

The little PSU has served me well for the past 7~8 years. It's one of the older models with a heat sink rather than a fan and a nice toroid to boot.

Would you guys point me to the right direction?

Thanks,

Yong

[Kleinstein]

The describtion sounds a little like a problem with the relais for switchung the voltage range.
A first test would be measuring the raw voltage as a funktion of output voltage. If reserves are to low, possibly one relay not working, this might explain the trouble. There shoud be no relay switching in the low voltage range (e.g. less than 10 V), no matter if CC mode active or not.

An alternative cause might be oscillation in CC mode, e.g. due to to much ESR of the output capacitor. However this should mainly be a problem with inductive load.

[Fidelis45]

Finally found couple of hours to work on my PSU tonight.

Relay Voltage Steps. There are 7 steps in total. My unit is 50 volts and there are 4 relays.

Output(V)   Raw(V)
   0      20.1
   6.6      28.9
   12.6   37.8
   19.2   46.5
   25.3   54.9
   30.3   63.6
   36.4    72.4
   43.3    81.0
   
The raw voltage was measured from the main capacitor. Later I desoldered both capacitors (the main one off the PCB and another one tied to the back of the output binding posts), measured the capacitance and ESR. They all look good. Based on multiple measurements, capacitance was within 10% to 15% and ESR was between .02 and .06 Ohms. I don't have a real ESR meter, the ESR measurements were taken with one of those "Fish8840 transistor testers"

An interesting observation. You had mentioned the load types, so I tied together six 68 Ohm, 10 Watt ceramic resistors, efffectivly creating 11.5 Ohm load (measured). To my surprise CC mode was working. I still can't set the CC with output dead short.
   
Up to now the CC <--> CV oscillation behavior was off the electronic load (EL) set for 'Resistor Set' or constant resistance mode.  On the EL when I set for 11.5 Ohm load, the PSU CV and CC mode still oscillates. And the relays on my EL is also clicking.
   
Electronic Loads do not truly emulate resistive load? or perhaps my EL (Array 3710A) is also broke?
   
   Yong

[singapol]

It helps if you understand how constant current and constant voltage power supply works.
Here is a project that explains how such a power supply works:

http://www.electronics-lab.com/project/0-30-vdc-stabilized-power-supply-with-current-control-0-002-3-a/

That said I would probably zoom into the current control circuit to check and this is commonly using opamps.

[Fidelis45]

Thanks for the link. It looks like an interesting project. Perhaps it time for me to actually build one to get more detail understanding.

I've used CC mode on my PSU few times in the past, mostly for some LED stuff, and battery charge tests, but not much else.

So far I can conclude that using resistive load of at least 11.5 ohms the CC mode works. What still does NOT work is setting CC with the output dead short. I'm going to test for the lowest resistance CC mode will work.

Based on comparison of schematics (ref: my 1st post)for HY3005 and my actual unit:
The LM324 to drive the relays is pretty much the same, but the wirings for discreet opamps (HA17741) for V and C are different.  My unit actually has pin5 (zero-offset) wired in via trimpots for both opamps. I strongly suspect some passives wired one the opamps to be the cause.

This little bugger's got me worked up. No more 'plug and pray', I guess it's time to takeout the  calculator, pencil and paper and start tracing and measuring.


Yong

 



 

 

[singapol]

Anything can happen after 7/8 years. There is no need to trace out the schematic as the one post in the 1st. post should do. I think you have missed a very important clue in the schematic...notice there
are voltage readings in green and red probably the  results of the switching the relays k1 and k2.

I would initially check for cold or broken solder joints around the control circuits. Sincev you say there is oscillations then check C8/N1 741 opamp and C11/N2 opamp741. The caps are for stability,check for value and solder joints. Also check C7/10.

Next measure voltage with respect to ground the plus/minus inputs of 714 pin 2/3.Compare it with the schematic. This should give a clue...( learned this from Signal path  ) they should be roughly equal if opamp is functioning properly.
                                                 Over to you...
Edit: Forgot to say do check the power supply to LM741 pin7= positive pin4= negative.