Mastech HY3020MR Constant Current

[osteichthyes]

Someone recently gave me a Mastech HY3020MR. It's nice, and fits my needs quite perfectly. I got the voltages calibrated pretty well, such that it's about 0.5mV high at low voltages and 0.5mV low at high voltages. I also have the meter pretty well calibrated to that. Same goes for the current meter. What I cannot get set for the life of me is the current. There are five trimpots in it, and the pot that (I think) is for current seems to do nothing when turned. The unit puts out about 450 mA, regardless of setting in constant current mode. No fuses are blown and all components look OK. I've tried using four meters and tried just shorting between contacts, I cannot set the current at all. Once  set, nothing changes, it just stays at 450mA.

Any thoughts of where to start fixing this?

[osteichthyes]

Picture 1 is a general overhead view. The next two photos are of the daughter board.

I'm pretty sure W1 is the volt meter adjustment. I'm not really sure what W2 does, but it seems to affect the current meter, I think it's maybe a zero adjust for the current meter? W3 is the output voltage adjustment and I think W4 is output current adjustment. I think W5 is current meter adjustment, but I'm not entirely sure. Both W2 and W5 seem to affect the current meter, but not the current output.

I'm having issues with photos, so here's a link.

https://photos.app.goo.gl/qe6D6FkCUNuumrEVA

[coromonadalix]

Did not find the schematics,  ask mastech for the front panel schematics, if they are willing to give you one.

The controller have 2 dac outputs, 1 for current, 1 for the voltages  as you guessed,  and the corresponding 2 return feedback lines,  must be 2 trims for the current and voltage offsets,   maybe the 5th is an voltage reference adjustments

It was advertised as an Arduino based "Atmega 8 or 328" controller,  you could try to make an hand drawing of the circuit

There projects of anrduino controlled psu floating around, it should give you some ideas

[osteichthyes]

I'll ask about the schematic and calibration manual. I think I asked for the service manual and  didn't get a response. I'll try again.

This one isn't arduino based, I believe they discontinued this model to replace it with an arduino based model.

Thanks!

[osteichthyes]

They sent the schematic. I understand what most of the pots do, I'm a little confused about a few of them. I'll figure out a  way to post it in a limited way.

[coromonadalix]

IF they told you to not disclose it .........  otherwise you could post it entirely for the eevblog members community here, it may help others

[osteichthyes]

Schematic attached.

[coromonadalix]

Do you have the other schematic section ? , that way we could help identify the trimpots functions   ?

I think
sp1 is the inputs for the controller, because of the signal path thru the op amps going to the cpu
sp2 is the output

The 4 trimpots are related to ground, we could say  levels adjustments of volts and currents,  w5 is the gain control of U0  lm741 you could try at first to set it in the middle.

Put the other trimpots in the middle and do this:

Try to output 10 volts no load and play with w3 or w4 trying to get the same output voltage, when you match or get pretty near, you may have found the good one, until we know what w5 does ??

Try putting an 10 ohms load 10 watts at 10 volts, it should consummate 1 amps, and try to play slowly with w1 or w2 trying to get the nearest read value ...  depends of w5 once again ...

[osteichthyes]

I asked for the full schematic, the portion that I posted is all they have. I don't think this is a calibration issue. It does not change from 450mA, even when shorted. It will not go down or up. This seems like it's broken.

[coromonadalix]

Ask at Hyelec for the full schematic, this is an smps power supply without an schematics it will be hard to service, i have an HY3010E-2  to service  it is not easy at all, and have an hy10010e to service too ... cheap supplies, badly assembled. lots of bad solders

maybe the HY3020M  may offer some similarities, but i did not find any schematics

[bitseeker]

The 3020 schematic I have is for the linear model. However, I have a 5020E schematic that might be of some use.

[coromonadalix]

On many smps they tend to use similar circuits, it may help,    on the bigger models its completely different

On the hy5020e,  they do an smps circuit up to the power mosfet  q401  (75nf75), just to save on a big transformer,  after that it operate as an linear power supply with the pcb's HY013 and HY014

[bitseeker]

Yeah, I wasn't sure since I've only seen the linear version of the 3020 and similar range supplies.

[osteichthyes]

On many smps they tend to use similar circuits, it may help,    on the bigger models its completely different

On the hy5020e,  they do an smps circuit up to the power mosfet  q401  (75nf75), just to save on a big transformer,  after that it operate as an linear power supply with the pcb's HY013 and HY014

I'm pretty sure  this works the same. I'm working on other projects right now, but I'll take a look ASAP. I'm leaning towards just reflowing any dodgy solder joints to start.

[osteichthyes]

Here's the schematic from the HY3020E, which they claim is very similar.

[coromonadalix]

For the 3020E schematics, I do think it work like this,  of course i may have some errors

U+ serves as a ground for the meters, floating +5 and -5vdc  supply for them with pin 9 and 10 of transformer T101
M1 is the current meter, w1 adjustment
M2 is the voltage meter, w2 adjustment

U401A is the voltage control circuit, W1 the front panel potentiometer (W2 fine adjust) VR2 voltage limit
U401B is the current control circuit, P1  front potentiometer, (W3 fine adjust) VR1 and W4  maybe the minimun, or an zero adjust or maximum current output ??

For a total of 7 adjustments

U401c and U401D are the CC (constant current) CV (constant voltage) mode led indicators

[osteichthyes]

I'm thinking maybe one of the daughter board connections may be bad. I'm going to go through and clean it and reflow any bad joints. I'll report back after that. Given the middling (at  best) build quality, a build issue seems likely.

[osteichthyes]

OK. I ran through the boards, cleaned all the connections, and reseated all the cables. Nothing has changed. The supply is still pushing about 0.5A. I noticed some odd resistances on the trimpots, they seemed a bit off.

BUT I noticed something really odd. I had a multimeter in amps on the supply. I then just started pushing the 1A+ buttons and noticed that when I pushed the button, the current increased by 10mA. When I pushed the -1A, it went down by 10mA. When I pushed +0.1A, it went up by 1mA, and down by 1mA when pushing -0.1A. It almost seems like it's working, but that the adjustment is off by two decimal places. How does that happen? I'm completely lost.

[coromonadalix]

How do you take your current measurements ?

Normally to do some tests

Example:

You set 10 volts out at 1 amp,  you put an 1 ohms say 50 watts (check ohms law) load on the output plugs, the current passing thru the load will be 1 volts per amps,  but the wattage will be 10 watts, V x I = W  that's why i told you 50 watts load, it can get pretty hot ... that way i read ac or dc current in voltage. ratio 1 : 1

An multi meter at 10 amps will be considered a short not always a load ... you can get current values too  but you may blow fuses and overheat the meter instead of an load.

[osteichthyes]

My first test was just to short the v+ to v- and look at the screen. It recommends that  you short the supply to get into constant current mode, then set the current, it should reach the value, then connect the load.

You're saying that since the short would create a near infinite draw, the supply is limited, hence the 0.5A. Perhaps I misunderstood the instructions, I thought it would reach the necessary current under a short. The short is just to set the current, then I won't see it without a load.

I'll try to find a load. I was just using the current shunt in my multimeter, set to 2V at 1A (I did test at some higher power settings as well).

It sounds like 10 ohm isn't sufficient to be seen as not a short? How much resistance do I need for it to be seen as a load?

[coromonadalix]

10 ohms may do   

It depends of how much current value you want ? 10 volts at 10 ohms will draw 1 amp,  at 10 watts    ohms law ...

[bitseeker]

Note that I have not been able to confirm this myself, but I have seen a complaint that some of these HY switching supplies go into short circuit protection mode despite the instructions saying that you should short the output to set the current limit. If that's true, then the instructions are wrong, some of these devices are incorrectly designed, or they have a defect.

Again, I haven't been able to confirm this, but it sounds similar to the short circuit behavior that you describe. Setting the limit with a load should avoid triggering the protection mode, if that is what's happening.

[coromonadalix]

I base my assumptions with the 3020e model  since we dont have any 3020mr schematics, the 3020e has standard settings, you could say "you do an short at the output to set the current limit"

Many power supplies are set this way ... some have an button to do this, you push it, set the values and there you go ...

I asked Hyelec for an schematic of one of my model,  try to ask them  they maybe give you the 3020mr model ?

Without the right schematic, it wont be easy to repair your psu,  since i cant do any comparison between them ...

On the 3020e
The smps section has a feedback line for the operation, and the linear section has op amp for the current protection / limit and one for the voltage regulation, But the linear section can and/or could shutdown the smps section with the Q305 2n5401 transistor on the schematic. The 3020E and 3030E have the same circuit,  the 3010E and 5020E are not the same in the smps feedback line.

The Hy60xxE and HY100xxE series seems totally different beasts, way more complex.

You should take many photos of your 3020mr psu insides and we could do some checking if they are similar,  i work with nothing here  loll

[osteichthyes]

I asked both Mastech and Hyelec. I got the two schematics that I posted. One is for the 3020E, the other is a small part of the 3020MR

I really appreciate all of the help. I've never really used a constant current supply. I didn't understand how they work, hence my confusion earlier.

I may have a motor that would fit the bill, loadwise. Unfortunately, I don't have an appropriate static load. I need a few big wire wounds for a different project so I'll order some in the future.

As an aside. In their email, Hyelec said,

And by the way, our power supply's C.C. mode is passive function, need enough load to make the C.C. mode working. 

I took this to mean what the manual suggests, that some short is necessary to set the CC, but perhaps it's more akin to what has  been suggested here.

[bitseeker]

The "need enough load to make the C.C. mode working" sounds like they're saying that you need a load will draw more than the set current limit in order to switch into CC mode. That would be normal.

What I'm unclear about is whether their supply does support shorting the output to set the current limit. Although many supplies do work that way, I don't know about Hyelec's in particular.