Confused with about my new Lab/Bench PSU C.C. & C.V. Is it working correctly?

[castingflame]

Hey all.

I just purchased a PSU from Amazon which is the following model...

https://www.amazon.co.uk/gp/product/B00BCPTG66/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

I am a little confused about the Constant Current (C.C) and the Constant Voltage (C.V.). I have read and looked at videos etc about using c.c . and c.v.   but my PSU is not behaving how I would expect.

It is always only ever showing 'C.V.' on the display. If I hit the button to switch between C.C. and C.V. The display does not change (Still displays C.V.) and the current is not constant. I can not change the current via the CUR ADJ buttons either.

This is with or without a load. The manual is really no help.

I am thinking it is faulty.

Thanks for any help.

[Circlotron]

Say you have the voltage set at 15V and the current set at 1A. If you load it more and more, it will stay in CV mode until the load current reaches the 1A setting. Any further increase in load will cause the voltage to decrease (not CV anymore) and the amps will go no higher than the 1A setting - CC.

[rstofer]

I have always had an issue with the definition of Constant Current as it relates to power supplies.  I prefer to think of it as Current Limit.

For example, if I set the current to 1A, will the power supply raise the output voltage enough to deliver 1A to a 1K resistor?  Of course not!  In fact, it won't raise the output voltage at all; I have to set a sufficiently high voltage to allow the device to throttle on current.  Simply put, all it does is reduce the output voltage to prevent more than 1A from flowing.  It does NOT maintain a constant current regardless of load.

Set the PS to 10V with a 100 Ohm load.  The current should be 100 mA.  Now, keep reducing the current setting until the voltage begins to drop.  That should put the PS into CC mode and, if you parallel another 100 Ohm resistor, the current will stay at 100 mA but the voltage should drop to 5V.

[jeroen79]

I have always had an issue with the definition of Constant Current as it relates to power supplies.  I prefer to think of it as Current Limit.

For example, if I set the current to 1A, will the power supply raise the output voltage enough to deliver 1A to a 1K resistor?  Of course not!  In fact, it won't raise the output voltage at all; I have to set a sufficiently high voltage to allow the device to throttle on current.  Simply put, all it does is reduce the output voltage to prevent more than 1A from flowing.  It does NOT maintain a constant current regardless of load.

By that logic there are no constant voltage supplies either.
Just voltage limited as none of the would increase the current enough to maintain 10V over a .1 ohm resistor.

[james_s]

How do you expect to get a constant current into no load? Do the math, as the load approaches zero (open circuit), the voltage required to maintain a constant current approaches infinity.

[IanB]

By that logic there are no constant voltage supplies either.
Just voltage limited as none of the would increase the current enough to maintain 10V over a .1 ohm resistor.

That's exactly right. "Constant voltage" is just as poorly chosen a description as "constant current".

A lab power supply has a "voltage limit" setting and a "current limit" setting. Most of the time such power supplies are operated at the voltage limit, but if you try to draw enough current you may hit the current limit.

[bson]

In CC mode you set the output current and compliance voltage.  Until you load it enough to reach its set current it will peg at the compliance voltage.  Any CC source or sink circuit will work the same.  It's not a misnomer.

[mikerj]

This PSU apparently has a button to switch from CV/CC mode which doesn't make a lot of sense.  The Circuit Specialists website doesn't explain things very well either:

In CC mode the user may set the CURRENT LIMIT as desired up to the full limit of the unit. The current is set by the UP and DOWN buttons that are labelled  "current-adj."

To set this variable current limit, connect a short (test lead or wire jumper) directly across the Power supply output terminals and then adjust the current limit value up or down using the buttons described.
When the meter displays the desired current limit value, remove the short from the power supply and the unit will now be set as desired.

If you remove Power from the unit (or turn it OFF) you must reset any current limit value you previously had set. It will default to the maximum current value.

Does this mean that the supply always uses the full output voltage compliance range with the button set to CC mode?

[ArthurDent]

Perhaps the supply should be described as automatic crossover from constant voltage to constant current. With no load connected to the output (or the output off), if you set the voltmeter to the maximum voltage you want the circuit to see then short the output leads and set the current to the maximum current you want it to see, when you turn the output on, the voltage will remain constant until the current reaches the max you set. The current will remain at this constant value as the load increases and the voltage will decrease to keep the current at this limit.

https://e2e.ti.com/support/power_management/isolated_controllers/f/188/t/445939?UCC28950-Constant-Current-Constant-Voltage-with-Automatic-Crossover

The other possibility is that if you select constant current and set the current to some value, the supply will try to output this current by adjusting the voltage from 0 to the max output the supply is capable of. It should be easy to try this to check it out.

[james_s]

That's exactly right. "Constant voltage" is just as poorly chosen a description as "constant current".

A lab power supply has a "voltage limit" setting and a "current limit" setting. Most of the time such power supplies are operated at the voltage limit, but if you try to draw enough current you may hit the current limit.

I would say that's splitting hairs. We all know that "constant voltage" means the voltage is constant over a range the device can achieve. Every real world device has limitations and boundaries.

[IanB]

I would say that's splitting hairs. We all know that "constant voltage" means the voltage is constant over a range the device can achieve. Every real world device has limitations and boundaries.

That's quite right. But it's equally true of "constant current" mode. The current is also only constant over a range the device can achieve.

Whether you call it "constant current" or "current limited" is just a matter of perspective.

[james_s]

Well yeah, I wasn't complaining about the use of the term constant current either. As with almost everything in life, it comes with a few strings attached and should not be taken in an absolutely literal sense.

[piguy101]

This PSU apparently has a button to switch from CV/CC mode which doesn't make a lot of sense.  The Circuit Specialists website doesn't explain things very well either:

Does this mean that the supply always uses the full output voltage compliance range with the button set to CC mode?

I have used this power supply in a lab before and I am not a fan of it. With the CC option off, the power supply outputs a constant voltage until it reaches the maximum current it can supply, just over 5 A. With the CC option on, the supply works as a typical CC/CV supply, but the buttons are used to set the current limit in steps of about 40 mA per press of the button. Of course, you have to short the output to see what the current limit is. 

I have no idea why you would ever want the CC option off. The first thing I always did when I turned on the supply was to enable the CC option, short the outputs and set my limit.

Anyways, the power supply is not great, but it has it two independent 30 V 5 A supplies that can be paralleled or put in series with the press of a button, which is nice.

[rstofer]

Shorting the output to adjust the current limit is typical in these older designs and I don't care for it. 

I really like being able to set and read out the voltage and current settings like on the Rigol DP832.  Most, if not all, of the digitally controlled modern supplies have this feature.

Having a known current limit BEFORE you turn on the output seems like a very good idea.  I have used it to good advantage when I was bringing up a Z80 project that turned out to have an address conflict.  Limiting the current to 100 mA before turning on the voltage probably saved a couple of chips.

[james_s]

I often just start the output current low and ease it up, stopping if it looks like it's rising to an unreasonable level while the voltage is still low.

[mikerj]

I have used this power supply in a lab before and I am not a fan of it. With the CC option off, the power supply outputs a constant voltage until it reaches the maximum current it can supply, just over 5 A. With the CC option on, the supply works as a typical CC/CV supply, but the buttons are used to set the current limit in steps of about 40 mA per press of the button. Of course, you have to short the output to see what the current limit is. 

I agree, that sounds like a pretty terrible feature and ultimately pointless.  Effectively CV mode is the same as CC mode with the current limit set to maximum.

[james_s]

It's kind of pointless to even have two separate modes. Makes more sense to just have a current limit knob and keep it turned up all the way if you want something as close to CV as possible.

[castingflame]

I really appreciate all your answers so thanks.

However, you all completely missed my comments "If I hit the button to switch between C.C. and C.V. The display does not change (Still displays C.V.) and the current is not constant. I can not change the current via the CUR ADJ buttons either"

I understood that C.C. is a limit but if I can not change any setting to set anything in C.C. mode and the CV does not change to C.C. mode , I would have to disagree and still think the unit is faulty. Is this still wrong?

Cheers

[IanB]

You cannot hit a button "to switch between CV mode and CC mode". The power supply will decide that for itself depending on the load. It is not something a button can change.

The button you are describing apparently switches between having an adjustable current limit and having no adjustable current limit. Others in this thread have suggested this is a pretty useless button.

If you want to see your power supply enter CC mode, then first you have to set a low current limit, and secondly you need to increase the load until it tries draw more current than the limit. If you do that then the supply will start limiting the current and the CC indicator will light.

[eblc1388]

On my Roband power supply, the output On/Off toggle switch is actually a three position center-off switch.

One position is output ON. On the other position it places a short circuit across the power supply outputs internally and enable user to adjust the current limit setting.

[castingflame]

"If you want to see your power supply enter CC mode, then first you have to set a low current limit, and secondly you need to increase the load until it tries draw more current than the limit. If you do that then the supply will start limiting the current and the CC indicator will light."

My point entirely!

With my load connected I want to set a ceiling of 2Amps using the C.C. mode. Hitting the CC mode lights up the CC LED but, I CAN NOT set the CC limit. The CC controls do nothing.

I have had a CV CC bench PSU before and that worked as expected.

Anyhow, It is being returned and I now have a different Lab PSU being delivered Monday.

[ArthurDent]

“However, you all completely missed my comments…”

No, several posters have told you how to set the current limit as I did in post #8 and the quote from your power supply manual in post #7 clearly describes. The output voltage is ‘potential’ and can be displayed and adjusted with no load but the current output is only non-zero if there is a load, either your load, or a short across the output terminals that is described as the way to set the current limit for your supply. Without a load, or a short to allow you to set the limit, you can turn the current knob from one limit to the other and there will be absolutely no change in the current display because there is no load. Some supplies like this will go from CV to CC without a load if the current adjust knob is set to zero but this will only show that the auto crossover works at zero and isn’t of much practical use. The power supply described above with a 3-position load switch that internally shorts the output does the same as externally shorting the leads.

Some higher end power supplies have the ability to set the current limit without shorting leads but your power supply doesn’t appear to have that feature built in. Here is a photo of a supply I have that can set overcurrent and/or overvoltage limits without a load. Unless you buy a power supply with the feature I described built in, or an output shorting switch, the next supply you get won’t work any differently.

[IanB]

With my load connected I want to set a ceiling of 2Amps using the C.C. mode. Hitting the CC mode lights up the CC LED but, I CAN NOT set the CC limit. The CC controls do nothing.

Did you install a short circuit across the output terminals before trying to adjust the CC limit, as you are required to do?

If you did not do so, then you are not using the instrument correctly.

[castingflame]

Thanks again for your replies.

Well my new power supply came today, this one...

https://www.ebay.co.uk/itm/DC-Linear-Bench-Power-Supply-Variable-Output-0-30V-0-5A-0-5A-0-30V/123219732357?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649


and I put it through its paces with my new DC Load ...

https://labtronix.co.uk/drupal/shop/electronicload/3715a


and everything worked as expected. Everything is great. All of the protections and limits are working and I am a happy bunny.


I had a 0.1R resistor on the other PSU and I still could not get a CC setting to change. I did not however short the leads together so you may well be correct but I tried with and without a load.

All in all I am much happier with the new PSU anyway as the old one also used to give a full channel output if I moved the dial sometimes. I saw it when I had 12v bulbs as a load, they would go full power for 0.5 of a second. That is not great.

Anyway, thank you all for being patient and taking your time to give input.

I am a happy to continue with my project

Paul

[ArthurDent]

The new power supply you chose is far superior to the last one you had. If you compare each half of your new power supply to the one I have that I showed in post #21 you will see that it has almost exactly all the same functions. Whether the previous one you had worked or not, I think you'll be much happier with this one. I think you made a wise choice.