Shorting swtich for cheapo power supply CC setting

[Jeff_Birt]

There are a plethora of cheapo power supplies out there that require shorting the output terminals to set the Constant Current level. I have always found this quite annoying, but hey it is a cheapo power supply. Yesterday I dug around in various parts bins and came up with a dual banana jack and a small switch that would fit (or nearly so) the recess in the middle of the banana jack.

The switch was about 1mm wider than the recess in the banana jack so the recess was milled out, a Dremel tool would have worked as well. A 1/4" hole was drilled for the switch shaft, switch mounted and soldered up to two 20AWG solid wires that are routed to the screw terminals of the banana jack. The 20AWG wire is fine for the 10A the supply can deliver given the wires are all of 30mm long; it also fit the holes in the switch terminals and was easy to shape to follow the counters of the switch/jack. Perhaps most importantly it is what I had at hand

The DPDT switch I found in the junk drawer was rated at 5A/10VAC and I had intended to wire it in parallel to distribute the current but as I was busy talking to a coworker as I was soldering this up it wound up in series. After some contemplation I decided this was not a mistake it was a feature! Now even if one contact is welded closed the other will (should) open thus opening the circuit.

The switch is oriented so that throwing the handle toward the power supply shorts the output and when the handle is toward you it is open. Functionally it works a treat. I had adjusted the switch mounting nuts so that the stem did not protrude over top of the nut when the switch was mounted. This leaves the bottom of the switch, and the switch terminals, dangerously close to the bench so the switch will need to be repositioned fully up. In addition I'll 3D print a cover for the switch terminals.

After going to all of this work I realized that if one were to use leads with stackable connectors you could just take a double jack like this and run a shorting wire between the terminals and plug it into the back of the leads at the power supply. It also seems a bit silly to need a junk box switch, a milling machine, and a 3D printer to fix an inherent draw back of a cheapo power supply. In that respect it would have been loads cheaper to just buy a better power supply but then I would not have had all of this fun and being able to make this improvement to the cheapo power supply helps justify having a milling machine and 3D printer! 

[Jeff_Birt]

More photos

A note to whom ever administers this forum: If your attachments wind up being too large you are taken to an error page, when you click the 'Back' button you are taken back to the post entry/editing page but the attachment list is gone. So you select fewer attachments this time and click post and get a screen that says, 'it has been less than sixty seconds since an post was made from this IP address'. So you click 'Back' again, attach photos again, wait more than 60 seconds and click 'post'. Now you get a page that says "You have already posted this message". Are you kidding me! All of this run-around and you still cant make a post, Luckily I copied the text so I could just start the process from the beginning. This makes for a very frustrating experience. 

[MosherIV]

Hi

Very nice 

However, please do not refer to 'Current Limiting' as 'Constant Current'
The 2 are very different things altogether.

I see many people, mainly newbies, refer to CC when they mean CL and it is WRONG !

[tatus1969]

please do not refer to 'Current Limiting' as 'Constant Current'
The 2 are very different things altogether.

In what sense?

@Jeff_Birt: you are shorting the output capacitor with your switch when you close it. If you use it often at high voltage setting I expect the switch to wear and eventually break soon because of arcing.

[MosherIV]

    please do not refer to 'Current Limiting' as 'Constant Current'
    The 2 are very different things altogether.

In what sense?

The PSU is a Constant Voltage PSU which has the ability to set and limit the current.
Hitting the Current Limit is not the same thing as a CC PSU.
Getting a CV PSU to do CC over a wide load (resistance) range is near impossible.

A Constant Current PSU is a totally different type of PSU.

[tatus1969]

    please do not refer to 'Current Limiting' as 'Constant Current'
    The 2 are very different things altogether.

In what sense?

The PSU is a Constant Voltage PSU which has the ability to set and limit the current.
Hitting the Current Limit is not the same thing as a CC PSU.
Getting a CV PSU to do CC over a wide load (resistance) range is near impossible.

A Constant Current PSU is a totally different type of PSU.

Hmmm. From the pictures this is a standard lab supply with two control loops, one CV and one CC. The classical way to combine them is that the one is in control that wants to restrict the output current more. In my view this always has been a device that you can put into CV (setting higher current than load demands), or into CC (setting higher voltage than load demands). Sorry, I still don't recognize the difference.

[Jeff_Birt]

@Jeff_Birt: you are shorting the output capacitor with your switch when you close it. If you use it often at high voltage setting I expect the switch to wear and eventually break soon because of arcing.

Yes, I expect the switch will not last forever. Ideally one would choose a switch with a suitable DC rating. AS I recall most arcing will occur when the contacts open as current is already flowing and thus you can ionize the surrounding air. I suppose one could fit a small arc snubber but it might be overkill when a proper switch would fit the bill.

I think the discussions on CC vs. CC is a bit pedantic as the supply will operate in both modes. Whether you view it as current limiting or constant current depends on how you are using the supply. If I am driving a device with a constant voltage then I would view the current control as current limiting. In this mode of operation you are driving a CV load and want to limit the current to a safe level. If I am driving a constant current load then the CV setting becomes a maximum drive voltage setting. In this mode I might want to apply 50ma to an LED but not exceed 3.2V.

Regardless of the CV/CC/CL/... discussion to set the constant current/current limiting setting you have to short the output of these types of supplies.

[madires]

Hmmm. From the pictures this is a standard lab supply with two control loops, one CV and one CC. The classical way to combine them is that the one is in control that wants to restrict the output current more. In my view this always has been a device that you can put into CV (setting higher current than load demands), or into CC (setting higher voltage than load demands). Sorry, I still don't recognize the difference.

CV = keep the voltage constant, i.e. adjust the current passing through the load
CC = keep the current constant, i.e. adjust the voltage across the load

You can't have both at the same time. Current limiting for CV simply limits the maximum current allowed to pass the load. If that limit is exceeded the voltage drops accordingly. But if the load draws less current again, the voltage rises to the set value. For CC you could add voltage limiting to protect the load from a too high voltage.

[madires]

What about using a MOSFET as short-circuit switch?

[tatus1969]

Yes, I expect the switch will not last forever. Ideally one would choose a switch with a suitable DC rating. AS I recall most arcing will occur when the contacts open as current is already flowing and thus you can ionize the surrounding air. I suppose one could fit a small arc snubber but it might be overkill when a proper switch would fit the bill.

Arcing when opening the contacts will most probably not happen, as the current is limited per definition. What I mean is the following situation: switch is open. You set the output voltage to a high value, lets say 30V. This charges up the output capacitor of the supply to 30V. As this is a switch-mode supply, the capacitance will be maybe several 100uF and low ESR. Now you close the switch. This will short circuit the capacitor. The current at this moment is only limited by the ESR and other parasitics, and can be hundreds of amps for a short time, damaging the switch contacts.

[tatus1969]

You can't have both at the same time.

Of course not, but the lab supply can instantly switch between the one or other depending on load condition (not taking into account the output capacitor charge). But that was not my point. I just see no difference between current limiting and current control. It is just different view angles to the same thing.

Current limiting for CV simply limits the maximum current allowed to pass the load.

Agree.

If that limit is exceeded the voltage drops accordingly.

That is what you can call current limiting or current control. Current is kept constant at the set value, and voltage follows the load condition.

But if the load draws less current again, the voltage rises to the set value.

Here the supply switches back from CC to CV.

For CC you could add voltage limiting to protect the load from a too high voltage.

That voltage limiting is again the voltage set point of the supply.

Example 1: when I want to charge my empty Lipo cell and have no charger at hand, then I adjust my lab supply's voltage to 4.2V, and the current to whatever my cell likes for charging. Then I can connect the battery and see it charging. The supply begins with CC charging, and at some time switches to CV charging. Besides of missing monitoring for e.g. temperature, there is no difference from what a dedicated Lipo charger does.

Example 2: powering my fresh built proto board that I want to protect from quick damage in case I did a mistake. I set the supply's voltage to what my board needs, and the current to a bit more than what I expect this board to draw. Then I connect the board. If I see the CC LED flash I can quickly react and see what is wrong. (In theory... here again this is a switch-mode supply with relatively large output caps. A possible short on my board sucks up all that stored energy, which is why I don't use them for sensitive circuits. And the correct way anyway is to start at 0V, ramping up slowly and watching current develop...)

Example 1 shows CC / current control, example 2 shows CL / current limiting, both with the same supply. Just wanted to explain my point of view, but as Jeff already pointed out this may be a bit pedantic.

[MosherIV]

I think the discussions on CC vs. CC is a bit pedantic

Yes it is but it is important so that someone new to the field understands the terms correctly.

Would you describe an Inductor as a resistor just because it has some resistance?

A true Constant Current PSU is nothing like a bench PSU.

Bench PSUs are Constant Voltage with a Current Limit. Yes the current limit can be used to force the PSU into current limited mode (or Constant Current mode) but technically the PSU is in current limiting mode.
The chinese deliberately miss-label their products in order to catch as many customers as possible, they do not care if their description is correct or not.
It is absolutely incorrect for someone new to the field to think that any bench PSU is a Constant Current PSU - IT IS NOT !

[madires]

Example 1: when I want to charge my empty Lipo cell and have no charger at hand, then I adjust my lab supply's voltage to 4.2V, and the current to whatever my cell likes for charging. Then I can connect the battery and see it charging. The supply begins with CC charging, and at some time switches to CV charging. Besides of missing monitoring for e.g. temperature, there is no difference from what a dedicated Lipo charger does.

Example 2: powering my fresh built proto board that I want to protect from quick damage in case I did a mistake. I set the supply's voltage to what my board needs, and the current to a bit more than what I expect this board to draw. Then I connect the board. If I see the CC LED flash I can quickly react and see what is wrong. (In theory... here again this is a switch-mode supply with relatively large output caps. A possible short on my board sucks up all that stored energy, which is why I don't use them for sensitive circuits. And the correct way anyway is to start at 0V, ramping up slowly and watching current develop...)

Example 1 shows CC / current control, example 2 shows CL / current limiting, both with the same supply. Just wanted to explain my point of view, but as Jeff already pointed out this may be a bit pedantic.

What you're describing is a fringe case. In those specific situations the current limiting acts like a CC as long as the load wants to draw more current than the limit you've set. A CC PSU pushes the set current through the load. If the load changes, the PSU changes the output voltage to keep the current constant.

This remembers me of the discussions about the difference of baud and bps

[tatus1969]

Did Dave recognize our discussion? 

https://www.eevblog.com/forum/blog/eevblog-919-how-to-charge-li-ionlipo-batteries-with-a-power-supply/?topicseen

[tatus1969]

The chinese deliberately miss-label their products in order to catch as many customers as possible, they do not care if their description is correct or not.

Maybe I found the point where we disagree. I was only talking about laboratory supplies all the time, the ones that the OP made pictures of. My point is that all lab bench supplies with the ability to adjust for accurate voltage and current set points are actually devices that can equally be used in CV and in CC mode. Just as Dave explains in his video.

I was not thinking/talking about simple PSUs that are designed to provide voltage, and that include overcurrent protection to keep them from start smoking. Some do approximate current limiting (with very high production / temperature / input and output voltage dependency in most cases), some have foldback behavior. Both can not be used for constant current applications. I think that is what you are relating to, correct? If we put it that way, I totally agree.

[MosherIV]

My point is that all lab bench supplies with the ability to adjust for accurate voltage and current set points are actually devices that can equally be used in CV and in CC mode. Just as Dave explains in his video.

This is exactly the opposite view to what I think.

Bench PSUs are CV devices with a Current Limit, we agree on that. They are not Constant Current PSUs, I think we agree on that.

What your examples show is how to use the Current Limit to force the PSU to output a steady current but the PSU is in Current Limit mode.
This does not make it a Constant Current PSU.

My main point and worry is that new people to the field of electronics are miss-led about PSU terminology. I think we both under stand what (and how) the PSU is doing/behaving, just the way we describe it is different.

I have not seen Dave's latest video yet.

[tatus1969]

just a bad example of Chinese engineering. This is a V/I curve of a 6S Lipo charger that I recently bought. It will work nicely... unless you short circuit it.

[tatus1969]

Bench PSUs are CV devices with a Current Limit, we agree on that. They are not Constant Current PSUs, I think we agree on that.

Apparently not. My assertion is that lab/bench supplies are mixed CV and CC devices. They do not implement current limiting but instead they accurately regulate for constant current, when setting and load condition puts them into CC mode. This makes them ideal CC devices, of course with limited output voltage capability. I use them to charge NiMH, Lipo, Liion, and supercaps every day. I do agree that you should not do this with regular PSU adapters.

[rollatorwieltje]

The biggest problem with CC mode on a bench power supply is the relatively large output capacitance. Even in CC mode it can still deliver very large currents. A true CC supply doesn't have that output capactor. This Keithley 6220 for example has an output capacitance in the pF range. Your average bench PSU probably in the order of 100uF.

[MosherIV]

My assertion is that lab/bench supplies are mixed CV and CC devices.

I disagree.

They do not implement current limiting but instead they accurately regulate for constant current, when setting and load condition puts them into CC mode.

No, they implement current limiting and once in current limit mode will not allow the current above that current threshold.


A true Constant Current PSU will deliver the set current, within the voltage upper limit, no matter what the load resistance - that is the very definition of a Constant Current PSU
https://en.wikipedia.org/wiki/Constant_current

[MosherIV]

I am not following what you're saying. I've seen power supplies with a current limit where they shut off after the limit was reached. But I've also seen power supplies which can provide constant current. Are you somehow saying these don't exist?

No, not at all, I am not saying they do not exist.

I am trying to say that describing a Lab or Bench PSU as Constant Current is wrong.

The 'effect' of Constant Current with a Lab PSU is achieved via the Current Limit function of said PSU.

Edit: Sorry, I meant to say DO EXIST

[tatus1969]

No, they implement current limiting and once in current limit mode will not allow the current above that current threshold.

When they do not allow the current to raise above this threshold, and this threshold is accurate, then what you get is a constant current. You just need to make sure that the supply's voltage set point is high enough so that it does not switch back to CV mode.

In the same way you could argue that, when the supply is in CV mode, it limits its output voltage and makes sure that it does not exceed the set point.

Here is the basic (incomplete) circuit of an old-school linear bench PSU:



There is one opamp for CV, another one for CC. That makes two control loops. The two diodes decide which mode to use depending on load condition and set points; as mentioned, the one wins that is restricting output current more.

But what I want to show by that drawing is, that both CV and CC loops are constructed equally: they regulate the power element to achieve a given set point. This is true CV, and true CC as well.

The biggest problem with CC mode on a bench power supply is the relatively large output capacitance. Even in CC mode it can still deliver very large currents. A true CC supply doesn't have that output capactor. This Keithley 6220 for example has an output capacitance in the pF range. Your average bench PSU probably in the order of 100uF.

Agree, I also pointed this out in a previous post. And it cannot be stressed enough. For example: you want to use your bench supply to test a low power LED. You adjust the current set point to the LED's rating of lets say 10mA. And because you think, better have enough voltage, you set the voltage set point to max, lets say 30V. You connect the LED, and zap, it is dead. The fully charged supply's output capacitor has killed it. Therefore: to test a LED, or any other sensitive device, start with 0V set point, and increase slowly. Or use a dedicated low-capacitance current source if you have. I don't 

[MosherIV]

tatus1969, I do understand how the Current Limit circuit works  , and it is a Current Limit circuit not Constant Current circuit.

"In electronics, a constant current system is one that varies the voltage across a load to maintain a constant electric current."

Even with your battery charging examples, when the battery charges up the PSU drops out of current limit mode and goes back into Constant Voltage mode.
A true CC PSU cannot do this, it will maintain the set current no matter what the load.

[madires]

Did HP get it wrong?

Presumably When the load powered by a constant voltage lab PSU in current limiting mode decreases, i.e. requires less current than the set limit, the PSU switches back to CV mode. A constant current source would increase the voltage to keep the current constant. Do you see the difference?

[tatus1969]

Did HP get it wrong?

Presumably When the load powered by a constant voltage lab PSU in current limiting mode decreases, i.e. requires less current than the set limit, the PSU switches back to CV mode. A constant current source would increase the voltage to keep the current constant. Do you see the difference?

A dedicated constant current source also has a maximum voltage that it can generate. This is comparable with the lab PSU's voltage set point. When you want to use the latter in CC mode, you adjust the voltage set point to a value that is greater than what you expect your load to need. If you don't know that, you set it to max. Except from the output capacitance, and in consequence of that the dynamic behavior, there is no difference left to a dedicated CC supply.