0
\$\begingroup\$

Before asking my question I would like to briefly clarify what I understand by a bench power supply opeartion:

Let's have a bench power supply which is set to 10V 1A CC. As far as I understand, this means upto a 1A current the power supply can supply 10V CV accross the load. So if I hook up a 10 Ohm resistor as a load to the terminals of this power supply I will measure 10V accross it and V/R = 1A current through it.(I'm assuming a special resistor which can handle enough power for the sake of this example) But let's say if I hook up a 5 Ohm resistor instead. In this case the current will not increase to 2A since it is set to 1A CC. The current regulator inside the power supply will not let the load to sink more than 1A current. Which means the current will remain the same as 1A. But this time since V = I*R, the power supply's voltage will accrdingly drop to 5V. This is what understand in case of bench power supplies. Correct me if I'm wrong.

Question:

Most of the power supplies (SMPS or linear such as 9V adapters ect.) in the market have two ratings always mentioned on them: Rated voltage output and max current. Are they operate the same way as the bench power supplies? Let me ask the question by an example. If we have a power supply such as the following adapter: https://www.alibaba.com/product-detail/10v-1a-power-supply-adapter-10w_60310531013.html?s=p

12V and 1A ratings written on it. If it were a bench supply with CC, I would say connecting a 6 Ohm load would sink only 1A and would drop supply's output voltage to 6V. But is that also what happens with most of the power supplies(not bench ones) in the market? I mean the same scneario happens as in bench supplies? Or it depends? How can we know it without trying?

Here are two examples of non-bench power supplies(typical ones used most commonly): https://www.alibaba.com/product-detail/10v-1a-power-supply-adapter-10w_60310531013.html?s=p

http://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/0413655/

Basically I'm asking what happens when we lower the load for a typical non-bench power supply and try to exceed its max current? Would there be a current and voltage regulation as in bench power supllies where CC activates or what would happen?

\$\endgroup\$
  • \$\begingroup\$ It's all down to the design - there are no general rules that say it should do one thing or the other. Most SMPSs will try and source the current demanded by the load, get too warm and start to shut down but some might blow a fuse or cheaper crappy ones might catch fire. \$\endgroup\$ – Andy aka Dec 28 '16 at 14:06
  • \$\begingroup\$ The main difference is that bench supplies have controllable current limits, which is very useful for not melting your board if you make a mistake. The mechanism is usually that there's a current monitor (e.g. over a shunt resistor) which can take action when the current limit is exceeded. That action can be an alarm sound, or just to cut power. \$\endgroup\$ – Polynomial Dec 28 '16 at 14:09
1
\$\begingroup\$

This is what I (and most EEs) would call a bench power supply or lab supply.

enter image description here

The currents and voltages written near the output terminals are the maximum ratings. These are the maximum currents and voltages this device can deliver.

Since this is a lab supply, it is posible to adjust the maxima for those values through the user interface. Then depending on the load the supply will regulate the voltage (the load does not draw more current than what you have set) or it will regulate the current (the load draws more current than what you have set so the voltage is lowered as to not exceed that current).

Devices like this:

enter image description here

and this:

enter image description here

Are what I call power adapters. They simply deliver a certain voltage. They can be used up to the current indicated. You cannot expect these devices to go into current regulation (lowering the voltage to maintain that maximum current) !!!

Depending on the power adapter, it might be overloaded and damaged or (better) switch off. Or the fuse might blow.

Power adapter are simply not suitable for current regulation.

\$\endgroup\$
  • \$\begingroup\$ Thanks the reason I asked this question a while ago a strange thing happened: There are two usual power adapters both with 12V voltage but one with 500mA max and the other is 2A max rated. There is a simple circuit LEDs in series with resistors. When this curcuit is hooked up to 12V 2A power supplies LEDs burn but when it is supplied by 12V 500mA one it works fine. I was thinking maybe max current is regulated in one of them which prevents it from burning. Could that be that a simple adapter can limit the current? \$\endgroup\$ – HelpMee Dec 28 '16 at 14:29
  • \$\begingroup\$ LEDs burn You mean catch fire ? Or "light up" or "become defective" ? Also provide a schematic. If the LEDs were used with the right series resistor, total current from supply is less than 500 mA and both power adapters are working properly, using either power adapter would give the same results. \$\endgroup\$ – Bimpelrekkie Dec 28 '16 at 14:34
  • 1
    \$\begingroup\$ And if that is your real question then ask that and be detailed about it. Now you asked a question assuming the difference between the supplies is the issue. If both supplies are working properly, there would be no difference. \$\endgroup\$ – Bimpelrekkie Dec 28 '16 at 14:37
  • \$\begingroup\$ Yes I meant become defective. I might be wrong about the exact currents but Im sure the max currents were different but the voltages of the power supplies were 12V. It was a while ago I dont have the adapters with me. Thats why didnt added to my question. \$\endgroup\$ – HelpMee Dec 28 '16 at 14:37
  • \$\begingroup\$ Just was curious about the differences between such supplies and lab ones. \$\endgroup\$ – HelpMee Dec 28 '16 at 14:38
1
\$\begingroup\$

The way to find out is to look at the specification of the power supply. It's whatever the designer chose to do. If it means 'constant current' it will generally say so explicitly. 'Max current' means no more than it will deliver that amount, and what happens if you try to draw more is unspecified.

If it's a linear power supply, it will have to dissipate more power than usual if it goes into constant current mode. A bench supply will often be designed to do this, as it can afford the extra heatsinking, and it's a useful function to have in the lab.

A non-lab power supply will generally not provide constant current limiting. Amongst the options are a trip, where it shuts down (it may stay shut down until the power is cycled, or try restarting periodically), or foldback limiting where it implements a lower current limit in overload, or it may go up in smoke (cheap ones), or it may blow a thermal fuse (many CE marked wall warts do this, it's safe, but it stops working, it took about 1 minute on the one I tested).

If it's a SMPS, then it doesn't cost power dissipation to add this feature, and it may well be designed in, or it may not.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.