Hi all
The space will be open tonight
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There was some talk of cheap lab power supplies (mainly myself and @Harvs) . I decided to look at what ebay has to offer and I am not clear on what/how they work. Maybe someone can enlighten me.
For example, looking at this one examining the front panel.
1 - I see a push button at the center between the V and A knobs, marked LO(mA) and HI(A) - what does this do?
I found a mention on the web saying “switch between 0.00A and 000mA measurement”.
2 - I think that I see two LEDs marked C.C and C.V. between each of the the two FINE/COARSE knobs. How do I select a mode?
3 - Is the current setting taken as a current limit or a constant current? Or maybe it has a different role in c.c. and c.v. mode?
4 - I see -, GND and + outputs. What do these provide? I guess GND is a chassis connection?
Beyond this, from looking at ebay and reading some eevblogs it seems that the name on the sticker means bugger all. You get what you pay for (around au$70?)…
Some can be had with 4 digits displays (so no A/mA switch required).
Re - and + outputs
Yes, the output floats, so you need to tie one to the earth if you want earth as a reference.
Re LO(ma) and HI (A)
I would suggest that is the meter scaling. If doing mA work, use the LO SETTING, ETC. 99.9ma->0.1A->5.0A?
Owen
I’ve got an old single rail disk smith supply you can have to help you decide whether you want one for real or not. It’s at least as good as those cheap ones. I can bring it along next week.
@Harvs Yes thanks - I am interested.
@owencook Agreed, I thought so much. It probably shifts the decimal point (.nnn?) for current under 1A?
CC is constant current; cv is constant voltage. Typically there is a knob for Amps (to set CC) and a knob for voltage (CV).
The mode is dependent on the load and how these two knobs are set.
With no load, voltage knob adjusts the output voltage as you would expect. This is constant voltage (CV) mode. You can think of it as a limit: voltage setting sets maximum output voltage.
With a load attached, the current knob may influence the voltage. It sets a maximum current you wish to be delivered to the load. As the current approaches or exceeds the desired setting, the output voltage will drop in order to satisfy the maximum current setting. This is constant current mode.
In practice, it’s quite useful for bringing up new circuits a bit more gracefully: you set a voltage, wind back the current, attach a load, and increase current until you’re happy that no smoke is coming out 
I’d recommend eevblog’s notes on cheap lab supplies. I can loan my dual cv/cc supply to the space, much like my Tek540A it doesn’t get much use any more
Separate chassis (floating) ground is very useful for the flexibility it allows, Eg. if you’re working on anything that might have some path (directly or indirectly) back to mains Earth; ground loops are a pain…
I did more reading. Had more looking around too. I am still not convinced that these cheap PSs do C.C. as I understand it. Or should I say “as I imagined it”?
For me, CC means that I set the required current and a max voltage, then attach a device. The device requires a specific current (think LED etc.) and it has a resistance that changes (e.g. as it warms, or when it changes modes). I want to see how the voltage varies while the current remains constant.
What I think these units do is provide a current limit, so voltage is affected (lowered) only if the current exceeds the limit. It will not raise the voltage if the current falls below the setting.
Am I correct?
Sort of correct.
You can set the voltage at max (usually around 30V per channel), then adjust the current. Assuming you’re load will keep the voltage under that limit, it will behave close to an ideal current source. And yes, very useful for testing higher power LEDs.