Control the current of a power supply using another variable power supply connected in series

Question

While disassembling a little electric oven (rated for 1300W at 230V), I scavenged one of the heating elements inside, which I measured its resistance to be about 40 Ohms (let's call it Rload).

Since it should be able to handle some power, I would like to use it as a "mild" load, for testing power supplies. For instance, let's say I have a 5V DC, 550 mA power supply (let's call it Ptest): once connected, Rload should draw around 125 mA, loading Ptest a bit.

I have a Rockseed RS310P programmable power supply, on which I can set voltage and current up to 30V and 10A (let's call it Pvar). I thought about connecting Pvar and Ptest in series with Rload, like this:

^{simulate this circuit – Schematic created using CircuitLab}

My idea is that, by setting the current limit of Pvar, the PSU will act as a constant current source and it will adjust its voltage to whatever is needed to mantain that current. The current will cause a certain voltage drop (Vload) at the load, and given that Vload must be equal to Vtest + Vvar, Pvar will adjust its voltage to be Vvar = Vload - Vtest.

For instance, given the current limit Iset = 250 mA, Vload = Iset * Rload = 10V, Vvar = Vload - Vtest = 5V: in order for 250 mA to flow through Rload, Vvar must be 5V. But, since everything is connected in series, this means that 250 mA are flowing through Ptest aswell, which is the effect I'm interested about: from Ptest point of view, it should be like a load which is drawing 250 mA.

Thus, setting the current limit could drive this makeshift "variable load" within a certain range: in this case, I should be able to go from a little above 125 mA up to 875 mA (by setting Vvar to 30 V, which means Vload = Vtest + Vvar = 35 V.

I know proper variable loads exist, but I would like to know if this could really work: theoretically it should, although I don't know much about power supply topologies (except really simple unregulated ones) so I'm not sure about practical pitfalls or even safety hazards involved.

How about connecting a battery instead of a power supply as test source? It should discharge the battery at a specific current.

Answer 1

This will work just fine as long as your variable power supply supports series connections (its output must be floating, not grounded). Keep in mind that you might damage your PSU under test if it too goes into current limiting, as the variable supply might drive a reverse voltage into it. It might be safer to use the variable supply in constant voltage mode, which will result in the overall current dropping if one of the two PSUs goes into current-limiting mode accidentally.