Electronics Load cannot sink current from 700mA current source?

[treez]

Hello,

We have a 700mA current source which we designed. It drives Banks of LEDs or resistors fine.

However, when we try to use the TTi "LD300" electronic load as a dummy load for our 700mA current source then it doesnt sink current from the 700mA current source.

Why is this?

TTi LD300 electronics load:
https://www.tti-test.com/products-tti/precision/electronic-load.htm

This electronic load works fine when we set it to a voltage (say 5V) and feed it with a 5R resistor, the other side of the resistor being connected to a 10V Voltage source, ..the LD300 sinks 1 amp as it should.

Why doesnt it work as a load for our 700mA current source?

We set the LD300 to 15V, so it should sink the 700mA and be 15V, but it doesnt do it. (we are trying to make the LD300 act like a 15V LED string..why won't it do it?)

 

[FvM]

I presume, a set of well considered measurements will answer the question.

You'll primarly ask why the current source doesn't deliver the claimed rated current. Either the controller is fooled by the load somehow, reducing it's output current, or the load doesn't keep the specified voltage.

 

[treez]

i worked at one of the worlds biggest lighting companies once, and when we wanted imitation led loads o for testing, we got many "led simulator" made up using Zener/resistor/Power BJT.

-it does sound to me that electronic loads cannot simulate led loads?

The LD300 is with our software consultant now, so i dont have access to it at the moment....its the software consultant who wants to use the LD300 as a "dummy LED load"

 

[FvM]

Without measuring load voltage and current waveforms simultaneously, you won't know which side is behaving unexpectedly.

 

[dick_freebird]

An E-load will not resemble a LED string, much. And a
mess of LEDs, perhaps with added relays to control
load current, ought to be a factor of 100 cheaper than
a good quality E-load.

If this is a closed-loop load that tries to enforce a current
then its frequency response could be fighting with the
loop response of the source.

I have had trouble with E-loads in power supply testing at
low voltages, they require headroom that can't be had at
1V and a few meters of cabling. Some of them have devils
in the spec details.

For simplicity's sake you might try a resistor load of roughly-
right value, to see which end is having the problem.

 

[treez]

The current source and the LD300 work ok when not connected to each other.

 

[albert22]

I agree that it can be a problem of closed loop responses of both the source and the LD300 combined.
Did you try to see what is the waveform at the input of the LD300 ?
In may be oscillating.