How do I make a constant current load?

[ITSME.ULTIMATE]

I'd like to design a constant current load that can handle 0-5A and
resolvable to 100mA or possibly 50mA for testing the mAh capacity of
batteries.

The circuit needs to be able to pull full 5A from 0.8 to about 15v.

I will use a data logger to monitor the voltage across input terminals
and multiply the current by the time it takes from start to 0.9v per
cell to get the capacity.

I will use a precision 0.05A resistor in series for monitoring current
and I don't mind tweaking a pot to set the current.

What is the easiest way to make a load that can load a varying voltage
source with a constant current? I'd like to use 12v light bulbs or a
power transistor for dissipating the power. I would prefer the latter,
and something to control the base current to maintain a constant
collector current.

Any idea?

 

[Tim Williams]

or a
power transistor for dissipating the power. I would prefer the latter,
and something to control the base current to maintain a constant
collector current.

You'll need a MOSFET to handle 5A at 0.8V (Ron < 0.15 ohm), assuming that
combination is in your area of operation. A couple MOSFETs paralleled would
be nice to spread out the heat, although one can handle the 15V * 5A = 75W
maximum you want to dissipate. Such MOSFETs are cheap, you'll spend a few
bucks at most.

A small source resistor to detect current, an op-amp with output connected
to MOSFET gate, inverting input connected to source and noninverting input
connected to a voltage that determines current (to be exact, Iout = Vin /
Rsource). You'll need a 10V supply for the op-amp, and around a 100 ohm
resistor in series with the MOSFET's gate to prevent oscillation. Remember
to get an op-amp that can operate with its inputs at very low voltage.

Don't forget the source resistor subtracts from the MOSFET's available
voltage. You might want as low as 0.05 ohm here. That leaves 0.15 - 0.05 =
0.1 ohm for the MOSFET, which is still easy enough. 0.05 ohm puts your
input at 0 to 0.25 volts for full range.

Tim

 

[Deefoo]

I'd like to design a constant current load that can handle 0-5A and
resolvable to 100mA or possibly 50mA for testing the mAh capacity of
batteries.

The circuit needs to be able to pull full 5A from 0.8 to about 15v.

I will use a data logger to monitor the voltage across input terminals
and multiply the current by the time it takes from start to 0.9v per
cell to get the capacity.

I will use a precision 0.05A resistor in series for monitoring current
and I don't mind tweaking a pot to set the current.

What is the easiest way to make a load that can load a varying voltage
source with a constant current? I'd like to use 12v light bulbs or a
power transistor for dissipating the power. I would prefer the latter,
and something to control the base current to maintain a constant
collector current.

Any idea?

There has recently been a design idea in EDN Europe for a similar device
(http://www.edn.com/article/CA629310.html, august 2005, a programmable
active load) but you'll have to modify it to go down to 0.8V.

--DF

 

[Deefoo]

There has recently been a design idea in EDN Europe for a similar device
(http://www.edn.com/article/CA629310.html, august 2005, a programmable
active load) but you'll have to modify it to go down to 0.8V.

--DF

Also (much simpler, referenced by the other article)
http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=3239

--DF