Building a 3A Constant Current Load Box on the Fly!

I needed to get one of the boards i made tested by a third party for certification.

I was surprised to find out that the company that was supposed verify that the board meet all the necessary standards did not have a constant current load box. Well, having a constant load was one of the criteria.

Renting one involved long lead times...

So what was the quickest solution? Build one of my own and ship it over! 

Here are the features of the load box that im going to share with you.

1) 3A constant current load.

2) 6.7V UVLO (it will require atleast 6.7V to turn on)

3) Does not require any battery.

To build this load box you will require the following

• 1x LT1635 (Micropower Rail-to-Rail Op Amp and Reference)
• 1x 6.2V Zener
• 1x 12V Zener
• 1x 4148 Diode
• 2x MUR460 Diodes
• 2x 2n3904 NPN Transistors
• 2x FQA140N10 MOSFET
• 1x 0.1uF Ceramic Capacitor
• 4x 100k 1/4W Resistors
• 1x 130k 1/4W Resistor
• 4x 10k 1/4W Resistors
• 2x 1k 1/4W Resistors
• 2x 10Ohm 1/4W Resistors
• 1x 1M 1/4W Resistor
• 8x 470mOhm 1W Resistors
• 2x Heatsinks atleast 3.1 C/W

Lets get to it!

I will assume that the reader interested in building a load box is familiar with electronics and can read schematics so i wont spend time on that. Instead ill provide you with the schematic and explain how the circuit works.

Be sure to put it in an enclosure. The heat sinks will get pretty hot, so its important that they get adequate cooling, if they stick out of the box like mine do then be sure to be careful around them. 

3A Constant Current Load Box

Our operating voltage is 12V. Q3 and Q4 each have half of the total current flowing through them (1.5A). R14 - R17 are chosen such that the opamp biases the FETs to always have 1.5A flowing through each one of them. 

To set the current in Q3 for example, we set the voltage at the source of Q3 to be 3V. Hence our sense resistors (R14 - R17) should have a total value of 3/1.5 = 2Ohms. Since 1.5A would be flowing through 2Ohms, the total power dissipation would be around 4.5W. So in order to get around that we pick smaller resistors with higher wattage. Four 470mOhm 1W resistors do the job nicely, they add up to 2V and the power dissipation in each resistor is now reduced to (1.5)^2 * (470mOhm) = 1W.

The same strategy is implemented for Q4. 

R8 and R9 provide feedback paths to the inverting terminal of the LT1635. In order for the Opamp to keep the source of these FETs biased to 3V we need to set the non-inverting terminal to 3V as well. This is done by the resistor divider at the output of the reference. If the output of the reference is at 3V, 140k and 10k drop this voltage down to 200mV and feed it to the reference Opamp. This makes the Opamp very happy indeed :)

So we just made a constant current source. However, what we want is a constant current source that needs no external power source except the load and that turns on once the V+ voltage is higher than 6-7V. We achieve this with the help of Q2, and Q1.

Q2 remains on for as long as Q1 stays off. This is due to the pullup resistor R2.  This shorts the supply of the Opamp to GND. As a result the Opamp cannot bias the two MOSFETs and they remain off. Once the voltage at VIN rises above the zener clamp voltage of 6.2V + 0.7V(of D3) the clamp turns on and biases Q1.

Q1 turns on and shorts the base of Q2 to GND. As a result Q2 turns off and a resistive divider between R3, R4 and R5 beings the voltage down to a safe value for the LT1635 to operate at. D2 prevents the input to the LT1635 from rising higher than 12V incase a transient event takes VIN to a high voltage.

R10 and R11 isolate the output of the Opamp from the gate capacitances of the FETs. The reason why those were put there were because i was seeing the op amp oscillate, the loop did not like seeing those capacitances :(

Oh and if your wondering what purpose D3, D4 and D5 serve, they are there to prevent the circuit from frying if you accidentally hook up the load box the wrong way. ;)

And there you have it. Your very own load box! 

I put mine in a plastic case, i think it looks much cooler when you can see the guts of everything. :D

Lets see it in action...

Notice how the current starts flowing after we cross the 6.7V threshold and ramps up till we reach our set voltage of 12V. Increasing the voltage further no longer makes a difference and the current remains constant.

You can always make the load variable by adding 10k potentiometer instead of R6. I plan on implementing that which is why i have the holes drilled in the front.