# Current limit of switch mode power supply

I have a premade SMPS, it is 12V and maxes out at 8.5A. I have a TEC1-12706 on one pair of terminals and there will be a fan on the other terminals (I only have a total of 4 terminals for out). I would like to limit the current for the fan. How would this be done?

The PWM controller inside of the SMPS is the OB2269AP along with a coupler ic.

• There are lots of ways to limit current, but what is the reason for needing the current limited? – Chris M. Jul 10 '17 at 16:48
• Are you saying that you want a separate "current limit" circuit placed in series with your SMPS, which will work to limit the current to some settable maximum value but otherwise doesn't limit current if the current is less? (It can't force current, you know.) It might have to dissipate a LOT of power. – jonk Jul 10 '17 at 16:48
• First we'd have to see a schematic or a part number, preferably a schematic. – Voltage Spike Jul 10 '17 at 16:50
• Nothing came with it, no model number serial etc.. Would it be on the PCB? – Kevin James Jul 10 '17 at 16:58
• Is it in an enclosure? If so, I wouldn't think it would be on the PCB. Are you worried about damaging the SMPS? – Chris M. Jul 10 '17 at 17:01

As outlined in some of the comments, there are several ways to limit the current. One of them is to force a constant-current operation (CC) directly at the secondary side. Your SMPS is naturally designed to operate in constant-voltage (CV) mode and goes into short circuit protection if the feedback information is lost in the primary side (the opto collector is open loop). A simple way to limit the output current and force a CC operation is to break the ground path in the secondary side and insert a shunt resistance ($R_3$) in the below schematic:
Then the voltage developed across the shunt drives a bipolar transistor $Q_1$. When this voltage reaches the transistor $V_{be}$, it will start pulling the optocoupler LED to ground, forcing current to maintain a "constant" drop across the shunt. I say "constant" because the gain is very poor and it is a cheap system. If we consider a 0.65-V $V_{be}$ voltage at 25 °C, then for a 1-A constant current, $R_3=\frac{0.65}{1}=0.65\;\Omega$. $R_2$ can be dropped (unless you want to fine-tune the CC point) and $R_1$ typical value is $100\;\Omega$. In normal operation, when the output current is below the CC value, $Q_1$ is blocked and the added block is silent: the SMPS operates normally in CV. As the output current increases, the two loops being ORed, the CC takes the lead while the CV gives up and stop pulling LED current as $V_{out}$ goes down. As I said, it was a cheap way of doing CC in cell-phones chargers at the time Nokia was still number one, so quite some time ago : ) but it did the job well.