MylarHornet

Hi, yes I'm in the UK too and you're right about prices. I'm shopping at the places you mentioned also. Sometimes bargains can be found in the advertisers in EPE Magazine. I'm planning on using an open frame toroidal Xformer for my PSU. Should be fine for you also. My design is for running variable speed DC motors at constant torque. So my current limiter is really a constant current source. If I have a motor rated at 12V 1A, I can supply it with any voltage over 12V and the current delivered will always be 1A. I could give it 100V and it would still only draw 1A. Without the PWM the motor would be trying to pull 8.33A at 100V and it would be dead very quickly. I'm not an electronics expert, but I've done alot of reading on the subject in relation to my particular application. Although you may find this approach useable, you may be better served with a design similar to the schematic you posted. My design does not require variable voltage rails. You could for example run an LED, directly connected to 40V as long as you set the current limit to the current draw of the LED, say 10ma, the pulse width would be modulated to supply just the right amount of power to the LED, without blowing it to bits. Pulse width modulation relies on having a higher voltage supply than that required to run the load. Regards MH...

I am also building a power supply with current limiting. For the current limiting feature I'll be using an SG-3524 Pulse width modulator and a low ohmic current sense resistor, 0.05 Ohms. The basic theory is: Voltage drop across a low ohmic resistor is directly proportional to the current flowing through it. The voltage drop across the 0.05 Ohm resistor is fed into a LM324 OP-AMP, which amplifies the voltage drop by a factor of 10. This is then passed onto the PWM and is compared against a reference voltage which is set by a potentionometer. This is what allows you to set the current limit. Once the two voltages have been compared the output pulse from the PWM is either increased (more current) or decreased (less current) to maintain the current limit set by the pot. The output pulse train then drives an N-Channel Power MosFET to switch the higher current supply. For my application this switching will occour 20,000 times a second, (20Khz). For example, if you set the current limit to 2A, the voltage drop across the resistor would be I*I*R 2*2=4 4*0.05=0.2 So that's a drop of 0.2V the LM324 then multiplies this by 10 to give 2V. That 2V is then compared against a reference voltage in the PWM which can be set to anywhere between 0V and 5V using the pot. As we have theoretically set the limit a 2A, the pot will be set to supply a 2V reference voltage. So we have 2V Ref and 2V from the LM324. These two voltages are equal so no action is taken to change the pulse width and 2A continues to flow. Now, if we were running a motor from this supply and it suddenly stalls, trying to pull 3A. The voltage drop is now 0.45V*10=4.5V. This 4.5V is now compared to our 2V Ref and the PWM has to take action to bring the 4.5V back to 2V. It does this by narrowing the width of the pulse to reduce the current flowing through the 0.05 Ohm resistor. The pulse width is narrowed until 2A and the resulting 0.2V drop *10=2V is achieved. These parts are inexpensive. The PWM chip and a low Ohmic resistor can be had for around