I am in the thought process for a new project where one of the modules is to design a Digitally controlled constant current source (50mA-1A in steps of 10mA).
I am thinking about using the Silab's C8051F021 with a 100ksps sampling rate 12-bit-ADC and 12-bit-DAC.
I am looking for some starters/pointers on designing the CC Digital control.
The DAC is fed into a voltage divider that feeds the opamp, as my DAC output was fixed at 5 volts, but if you have a controllable reference you can scale the DAC itself directly without the need for the attenuator.
I asked about the frying thing since I am going to use this as a switched (not SMPS) current of set currents for a set time (50msec to 500msec) ...one time only..
So i was thinking about the response...Will it settle in that time?
The settling speed will be determined by the opamp you use and any delay in the feedback loop, though I'm not sure how it'll react to highly reactive loads. What will you be feeding the current into? Actually, as was pointed out to me technically this is a current sink, so what will you be pulling it out of?
I(R1) {your load} is equal to I(R2) {because FET's gate current is ≈ 0}.
The voltage divider R4-R3 scales the DAC's voltage to 1V at V(c).
The linearity is as good as the linearity of the DAC itself.
With an 8 bit DAC, you would be able to produce 2^8=256 steps. If you need higher resolution, get a 10bit or 12bit DAC.
Why do you want variable current and pulse width?
You're better off simply PWM'ing the nichrome at the maximum voltage/current you can provide and changing the duty cycle to adjust heating. Losses in the fet are less then, and it's FAR more effcient.
Why do you want variable current and pulse width?
You're better off simply PWM'ing the nichrome at the maximum voltage/current you can provide and changing the duty cycle to adjust heating. Losses in the fet are less then, and it's FAR more effcient.
He will still want something like my circuit unless his primary power supply provides the current limiting. He can do the PWM by loading the DAC with a finite current value followed by a "zero" value at a rate determined by the controlling processor. Reducing the duty-cycle using processor-controlled PWM would proportionally reduce the dissipation in the NFET.
I don't get it, the only thing using a variable current source (like the one posted so far) is to limit current, but it DOES NOT limit current, it bypasses it to the FET, it's still used. Straight PWM to a heating element actually reduces power consumption. If it's nichrome wire, it's a heater, the thermal constant of the element will act as a 'low pass' So there's no reason to adjust the current at all, you just adjust the PWM width.
Ok, lets make up some numbers. Suppose the resistance of his nichrome wire is 0.01Ω. Suppose his primary power supply is 12V. Based on Ohm's law (other thread ), the peak current would be 12/0.01 = 1200A. If you consider the on-resistance of a large NFet, it might add 50mΩ so that would reduce the current to 12/0.06 = 200A.
Seems to me, you want some current limiting somewhere
No. Just pulse the led VERY shortly.
The power supply itself will limit the current. You're assuming wrongly that a power supply will supply infinite current.
Ok, lets make up some numbers. Suppose the resistance of his nichrome wire is 0.01Ω. Suppose his primary power supply is 12V. Based on Ohm's law (other thread ), the peak current would be 12/0.01 = 1200A. If you consider the on-resistance of a large NFet, it might add 50mΩ so that would reduce the current to 12/0.06 = 200A.
Seems to me, you want some current limiting somewhere
Yesterday my 7 month son was down with fever so couldn't reply.He is still cranky..so I am just giving you some details of what I need.
I would not worry with the dissipation since as I said earlier that this is going to be on for a max of up to 500ms.
This is NOT a heater as you have misinterpreted.This is a test instrument for a wire that has the profile of a nichrome wire.
For this i set up a current from 50mA to 1000mA and the time period for which this flows and then press fire.
I get some data through an ADC to the computer terminal where some graphs are plotted and some calculations are done and a result a printed or stored.
Depending on the total wire loop resistance, you could reduce the 12V battery in the circuit I posted to something like 5V, thereby reducing the dissipation in the NFET.