| View previous topic :: View next topic |
| Author |
Message |
dima2882
Joined: 13 Jul 2005
Posts: 25
Location: Maryland
|
| Creating a programmable current load |
 Posted: Tue Jan 24, 2006 9:15 am |
 |
|
Hi all,
I'm trying to use the PIC in an application that tests a commercial DC-DC converter at diffrent loads. I wanted to avoid using load resistors and instead have the PIC use a digital trimmer, like the MCP42010, to control a FET circuit that pulls a specified current from the DC-DC and loads it down. I want the circuit to be sensetive to variations in its supply, so that a low voltage out of the DC-DC causes lower current loads. Does anyone know what that kind of circuit might look like? Any suggestions are appreciated.
-Dima |
|
 |
ruediw
Guest
|
| Re: Creating a programmable current load |
| Posted: Tue Jan 24, 2006 10:17 am |
|
|
| dima2882 wrote: | Hi all,
I'm trying to use the PIC in an application that tests a commercial DC-DC converter at diffrent loads. I wanted to avoid using load resistors and instead have the PIC use a digital trimmer, like the MCP42010, to control a FET circuit that pulls a specified current from the DC-DC and loads it down. I want the circuit to be sensetive to variations in its supply, so that a low voltage out of the DC-DC causes lower current loads. Does anyone know what that kind of circuit might look like? Any suggestions are appreciated.
-Dima |
I would create a voltage controled current source (sink).
And control this from a D/A-Converter attached to the PIC.
You may build this current source with little component count:
OP-Amp with a fiew resistors and a transistor. Type and cooling of the transistor depends on the voltage and current you want to test.
HTH
Ruediw |
|
|
dima2882
Joined: 13 Jul 2005
Posts: 25
Location: Maryland
|
|
| Posted: Tue Jan 24, 2006 10:34 am |
|
|
I'm actually looking to sink quite a few amps. The DC-DC needs to be walked over its range of 0-7A. I'm a bad analog guy :( If anyone has more details, I'd love to know!
Last edited by dima2882 on Tue Jan 24, 2006 12:43 pm; edited 1 time in total |
|
|
Humberto
Joined: 08 Sep 2003
Posts: 1215
Location: Buenos Aires, La Reina del Plata
|
|
|
kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
|
|
| Posted: Tue Jan 24, 2006 2:18 pm |
|
|
In the circuit shown below the op-amp with a feedback will drive the MOSFET in such a way that the voltage at the negative input will be the same as the on the positive input, which you are driving with a DAC. The voltage on the negative input is I*R1 - proportional to the current.
| Code: |
to the circuit you are loading
|
|\ U1 |
DAC or PWM+LPF | \ |
--------------------|+ \ ||-- Q1
| \----------------------|| MOSFET rated for 7A
| / | ||--
---|- / --- C1 |
| | / --- 100pF |
| |/ | |
| | |
----------------------------------
|
/
\ R1
/ Current-sense resistor
\ should be able to dissipate I^2*R
/ 1%
\
|
___
/// GND
|
|
|
|
SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
|
| Re: Creating a programmable current load |
| Posted: Tue Jan 24, 2006 3:41 pm |
|
|
| dima2882 wrote: | | I want the circuit to be sensetive to variations in its supply, so that a low voltage out of the DC-DC causes lower current loads. |
So you don't want just a current sink. You want something more like a simulated resistor. That is a little more complicated, but if you already have a uP the easiest way is probably to just measure the voltage and calculate the current it should draw. Then use the current sink others have described to draw the current you calculated.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
|
|
kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
|
| Re: Creating a programmable current load |
| Posted: Tue Jan 24, 2006 5:08 pm |
|
|
| SherpaDoug wrote: | | dima2882 wrote: | | I want the circuit to be sensetive to variations in its supply, so that a low voltage out of the DC-DC causes lower current loads. |
So you don't want just a current sink. You want something more like a simulated resistor. That is a little more complicated, but if you already have a uP the easiest way is probably to just measure the voltage and calculate the current it should draw. Then use the current sink others have described to draw the current you calculated. |
If you can sample the voltage fast enough, then Doug's method is the easiest.
If you are less tolerant to the delay, Humberto's circuit would work best.
To add digital control to it, you migh alternatively replace R1 and/or R2 with a digital POT. |
|
|
dima2882
Joined: 13 Jul 2005
Posts: 25
Location: Maryland
|
|
| Posted: Wed Jan 25, 2006 7:21 am |
|
|
| Yes, Doug's simulated resistor concept is exactly what I'm looking for. I will try to implement this in a 40 MHz PIC and external A/D, that should be fast enough. Thanks for all your help. |
|
|
|
FAQ 
Search
Register
Profile 
Log in to check your private messages 
Log in
