I need help with a basic circuit - Page 2

mogwai · 09-17-2005, 11:10 PM

Quote:

Originally Posted by Vector86

I breadboarded it up with an lm339 but I couldn't get it to work the way I want it to. Is there anything I should look closely at?

Can you post more details? What component values are you using? How is it working that is different from what you want?

The output of the comparator should go high when Vc is lower than Vr and Low when Vc is higher than Vr. You need to use a value for the resistors and capacitor that makes it take long enough to charge/discharge to the triggering voltage that you set. You would probably want to use a cap that is a few uF instead of something in the nF or pF range. Small capacitors are more easily influenced by noise, and the leakage current from the diode and comparator would mess up your timing calculations. You may want to look up the formulas for RC circuits if you haven't already.

One more note: with this circuit as is, if the triggering voltage Vs varies, it will change the timing.

Vector86 · 09-18-2005, 10:26 PM

For the RC combo I got 100uF and 1kohms at 12v for a 5 sec charge, It charges fine but it jumps right up to 11v when I discharge it and turn the power back on. Also, what voltage should I be seing on Vo?

Vector86 · 09-21-2005, 11:48 AM

Quote:

Originally Posted by Vector86

For the RC combo I got 100uF and 1kohms at 12v for a 5 sec charge, It charges fine but it jumps right up to 11v when I discharge it and turn the power back on. Also, what voltage should I be seing on Vo?

I'm an idiot. I figured out what I was doing wrong and the circuit (the charging part at least) works fine now. The biggest part of my problem was that the original values I came up with were completely wrong. After calculating the RC values it works.

r1 = 1k
r2 = 200k
c1 = 1000uF or 1mF

superart · 09-21-2005, 01:03 PM

hey, just cureous,

what are you guys using for ECU and fuel injection and the likes?

You dont have to answer if you wanna keep it secrete.

Im just asking cus I'm doing FSAE also, and Im looking around for a ECU solution. Right now I think I'm leaning more towards PowerSquirt v.3, but not really sure yet.

Vector86 · 09-21-2005, 03:37 PM

All I can really say is that we have a Motec ECU, other than that, I know absolutely nothing about it.

Vector86 · 09-30-2005, 04:47 PM

I couldn't find any opamps that will take and output 15v. I only need 12, but I don't want my circuit to die if it goes over. I have the above schematic wired up with an lm339 quad comparator and I only get 3v output. I could wire up all the comparators but that seems like overkill. Can someone recommend an opamp?

mogwai · 09-30-2005, 05:19 PM

I did a parametric search on TI and found this: http://focus.ti.com/docs/prod/folders/print/opa251.html

The opa251 is single supply, can handle 36v, and has rail to rail output. (meaning that the output should go to the same voltages as the supply.) Apparently you can get free samples of this chip from TI. If you are worried about overvolting it you can always use a resistor and a zener diode to protect it.

Brians256 · 09-30-2005, 06:18 PM

Vector86, you could also use a voltage divider to use an opamp with a smaller range on input. Spike protection isn't a bad idea either, though

Vector86 · 10-01-2005, 05:07 PM

Thanks mogwai, I ordered a free sample from TI so we'll see how that works out.

Vector86 · 10-15-2005, 08:04 PM

When I have the opa251 wired in, the output doesn't behave as expected. When the input voltage reaches the refference voltage, the output stays high instead of going low. What am I doing wrong?

mogwai · 10-15-2005, 09:52 PM

Does your input signal, vref, and the op amp have a common ground?

When the voltage is higher on the - input than on the + input the output should be low.

Vector86 · 10-18-2005, 11:22 AM

yes, they all have a common ground.

Vector86 · 10-18-2005, 09:51 PM

I think I need to drop the input signal b about 1 volt. It seems that the comparator won't function properly if the input signal is within .8 volts of the supply voltage.

09-21-2005, 01:03 PM	#29
superart Cooling Savant Join Date: Mar 2002 Location: FL Posts: 787	hey, just cureous, what are you guys using for ECU and fuel injection and the likes? You dont have to answer if you wanna keep it secrete. Im just asking cus I'm doing FSAE also, and Im looking around for a ECU solution. Right now I think I'm leaning more towards PowerSquirt v.3, but not really sure yet. __________________ When you do things right, people won't be sure youv'e done anything at all. Looking to buy/trade for used Deep Fryer and Vacume Pack Sealer.

09-18-2005, 10:26 PM	#27
Vector86 Cooling Savant Join Date: Jun 2002 Location: KANSAS "it's not just cows"! Posts: 211	For the RC combo I got 100uF and 1kohms at 12v for a 5 sec charge, It charges fine but it jumps right up to 11v when I discharge it and turn the power back on. Also, what voltage should I be seing on Vo?

09-21-2005, 03:37 PM	#30
Vector86 Cooling Savant Join Date: Jun 2002 Location: KANSAS "it's not just cows"! Posts: 211	All I can really say is that we have a Motec ECU, other than that, I know absolutely nothing about it.

09-30-2005, 04:47 PM	#31
Vector86 Cooling Savant Join Date: Jun 2002 Location: KANSAS "it's not just cows"! Posts: 211	I couldn't find any opamps that will take and output 15v. I only need 12, but I don't want my circuit to die if it goes over. I have the above schematic wired up with an lm339 quad comparator and I only get 3v output. I could wire up all the comparators but that seems like overkill. Can someone recommend an opamp?

09-30-2005, 05:19 PM	#32
mogwai Cooling Neophyte Join Date: Jun 2003 Location: Miami Posts: 23	I did a parametric search on TI and found this: http://focus.ti.com/docs/prod/folders/print/opa251.html The opa251 is single supply, can handle 36v, and has rail to rail output. (meaning that the output should go to the same voltages as the supply.) Apparently you can get free samples of this chip from TI. If you are worried about overvolting it you can always use a resistor and a zener diode to protect it.

09-30-2005, 06:18 PM	#33
Brians256 Pro/Staff Join Date: Oct 2001 Location: Klamath Falls, OR Posts: 1,439	Vector86, you could also use a voltage divider to use an opamp with a smaller range on input. Spike protection isn't a bad idea either, though

10-01-2005, 05:07 PM	#34
Vector86 Cooling Savant Join Date: Jun 2002 Location: KANSAS "it's not just cows"! Posts: 211	Thanks mogwai, I ordered a free sample from TI so we'll see how that works out.

10-15-2005, 08:04 PM	#35
Vector86 Cooling Savant Join Date: Jun 2002 Location: KANSAS "it's not just cows"! Posts: 211	When I have the opa251 wired in, the output doesn't behave as expected. When the input voltage reaches the refference voltage, the output stays high instead of going low. What am I doing wrong?

10-15-2005, 09:52 PM	#36
mogwai Cooling Neophyte Join Date: Jun 2003 Location: Miami Posts: 23	Does your input signal, vref, and the op amp have a common ground? When the voltage is higher on the - input than on the + input the output should be low.

10-18-2005, 11:22 AM	#37
Vector86 Cooling Savant Join Date: Jun 2002 Location: KANSAS "it's not just cows"! Posts: 211	yes, they all have a common ground.

10-18-2005, 09:51 PM	#38
Vector86 Cooling Savant Join Date: Jun 2002 Location: KANSAS "it's not just cows"! Posts: 211	I think I need to drop the input signal b about 1 volt. It seems that the comparator won't function properly if the input signal is within .8 volts of the supply voltage.

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