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### Author Topic: Need help figuring out what I need in this circuit  (Read 7208 times)

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#### whitewolf103187

• Guest
##### Need help figuring out what I need in this circuit
« on: April 23, 2012, 01:13:27 AM »
I'm looking at making a circuit for an injector driver box that is based in the lm1949 circuit found below http://www.ti.com/lit/ds/symlink/lm1949.pdf

The problem is I don't know what to use for the timing resistor on the right of the schematic. I have very little PCB knowledge and don't mind learning a little bit, I do however have some air force electrical training but it's mostly just the basics such as resistors, diodes, relays, ohms law, ect... When it comes to complex circuits with zener diodes, transistors or anything in between I can get the gist of it but I don't really have any clue with anything with a chip in it.

Can anyone point me in the right direction or anything with this?

#### kam

• Hero Member
• Posts: 1849
##### Re: Need help figuring out what I need in this circuit
« Reply #1 on: April 23, 2012, 08:55:26 AM »
i cannot open the link though

#### whitewolf103187

• Guest
##### Re: Need help figuring out what I need in this circuit
« Reply #2 on: April 23, 2012, 16:50:12 PM »
Its a PDF file so you need something like adobe reader to view it.

#### cheerio

• Sr. Member
• Posts: 306
##### Re: Need help figuring out what I need in this circuit
« Reply #3 on: April 24, 2012, 20:28:01 PM »

#### cheerio

• Sr. Member
• Posts: 306
##### Re: Need help figuring out what I need in this circuit
« Reply #4 on: April 27, 2012, 14:22:43 PM »
The useful range of Rt is approximately 1k to 240k. The capacitor Ct  is limited only by stray capacitances for low values and by leakages for large values.
The timing capacitor is reset (discharged) when the IN pin (Pin 1) is below the VOL(MIN) threshold. The capacitor reset time at the end of each controller pulse is determined by the supply
voltage and the timing capacitor value. The IC resets the capacitor to an initial voltage (VBE) by discharging it with a current of approximately 15 mA. Thus, a 0.1 µF cap is reset in approximately 25 µs.

i would go for 1k and 0.1uF. Then i would check if the timing is alright. If not i would increase Rt or Ct. Depends of the timing result i want to have.