Video Polarity and Direct Injection

[Findm-Keepm]

Or with a single transistor:

Video Inverter near the bottom of the list.....

http://www.hobbyprojects.com/A/video.html

I built a crosshatcht generator using an EDN magazine "Design Idea" and tried the video out with a salvaged Astec modulator - no joy, so I rigged up a 2N2222A as an inverter, and voila....


BTW, it still works, and is more stable than my RCA portable unit...

[vts1134]

Would a 2N4401 work in that circuit?

[Findm-Keepm]

Sure - any low gain (hfe<500) NPN should work. If you get any ringing, try ferrite beads on the base and collector leads.

I built my 2N2222A inverter on a double sided PCB, with the bottom as a ground plane, and "Manhattan" style construction on the top. The circuit I used was in "Circuits Cookbook" from 1967....

The whole circuit takes advantage of the common emitter configuration for inversion - and - the common collector configuration gives you the non-inverted video on the emitter. Two in one!

https://en.wikipedia.org/wiki/Common_emitter
https://en.wikipedia.org/wiki/Common_collector

[kf4rca]

http://www.circuitdiagramworld.com/a...fier_4380.html

[vts1134]

Can anyone verify that the circuit I posted should work? I can't get anything out of it.

[old_coot88]

Originally Posted by vts1134

Can anyone verify that the circuit I posted should work? I can't get anything out of it.

You mean you've built it already? If it's not working, the transistor may not be biased correctly. First measure the emitter/base voltage, which should be about 0.7 V. If the voltage is too low or missing, you need to increase value of the base-to-ground resistor.

Or, if the emitter/base voltage is too high, the transistor will overconduct and saturate. So lower the voltage by increasing value of the base-to-12V resistor.

This is all assuming the transistor is NPN silicon type and supply is +12V.

BTW, the circuit shown will deliver only half the output level that it would if the non-inverting feature (which you don't need) were eliminated. The mod would be easy to do if the output level turns out to be inadequate. Here's a neat little tutorial on transistor biasing..
http://www.electronics-tutorials.ws/...r-biasing.html

[vts1134]

Originally Posted by old_coot88

...the circuit shown will deliver only half the output level that it would if the non-inverting feature (which you don't need) were eliminated. The mod would be easy to do if the output level turns out to be inadequate...

How would I accomplish that?

[old_tv_nut]

Originally Posted by vts1134

Can anyone verify that the circuit I posted should work? I can't get anything out of it.

The problem is that the base is biased at about 1/2 of B+, which makes the emitter that (6 v) minus 0.7 volts = 5.3 v, and the collector drops to 12-5.3 = 6.7 v, so the transistor is almost saturated at one end of the circuit's output range. You need to reduce the lower base resistor or increase the top one to get about 3.7 volts at the base. Then the emitter will be at 3 volts and be able to swing almost from ground to 6 volts, while the collector will be at 9 volts and be able to swing almost from 6 to 12 volts.

You could try leaving the lower base resistor as 4.7k and increasing the upper one to 15k or 18k.

[kf4rca]

say 100K or so and adjust it till I get what I want with a scope on the output. Then measure the pot and substitute fixed resistors.