Delaying the saturation of a transistor

I don't think you means saturation. Why do you ?

Graham

But what is the minimal delay required? If it is in miliseconds or parts of it then a 5V relay shorting the base to ground and activating on the power rail would create such delay. Reed relays may be too fast and mechanical relays may draw too much current, so give us some info as to what are the limits in which to move.

Have fun

Stanislaw Slack user from Ulladulla.

In message , dated Sat, 19 Aug

Mr. J D may not be a native English speaker. He means 'collector current'.

I suspect you're on the right track but if so why doesn't he want the collector current to go up and down ?

Maybe he wants a delayed swich on with a 'snap' action ? In which case a comparator or dedicated reset type circuit would be appropriate.

Perhaps he can advise.

Graham

You can prevent the transistor from going into deep saturation by connecting a diode with low voltage drop from base to collector; cathode to the collector. You can trim that even more by adding a low value resistor between directly in series with the base of the transistor. The ON voltage of the transistor will be slightly raised.

Tam

Baker Clamp !

Graham

At least you recognized that wrapping that delay into the single transistor will also slow the rise/fall times. The R+C method shunts the high frequency transistor drive and slows it down. The only way to prevent this is to Schmitt trigger the transistor input, with either a tiny logic Schmitt using an RC on its input, or another transistor. A tiny logic Schmitt is probably the cheapest, most compact, and most consistent performer of your choices. Of course, this answer is given in isolation and without knowledge of your specific circuit. If "safety" is critical, the *best* way is to use the XOR output as an input to whatever eventually drives the transistor. This may get complicated with an XOR and I won't waste time on conjecture without additional information.

Thanks for all the replies. I am a native english speaker. To clear up what I meant: I only need a few milliseconds of delay time, and then stay at the peak. Like a capacitor but with only one rise, and no fall. Someone mentioned using a diode to do this, can you explain more in detail?

In message , dated Sat, 19 Aug 2006, Mr. J D writes

Apologies. The use of the word 'saturation' instead of 'collector current' has echoes of French terminology.

You could do it logically. See...

For your case just use the "TAU" output and drop the XOR portion.

...Jim Thompson

Thanks for the schematic. I don't want to do it logically, as it require an excess of parts, I have very limited space. I was almost out of space when I decided to add an XOR gate.

Can't you use a gate with schmitt inputs ?

Graham

The diode suggestion has to do with limiting the excess minority carrier charge in the transistor base by shunting base drive current around the base and directly into the collector at small BC forward bias voltage, which is why a Schottky diode is best and sometimes this combination is called a Schottky transistor. It is a speedy switch because the storage time, or roughly delay in time after removal of base drive until collector current begins to fall , is a function of the excess minority carrier base charge. This has little to do with your problem. You're not going to beat a tiny logic Schmitt trigger used to drive the transistor: View in a fixed-width font such as Courier.

. . . . dual .---> OUT . schmitt | . .--|--+--[R]--+----| o--| o-[Rb]-| . | |/ |/ |>

. === | | | . | C | | | . gnd1---------+-----+----' --- . gnd2 . . . RxC~ 1.5ms . . . .