# Discrete Current Boost/Pulse circuit

Discussion in 'Electronic Design' started by Neil, Sep 25, 2007.

1. ### NeilGuest

Hello,

I'm seeking help to design a discrete BJT current boost circuit which
can source a 600ma pulse for a duration of 200ms.

Any help would be great...
Thanks.

-Neil

2. ### Tim WilliamsGuest

Over what voltage range? Is current only forward?

Tim

3. ### Rich GriseGuest

At least this year, they're getting started on their class projects
at the _beginning_ of the term, rather than waiting 'til the last
minute. ;-)

Cheers!
Rich

4. ### Martin GriffithGuest

At what tyre pressure?

Martin

5. ### NeilGuest

It's not a university electronics engineering project, or any post
secondary electronics course related material either.
I'm on my own on this one. OK.

-Neil

6. ### NeilGuest

Over what voltage range? Is current only forward?

Tim

Tim, Well that would be between 10 to 12 volts.
Current forward only. Yep

-Neil.

7. ### NeilGuest

Martin, Pressure?..this is a electronics related question, it's a 600
milliamp pulse at 200 milliseconds.

8. ### WinfieldGuest

Why do you ask for a "current boost", as opposed to
a current source, or mirror, etc. Do you possibly
mean a current amplifier, getting say 6mA IN, and
delivering 100x more or 600mA OUT.

What do you mean by the phrase, current boost?

9. ### Tim WilliamsGuest

Then something like this will suffice.
/---o Current sink
R1 1k (1.2V) |/
Logic in o---/\/\----+-----+---------|
| | |\
R2 | _V_ v
1k > | 2 x | (0.6V)
| | >
| | |
+-----+------------+
_|_ GND

R3 = 0.6 / Iout.

Or reverse the transistor and diodes for PNP (change GND to +V).

A transistor like TIP29 should suffice. Actually, a darlington (TIP120,
etc.) might not be a bad idea, in which case you will need three 1N914's in
series. The transistor will need a small heatsink, as you will be
dissipating about 12V * 0.6A = 5W.

Rise and fall time will be under a microsecond. If you need faster, put a
470pF capacitor across R1 and use a faster transistor.

If you need more accuracy, you can use more base voltage (i.e., more diodes
in series, or use a reverse-biased zener instead), in which case R3 needs
to change accordingly.
You can also cascode it:

o Iout
|
2.2k (3.3V) |/
+V o---/\/\---+-----|
_|_/ |\
3.3V //_\ v (2.7V)
Zener | |
_|_ GND o To above collector

Again, flip if you need a source.

Tim

10. ### John FieldsGuest

---
1. When you say "current forward" do you mean that you want the
circuit to send current to the load (source current) or do you mean
that you want current to come from the load into the circuit (sink
current)?

2. How often do you want this current pulse to appear?

3. Is the amount of current to be determined by the load resistance
or by the circuit?