Re: Current source design (tricky?)

From: Winfield Hill (hill_a_at_t_rowland-dotties-harvard-dot.s-edu)
Date: 03/12/05 

Date: 12 Mar 2005 08:26:27 -0800

Fred Bloggs wrote...
>
> He doesn't require any precision whatsoever- and can hack together
> an acceptable current source with readily available Radio Shack and
> NTE stuff- a simplified circuit w/o protection would be like so:
>.
>. +--------------------+
>. | | PMOS
>. | I-> | +-------+
>.VBATT>---+------+---/\/\-----+-------------+-------|s d|---->
>. | | Rs 100m | | | | |
>. / / / | | | g |
>. 680 10k 10k | / +-------+
>. / / / | 4.7k |
>. \ \ \ LM741| / |
>. | | | |\ | \ |
>. | +------------|----|+\ | 330 |
>. | | | | >-----+---/\/\----+
>. | | +----|-/ |
>. | | | |/ | |
>. | | | | |
>. | | +--||---------+
>. | | | 1.5n |
>. | / / |
>. | 10k 10k |
>. | / / |
>. | \ \ |
>. | | 330 | |
>. Vd +-----------/\/\----+ |
>. | | | |
>. --- | / |
>. \ / | 100 |
>. --- | / |
>. 1n4148 | \ |
>. | | | |
>. +------+------------+-------+
>.
>. (Vbatt-I*Rs)*10.08 + 0.23*Vd*10.0
>. --------------------------------- = 0.5* Vbatt
>. 20.08
>.
>. (0.002*Vbatt + 0.114*Vd)*2
>. I= ---------------------------
>. Rs
>.
>. I I
>. Vbatt calculated simulated
>. ----- --- ---
>. 8 2.0 1.9
>. 10 2.0 2.0
>. 12 2.1 2.1
>. 14 2.2 2.2
>. 16 2.3 2.3

 Whew! That's supposed to be a simple way to get the moderately-
 precise current the O.P. needs? Ten resistors, a cap, a diode
 and an opamp to drive the FET? Nah, come-on man, let's get down!

 The single-transistor regulator is a simple yet effective rough
 current source, requiring only two resistors with a transistor.

. (O)----o-- Rs --o----S D----------(O)
. | | _|__|_
. e\| | ---- Q2
. |------' |
. /| Q1 G
. | |
. '-------------o---- Rg ---- gnd

 But this circuit needs about 0.65 to 0.7V across the current-sense
 resistor Rs, whereas we'd like that to be about 0.25V for 2.5A through
 a 0.1-ohm resistor, etc. So we need to subtract a little voltage from
 the Q1 Vbe drop. We can use an old Robert Widlar trick, and get this
 from a fraction of the pass-transistor's drive voltage, like this:

. simple low-drop-out current source
.
. (O)----+-- Rs --+----S D----------(O)
. | 0.1 | _|__|_
. | R1 ---- Q2
. e\| | |
. |------+ G
. /| Q1 | |
. | R2 |
. | | |
. '--------+----+--- Rg ---(O) gnd

 That's pretty simple, only three resistors. The current is given by

 Io Rs = Vbe (1+R1/R2) - R1/R2 (Vgs + Io Rs)

 R2/R1 = (Vgs - Vbe + Io Rs) / (Vbe - Io Rs)

 Trying for a 2.5A CS, so the last term is 0.25V, assume Vbe = 0.65
 and Vgs = 3V, so we can choose some values. R2 has 3 -0.65 + 0.25
 = 2.6V across it and with 0.4V across R1 we have R2 = 6.5 R1, so if
 we pick R1 = 220 ohms, then R2 = 1430 ohms, which we can make from
 1k plus a 1k trimpot.

 The OP can assemble the circuit, and adjust R2 to get his desired Iout.
 There will be a little variation with load because Vgs changes a little
 with Id = Io and because the FET's junction is heating up, but it after
 trimming it may be all the OP needs.

 Given that the 12V battery is probably pretty constant (near 12V if
 running alone, or near 13.8V if in an operating automobile) we could
 choose to ignore the issue of changing supply voltage. But if not,
 there's a simple one-resistor improvement to the circuit to cancel
 the transistor's change in Vbe due to varying Ic current through Rg.

. simple low-drop-out current source
.
. (O)----+-- Rs --+----S D----------(O)
. | 0.1 | _|__|_
. | R1 ---- Q2
. e\| | |
. |------+ G
. /| Q1 | |
. | R2 |
. | |___ | _______
. | | |
. '-------------+-- Rc --+-- Rg ---(O) gnd

 Properly chosen, the higher voltage across Rc at high battery voltages
 cancels Q1's higher Vbe voltage at higher Ic currents. Readers who
 are familiar with re = kT/qIc, etc. (see AoE page 80), may recognize
 that choosing Rc = R2/R1 re, will do the trick.

. simple 2.5A 10-15V low-drop-out current source
.
. (O)----+-- Rs --+------S D----------(O)
. "12V" | 0.1 | _|__|_ --> 2.5A
. | 220 ---- Q2
. e\| | | IRF4905
. Q1 |------+ G
. 2n4403 /| | |
. | 1k +1k pot |
. | |_____ | ________
. | | |
. '---------------+-- 160 --+-- 10k ---(O) gnd

 Still pretty compact, only four resistors. Now that's getting down!

 BTW, watch out. Anyone trying to simulate this circuit with spice
 should realize that most power MOSFET models will provide incorrect
 Vgs vs Id values at these low current densities. 

-- 
 Thanks,
    - Win
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