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Just use "matched transistor pair" with any search engine.On Apr 19, 5:33 pm, JosephKK <quiettechb...@yahoo.com> wrote:
On Fri, 18 Apr 2008 09:18:41 -0700 (PDT), gearhead
nos...@billburg.com> wrote:
On Apr 17, 9:16 pm, JosephKK <quiettechb...@yahoo.com> wrote:
On Fri, 11 Apr 2008 19:54:06 -0400, "NewsGroups" <spar@plaus> wrote:
"gearhead" <nos...@billburg.com> wrote in message
About temperature matching. I calculate that one tenth of a degree
centigrade difference between the transistors in a mirror would add
about one percent error to the current mirror. I wouldn't want
temperature vagaries causing any more error than that.
For C> sakes! You are talking about a motorcycle here. 1% up or
down will not make an ioata of difference to the battery charging on the
bike.
I'm wondering
if you could count on discrete transistors on a board to stay within a
tenth of a degree to each other. More fodder for the experts.
It is not 1 percent, it is 1 percent per degree C. if you have a 10
degree C temperature difference change you get a 10 percent unbalance.- Hide quoted text -
- Show quoted text -
At 300K and Vf=.6, a change of 0.1 degree would result in delta Vf =
200 uV.
At a current decade for every 60 mV, that introduces an error (added
to existing offset) of 10^.003333 = 1.0077 or about eight tenths of
one percent.
At Vf = .7, it comes out closer to 0.9 percent.
So I get a round figure of about one percent error in the current
mirror for each tenth of a degree temp mismatch centigrade in the
transistors.
So you are paying attention after all. Good. Transistors not on the
same die are difficult to keep within a few degrees of each other. In
this case thermal design matters.- Hide quoted text -
- Show quoted text -
Pickings for current mirrors are slim. I found BCV62, but it is not
what you would call well matched at all.
The 100 ohm base-loading resistors are there to take care of gain andOn Apr 8, 2:13 pm, Wimpie <wimabc...@tetech.nl> wrote:
On 8 abr, 22:34, gearhead <nos...@billburg.com> wrote:
On Apr 8, 10:03 am, Tim Wescott <t...@seemywebsite.com> wrote:
gearhead wrote:
Hi all, I'd like to help out some friends on an antique motorcycle
forum that have a 6 volt regulator project. One of them built a
regulator on perfboard and posted the schematic. It has a couple of
things I'd tweak, but it works.
The main challenge: his regulator design doesn't incorporate current
limiting. Even the original relay-style mechanical regulators
incorporated current limiting, because the generators required it.
Somebody on the forum suggested the zxct1009, but it only comes in
surface mount which means we can't rejigger the existing perfboard
project to include the new chip. It would mean having a custom-
printed circuit board, and soldering techniques perhaps a little too
demanding for somebody building his first circuit -- which probably
describes a lot of the guys on the forum.
pdf of the circuit:
http://www.hydra-glide.com/phpBB2/download.php?id=1173
I've tried to find a through-hole component that fits the bill, but
can't seem to come up with anything just right. I saw the micrel
MIC5021, but it supposedly operates on 12 volts and up.
http://www.micrel.com/_PDF/mic5021.pdf
Now, perhaps they make that statement on the datasheet because the
chip actually also has a voltage doubler and the high side drive needs
that kind of voltage to turn on a mosfet, and it's possible the chip
itself will actually run on a much lower voltage. But the datasheet
doesn't say anything to this effect. Has anybody used the MIC5021 and
know if it might actually turn on at say 5 volts or perhaps know of
any other through hole chips for current sensing?
This is just a suggestion, but consider the circuit below. If the
transistors are well matched Vchg should come pretty close to Vgen/10 +
47 * 0.02 * Ibatt -- and you can jigger your component values around to
change the numbers. You'll _probably_ be able to make this work OK with
any two unmatched (but like part-number) PNP transistors, like 2N3904's,
but you'd do better to get a matched pair (DigiKey has some Zetex parts
that would do, if there are any through-hole ones).
Try this out with SPICE before you run with it: I just threw it down off
the top of my head, it's not exactly like anything I've done before nor
is it tested at all.
Vgen ___ Vbatt
o-----o---|___|---o-----o
| | -----
| 0.02 | Ibatt
.-. .-.
| |100 | |100
| | | |
'-' '-'
| |
| |
| |
|---o---|
/| | |\
| | |
| | |
Vchg | '-----o
o-----o |
| |
.-. .-.
| |4.7K | |47K
| | | |
'-' '-'
| |
| |
=== ===
GND GND
I've done plenty of scheming about how to do it with discretes. But
like you said, I'd still want to use a chip in the end, to get matched
transistors. Pairs and arrays, I see ony surface mount, including
zetex.
If I have to go that way, might as well use the ZXCT1009F in SOT-23
with pins 1.9 mm apart according to the diagram. Almost a tenth of an
inch, could probably solder it right onto the pads of a perfboard, eh?
Hello,
When you buy about 10 general purpose PNP transistors from one batch,
you will probably find several transistors that are within some mV
with same Ic.
Doesn't gain determine the balance in a current mirror? If so, one
would have to match for gain. I did match for gain last time I
experimented with current mirrors. I'd like to hear from the experts
about the need for this.
When you design the 2 transistor current sensor at low
bias current, self heating can be neglected and it saves you from
soldering SMD devices like NXP's BCM857. When matching is impossible,
you might add a trimmer potentiometer for nulling.
best regards,
Wim
PA3DJSwww.tetech.nl
About temperature matching. I calculate that one tenth of a degree
centigrade difference between the transistors in a mirror would add
about one percent error to the current mirror. I wouldn't want
temperature vagaries causing any more error than that. I'm wondering
if you could count on discrete transistors on a board to stay within a
tenth of a degree to each other. More fodder for the experts.