Current flows from collector to base of PNP transistor
- From: Phil <spam_from_usenet08@xxxxxxxxxxxx>
- Date: Fri, 29 May 2009 15:02:16 +0100
Dear Experts,
I have built the following current-regulating battery charger circuit:
+ o-----------------+
|
| /
____|L TIP32A
| |\
| | \
| |
+-W--||-+
| |
| =
| ---
| |
| =
| ---
LM334 O |
O<------+
| |
| R R = 0.3 Ohm
| R
| |
- o---------+-------+
The idea is that I can connect any convenient supply to the +/- connections on the left and a maximum current determined by the 0.3 ohm resistor will flow into the batteries. This seems to basically work as intended.
I was surprised, however, to find that when no power source is connected the batteries seem to discharge into the circuit at about 20 mA. I am struggling to understand how this can be. When no power source is connected the emitter of the transistor is floating and, ignoring the R-C that's required to prevent oscillation, the circuit boils down to:
|
| /
____|L TIP32A
| |\
| | \
. |
. |
. =
---
|
=
---
.
.
In my (perhaps naive) understanding of how transistors work, if the emitter is floating then no current can be flowing between the base and the emitter, and if no current is flowing between the base and the emitter than no current should be flowing between the collector and anywhere. What I observe is that 20 mA is flowing from the batteries (multimeter in series between the two cells) and that that emitter is at the same voltage as the collector (about 3.6 V).
Possibilities that have occured to me include
- There is some sort of "parasitic" effect in the transistor that I need to consider. (I'm vaguely aware of parasitic diodes in MOSFETs, but I'm too rusty to remember if there is something similar for bipolar transistors).
- One of the components is duff. E.g. the capacitor.
- The circuit construction is duff, e.g. a solder bridge somewhere. (I've looked, and simple resistance measurements between the pins of the transistor give the expected results.)
Does anyone have any suggestions?
I have another concern about the circuit. The idea is that I can charge e.g. 3x AA NiMH cells from a nominal 6 V solar panel, or from a 5 or 6 V power brick. The solar option is the more important one; the panel I have is nominally 250 mA but in practice produces about 150 mA where I live. This is below the circuit's current limit so I would like the transistor to be 100% on, i.e. saturated. But thinking about it I now believe that the LM334 will keep drawing more current from the transistor's base because its reference pin will never reach Vref. So (a) I will waste current that could be going into the battery, and (b) I potentially have more current flowing into the LM334 than it is designed to cope with. Is this a real problem, or is there some fault in my reasoning? Maybe I have already damanged the LM334 by doing this.
Thanks for any ideas,
Phil.
.
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