Re: Strange transistor driver circuit

On Tue, 31 May 2005 08:22:55 -0700, the renowned "Larry Brasfield"
<donotspam_larry_brasfield@xxxxxxxxxxx> wrote:

>"Philip Pemberton" <philpem@xxxxxxxxxxxxx> wrote in
> message news:a191b9734d.philpem@xxxxxxxxxxxxxxxx
>> Hi,
>Hi.
>> I've just been reverse engineering a printer controller PCB and I've come
>> across a rather unusual (to me anyway) transistor driver circuit in the
>> printhead solenoid driver.
>> Forgive my naff ASCII art, but here's the schematic:
>>
>> +V +V
>> --- ---
>> | |
>> | 3 Solenoid
>> | 3
>> e | 3
>> b | / PNP +--|>Z---*
>>>-----[ 4k7 ]------|< | | c
>> | \ | b | /
>> +-----*-----|< NPN
>> | \
>> | e
>> |
>> ---
>> GND
>>
>> To me, that looks like a pretty normal complementary Darlington circuit.
>
>Those transistors are not Darlington connected. (But I have never
>heard of a "complementary Darlington", so maybe you're right.)

Probably thinking of Sziklai connection, which is a bit different. The
base current in this case comes from the supply rather than through
the load, which means it uses more current from the supply and
(potentially) has a far lower "on" voltage.

>I suspect a current setting or limiting resistor is missing somewhere.
>Either the emitter or collector of the PNP must have a resistor in
>series, otherwise the NPN base current is poorly determined and
>likely too high.

Definitely. A resistor is required.

The zener connection is not uncommon, especially in chips. The SOA of
the BJT has to be about good enough for this service to safely switch
the inductive load in the first place, so it's trading off a small
3-cent glass zener diode against a larger power zener.


Best regards,
Spehro Pefhany
--
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