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Home > Archive > Electrical Engineering > May 2007 > transistor relay circuit
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transistor relay circuit
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| I'm confused about something.
I'm using the following relay: DSP1-DC5V
Originally I had 5-volts to the top side of a relay coil, the bottom side
to the collector of a 2N3904, the base is being driven by a 10k with 5-
volts, and the emitter grounded.
I was unable to energize the coil, so I placed a 9-volt battery on the top
side of the relay coil while still using 5 volts through 10k on the base
and still was unable to energize the relay.
The datasheet states the relay coil is 83 ohms with a normal operating
current of 60mA. If the transistor is on full, why wouldn't my 9-volt
battery through the 83ohm relay coil energize when that is passing about
108mA through it?
Also, in this case, my collector current doesn't increase as I increase the
base current. I thought the collector current would increase as I increase
base current.
BTW, I'm working on a school project and I have all the circuits built and
the only two circuits not working are my relay coils. I've spent most of
the day on this problem and thought I could use x2 2n2904s but again, the
base current doesn't drastically affect the collector currrent - all it
does is decrease my beta ratio.
Any assistance would be appreciated.
Thanks in advance.
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| Peter wrote:
> I'm confused about something.
>
> I'm using the following relay: DSP1-DC5V
>
> Originally I had 5-volts to the top side of a relay coil, the bottom side
> to the collector of a 2N3904, the base is being driven by a 10k with 5-
> volts, and the emitter grounded.
>
> I was unable to energize the coil, so I placed a 9-volt battery on the top
> side of the relay coil while still using 5 volts through 10k on the base
> and still was unable to energize the relay.
>
>
> The datasheet states the relay coil is 83 ohms with a normal operating
> current of 60mA. If the transistor is on full, why wouldn't my 9-volt
> battery through the 83ohm relay coil energize when that is passing about
> 108mA through it?
>
>
> Also, in this case, my collector current doesn't increase as I increase the
> base current. I thought the collector current would increase as I increase
> base current.
>
>
> BTW, I'm working on a school project and I have all the circuits built and
> the only two circuits not working are my relay coils. I've spent most of
> the day on this problem and thought I could use x2 2n2904s but again, the
> base current doesn't drastically affect the collector currrent - all it
> does is decrease my beta ratio.
>
>
> Any assistance would be appreciated.
>
> Thanks in advance.
Diode
+ 9 -----------+---|<---+
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[Relay] |
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+--------+
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/c
+5 ---[1K]---| 2N3904
\e
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Gnd -----------+
The diode is not needed to make the circuit work,
it is needed to protect the transistor.
Note the *1K* base resistor.
With good parts the above circuit will work,
*provided the grounds from the 9V and 5V*
*suppiles are connected together*
Ed
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