Feedback from: Problems with transistor driving relay
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At 1:14 PM -0700 9/6/00, Mike Hardwick wrote:
>I would guess that your 555 is damaged, because the circuit should work as
>described at any reasonable supply voltage. Check the 555 output (pin 3)
>voltage when it's low. It should be less than 0.25V.
Changed it out with two others, no luck. Checked the voltage at pin
3, it's about .025V, so that should be OK.
>
>By the way, you could easily drive a 30mA relay directly from the output
>pin of a standard 555-- there's no obvious reason (to me, anyway!) to use
>the coil drive transistor in this case...
Like gosh that makes sense. I'm use to dealing with stamps and I
never thought about it.
At 12:02 AM +0200 9/7/00, Philippe Derenne wrote:
>If you want to use a transistor, you may try to put a 1K resistor between
>pin 3 and ground.
I had tried this but it didn't make any difference. Still works with
the BC547 but not the others.
One thing to note that I just found. If I disonnect the relay coil
after it sould have been open (output has gone low), and then
reconnect it it will then stay open as it should until the next time
the output goes high and it closes.
Thanks for the feedback so far. I'll just go with the 555 direct for
the project, but would still love to know what's going on.
Mike
>I would guess that your 555 is damaged, because the circuit should work as
>described at any reasonable supply voltage. Check the 555 output (pin 3)
>voltage when it's low. It should be less than 0.25V.
Changed it out with two others, no luck. Checked the voltage at pin
3, it's about .025V, so that should be OK.
>
>By the way, you could easily drive a 30mA relay directly from the output
>pin of a standard 555-- there's no obvious reason (to me, anyway!) to use
>the coil drive transistor in this case...
Like gosh that makes sense. I'm use to dealing with stamps and I
never thought about it.
At 12:02 AM +0200 9/7/00, Philippe Derenne wrote:
>If you want to use a transistor, you may try to put a 1K resistor between
>pin 3 and ground.
I had tried this but it didn't make any difference. Still works with
the BC547 but not the others.
One thing to note that I just found. If I disonnect the relay coil
after it sould have been open (output has gone low), and then
reconnect it it will then stay open as it should until the next time
the output goes high and it closes.
Thanks for the feedback so far. I'll just go with the 555 direct for
the project, but would still love to know what's going on.
Mike
Comments
Very strange... Pray tell, what voltage do you measure on the low side of
the relay coil when the 555 output is low? If it's any lower than the
supply voltage, then your transistor must be leaking current when it should
be completely cut off. It's possible that your 4.7K base resistor is
actually much higher in value, which could permit normal collector-base
leakage to bias the device slightly into conduction.
>One thing to note that I just found. If I disonnect the relay coil
>after it sould have been open (output has gone low), and then
>reconnect it it will then stay open as it should until the next time
>the output goes high and it closes.
>
>Thanks for the feedback so far. I'll just go with the 555 direct for
>the project, but would still love to know what's going on.
Mike Hardwick, for Decade Engineering -- <http://www.decadenet.com>
Manufacturer of the famous BOB-II Serial Video Text Display Module!
>after it sould have been open (output has gone low), and then
>reconnect it it will then stay open as it should until the next time
>the output goes high and it closes.
Maybe you have some kind of special latching relay?
What are some measured circuit voltages or currents when the relay is ON
but should be OFF?
-- Tracy
>after it sould have been open (output has gone low), and then
>reconnect it it will then stay open as it should until the next time
>the output goes high and it closes.
Relays have two important operating parameters: pull-in current and holding
current. It takes much more current through the coil to attract the
armature than it does to keep it once it has been caught. This is where you
problem lies. Your transistor is leaking enough current to maintain the
holding current of the relay. You must make certain that the transistor is
turning completely off (hence the pull down resistor suggestions). You
could also bend the tangs that the pull-off spring is attached to. I have
also found that some relays can become magnetized and need to be introduced
to a tape head de-magnetizer.