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LC Meter Relay

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 26 September, 2010 » LC Meter Relay « Previous Next »

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muskrat
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Username: muskrat

Post Number: 286
Registered: 06-2009


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Posted on Wednesday, 01 September, 2010 - 05:26 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I intend building the LC meter as published recently by EPE.

As usual I have component problems.

I can only find a relay with a 3V (24mA) coil.

Will it work as is, or should I get a 12V coil version and drive it with a transistor?
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alec_t
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Username: alec_t

Post Number: 293
Registered: 03-2009

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Posted on Wednesday, 01 September, 2010 - 06:35 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I don't see why you can't just use a dropper resistor to limit the current to 24mA.

HTH
Alec
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muskrat
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Username: muskrat

Post Number: 287
Registered: 06-2009


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Posted on Thursday, 02 September, 2010 - 04:55 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

That was my initial thought. But the current capability of the PIC pin is between 8.5 and 10mA. The relay I have found for this job is a bit too high at 24mA.

As I have to modify the original PCB design to accomodate this relay form factor, I think a driver transistor would be an advantage, regardless of the relay type fitted. It would certainly reduce the load on the PIC pin.

I guess I've answered my own question!

Thanks anyway Alec.
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terrym
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Username: terrym

Post Number: 828
Registered: 05-2005

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Posted on Thursday, 02 September, 2010 - 08:06 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Umm? If you are using the 16F628A as specified for the project, it has a maximum current sink/source capability of 25mA per output pin.
Well able to drive the relay.
I opened up my LC Meter (the original homemade AADE version)and I am driving the relay direct. A drive transistor wont hurt if you're worried about it though and if it is a 3v relay, I wouldn't include a series resistor.

TM
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epithumia
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Username: epithumia

Post Number: 783
Registered: 06-2006

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Posted on Thursday, 02 September, 2010 - 09:14 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The difference between 10mA (muskrat) and 25mA (terrym) is that the former is guaranteed to produced a certain output voltage while the later is an absolute maximum rating with no guarantee about the voltage on the pin.

I looked at the I/O characteristic graphs - if the relay is between the pin and 5V, the PIC should be able to drive it and I'd include a 82 ohm resistor. Leave out the resistor and you'll surely bust the absolute maximum rating of the PIC.

If the relay is between the pin and 0V, I'd definitely use an emitter follower transistor buffer because the PIC's voltage drop is far higher when driving high. That would make it less certain to work, and more difficult to decide what resistor to fit, and would also cause the PIC's output driver to run hotter.

Epi
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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terrym
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Username: terrym

Post Number: 829
Registered: 05-2005

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Posted on Thursday, 02 September, 2010 - 02:27 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I could be in trouble then . Have a device that was made for a customer that uses a 3v relay driven directly by the PIC. (The 3v relay 'cause he had 1000's of them). No dropping resistor, just direct from pin to ground.
It's been in use for a little over 12 months now.

TM
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microguy
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Username: microguy

Post Number: 205
Registered: 01-2010

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Posted on Thursday, 02 September, 2010 - 04:09 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Reliability. Just because it's working now, doesn't mean it'll still be working 10 microseconds from now.

(read it again in 10 microseconds)

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