Confirming/correcting circuit a design

malc9141 said:

Hello:

I have been given a diagram but wonder if there is a mistake. It is meant to deliver a short but variable pulse to a FET, and I want confirmation that the RC timer is correct.
In fact there are two of these, one for a very short pulse (Fire), the other, longer, up to about 5 ms. (Hold)

In particular, is R1 correct or should it be a capacitor? Also is the timer around C5 correct?

Thank you for any help.

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malc9141 said:

In fact there are two of these, one for a very short pulse (Fire), the other, longer, up to about 5 ms. (Hold)

In particular, is R1 correct or should it be a capacitor? Also is the timer around C5 correct?

Click to expand...

malc9141 said:

To JRW, thanks. Someone said the R1 should be a cap.

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malc9141 said:

To Nigel:
He was on this board!!

Malc

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malc9141 said:

To E and T, there is nothing to add to the diagram. I haven't a clue how to send an asc file.

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malc9141 said:

Hello:

I have been given a diagram but wonder if there is a mistake. It is meant to deliver a short but variable pulse to a FET, and I want confirmation that the RC timer is correct.
In fact there are two of these, one for a very short pulse (Fire), the other, longer, up to about 5 ms. (Hold)

In particular, is R1 correct or should it be a capacitor? Also is the timer around C5 correct?

Thank you for any help.

Click to expand...

eTech said:

Since the "Hold" and "Fire" outputs trigger on the same signal, the "Hold" output time is always greater than or equal to the Fire output, so the "Hold" output always overrides the "Fire" output. So, basically, the Fire output doesn't do anything. Lcoil is always energized by "Hold".

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rjenkinsgb said:

It's a power reduction or energy saving system,

The solenoid gets double current for a short time to pull in, then the fire FET turns off and the Hold one keeps it engaged, at half current, for the rest of the time.

It's quite common, in some form or other, with DC operated solenoids and contactors etc., to give positive operation without excess long term current or coil heating.

The simplest form is just an under-voltage rated coil plus a series ballast resistor to set the correct current, and an appropriate size capacitor across the resistor to give the appropriate high current pull-in pulse.

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