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Old 13th August 2014, 01:04 PM   #1
akis is offline akis  United Kingdom
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Default 555 buffering

The old 555 has a supposed capability to source and sink up to 200mA, or more, but in practice the output drops significantly with load.

For example powering the ICM7555 or SE555 with 10V and drawing about 20mA results in 7.44V at the output, whereas at 1mA gives 9.84V at the output.

Is there a simple way to provide some sort of a buffer to give us say 50mA source and sink without any voltage drop issues?

I have been working on a full NPN push-pull stage on the simulator, three transistors in total, but if you have some better ideas please let me know.
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Old 13th August 2014, 01:40 PM   #2
Mooly is offline Mooly  United Kingdom
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How about a simple N and P channel FET pair such as IRF240 and 9240. No resistors needed, just the two transistors.
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Old 13th August 2014, 01:42 PM   #3
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The 555 already has a totem pole output, using an additional totem pole results in both transistors being on at the same time.
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Old 13th August 2014, 01:50 PM   #4
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What are you trying to drive?

I have found that a simple npn/pnp push pull stage works the best.

I used it for driving single FET's directly and I used it to drive a Gate transformer for use in a FET Class B stages.

Here is my implementation,

how can test the stator insulation and mylar coating?

FWIW

jer
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Old 13th August 2014, 02:25 PM   #5
akis is offline akis  United Kingdom
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Quote:
Originally Posted by Mooly View Post
How about a simple N and P channel FET pair such as IRF240 and 9240. No resistors needed, just the two transistors.
That has worked better than ever imagined. I used what I had in the draw, so IRF9540 (P-channel) at the top and PSMN017-80PS (N-channel) at the bottom, switching happily from rail to rail with 0 load on the 555.
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Old 13th August 2014, 02:30 PM   #6
Mooly is offline Mooly  United Kingdom
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There you go then
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Old 13th August 2014, 02:31 PM   #7
akis is offline akis  United Kingdom
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Quote:
Originally Posted by geraldfryjr View Post
What are you trying to drive?

I have found that a simple npn/pnp push pull stage works the best.

I used it for driving single FET's directly and I used it to drive a Gate transformer for use in a FET Class B stages.

Here is my implementation,

how can test the stator insulation and mylar coating?

FWIW

jer
I need a level shifter and a current buffer. What I had overlooked was that with the buffer the requirements on the 555 would be minimal and thus the amount of level shifting needed would also be minimal. So maybe I do not need a level shifter, just a buffer. Anyway I just tried the MOSFETs and they switch from rail to rail, no need to use transistors.
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Old 13th August 2014, 02:32 PM   #8
Mooly is offline Mooly  United Kingdom
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Do you not mean N channel at the top ? I did
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Old 13th August 2014, 02:47 PM   #9
akis is offline akis  United Kingdom
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No, it is like this
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Old 13th August 2014, 04:07 PM   #10
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This makes the output 180 out with gain. May not be a real problem for your application, but, may want to look out for ringing or other signs of instability.
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