M
Michael A. Terrell
- Jan 1, 1970
- 0
Fred said:Jim Thompson wrote:
Forget that idiot- he is clearly wrong about that hokus pokus 50% duty
circuit National publishes. I see no practical use for that touchy thing:
View in a fixed-width font such as Courier.
.
.
.
.
.
. Vcc
. |
. RA TH=069*RA*C
. |
. DIS-RB--+ RA*RB RB-2RA
. | TL= ------- * C *Ln( ------ )
. | RA + RB 2RB-RA
. ===
. |
. |
. --- substituting RA*C=1.44*TH
.
.
.
. TL 1.44*RB RB-2RA
. -- = ------- *Ln( ------ )
. TH RA + RB 2RB-RA
.
.
. RA TL 1.44 1-2r
. substituting r= -- => -- = -------*Ln( ---- )
. RB TH 1 + r 2-r
.
.
. TL ~
. D=50% => -- = 1 , makes r=2.3 , RA and RB always in same ratio.
. TH
.
.
. TL
. (--)
. TH - r 4.32
. Then S = ----- *( 1 + ------------)= -3.5
. r 1 + r (1-2r)*(2-r)
.
.
.
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. For the case:
.
.
. Vcc
. |
. RA
. |
. DIS---+
. |
. RB
. |
. ===
. |
. ---
.
. TL RB 1
. where RA<<RB or r->0, -- = ------ = ----
. TH RA + RB 1+r
.
.
. TL
. (--)
. TH - r
. => S = ----- ->0 also
. r 1 + r TL
. (--)
. TH
. Example case of C=0.001u RA=10K RB=720K makes S =-0.014
. r
.
. Looks to be 3.5/0.014=250x better with initial accuracy.
.
.
. The "standard ckt" is patently inferior.
The simple way to ensure a 50% duty cycle is to run it at twice the
desired frequency and shove it through a flip-flop.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida