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DC motor soft start

Started by Bob Engelhardt November 22, 2021
I found this neat soft-start circuit on the eevblog forum.  Its neatness 
is that the gate threshold voltage occurs immediately and then the RC 
softness kicks in.

V+ ________________________ V+
        |
        |
       1M        ___________ V-
        |___1u___|
        |        |
        |      |--
        |------|
               |--
                 |
  V- ____________|


As neat as that is, I have a situation where the motor may be run in 
either direction (either polarity on the input) and the soft start 
circuit won't handle that.  To use it with either polarity, I plan to 
use 2 of them & isolate with diodes:

                   _______
  V+/- -x-----D>--|       |---------x-----V+/-
        |         |       |         |
        |         |       |-----x---------V-/+
  V-/+ -----x-<D--|_______|     |   |
        |   |                   |   |
        |   |      _______      |   |
        |   |_D>__|       |_____|   |
        |         |       |         |
        |_____<D__|       |_________|
                  |_______|

It looks OK to me, but what do I know & I'd like to hear what you think. 
  Am I missing anything?

Thanks, Bob

BTW - anybody know of software that will do schematics with keyboard 
characters?
On 2021-11-23, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
> I found this neat soft-start circuit on the eevblog forum. Its neatness > is that the gate threshold voltage occurs immediately and then the RC > softness kicks in. > > V+ ________________________ V+ > | > | > 1M ___________ V- > |___1u___| > | | > | |-- > |------| > |-- > | > V- ____________| > > > As neat as that is, I have a situation where the motor may be run in > either direction (either polarity on the input) and the soft start > circuit won't handle that. To use it with either polarity, I plan to > use 2 of them & isolate with diodes: > > _______ > V+/- -x-----D>--| |---------x-----V+/- > | | | | > | | |-----x---------V-/+ > V-/+ -----x-<D--|_______| | | > | | | | > | | _______ | | > | |_D>__| |_____| | > | | | | > |_____<D__| |_________| > |_______| > > It looks OK to me, but what do I know & I'd like to hear what you think. > Am I missing anything?
could do this perhaps? __________ V +/- .--S<--| | | | --| | |------. | --| | | |___1u___| V+/- ___L______| | | | | 1M 1M | | _________L__ V-/+ |___1u___| | | | | | |-- | |------| | |-- | | | V-/+ ____________|-->S--' maybe add a diode parallel with the 1Ms to make faster resets.
> BTW - anybody know of software that will do schematics with keyboard > characters?
None that's worth bothering with. -- Jasen.
On 11/23/2021 2:11 AM, Jasen Betts wrote:

> > could do this perhaps? > > __________ V +/- > .--S<--| > | | > | --| > | |------. > | --| | > | |___1u___| > V+/- ___L______| | > | | > | 1M > 1M | > | _________L__ V-/+ > |___1u___| | > | | | > | |-- | > |------| | > |-- | > | | > V-/+ ____________|-->S--' > > > maybe add a diode parallel with the 1Ms to make faster resets. > ...
That would work ... clever, too. Thanks. I'm guessing that the "S<" is a Shottky diode. Preferable for the lower forward drop?
On 2021-11-24, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
> On 11/23/2021 2:11 AM, Jasen Betts wrote: > >> >> could do this perhaps?
...
>> | | >> V-/+ ____________|-->S--' >>
> That would work ... clever, too. Thanks. > > I'm guessing that the "S<" is a Shottky diode. Preferable for the lower > forward drop?
Yes. mainly to have a lower drop than the substrate diode of the MOSFET. yes. -- Jasen.
On 11/22/2021 9:20 PM, Bob Engelhardt wrote:
> I found this neat soft-start circuit on the eevblog forum.&nbsp; Its neatness > is that the gate threshold voltage occurs immediately and then the RC > softness kicks in. > > V+ ________________________ V+ > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1M&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ___________ V- > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |___1u___| > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |-- > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |------| > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |-- > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | > &nbsp;V- ____________| >
... OK ... I built this and tried it out. Didn't work as expected & when I put my scope (Tek 465) on it, I couldn't see the trace. It's a 1-shot sort of event & the scope doesn't have the persistence (let alone storage) to see it. Any good tricks for seeing non-repetitive events? Thanks, Bob
On 2021-11-25, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
> On 11/22/2021 9:20 PM, Bob Engelhardt wrote: >> I found this neat soft-start circuit on the eevblog forum.&nbsp; Its neatness >> is that the gate threshold voltage occurs immediately and then the RC >> softness kicks in. >> >> V+ ________________________ V+ >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1M&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ___________ V- >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |___1u___| >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |-- >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |------| >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |-- >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | >> &nbsp;V- ____________| >> > ... > > OK ... I built this and tried it out. Didn't work as expected & when I > put my scope (Tek 465) on it, I couldn't see the trace. It's a 1-shot > sort of event & the scope doesn't have the persistence (let alone > storage) to see it. Any good tricks for seeing non-repetitive events? > > Thanks, Bob
long exposure photography on your scope screen. take a video recording of the scope get a storage oscilloscope. use a sound-card and software PC oscilloscope -- Jasen.
On 11/25/2021 9:42 PM, Jasen Betts wrote:
> On 2021-11-25, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
>> ... Any good tricks for seeing non-repetitive events? >> > > long exposure photography on your scope screen. > take a video recording of the scope > get a storage oscilloscope. > use a sound-card and software PC oscilloscope >
I have an iPhone camera - no shutter control. But I just found a feature that does a virtual long exposure (Live photos & Effects). I'm gonna' try it.