DC motor soft start...

B

Bob Engelhardt

Guest
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?
Click to expand...

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?
Click to expand...

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.
...
Click to expand...

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?
Click to expand...

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.  Its neatness
is that the gate threshold voltage occurs immediately and then the RC
softness kicks in.

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

....
Click to expand...

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.  Its neatness
is that the gate threshold voltage occurs immediately and then the RC
softness kicks in.

V+ ________________________ V+
       |
       |
      1M        ___________ V-
       |___1u___|
       |        |
       |      |--
       |------|
              |--
                |
 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
Click to expand...

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
Click to expand...

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.
 
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