DC motor soft start

16 views
Skip to first unread message

Bob Engelhardt

unread,
Nov 22, 2021, 9:20:31 PM11/22/21
to
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?

Jasen Betts

unread,
Nov 23, 2021, 2:31:00 AM11/23/21
to
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.

Bob Engelhardt

unread,
Nov 24, 2021, 7:27:49 AM11/24/21
to
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?

Jasen Betts

unread,
Nov 24, 2021, 9:01:02 AM11/24/21
to
On 2021-11-24, Bob Engelhardt <BobEng...@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.

Bob Engelhardt

unread,
Nov 25, 2021, 12:01:22 PM11/25/21
to
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

Jasen Betts

unread,
Nov 25, 2021, 10:00:52 PM11/25/21
to
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.

Bob Engelhardt

unread,
Nov 27, 2021, 7:39:26 AM11/27/21
to
On 11/25/2021 9:42 PM, Jasen Betts wrote:
> On 2021-11-25, Bob Engelhardt <BobEng...@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.
Reply all
Reply to author
Forward
0 new messages