On 9 Aug 2015 at 15:21, IVP wrote:

> Hi all,
>=20
> I've got 12VDC geared motors here which a client wants to use in
> his product
>=20
> There are two sizes, both from Shayang Ye
>=20
> http://www.shayangye.com/english/products.html
>=20
> The IG420294-25111R and IG520150-40101R
>=20
> The IG42 is what I'm preparing the PCBs for now, the IG52 is what
> he (might) want to use in the future
>=20
> The run current of the IG42 is ~0.8A, which the planned relay can
> handle, but the start current does seem quite "sparky" when I test
> with a crocodile clip, and I worry that this may in time weld or pit
> the relay contacts. The IG52 run current is ~1.5A, so its starting
> current will be even higher.
>=20
> Three options
>=20
> (1) a heavier relay, although that too may get damaged, and space
> is limited for anything bigger than a 16-pin DIL type
>=20
> (2) "something" to absorb the starting spark
>=20
> (3) PWM the motor from 0 to run current
>=20
> Option (2) would be the most convenient at this stage. What could
> I use ? An RC snubber across the motor and/or relay ?
>=20
> Option (3) is doable. Product has PIC content with a spare output,
> would mean changes to the PCB and s/w

Hi Joe,

An old but true rule of thumb for motor startup current is at least 6 x the=
 full load=20
rated current, so that might help you in finding an appropriate relay. Expe=
ct similar=20
when stalled. RC snubber prevents/reduces commutation noise and inductive=20
switch off spikes but won't affect the current.

Otherwise yes PWM with a transistor of some type (plus flyback diode). A si=
mple=20
ramp up from 0 to 100% over a period of time could keep the startup current=
 in=20
check, but obviously will reduce torque. Current feedback to your micro add=
s=20
complexity but would allow for more smarts, such as stall/overload detectio=
n and=20
protection. Keep relay if you need to reverse the motor, else use a full br=
idge.

Another option could be to use an intelligent MOSFET device with overload=20
protection circuitry built in to limit the current (eg. something like=20
BTS4140/41414142). Possibly even keeps PWM an option, though frequency woul=
d=20
have to be fairly low (100's of Hz) as such devices have significant switch=
ing delays.



--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.