Ruth Hendrix

Regarding Driving Power Mirror

Discussion created by Ruth Hendrix Employee on Jan 26, 2006
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Posted: Jan 10, 2006, 8:47 a.m.
 
Hi,
I am currently developing the power mirror unit in a automotive car using EY16 motorola board.
 
Can anyone have idea about the motors inside the power-mirror unit.
 
What type of Relay should I use?
 
Posted: Jan 10, 2006, 12:46 p.m.
 
I build servo drives for very similar motors to those used in power mirrors. I suggest an FET H bridge rather than a relay. Motors I've seen in this application have only a few Amps at the most stall current at 12 Volts. There are some cheap very low on resistance logic level FETs available. Considering the low current involved a pair of dual p/n channel soic8 FETs would be ideal. But there are plenty of other options.
 
Posted: Jan 11, 2006, 12:16 p.m.

I agree. We have worked on mirror modules in the past. The maximum (inrush, stall) currents for mirror motors can readily be handled with solid-state FETs. There are also ICs that can handle small DC motors of this size. There are challenges to face -- including load dump specs, reverse battery protection, etc.
 
Last time I looked the crossover point as between solid-state and relays is somewhere north of 10 amps -- solid-state parts keeping getting better and cheaper, but automotive relays still ship in the millions because the specialist automotive relay manufacturers have figured out how to make full-H automotive relays for amazingly low costs.
 
Posted: Jan 11, 2006, 12:00 p.m.
 
I agree. Technically speaking MOSFETs are a nice choice, but if someone is not located in US, maybe the crossover point could be different (e.g., high import taxes in India could make locally produced relays cheaper).
 
Anyway, my cheap-and-not-so-dirty suggestion would be Mosfet w/active protection (use Ey16 to monitor the load and do the overcurrent cutoff) or a relay w/ some passive protection like a polyswitch (or prepare PCB tracks to blow some car fuse).

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