> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Jinx
> Sent: 24 November 2009 09:13
> To: Microcontroller discussion list - Public.
> Subject: [EE] DC multiplier/amplifier
> 
> I've got a friend (yes, really) who has an old vehicle dynamometer. At
> the moment he uses a DC tacho generator attached to the floor roller
> which puts out 8V per 1000rpm into the dynamometer, up to a limit
> of 6000rpm. Doing it this way means the feedback is speed
> 
> He's wanting to use a dashboard tacho, to use rpm feedback, an
> example of which he has and thinks puts out 1V per 1000rpm. The
> active component is an SAK215
> 
> http://www.datasheetcatalog.com/datasheets_pdf/S/A/K/2/SAK215.shtml
> 
> The meter in this dash tacho is presumably a 10mA movement
> 
> What he's asked me to do is amplify the 0 - 6V (ie 0 - 6000rpm) range
> of the dash tacho to 0 - 48V to feed to the dynamometer
> 
> One thought I have is to boost the 12V car supply to 48V with an SMPS,
> and use a PIC to measure the voltage from the tacho and chop/integrate
> the 48V to 8x that voltage
> 
> I think opamps are out because of the 48V, but would consider any
> other linear suggestions anyone has, perhaps a modified audio amp
> for example

Personally there is no way I would be resorting to complex digital
techniques for such a simple function.  A low voltage op-amp is easily
useable by powering it from a regulated supply (a simple zener shunt
would likely be adequate) and following the op-amp by a simple
transistor driver stage and either a series pass output transistor, or
push-pull pair if you need to sink significant current.  Essentially the
same as a classical regulated supply design.

Alternatively you could simply pay the money for a high-voltage op-amp,
e.g. the TI OPA445.  Since you will have a fairly modest gain, the
relatively poor offset performance of these amplifiers is unlikely to
cause problems.

A switch mode design seems like massive overkill in this case.

Regards

Mike

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