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[DPRG] More efficient voltage regulator?

Subject: [DPRG] More efficient voltage regulator?
From: Sluggy slugmusk at home.com
Date: Thu May 25 00:13:44 CDT 2000

Kipton moravec wrote:

> So what you select depends on your battery voltage, surge of the 
> circuit, whether you need really clean voltage, cooling ability, and 
> cost.

All good information!

The longterm operational cost of a system is an important factor that is
often overlooked. I know, I've overlooked it often!

Linear regulators are a very attractive option at construction time.
It's hard to ignore that, from the microcontroller's point of view, a
7805 and a DC-DC converter look pretty much the same, while the 7805
costs about 2% of what a DC-DC converter would cost.

A $0.99 7805 running on a $20 12 volt gelcell battery is only 41.67%
efficient, but it's real cheap to build. The lost power means lost
runtime, lost recharge time and the early retirement of the battery due
to an increased number of charge/discharge cycles. Cooling might become
an issue, too, especially if you need a fan to cool the regulator
because of packaging or placement.

Using a better matched input voltage helps a lot, but now instead of an
off the shelf commonplace 12V battery, we need 8 volts. Not a lot of 8
volt batteries on the shelf at Tanner's, although there are 2 volt cells
that we could build one out of. Might even be a tiny bit cheaper, that
would only apply at Tanner's, for as long as they have the cells. No
telling if they will have them next time or not. In general, a non-12v
battery is going to cost more.

The slightly more expensive (not much) low dropout voltage regulator
helps us a lot by letting us dip below that output voltage + 3
limitation. Using such regulator and a 6 volt battery gets us a pretty
decent efficiency and we get to keep our construction costs down. 

The problem is that we probably need 12V for other parts of the robot,
so now we either need two batteries, a 6 and a 12, or we lose much of
the efficiency gain afforded by the low dropout regulator by feeding it
>from a 12V battery.

The $18 switching regulator (PT78ST105) and it's required output
capacitor running at the published 85% efficiency would allow the same
cheap 12v battery to last about twice as long as the 7805 per charge,
with the added advantages of being relatively flexible on the input
voltage and available in a variety of output voltages and currents (with
very little change in efficiency). Two cheap 12 volt batteries might
last as long as 4 using the linear regulator. You get to play with your
robot for longer times, you have less downtime waiting for batteries to
charge. You can charge the batteries at more conservative rates which
increases their lifespan, so you have to replace them less often.

So, we take into account the intended lifespan of the robot, the
contruction and maintenance budgets and select our power supply
components accordingly.

power 		construction 	maintenance	batt replacement
----------------------------------------------------------------
7805/12V	| low		| medium	| highest
LDV 7805/6V 	| slight higher	| medium	| medium
DC-DC/12V	| highest	| low		| lowest

It becomes a matter of whether you want to pay up front or over the life
of the robot. That's why it's important to realize the intended lifespan
of the robot. For about the cost of an extra battery up front, you can
make the batteries you have last longer, but if the robot is an
experimental stepping stone or a rough draft, it might be better to go
with the cheaper, though less efficient, parts.

Sluggy!

-- 
------

Why Windows NT Server 4.0 continues to exist in the enterprise
would be a topic appropriate for an investigative report in the
field of psychology or marketing, not an article on information
technology.  - John Kirch     http://www.unix-vs-nt.org/kirch/


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