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Still looking at power supplies

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Scarr

Member
Hi,
I have a 5v 3amp reg, and I need the following supplies on my project

5v @ 200ma (max)
3.8v @ 2amp (max)
3.3v @ 200ma (max)

I currently use three regs to achive this the 3.3v & 3.8v being fed from the 5v reg. (saves smoothing caps and three wide voltage range regs)

Could I use voltage dividers instead of the 3.3v & 3.8v reg bearing in mind that the current drawn from the 3.8v device varies from a few ma to 2amp. and the 3.3v device stays pretty much constant at 120ma

P.S. Main supply is 12v-24v (vehicle / truck)

Thx
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Scarr said:
Hi,
I have a 5v 3amp reg, and I need the following supplies on my project

5v @ 200ma (max)
3.8v @ 2amp (max)
3.3v @ 200ma (max)

I currently use three regs to achive this the 3.3v & 3.8v being fed from the 5v reg. (saves smoothing caps and three wide voltage range regs)

Could I use voltage dividers instead of the 3.3v & 3.8v reg bearing in mind that the current drawn from the 3.8v device varies from a few ma to 2amp. and the 3.3v device stays pretty much constant at 120ma
No, particularly for the 3.8V rail - if the current drawn varies you need a regulator. If the 3.3V rail is constant at 120mA (and not 'pretty' constant), you could use a 14 ohm resistor (fed from the 5V rail), it will dissipate about 0.2W - however, if the current drawn varies, the voltage will vary as high as 5V, and as low as 0V. If you fed it from the 3.8V rail you could use a 4 ohm resistor, and dissipate only 0.06W - and the voltage couldn't go any higher than 3.8V.

Using a potential divider would help reduce the maximum voltage on the rail (rather than a straight dropper resistor), but would increase heat dissipation considerably - as a 'rule of thumb' a potential divider should pass five times the current that you are drawing from it.

What are the three rails actually for?.
 

Scarr

Member
Thanks

I have a GSM and GPS engines, the GPS is pretty constant but I don't think constant enough having read your post, and the GSM when just sitting there doing nothing draws little current but can peak at 2amp whne making a call in bad area.

All I was trying to do is save PCB space and cost :eek:) but I do need three supplies.

Is there no tri-supply adjustable voltage reg, where I can decide what volts I want from each output. or a 5v,3.8v,3.3v fixed? Probably not but worth asking.
 

Sebi

Active Member
Another problem the small voltage difference: not enough for normal regulator. The 2A adjustable LDO not too cheap...
 
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