View Full Version : Prof. Devt Board - 5v capacity?

05-18-2010, 08:29 PM
Hi all, simple Q:

I have a backlit parallel LCD module (2x16) connected to my Prof Devt Board,
and it seems like it is drawing too much current from the 5v supply, causing
the 5v regulator to get warm and then the propeller goes unstable (I presume
because the input supply goes out of spec)

I took a look at the cct for the Devt board, and the 5v regulator should be good
for an amp or more. The LCD backlight should draw no more than 100mA or so.
And the plugpack I am using is rated 1A at 9v.

I have a resistor in series with the +5v connection to limit the current to the backlight
to 100mA.

Will the Prof Devt Board really get this upset if I only pull 100mA from the 5v rail?

There is nothing else connected up, and the propeller is only driving two of the
surface mount LEDs on the board, so it's not like I am making any huge demands here.

From a cold start, it will run for a few minutes before failure, so I am guessing that
I am only just at the limit, otherwise the failure would occur much quicker.

Can anyone shed any light (backlight or otherwise) on the 5v rail rating for the Prof Devt board?


05-18-2010, 08:45 PM
Hi Mark,

I would check several things:

1. Try to measure the current. Is it really 100 mA? Perhaps there is a short circuit somewhere.

2. The voltage difference is rather big. With the 9V supply you will get 0,4 W of losses @ 100 mA. If you have the possibility, try to use for example a 7,5 V supply.

3. Is the power supply you are using a switching power supply?
I had one which was broken: At first everything was fine (7.5V output), but after several minutes the voltage dropped to values below 5V and as my 5V regulator wasn't able to keep the voltage constant anymore, I had the same instability you described.

Toby Seckshund
05-18-2010, 09:10 PM
If the PSU is of the unregulated, linear type, then the actual output volts are quoted at the quoted Amperage draw. So at 200mA or so there might be somewhere between 9V and 13V actually being applied to the Reg, more volts to drop = more heat.
The reg might be quoted as "good for an Amp" but that will be another of those "in a perfect world" statements. It will have to be bolted to an infinite heastsink. You know, a bit like the titanic at the bottom of the coldest bit of the north Atlantic.

Style and grace : Nil point

05-18-2010, 09:29 PM
Sounds like it needs a heatsink. Bigger = better, and ideally it should not get much warmer than body temp. Have you got a bit of aluminium you can bolt to the regulator, eg something 8x5cm or so?

www.smarthome.viviti.com/propeller (http://www.smarthome.viviti.com/propeller)