Re: 3A adaptor for driving LCD
- From: paul$@pcserviceselectronics.co.uk (Paul Carpenter)
- Date: Thu, 09 Feb 2006 22:01:34 +0000 (GMT)
On Thursday, in article
<Xns976586D37CF54vaj4088ianshef@xxxxxxxxxxxxxxx>
invalid@xxxxxxxxxxxxx "Ian Shef" wrote:
"chris_ivan" <chris.ivan@xxxxxxxxx> wrote in news:1139505907.756795.187500
@g43g2000cwa.googlegroups.com:
Hi everybody,
I have a project to build 14V/3A adaptor to drive an LCD monitor.
the block diagram can be described as follow:
main plug (220VAC) --> step down transformer (18VAC) --> diode bridge
using 1N5402 --> cap 4700uF/25V --> voltage regulator LM350T -->
Voltage Divider --> DC Jack
What no bypass or output filtering caps?
Please provide your schematic. Does it look anything like the figures in
the datasheet, especially either Figure 1 or "1.2V?25V Adjustable
Regulator" ?
What are the resistor values that you used?
Sounds like you are trying to use a voltage divider to directly drive your
Using my DMM, I measure the unloaded output voltage is 20VDC. After I
load the adaptor with rhe LCD monitor, the output voltage of the
adaptor drops to 2.5VDC.
load. A voltage divider only provides the rated voltage if the load on it
is small compared to the load of the resistors in the divider. When used
in conjunction with a three terminal regulator (LM350T), the divider should
be hooked up like Figure 1 in the datasheet.
Also as stated in the paragraph below Figure 1, if insufficent load the output
will rise. As someone else has said put an LED across the output with a
resistor calculated on at least 21V at output for 5 to 10mA LED current.
I think he should scope his inputs and outputs as I doubt his levels being
measured are truely DC (within normal noise parameters).
Caution: It is risky to use your expensive LCD monitor for testing. It
would be better to use a 5 ohm 50 watt (!) resistor.
At first I think it's because the regulatorWrong. The datasheet guarantees 3.0 Amps. The 4.5 Amp value is what the
can't drive upto 3A, but the LM350T datasheet states that the device
can drive output current max 4.5A.
"typical" part is capable of. Your part may or may not be typical. In any
case, your design is marginal, you are on the edge of the guaranteed limit
of the part. Oh, and this limit only applies if the difference between
input voltage and output voltage is less than or equal to 10 volts.
Otherwise, the limit is worse (lower current). More on this below.
does anybody have suggestion how to solve this? Any comment is
appreciated. Thank you.
Another concern: Your transformer puts out 18 VAC Root Mean Square. When
rectified, this is about 25.5 volts peak. You are stressing your 4700 uF
filter capacitor - verify by using a DC voltmeter to measure the voltage
across it.
If your input voltage is 25.5 volts and your output is 14 volts, you are
beyond the 10 difference that limits the output current. The guarantee of
3.0 amps no longer applies.
Another indication of marginal design is if he could get 14V out at 3A from
25.5V input that regulator is going to COOK without a very good heatsink.
(25.5 - 14) * 3 = 34.5W
Nice little heater he is designing, if the cap is still working.
--
Paul Carpenter | paul@xxxxxxxxxxxxxxxxxxxxxxxxxxx
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