problem with my microcontroller power supply

• Jul 14, 2004

• #1

A

andrasvst

Newbie level 2

Joined

Jul 14, 2004

Messages

2

Helped

0

Reputation

0

Reaction score

0

Trophy points

1,281

Activity points

32

I HAVE AN EVALUATION BOARD WITH DS89C420 AND I MADE A POWER SUPPLY FOR IT.
Vout = 5V REGULATED(7805) / 1.5A.
WHEN I CONNECT MY LCD (2x16) WITH DS89C420 EVERYTHING IT'S OK
BUT Vout = 4.3V AND I WANT TO PUT A LOT OF OTHERS PERIPHERALS TOGETHER IN MY BOARD AND I THING THAT MAYBE THIS WILL BE A PROBLEM.
I THINK THAT I SOULDN'T HAVE TO HAVE A PROBLEM WITH POWER SUPPLY OUTPUT VOLTAGE AND I CAN NOT UNDERSTAND WHY?
CAN I HAVE A SOLUTION TO THIS?

THANKS!!!

 

• Jul 14, 2004

• #2

E

E-goe

Member level 5

Joined

Jan 10, 2003

Messages

84

Helped

2

Reputation

4

Reaction score

2

Trophy points

1,288

Activity points

715

Hi

The input voltage to the 7805 must be higher than 5V ( I think at least 2V).
Maybe this is the solution to your problem?

Greetz E-goe

 

• Jul 14, 2004

• #3

H

hill

Full Member level 5

Joined

Jun 17, 2004

Messages

277

Helped

10

Reputation

20

Reaction score

1

Trophy points

1,298

Activity points

2,334

andrasvst said:

Vout = 5V REGULATED(7805) / 1.5A

Click to expand...

Is that the output that you intend or it's the specification of 7805?

Normally 7805 rated to output 1A only. There are other package that output even fewer than 1A.

Also tell us what other component(s) that you have connected to 7805?

 

• Jul 15, 2004

• #4

N

nenad_p

Newbie level 4

Joined

Jun 4, 2004

Messages

5

Helped

0

Reputation

0

Reaction score

0

Trophy points

1,281

Activity points

38

andrasvst said:

I HAVE AN EVALUATION BOARD WITH DS89C420 AND I MADE A POWER SUPPLY FOR IT.
Vout = 5V REGULATED(7805) / 1.5A.
WHEN I CONNECT MY LCD (2x16) WITH DS89C420 EVERYTHING IT'S OK
BUT Vout = 4.3V AND I WANT TO PUT A LOT OF OTHERS PERIPHERALS TOGETHER IN MY BOARD AND I THING THAT MAYBE THIS WILL BE A PROBLEM.
I THINK THAT I SOULDN'T HAVE TO HAVE A PROBLEM WITH POWER SUPPLY OUTPUT VOLTAGE AND I CAN NOT UNDERSTAND WHY?
CAN I HAVE A SOLUTION TO THIS?

THANKS!!!

Click to expand...

7805 if it is in TO-220 package can give up to 1.5A
so I think that 220V thansformer in your AC/DC adapter
is too small

 

• Jul 15, 2004

• #5

W

whitenoise

Member level 1

Joined

Aug 21, 2001

Messages

34

Helped

0

Reputation

0

Reaction score

0

Trophy points

1,286

Activity points

235

When Vout=4.3V measure the Vin (so you can understand if the 7805 is working correctly), and measure also the current consumption of the board.

Remember that a 7805 really sourcing 1.5A could be very hot !! :twisted:
with few components (e.c. 1 controller lm259x from NatSemi, 1diode, 1 inductor and some capacitors) you can realize a switching solution.

bye, wn

 

• Jul 15, 2004

• #6

T

tgq

Full Member level 5

Joined

Jul 15, 2002

Messages

289

Helped

7

Reputation

14

Reaction score

4

Trophy points

1,298

Activity points

2,360

Maybe first capacitor (after rectifierbridge) is bad
Or 7805 is oscillating, check for 2 little caps normally close to it

 

• Jul 15, 2004

• #7

mrcube_ns

Advanced Member level 1

Joined

Apr 10, 2002

Messages

452

Helped

48

Reputation

96

Reaction score

34

Trophy points

1,308

Location

Europe

Activity points

3,813

7805 if it is in TO-220 package can give up to 1.5A

Click to expand...

To nenad_p:

7805 in TO-220 has 1A, in TO-3 case they can give 1.5A.

7805 has internal overcurrent protection and if load need more than 1A 7805 put down his output voltage to keep current max 1A.

Mr.Cube

 

• Jul 15, 2004

• #8

checkmate

Advanced Member level 3

Joined

Feb 25, 2004

Messages

832

Helped

178

Reputation

356

Reaction score

125

Trophy points

1,323

Location

Toilet Seat

Activity points

7,811

Most commercial evaluation boards would have come with an on-board regulator. Is there really a need to construct a power supply? If so, how did you construct the power supply? It is common practice to use a 9V adaptor to power a 5V circuit. Most adaptors should be sufficient for digital circuits. But if your circuit consists analog components too, you would have to consider your current requirements.

 

• Jul 15, 2004

• #9

M

manu

Full Member level 3

Joined

Mar 14, 2004

Messages

179

Helped

11

Reputation

22

Reaction score

2

Trophy points

1,298

Location

France

Activity points

1,297

May you give us the reference or a pic :? of your evaluation board ?
thanks

 

• Jul 15, 2004

• #10

E

electronrancher

Advanced Member level 1

Joined

Mar 24, 2002

Messages

447

Helped

46

Reputation

92

Reaction score

11

Trophy points

1,298

Activity points

6,935

i think you have a bad connection somewhere. i use ds89c420 and LCD and they take NOWHERE close to 1A.

something should be getting hot - if nothing is getting hot i think you have a short in your board. try putting a 10Ohm in series with the 5v line. measure the voltage drop across it to get an idea of the current consumption of your board.

 

• Jul 16, 2004

• #11

A

alphi

Advanced Member level 1

Joined

Dec 23, 2001

Messages

413

Helped

11

Reputation

24

Reaction score

10

Trophy points

1,298

Activity points

3,212

Maybe input power is not enough.for LM7805,input voltage must >7.5V for any load.
Pls measure the input voltage.

 

• Jul 20, 2004

• #12

A

albert22

Full Member level 6

Joined

Jul 20, 2004

Messages

332

Helped

68

Reputation

136

Reaction score

65

Trophy points

1,308

Activity points

5,005

You should check your input voltage at max load. If it is too low, the 7805 will not have the minimun dropout voltage to work properly. If it is too high, the power dissipated at the 7805 will be excessive and the regulator will enter thermal limiting, reducing the current.
Keep in mind that with 12vdc in and 1.5amp your power dissipation at the 7805 is 10watts (at 5vdc out).
K

 

• Jul 21, 2004

• #13

glenjoy

Banned

Joined

Jan 1, 2004

Messages

962

Helped

72

Reputation

146

Reaction score

20

Trophy points

1,298

Location

Philippines

Activity points

0

I think you have over current, if your load asks for more the current the 7805 can handle, you are in a problem, it will pull down your supply voltage.

Another, your powwer supply (transformer) is giving not enough voltage for the 7805 or cannot give enough current. Check your supply and load ratings.

It is easy, just series an ammeter for the current.