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Author Topic: i need help in this circuit plz  (Read 1048 times)

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Offline ammeerTopic starter

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i need help in this circuit plz
« on: September 14, 2010, 10:26:06 PM »
i am trying to use the lp2951 as a low dropout regulator , i read the data sheet and i got the circuit attached
but i have a small problem that i cant understand the values of the components of the resistors and and capacitors (the unites are not mentioned) , so please if any body can help me with that , i numbered the components in the attached circuit to make it easy for help ..
thnx in advance

Offline galannthegreat

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Re: i need help in this circuit plz
« Reply #1 on: September 14, 2010, 11:53:26 PM »
The two resistors marked "R1" and "R2" are a voltage divider that is set by the user to get a desired operation (there should be a formula in the datasheet to help you find out what you need to do). The unmarked resistors should be in ohms, as the others have kilo and mega markings, and the capacitors should be micro farads.

Hope this helps!
Kurt

Offline ammeerTopic starter

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Re: i need help in this circuit plz
« Reply #2 on: September 14, 2010, 11:56:17 PM »
can u please have a look on R3 :(
is it 0.05 ohms !!
thnx for care

Offline galannthegreat

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Re: i need help in this circuit plz
« Reply #3 on: September 15, 2010, 12:07:21 AM »
Yep, I know it looks crazy but it's right. I would go into circuit analysis mode here, but I'm tired from learning all this CCNA stuff I'm doing right now :(
« Last Edit: September 15, 2010, 12:09:51 AM by galannthegreat »
Kurt

Offline Soeren

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Re: i need help in this circuit plz
« Reply #4 on: September 16, 2010, 09:36:11 PM »
Hi,

[...] but i have a small problem that i cant understand the values of the components of the resistors and and capacitors (the unites are not mentioned) , so please if any body can help me with that , i numbered the components in the attached circuit to make it easy for help ..
As galantthegreat told you, the values are in ohms and F where no multiplier is mentioned, making the one you call R6 = 680 Ohm etc. The 0.05 Ohm resistor is used in the current limiter section and it should be 0.05 Ohm (50mOhm), which at 2.2A will develop a potential of 110mV which is going directly to the current limiter.

C1 is 0.033 F = 33nF
C2 is 4.7F Tantalum, but unless you can get a dry electrolyte tantalum (they are easy discernable from electrolytes by being much heavier - don't use tantalum drop shaped capacitors, they'll die soon) a 10F to 22F regular electrolyte will somewhat replace it. However, tantalum is used to get a low ESR (Equivalent Serial Resistance) and since there is a 100F cap as well, it would be better to replace these two caps with 4 x 33F electrolytes (sharing values over more caps will lower the ESR).

The formula for selecting the resistors in the feedback loop to set the output voltage cannot be calculated without knowing what output you want. The formula is right under the schematic in the pic you posted. Values around 100kOhm for R1 is a good place to start, but you could just replace them both with a trimming potentiometer of 100kOhm to 250kOhm and adjust it to the voltage you want.

All that aside, I don't see why you would bother boosting a low power regulator when you can just get a 3 pin (LDO if needed) regulator that can handle the current??
Regards,
Sren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline ammeerTopic starter

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Re: i need help in this circuit plz
« Reply #5 on: September 18, 2010, 02:53:40 PM »
thnx very much for ur help , am going to try the circuit in the next few days ..
unfortunately , we dunt have a 3-pin LDO in our region :(

 


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