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Author Topic: What will burn my 7805?  (Read 1304 times)

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Offline unicoderTopic starter

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What will burn my 7805?
« on: May 09, 2009, 10:25:55 AM »
Heya ive been reading that planning ahead in regards to using a 7805 for how much amp draw your circuit will have is crucial. Im still sorting out datasheet info. I wanted to know if when doing noob projects like making a blinking LED or powering oscilators and an LED, will I near that threshold?

I know that 1 Amp is about it for them.
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Offline SmAsH

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Re: What will burn my 7805?
« Reply #1 on: May 09, 2009, 10:52:03 AM »
one standard led normally draws ~20mA, 1000mA makes up one amp so in theory you could have 50 leds in parallel...
but most still operate at ~3V, what are you regulating the voltage for? the oscillator or the leds?
Howdy

Offline unicoderTopic starter

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Re: What will burn my 7805?
« Reply #2 on: May 09, 2009, 11:07:40 AM »
everything lol. I drained all the batteries i had hooking a dc motor to them. So I pulled a 24vac wall plugging thing out and have rectified it to DC and then smoothed-regulated-filtered it to 5 vdc.

Im using some 1N4005's I pulled from an old VCR to make
full-wave bridge rectifier -> 22.6 VDC -> Smoothed with 100uF Elec.Cap -> 23.999VDC -> 7805 Regulated -> 4.6 VDC -> Filtered 10uF Elec.Cap -> 4.9999VDC

I have read that I can toss a L741 before the 7805 to help protect it. Which makes me wonder if I can(when i figure the dtasheets out) use a TLC272CP in its place.
« Last Edit: May 09, 2009, 11:14:17 AM by unicoder »
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Offline Soeren

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Re: What will burn my 7805?
« Reply #3 on: May 10, 2009, 05:19:47 AM »
Hi,

full-wave bridge rectifier -> 22.6 VDC -> Smoothed with 100uF Elec.Cap -> 23.999VDC -> 7805 Regulated -> 4.6 VDC -> Filtered 10uF Elec.Cap -> 4.9999VDC
It should have been more like (given an ideal transformer with a sine wave output):
24C AC -> rectified and peak rectified to 24*sqr(2)= 34V DC which is all too much for a 7805!
You're probably overloading the transformer or it's bad, since you get that low a voltage reading (or you measured it wrong?).

You need a capacitor of (rule of thumb) minimum 2,000 F/A   (i.e. 4,000 F minimum for a 2A supply)
2,000 F @ a 1A current drain is around the pain limit regarding ripple voltage.
Using 4,700F with your 1A supply wouldn't hurt

Do you know anything about what current the wall wart can supply?
(If the supply can only deliver 100mA, it's a bit too much expecting 1A after the 7805).


I have read that I can toss a L741 before the 7805 to help protect it. Which makes me wonder if I can(when i figure the dtasheets out) use a TLC272CP in its place.
Not really sure what you're getting at here?
"before", I read as, in between the transformer/rectifier and the 7805?
A power resistor to suck up and waste some of that power and drop the voltage would be the best protection.

If you're talking about a current limiter instead... Like a cat, that can be skinned in so many ways and op-amps of all kinds will come in handy for that.

But... Better start off by finding another wall wart given off, say, 6V to 9V (max.) if it's an AC output (8V to 12V if it's giving off DC).

Or better yet... Make a small lab supply with variable voltage, variable current limiter and metering - that really ought to be one of the first projects for everyone starting out in electronics, since you just cannot live without it.
Batteries and ad hoc mock up supplies are poor solutions, even if you don't take your hobby too serious ;D
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 unicoderTopic starter

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Re: What will burn my 7805?
« Reply #4 on: May 11, 2009, 05:05:52 PM »
My multimeter actually measured 25vac to 27vac prior to connecting the wall wart to anything. I think the leads on this MM are crap. At least the clamp meter seems accurate enough.
I actually just did this rough estimate:
SupplyVoltage - 2(DiodeVoltageDrop) = RectifiedVoltage
24 - 2(0.7) = 22.6
After that I measured again and came out around 23.2 or so. Then I tried two or three capacitors that gave an output I liked.
I'm not actually trying to draw more than 600 mA off of this hodge-podge circuit.

Ive got the parts I need on order to put together a proper lab supply.  I've also got some op-amps on order.

Thanks a ton for your input!
Industrial Electrician / Maintenance / Motor Controls / PLC's : 3572 hours logged.
C++, C#, MASM32 profficient. Experienced (5 years) DirectX / HLSL / General Graphics and IK programmer.

 


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