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Old 04-23-2013, 07:34 AM #1
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Default 7805 help

If I want to use a 7805 regulator to drive one or more ttl inputs to lasers do I really need both input and output caps?? If so what size??


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Old 04-23-2013, 07:38 AM #2
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Default Re: 7805 help

100nF-1uF decoupling capacitors on both sides will do in most cases, for more exact values you need to look at the datasheet for your IC if you have it.
You likely don't need the caps for this, but they are dirt cheap so it doesn't hurt to add them.

Edit: You can always buy something like this if you want it quick and easy.
http://numato.com/7805-5v-voltage-re...breakout-board

Last edited by ARG; 04-23-2013 at 07:39 AM.
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Old 04-23-2013, 09:55 AM #3
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Default Re: 7805 help

The input cap is only absolutely critical when using certain power sources where inductance of sharp rise/fall times will cause ringing on the output. This is a decoupling cap. The output cap is needed to prevent runaway over-correction to slight variances in the output voltage due to load changes that cause destabilization and eventually oscillation within the regulator. This cap is always needed.

If you're running a fairly constant load off of a battery or a nearly ripple free (<10mV/A) supply then you can successfully leave off the input cap. The moment your load ceases to be constant you can no longer omit the cap though. 0di/dt requires no capacitive correction.
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Old 04-24-2013, 03:05 AM #4
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Default Re: 7805 help

TTL gives a lot of leeway on the input level. There's really no need to have a regulated voltage source, let alone one capable of 1.5A. A voltage divider or zener will do fine.
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Old 04-24-2013, 05:47 AM #5
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Default Re: 7805 help

Thanks all. I was a little unclear on on the exact reason for the caps. Online I found several different diagrams, some with different values and some with none at all. Was just a little confused.

I have one of the laserlands RGB units and wanted to put it in a project box and have some control over the colors using TTL and will be using either a 9VDC or 12VDC wall wart.
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Old 04-24-2013, 04:34 PM #6
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Default Re: 7805 help

Why not use an old cell phone charger? (most of) Those are already well regulated to 5V.
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Old 04-24-2013, 05:02 PM #7
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Default Re: 7805 help

If your wallwart is the old clunky type with a real transformer and rectifier inside it check the voltage with a good meter. I've had "9V" units put out 16V unloaded and even a "12V" unit put out 21V unloaded. That 12V one only dropped down to 14V after over an amp of draw was loaded on to it.

A voltage regulator is a good idea if you can't find a 5V smps type adapter. Also note that some versions of 7805 are only good for 500mA (TTL only needs a couple mA at most), not the 1A their bigger cousins are rated for. I had to search for a while for an analog which could handle the 1.5A I needed for a project. There are also LDO versions of the standatd 78XX series that are good for low power projects, as less energy is wasted as heat.
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Old 04-25-2013, 03:42 AM #8
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Default Re: 7805 help

Thanks folks. I'm hoping to get this all wired together next week. It should be quick land easy to put it together.
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Old 04-25-2013, 03:54 AM #9
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Default Re: 7805 help

Note that most of these voltage regulators also have a minimum load rating. TTL input impedance is usually very high, so I'd be surprised if you were drawing any more than 5mA. Might need to be careful with that, as some voltage regulators can't regulate until a minimum current draw. Might have to use a load resistor, or maybe put a few LED's on the box. I think for most voltage regulators it's around 50mA? But you'd have to check the datasheet.
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