REG001 78L05 Fixed Voltage Regulator ... or .. REG005

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ed1066

Member
I need to decide whether to use REG001 78L05 Fixed Voltage Regulators or the considerably more expensive REG005 5V voltage regulators which the tech-supplies website say are "low dropout voltage regulator (higher quality than 78L05)."

I want a fully reliable circuit but don't want to waste money and I'll need to make several. The circuit diagram is near the end of this thread :

http://www.picaxeforum.co.uk/showthread.php?22574-DS18B20-with-hpwm-pwmout-with-the-20X2-chip

The standard REG001 78L05's have served me just fine so far and I have measured the current draw on the completed circuit, 30mA, which is comfortably less than the 100mA rating.

(I'm away for a few days so I won't follow up replies immediately)

Thanks

Ed
 
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nick12ab

Senior Member
Using a low dropout regulator won't be beneficial in your project due to the input voltage.

The quiescence current consumption of a 7805 is also low, and even if the other regulator has it even lower then it doesn't matter since the project isn't battery powered.
 
fernando_g

fernando_g

Senior Member
Low dropout regulators are justifiable in battery powered projects, where one requires to extract every drop of juice from them.
 
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alien8r33d

New Member
Standard regulators require an input voltage of around 2.5v above the output voltage. Low dropout regs can supply a regulated output when the input voltage is as low as 0.5v above the output rating. This is especially useful in uP controlled battery powered circuits because you can monitor the unregulated voltage and detect low battery charge.
 
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alien8r33d

New Member
Maybe so, I was making reference to a generic part to illustrate the differences between standard regs and low dropout types.
 
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srnet

Senior Member
alien8r33d said:
Maybe so, I was making reference to a generic part to illustrate the differences between standard regs and low dropout types.
Click to expand...
Appreciate that most people reading post #4 would assume that LDOs need at least 0.5v difference to regulate.
 
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boriz

Senior Member
78L05 is fine. You want to save a little power? Go for the LDO type.

Sorry to be a bore, but...

That circuit diagram is not complete. It says nothing useful about the power supply. Important information for this kind of question. Or about the motor. And it's hard to read because the power is going right-to-left instead of the generally accepted left-to-right. And you should have a resistor (say 470R) in the Picaxe-to-MOSFET gate line to limit any failure currents. And it says "Pins 0,1,2 to DS18B20"? There is no PIN 0!. And it says "Pin 5 ready for flow meter."? Do you mean Pin 5, or in5, or out5? All different.
 
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ed1066

Member
Thanks all for your help on this and the answer is clear.

Cheers Boriz for forthright comments. Perhaps the PICAXE people might respond to this point you make

boriz said:
...

And you should have a resistor (say 470R) in the Picaxe-to-MOSFET gate line to limit any failure currents. ................
Click to expand...
because the 18M2 High Power Board seems to omit the resistor in question. Here is the circuit from the relevant datasheet.

High Power Board Circuit.jpg

Boriz - Could you let me know the kind of failures that you have in mind?

Thanks again, Ed
 
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srnet

Senior Member
ed1066 said:
because the 18M2 High Power Board seems to omit the resistor in question. Here is the circuit from the relevant datasheet.
Click to expand...
Whether you want a resistor there, or not, depends on exactly what you are doing with that particular PICAXE pin.

So it makes sense that its not fitted on the board.
 
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ed1066

Member
srnet said:
Whether you want a resistor there, or not, depends on exactly what you are doing with that particular PICAXE pin.

So it makes sense that its not fitted on the board.
Click to expand...
What am I doing that requires the resistor? Basically all I'm doing is connecting a motor, which is surely the standard application in mind behind the Power Board.. ?
 
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boriz

Senior Member
Do you know the magnitude of the back EMF spikes produced by the motor? Have you factored that into your choice of flyback diode?

What if the diode fails open circuit, and the full back EMF is applied to the MOSFETs Drain causing it to also fail? And if it fails with a short between the Drain and the Gate, you could have 12V going into the Picaxe output. A 470R resistor will limit the failure current to 25mA max. (actually a little less because the 10K forms a divider).

If the diode were to fail closed circuit, it could either 'pop' and go open circuit, or the MOSFET could be forced into failure by operating outside it's maximum limits.

Either way, the Picaxe is probably doomed without some sort of protection to 'intercept' the failure cascade.

There are other examples, but you get the idea. It's like a lifebelt. Most of the time you don't need it and you just look silly. But on that one occasion when you do need it, you better be wearing it.
 
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ed1066

Member
boriz said:
Do you know the magnitude of the back EMF spikes produced by the motor? Have you factored that into your choice of flyback diode?

.............
Click to expand...
I've no idea the magnitude of the back EMF spikes but I thought the motor capacitor sorts them out. Again, if the diode is good enough for the Power Board which covers a wide range of applications its suitable for my pretty standard one.

The motor to the pump is 12V, 'brushless' and has worked well so far, that's all I know.

If my board fails then I'll chuck the whole thing and replace it with an exact copy, I wouldn't want to risk replacing one diode and then putting it back to use in case the root cause of the problem was not that component but another fault producing undue stress on that component.
 
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boriz

Senior Member
Brushless? What's the flyback diode for? Why the hell am I talking about back EMF then? *rolls eyes*

Like I said. Inaccurate schematic, insufficient information. Wasting my time. Good luck.
 
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inglewoodpete

Senior Member
ed1066 said:
That sounds like good news. Can I dispense with the motor capacitor as well?
Click to expand...
It's probably a good idea to leave the capacitor in. Maybe not critical.

Brushless motors use electronics to create a rotating magnetic field to turn the rotor. Some brushless motors can be sensitive to power bus noise but others not. I have successfully used PWM to control the speed of a brushless motor, but that was with a capacitor across it. I wouldn't recommend PWM with your FET setup, though.
 
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inglewoodpete

Senior Member
The diode is not essential, since the 'commutator' is electronic, not mechanical. I noticed that the CHI035A board has a diode anyway.

If you want to use PWM to control the motor speed then some modifications are probably required. The problem with using a (relatively) large MOSFET is the way it is constructed. To control a large current (the IRL520 can handle 9 Amps), the Gate area of the MOSFET is quite large and insulated from the silicon "switching" material that is connected between the Source and Drain leads. So the gate forms a capacitor that must be charged and discharged on every on and off transition of the PWM. This can stress and possibly damage the drivers inside the PICAXE. To the PICAXE output pin at the instant when the transition occurs, the capacitor looks like a short circuit since there is no series resistance between them.

The solution will depend on the current that the brushless motor draws at the speeds that you want to run it at. You can add a series resistor between the PICAXE output and the gate. That will protect the PICAXE by reducing the surge current as the PWM switches. However, this can result in heating of the MOSFET if the current is heavy, due to the slower switching times. (The gate capacitance takes longer to charge and discharge and thus turns on and off more slowly.) For a small motor, this heating could be negligible.

One option that I've used is a smaller MOSFET like a 2N7000 (200mA max): smaller MOSFET means smaller gate capacitance.
 
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ed1066

Member
inglewoodpete said:
The solution will depend on the current that the brushless motor draws at the speeds that you want to run it at. .
Click to expand...
I really appreciate your help inglewoodpete. The motor draws 1.2A when going flat out, and this current reduces down to a fraction as the pwm does its thing. I want to run it over that whole range


inglewoodpete said:
One option that I've used is a smaller MOSFET like a 2N7000 (200mA max): smaller MOSFET means smaller gate capacitance.
Click to expand...
I looked up the datasheet on the TRT003 tech-supplies version of the FET and it said:

"This Power MOSFET series realized with
STMicroelectronics unique STripFET™ process
has specifically been designed to minimize input
capacitance and gate charge. It is therefore
suitable as primary switch in advanced highefficiency,
high-frequency isolated DC-DC
converters for telecom and computer applications.
It is also intended for any applications with low
gate drive requirements."

So does that solve the problem?
 
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MartinM57

Moderator
http://www.techsupplies.co.uk/TRT003 is not a 2N7000

You have 2 problems:
- TRT003 (actually a IRF540) is not suitable - it's only fully specified (http://www.vishay.com/docs/91017/91017.pdf) to have its lowest RDSon (and then its a relatively high 0.27 ohms) at a gate voltage of 10v - which you obviously won't get direct from a PICAXE
- 2N7000 (http://www.fairchildsemi.com/ds/2N/2N7000.pdf) is only rated to 200mA continuous

I think you should seek out an IRL540 (http://www.vishay.com/docs/91300/91300.pdf) - a "logic level" MOSFET that has a RDSon of 0.077 ohms at a gate voltage of 5v.

Unfortunately, IRL540 is not available from Tech Supplies - and I have no idea why :(
 
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ed1066

Member
MartinM57 said:
............... its lowest RDSon (and then its a relatively high 0.27 ohms) at a gate voltage of 10v - which you obviously won't get direct from a PICAXE
Click to expand...
Thanks for this. It's got me in rather deeper than I imagined. I am seeing a friend tomorrow who knows this stuff . I appreciate the steer.

I have done some tests and the tech-supplies MOSFET only dropped the voltage from 11.8V (power to ground) to about 11.5V when the pump was on full power. It didn't get hot. I have a diode to protect from connecting the 12v supply the wrong way round and that had a greater voltage drop and did get too hot to touch.

MartinM57 said:
I think you should seek out an IRL540 (http://www.vishay.com/docs/91300/91300.pdf) - a "logic level" MOSFET that has a RDSon of 0.077 ohms at a gate voltage of 5v.
:(
Click to expand...
I looked at the datasheet but was not sure if it has the super fast response which makes the tech-supplies MOSFET a potentially attractive option.

Thanks again

Ed
 
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