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Koford (Slot Car) Bench Power Supply Project

TheChad

Sep 23, 2014
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...it's impossible to set the output to 0 unless you start adding more electronics.


Which is probably why the put a potentiometer with built in switch! Which I Bypassed because I wanted power when the knob was off to power the Digital volt/amp meter I just installed!

Looking at it, I think If I just install the positive test lead to the switch, It'll cut power as needed!

Is it possible to set output to anything less than 4v?

-TheChad
 

Gryd3

Jun 25, 2014
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Which is probably why the put a potentiometer with built in switch! Which I Bypassed because I wanted power when the knob was off to power the Digital volt/amp meter I just installed!

Looking at it, I think If I just install the positive test lead to the switch, It'll cut power as needed!

Is it possible to set output to anything less than 4v?

-TheChad
Yes, connecting the positive lead to a switch will allow you to cut power as needed, but be mindful that if you are powering something that is drawing 5-6A, you will need the switch to be able to handle that. If the switch is not rated to handle the current, it will begin to degrade and eventually fail as it is switched on/off.

If you have a data sheet handy for the regulator you are using, it should show you the lowest possible voltage output that it will regulate. If you want to go lower, and still have a regulated voltage, you will need to redesign a portion of the circuit.
 

Colin Mitchell

Aug 31, 2014
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Don't forget, the 12.6v is AC voltage, and the transformer is designed to drop to 12.6v AC on FULL LOAD.
This means the unloaded voltage is about 2v higher . Taking 14.6v AC and converting this to DC comes to 20.5v. DC Each bridge drops about 2v on full load. You need 4v across the regulators.
5v output at 8 amps produces a surplus of 9v x 8 amps = 70 watts of unwanted heat.
This is the minimum heat that has to be removed and that's why your case is as hot as a soldering iron !!
Adding 10 amp bridges will remove more than 10 watts per bridge from the regulators and this will prevent them burning out as you are now over-driving the whole system.
 

Colin Mitchell

Aug 31, 2014
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I forgot to mention that the 8 amps is AC amps. This has to be recalculated to 5.7 amps DC and if you are drawing 8 amps DC, you are overloading the transformer.
 

TheChad

Sep 23, 2014
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Okay, the transformer that came installed on this power supply was 6.3v 6amps. It did not have enough power, this the reason I installed the larger transformer..

I am never pulling more than 5 amps DC, typically I'm using it in the range of 3-6 volts DC, but need the ability to go up to about 10v DC...

Probably the most typical setting I use is 5v in which I'm drawing 2-4 amps depending on the motor..

I thought it didn't have enough power because of the voltage, it seems maybe the voltage was okay and it was the amps that was not high enough.. because 6A A/C is only what? Like 4A DC?

-TheChad
 
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TheChad

Sep 23, 2014
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Yes, connecting the positive lead to a switch will allow you to cut power as needed, but be mindful that if you are powering something that is drawing 5-6A, you will need the switch to be able to handle that. If the switch is not rated to handle the current, it will begin to degrade and eventually fail as it is switched on/off.

The switch is built into the potentiometer, it clicks off at the lowest setting on the potentiometer. So typically the amp draw at that setting will be less than 1-2amps..

There is a physical on/off switch that cuts the A/C feed between the wall and the transformer to shut the power off completely, it's rated at 30 amps.

-TheChad
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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because 6A A/C is only what? Like 4A DC?

When you bridge rectify 10V AC you get 10 *1.414 V DC (peak, and there are losses)
So, if you draw 1A at that 14V, you are effectively drawing an average of 1 * 1.414 A from the transformer.

Thus Colin divided the 8A rating by 1.414.

For different rectifier arrangements this calculation can actually differ, but bridge rectification with 4 diodes is pretty common, so this is one worth remembering.
 

TheChad

Sep 23, 2014
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Soo, if I were to change out my regulators for this: http://www.mouser.com/ProductDetail/STMicroelectronics/MD1803DFP/?qs=sGAEpiMZZMshyDBzk1/Wi50akZ1xmvG3YC1gaKNKA7w=

Which is a 10a switching regulator, would that work? or is there more to it then that?

No matter what I am going to purchase the above listed transformer, because I don't need or want my output voltage above 12v. Currently I'm at 14v, so I am going to purchase the smaller 10v 8a transformer which should put me about 12v output.

Still if it's as simple as changing out the regulators network for that switching regulator, then that's a no brainier!


Also I was doing some testing on the power supply after changing out the analog panel meter's for a digital one, and the voltage jumps around, +/- 0.10v sometimes spikes +/- 0.20v range.. Obviously this wouldn't have been detectable with the analog meter's, but it's obvious on the digital meters. Is that normal? Does that mean my capacitor isn't big enough? Can I fix that so it's consistent?

Thanks!


-TheChad
 
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Gryd3

Jun 25, 2014
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Soo, if I were to change out my transistor network for this: http://www.mouser.com/ProductDetail/STMicroelectronics/MD1803DFP/?qs=sGAEpiMZZMshyDBzk1/Wi50akZ1xmvG3YC1gaKNKA7w=

Which is a 10a switching transistor, would that work? or is there more to it then that?
well.. it's not that easy :p
What you have in there are linear adjustable voltage regulators. They are not transistors.
They come in all sorts different packages, so it's not uncommon that they can look alike.
A switching regulator will require an inductor, capacitor, and some resistors in addition to the actual switching regulator component, so it won't be as easy as a quick swap unless you can find a small regulator board that is pre-assembled.

Also I was doing some testing on the power supply after changing out the analog panel meter's for a digital one, and the voltage jumps around, +/- 0.10v sometimes spikes +/- 0.20v range.. Obviously this wouldn't have been detectable with the analog meter's, but it's obvious on the digital meters. Is that normal? Does that mean my capacitor isn't big enough? Can I fix that so it's consistent?
This is normal during operation, as it is operating with a noisy load. This is not ideal, but normal for the setup you have.
This simply means that you need an additional capacitor. There are two distinct sides of your current regulator. You have the 14V side from the transformer which currently has the large capacitor. This will help smooth out the input voltage to the regulators.
You do not have a capacitor for the lower voltage half of your circuit though. Try not to overdo the size on the output... it could lead to some nasty surprises when you first connect anything, as the capacitor has the ability to unload a LOT of current instantly if you connect it to a low resistance load. In addition to a smaller rated capacitor, you should also add a large value resistor like a 1MegΩ in parallel to help drain the capacitor when power is shut-down.
 

TheChad

Sep 23, 2014
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This is normal during operation, as it is operating with a noisy load. This is not ideal, but normal for the setup you have.
This simply means that you need an additional capacitor. There are two distinct sides of your current regulator. You have the 14V side from the transformer which currently has the large capacitor. This will help smooth out the input voltage to the regulators.
You do not have a capacitor for the lower voltage half of your circuit though. Try not to overdo the size on the output... it could lead to some nasty surprises when you first connect anything, as the capacitor has the ability to unload a LOT of current instantly if you connect it to a low resistance load. In addition to a smaller rated capacitor, you should also add a large value resistor like a 1MegΩ in parallel to help drain the capacitor when power is shut-down.

Okay - The large capacitor: Is it too big? How do I know what the proper size is? I think it was like a 3500uF or something, I changed it to a 4700uF (I don't remember why).

What size capacitor would you recommend for the low voltage side? I assume the resistor would be installed on the output side of the capacitor in between the capacitor and the load?

Is having a capacitor here going to affect the operation, in other words, The power supply in addition to breaking in motors at consistent voltages is also used to do quick rev's on motors for testing. Is having a capacitor there going to cause the response from the potentiometer to not be instant? (I know a Capacitor acts like a battery, so I'm thinking if I turn the potentiometer up for a second (to rev a motor) it's going to charge the capacitor, then when I turn the potentiometer back down the capacitor will still be charged up causing the motor to have to drain the capacitor down before dropping back down) Which I understand is the point of the resistor... I guess what I am thinking is, will the capacitor and the resistor make it so it works the same as it does now being able to instantly rev the motor or is it going to cause a sluggish response when I turn the potentiometer quickly?

(I am wondering why they wouldn't have put the capacitor in there in the first place)

Thanks!

-TheChad
 

Gryd3

Jun 25, 2014
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Using a larger capacitor will increase the sluggish behavior, you just need one to handle the little spikes that you are reading on your meter. The capacitor on the input side of that circuit is fine for where it's at.

The resistor would be connected in parallel to the capacitor. A high value resistor will not drain the cap right away, and should not effect the output. It will help when leaving the power supply turned off to allow for it to drain quicker.

I am sorry, but I don't have a good number for you. I understand 'why' and 'how' but not enough to provide values.
 

TheChad

Sep 23, 2014
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Can any one else suggest a capacitor size ?

Thanks again!

-TheChad
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Yeah, 100uF sounds right. And no, changing the 2200uF cap for a 4700uF cap is not going to make a lot of difference in this application. It will probably help make the output voltage a little cleaner by filtering the input voltage more.
 

davenn

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hi ya

yup, that's good info on that www site :)

the big cap value after the rectifier is chosen mainly on the current rating you are likely to draw from the supply
The rule of thumb is 1000uF / Amp
so for a say a supply capable of 4 Amps, the minimum would be a 4700uF electrolytic cap

the 100uF electrolytic cap on the output as shown is good. Its also common practice to put small value caps on the input and output of regulator chips .... commonly around 0.01 to 0.1 uF .... these smaller values are there for 2 main reasons
1) to stabilise the regulator to avoid high freq oscillations
2) they also ground any high freq oscillation interference coming from other sources


EDIT ... dang Steve ... I was still typing haha


cheers
Dave
 

TheChad

Sep 23, 2014
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the big cap value after the rectifier is chosen mainly on the current rating you are likely to draw from the supply
The rule of thumb is 1000uF / Amp
so for a say a supply capable of 4 Amps, the minimum would be a 4700uF electrolytic cap

cheers
Dave

With that information, then my 4700 uF capacitor is too small..

Colin said earlier that I'm at about 5.7a DC after rectification, so I should have a 5800uF capacitor?

Okay, I went and picked up a 100uF capacitor and a 1MegOhm resistor...

Where should they be installed? Directly after the regulators?

Install the capacitor inline (out from regulators->Capacitor->)? Install the resistor between the in and it of the capacitor?

Thanks,

-TheChad
 
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Gryd3

Jun 25, 2014
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Earlier in the thread Gryd3 suggested a switch mode supply.

Might this be your answer? Any output voltage up to within 1V of the supply and with a maximum output of 15A.
http://www.ebay.co.uk/itm/15A-DC-DC..._Measurement_Equipment_ET&hash=item2c859f5986
Good find @pebe, this would be helpful and should run a lot cooler than the OPs current linear regulators.


With that information, then my 4700 uF capacitor is too small..

Colin said earlier that I'm at about 5.7a DC after rectification, so I should have a 5800uF capacitor?

Okay, I went and picked up a 100uF capacitor and a 1MegOhm resistor...

Where should they be installed? Directly after the regulators?

Install the capacitor inline (out from regulators->Capacitor->)? Install the resistor between the in and it of the capacitor?

Thanks,

-TheChad
@TheChad, I overlooked something regarding that resistor... The potentiometer and resistor that you have in your power supply is already going to do what I intended that resistor to do.
The 100uF Capacitor you have now will be connected similar to how the existing one is connected.
The existing capacitor is connected to the negative and the positive after the bridge, but before the regulators.
The new capacitor will be connected to the negative and and positive after the regulators. You are essentially connecting it directly to the output leads.

@(*steve*) and @davenn, thank you for jumping in. Where would one find these rules of thumb to properly size capacitors for inputs or outputs? Other capacitor sizing is usually done for timing (like a 555) and has a formula for it in the data sheet.
 

TheChad

Sep 23, 2014
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@TheChad, I overlooked something regarding that resistor... The potentiometer and resistor that you have in your power supply is already going to do what I intended that resistor to do.
The 100uF Capacitor you have now will be connected similar to how the existing one is connected.
The existing capacitor is connected to the negative and the positive after the bridge, but before the regulators.
The new capacitor will be connected to the negative and and positive after the regulators. You are essentially connecting it directly to the output leads.

Great! One less component to worry about!

I was doing some more testing today and it looks like the voltage jumping around may be due to a loose connection on the potentiometer... One of the colder tabs is loose (Which is why I am ordering a new one), and I put a drop of solder on it to hold it in place until the replacement shows up.. I was testing today and the voltage would just completely at random spike to like 14 volts..

So I messed around with that wire and when it was making good contact, the voltage was solid, didn't move more than .01v over a 10 minute period..

So it seems I may not even need the 100uF capacitor, I may throw it in there anyway, but it looks like that wasn't the underlying problem..

I changed the original capacitor from I believe 3500uF to 4700uF, which would have made since I went from a 6A Transformer to an 8A transformer..

But maybe I didn't go large enough? Though maybe it's not affecting anything... I assume if the capacitor was not large enough it would cause pretty noticeable voltage droops?


Before I go and order the new 10v transformer... This idea of installing another rectifier.. I can pick one of those up at Radio Shack.. If that a viable solution? From the sound of it, it would drop my current 14v down to 12v which is what I hoped to accomplish with the 10v transformer. How would I hook that up? Wouldn't that effectively make it a 1/2 wave rectifier instead of a full wave rectifier, thus creating bigger droops between phases?

-TheChad
 
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