Newbie - 12V Regulator Question

[john2k]

Hello,

I'm a newbie here, I have a quick question that i'm hoping someone can help me on.

I've got a device that needs powering from a car power source and has a 12V max input voltage. It draws 650mA of current. So i've created a little circuit board that consists of the following:

IN =>>>

0.33uF capacitor
...12V regulator without heatsink
......0.1uF capacitor

OUTPUT =>>>>>​

It seems to be all be working fine, just wondering if anyone has any suggestions on it? specially the capacitors for the load of current, should i use different capacitors maybe? Should i use a heatsink on the regulator? it doesnt seem to be getting too hot with a 650mA draw.

any suggestions or help would be greatly appreciated.

thanks

 

[Harald Kapp]

The capacitors are o.k. I see another potential issue:

What kind of regulator did you use?
A car battery has anywhere from 11 V to 14 V (approx.). A linear regulator cannot produce an output voltage higher than the input voltage. On the contrary, some voltage will drop across the regulator. For a low drop regulator this voltage can be as low as ~ 200 mV.
This means that the input voltage has to be at least 12.2 V for correct operation of the regulator. This is o.k., if you battery is always fully charged. But as soon as the battery's voltage drops below that value due to discharging, the regulator will no longer operate correctly.

Harald

 

[john2k]

The capacitors are o.k. I see another potential issue:

What kind of regulator did you use?
A car battery has anywhere from 11 V to 14 V (approx.). A linear regulator cannot produce an output voltage higher than the input voltage. On the contrary, some voltage will drop across the regulator. For a low drop regulator this voltage can be as low as ~ 200 mV.
This means that the input voltage has to be at least 12.2 V for correct operation of the regulator. This is o.k., if you battery is always fully charged. But as soon as the battery's voltage drops below that value due to discharging, the regulator will no longer operate correctly.

Harald

Thanks for your reply. I used a L7812CV regulator. What will happen if the voltage of the car drops? although the regulator will stop working, wouldnt the regulator then just output whatever is coming in? so for example i'm trying to get 12V or less into my device, and since the car voltage is above 12V most of the time the L7812CV will bring it down to 12V but then if car voltage drops below 12V and the L7812CV doesnt operate, will I not just sitll receive whatever volts is coming in?

Also, since i am using a max of 650mA at 12V, can I not just use a 5W zener diode?

Thanks

 

[Harald Kapp]

Chances are that the regulator will just follow the input - minus the regulator's voltage drop. But there is a risk that the regulators starts oscillating. Easy to try...

The 7812 is a particularly unsuitable chip because it has a dropout voltage of 2V - 2.5 V. A LM2940 would have only 0.5V dropout voltage

A zener diode can't be used alone. You need a current limiting series resistor. But the problem remains the same as with the L7812CV.

A good solution would use a buck-boost regulator, e.g. this: http://cds.linear.com/docs/Datasheet/4605fc.pdf
Unfortunately you'd have to do some dimensioning and build the complete regulator from this chip. But it doesn't look difficult.

Harald

 

[john2k]

Thanks for your reply Harald. I made a mistake, i am using a TS7812CZ 12V regulator. I understand that this regulator also has a high voltage drop. But the thing is i am using it to power an LED driver which requires 12v or under to drive LED's in parallel which ends up giving a 3.2v output because the LED's are in parallel. So based on the fact that the driver will take whatever the regulator gives it and the output 3.2v, doesn't that mean it should be ok? I'm using one regulator with a 0.33uF and a 0.1uF capacitor to power one content current driver that outputs 350mA (1-2W output)

 

[CocaCola]

i am using it to power an LED driver which requires 12v or under

What is the "or under" minimum of this driver? If it's say 5 volts then use a 5 volt regulator to power it and avoid all drop out concerns...

Or do the quick and dirty of using 4 (or 5) power diodes in series to drop the maximum 14.5v or so volts to a safe below 12 volt level, but still above say 8 or 9 volts worst case if the battery is in the 12 volt range...

 

[john2k]

What is the "or under" minimum of this driver? If it's say 5 volts then use a 5 volt regulator to power it and avoid all drop out concerns...

Or do the quick and dirty of using 4 (or 5) power diodes in series to drop the maximum 14.5v or so volts to a safe below 12 volt level, but still above say 8 or 9 volts worst case if the battery is in the 12 volt range...

There is no minimum rating on it, the input says 12vdc. but i've run the driver on 9vdc and it works.

If my driver draws lets say 400mA current, and i regulate with a 12V regulator that has a 2V dropout. then the lowest it would go to would be 10v which the driver still works with. However, is the 0.33uF capacitor before the regulator and the 0.1uF capacitor after the regulator the correct size capacitors to be able to deal with 400mA of current?

 

[CocaCola]

i regulate with a 12V regulator that has a 2V dropout. then the lowest it would go to would be 10v which the driver still works with.

That is not how dropout works, a 2v drop out means that the 12 volt regulator is going to want 14 volts minimum, to put out 12v anything bellow that +2v threshold will result in irregular behavior... It might just pass the lower voltage or it might go all sorts of wonky and/or cut out...

Do you have any additional information on this 'driver' you are using? If it's not sealed tight have you looked inside to see if it already has voltage regulation?

 

[john2k]

That is not how dropout works, a 2v drop out means that the 12 volt regulator is going to want 14 volts minimum, to put out 12v anything bellow that +2v threshold will result in irregular behavior... It might just pass the lower voltage or it might go all sorts of wonky and/or cut out...

Do you have any additional information on this 'driver' you are using? If it's not sealed tight have you looked inside to see if it already has voltage regulation?

Oh that makes sense now. I do not have any more info on the driver its a sealed box that says 12vdc input 3-9v output 1-2W. Definitely doesn't have a regulator inside. way too small to have a regulator as well. So is a safer bet to maybe use a 9V regulator or a 12V low dropout regulator? because a 9V with a 2V dropout will mean it would want minimum of 11V in which should be fine in a car?

 

[CocaCola]

Definitely doesn't have a regulator inside.

I wouldn't bet on that without further investigation...

way too small to have a regulator as well.

There are some pretty small smd regulators out there...

So is a safer bet to maybe use a 9V regulator or a 12V low dropout regulator? because a 9V with a 2V dropout will mean it would want minimum of 11V in which should be fine in a car?

Yeah a 9 volt regulator would be idea if the device runs on that voltage fine...

 

[john2k]

Yeah a 9 volt regulator would be idea if the device runs on that voltage fine...

The thing is that the device says the input is 12vdc but it does run on 9V as I have been running it on a 9V regulator as well for 4 months. Will powering it at lower voltage cause any damage to the device? I am thinking it would either not work with a lower voltage and if it does then it should be safer?

 

[CocaCola]

The thing is that the device says the input is 12vdc but it does run on 9V as I have been running it on a 9V regulator as well for 4 months. Will powering it at lower voltage cause any damage to the device? I am thinking it would either not work with a lower voltage and if it does then it should be safer?

It depends, I have no idea of what the mystery driver is let along what is inside it, basically I know nothing that would even begin to allow me to take an educated guess...

There are a ton of devices that have a stated input, but they will work fine at other voltages due to regulation inside, but there are others that have not regulation and expect the power supply to regulated... Pretty much a flip of the coin without any actually knowledge of the circuit...

 

[john2k]

It depends, I have no idea of what the mystery driver is let along what is inside it, basically I know nothing that would even begin to allow me to take an educated guess...

There are a ton of devices that have a stated input, but they will work fine at other voltages due to regulation inside, but there are others that have not regulation and expect the power supply to regulated... Pretty much a flip of the coin without any actually knowledge of the circuit...

Oh, I will try and see if i can dig up some info about it. but because its a sealed little plastic box with 4 wires coming out of it with a sticker on it that says Input 12Vdc output 3-9v. So by provided voltage under the stated input e.g. 9V instead of 12V does a device with for example no regulation not simply just work or does it also have a chance of failing with under-voltage as well as over voltage?

 

[CocaCola]

So by provided voltage under the stated input e.g. 9V instead of 12V does a device with for example no regulation not simply just work or does it also have a chance of failing with under-voltage as well as over voltage?

It could work, it could not, it could damage it, it could not...

Every device is unique in design you can't generalize an answer unless you are working with known devices/circuits...

 

[john2k]

It could work, it could not, it could damage it, it could not...

Every device is unique in design you can't generalize an answer unless you are working with known devices/circuits...

Thanks. very much appreciate your help. I've emailed the company i bought it from to see if i can get the technical datasheet for it.

 

[john2k]

are the capacitors i am using with the regulators good enough for a 400mA current draw?

 

[davenn]

are the capacitors i am using with the regulators good enough for a 400mA current draw?

from a battery supply yes they are OK, its really when you have a supply that is derived from a transformer and bridge rectifier that you need decent a sized cap on the input to the regulator tule of thumb is ~ 1000uF per Amp
in your case you have a 0.33uF on the input probablty wouldnt hurt to increase it to a few uF say somewhere between 1 and 10 uF

cheers
Dave

 

[CocaCola]

are the capacitors i am using with the regulators good enough for a 400mA current draw?

The datasheet for TS7812CZ suggest 0.33µF on the supply side and 0.1µF on the output, but it also states they are not always necessary but if you use those values you should be fine...

 

[john2k]

Thanks for your replies. I was able to get the following information from the supplier about the LED driver (my driver is the first one which is 1-2W)

Note:
Supply up to 2pcs of 1W 350mA LEDs wired in series and total value of Vf should be under rated DC Voltage.


But the thing is the LED's I am powering with this consist of 3 LED's wired in parallel. And because they are wired in parallel when I check the voltage draw with a multi-meter on the output of the driver, the voltage draw reads 3.4V and if I wire only 2 LED's in series as per the datasheet, the voltage draw reads 6.8V. Therefore, I understand why they say 2pcs in series because 3pcs in series will add up to 10.2V which is higher than the output of the driver. However, since my LED setup consists of 3 LED's in parallel, the driver is only using 3.4V and 350mA. so it does seem like the driver can adjust voltage depending on parallel or series. It's as it is treating parallel led's as a single LED. Which I assume is fine because in a parallel setup the 350mA of current will get divided by the 3 LED's.

Just wondering why they say to wire in series. Also they say to supply only 2 pcs of led's wired in series but adding an extra LED will only reduce the intensity but should still work shouldnt it?

 

[john2k]

I'm facing a bit of a dilemma here and hoping someone can help.

My LED Constant Current driver has a Constant Current output of 350mA. The input for the driver is specified as 12Vdc (not a range, just 12v). I am using it on a car DC supply and because a car voltage is usually 13.8 - 14.4 volts, i connected it via 12V regulator L7812CV. Problem is that i've just learnt that the regulators have a 2V voltage dropout. So that means for the 12V L7812CV to work it needs 14V minimum. but a car voltage is not always going to be at 14V. I've just learnt, actually from a test i done over a period of driving my car the voltage is at 13.8 more often. So from what I am told, my 12V regulator will not function as it needs 2V above the output voltage. I measured the voltage in and out and when voltage drops below 13.8v on the input of the regulator the output ends up lower than 12v. So it does seem like the 12V regulator still works but just outputs less.

I was thinking of playing it safe and using a 9V regulator instead. A 9V regulator will only need a minium of 11V which means even when car is at its lowest voltage it will still be ok. Driver seems to work fine with 9V even though it says input 12V. But problem is when I use a 9V regulator with my driver, the regulator warms up quite a bit. So I have calculated that if the car voltage is at 13.8V and the output voltage is 9V and the current draw is 0.35A then the Power lost (in watts) is 1.68W. I think a 9V or 10V regulator is good option for me but I am assuming a power loss of 1.68W via a regulator will probably need a heatsink. Whereas the closer the regulated output is to the input voltage the less heat would be generated, so if I use the 12V version the power loss in watts with be a max of 0.84W which I dont think would need a heatsink. The problem is that my regulator and driver is stored inside a water proof plastic box. I was told by my local electronics store that attaching a heatsink to a regulator sealed inside a plastic box is fine as long as the heatsink is not going to need to disipate too much heat as the main purpose is to transfer heat away from the regulator to a larger surface area. I am concerned about the heatsink in a plastic box.

Can anyone please help?

Thanks