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Old 25th August 2003, 01:22 PM   #1
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Default Regulator for fan, relays...

Hi,

I need to derive 12Vdc from my transformer which has a rectified/filtered output of 55V. Will the attached regulator work? It will only be used to power the fan and relays for the protection circuit. Maximum output current must be around 750mA which I think the regulators can easily handle.

Any comments?

JojoD
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Old 25th August 2003, 01:25 PM   #2
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Check the max voltage of the 7824! Some types can take 60 volts other take less....

Use a LM317HV for example. You could also use a series resistor to burn power, together with a 7812.
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Old 25th August 2003, 01:52 PM   #3
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Default Oooopppssss!

Hi,

The 7824 would probably "eject" itself out of the pcb!

Okay here's a new one.

LM317T Input Output Voltage Diff. (Vin-Vout) = 40V

In this case, my LM317 output is 28V which is set by 220Ohms and 4.7K ohms resistors,

then 55V-28V=27V so I'm below the max input for the LM317T which is 40V.

LM7812T Max Input Voltage = 35V;
in this circuit my input to the 7812 is the output of the 317 which is 28V.

Will this work?

JojoD
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Old 25th August 2003, 02:36 PM   #4
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Why not use some cheap darlington, with zener? Use 13V zener, and one darlington power transistor, such as BD651. You don't need two regulators, and the quality of the 12V is no very critical.

Sajti
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Old 26th August 2003, 08:47 AM   #5
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sajti,

I tried your suggestion and it worked. i used a tip29c since it was readily available. However, the transistor was very hot, even with a large heatsink. I was only drawing about 500mA.

JojoD
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Old 26th August 2003, 09:35 AM   #6
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Quote:
Originally posted by JojoD818
sajti,

I tried your suggestion and it worked. i used a tip29c since it was readily available. However, the transistor was very hot, even with a large heatsink. I was only drawing about 500mA.

JojoD
This is OK. If You check the dissipation is: (55V-12V)*0.5A=21.5W But the overall dissipation will be same with the regulators too. You can reduce the dissipation if You put some series resistor before the series-pass transistor. 33-47ohms 15-20W looks OK.
And You can avoid all the regulators with one series resistor:
R=(55V-12V)/0.5A=86ohms. The real value can be 91ohms, with 30W.

Sajti
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Old 26th August 2003, 12:51 PM   #7
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jojo,
for your circuit with lm317 you need zener diode protection for the lm317 because at switch-on the 40V rating is exceeded until the cap charges up to 15V !

even with resistors you get the same power dissipation, just more in the resistor and less in the device, just to clarify.
you can also use more transistors in parallel to ease dissipation requirements per transistor and get improved heat transfer to the heatsink.

sajti is right, you don't need regulators for fans and relays. these devices are very tolerant of voltage changes and work perfectly with unregulated voltage.
but burning up so much power in a resistor is not very elegant..
better use a second transformer!
or a buck converter.. !
regards
keyne
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Old 26th August 2003, 01:27 PM   #8
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There are some 24V, or 48V fans. They are better for this application!

Sajti
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Old 26th August 2003, 02:27 PM   #9
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Default Just what I thought...

Hi guys!

Adding another transformer for this application is exactly what I'm trying to avoid but still considers it if all else fails.

As of now, I used two tip29c with a resistor and it seems to be fine. Heatsink is hot but can be touched. Running for 2 hours now with a fan and a couple of relays, drawing around 450mA. Seems ok now.

Thanks guys!

JojoD
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Old 26th August 2003, 05:40 PM   #10
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Default Maybe this is simpler

I don't like this solution of generating heat to move a fan whose purpose is to remove heat -- not particularly a "green" solution.

National Semi has an application note on using the LM3524 controller for fan speed

http://www.national.com/an/AN/AN-292.pdf

with a little creativity you can configure the LM3524 to both act as a buck regulator, or a flyback converter and fan speed controller. Just use an ohmic sensor in the feedback loop.

The garden variety LM3524 will take up to 40 Volts so you will need to use a little beefier transistor to stand off the higher voltages.
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