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Neeed to generate 5-6DCV @10ma from a 75V DC supply?

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Newbie level 6
Jul 12, 2001
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Hi all,

Anyone have a suggestion on how to create a low amp (10-20ma) 5-6Vdc reference from a 40DCV - 76DCV supply. Regulation does not have to be that great.

Thanks in advance


I would bring the voltage down using a resistor and a zener. Then use a constant current generator off the zener, Us a regulator as they are more stable than a descreat Constant Current generator

I agree with barrybear, although if you don't have to have very precise current you can even omit the current regulator and go with a simple resistor and a zener.

This is the cheapest solution and is enough for most microcontrollers or logic ic's if this is what you want to use it with.

Personally, I've never felt comfortable with
zener diodes. I always try to use the 78xx
family when I want a stabilised voltage.
Although I don't know if this is the case,
because the voltage is relatively high and you'll have to check the '7805'(+5V DC) breakdown voltage


Ok for the zener solution but be carreful to well dimension the resistor and the m@x current supported by the zener according to the current consumption of your board.
Because if your board doesn't sink enough current, this current will be dissipated in the zener and then this one can burn !!!


I think zener and resistor is the best solution as 78xx will not work at 76dcv
so you can use zener resistor or series pass transistor pluse zener

Be extra careful if u use 7805. It cannot take 48Vdc in...nor 70VDc..
Zener is a better bet..
How about a simple switcher?


it's not so easy to use a switcher from 75V to generate 5V. Most switchers work from 24V or 35V.
I think you have 2 amother ways:
a) if you have a another source voltage with more wattage then use step up switcher.
b) use a good MOSFET and make a DC from your own PWM with MOSFET +R +C and step down to 24V or 35V - then use switcher (or 78xx) to 5V.


I forget to tell that you can use any dc/dc converter topolgy

for step down you need buck or buckboost topology

m@xim hzve alot of solution (chips) that perform these topologies with just resistor and transistor (sometimes not needed) ,diod and coil

not more it's easy i used it so many times

You can also look on the web site from or the website fron national.
They have solutions for dc-dc converters wich have a supply of 75VDC.

Good luck

Personally, I've never felt comfortable with
zener diodes. I always try to use the 78xx
family when I want a stabilised voltage.

Use BOTH: A resistor + 20V zener, followed
by a normal 78L05!

I agree to use both zener and 7805. Use zener to bring down the input voltage and use 7805 to regulate the output. For this low power application, switching DC-DC may cause more trouble, more cost and not worth the effort.

I agree with the last two members,use both 20V zener followed by a normal 7805 (it suport only 35V in the imput) but remember that if you need 10mA for you circuit and you want your zener working in an approximatly linear zone it should take other 10mA.So your resistor will support 55V and will carry 20mA,it makes 1.1 Watts so it should be at least a 3 watts resistor.Also you need just a regular 1 Watt zener (20V*0.01A=0.2Watts)

sorry for language

I think zener and resistor is the best ,
so you can use zener and resistor series in two pass ,a zener with resistor a 40v by example y other a 5v. But remember that if need 10mA for you circuit, and you zener working ,take other 10mA.your resistor will support 20mA,so it should be calculed the power resistores necesary.

I've have some experience with the 78xx-series of voltage regulators. They are *very* sensitive for over-voltage. I've successfully used the power resistor + zener + 7805 regulator approach. But I use to dimension the circuit so that the zener would only conduct during "worst case" scenario, meaning that under regular operation the voltage drop over the resistor should be so high that the zener do not conduct. The zener is merely only for protecting the 78xx regulator. The above described approach also protects the regulator from any voltage spikes on the input of the circuit.

Also, Micrel do have 5 V-regulators that can take up to 60 V on the input intermittently without going defective. Furthermore, I recall that Texas Instruments did some chip named TL783 that was intended as a high-voltage regulator, it could withstand higher voltages than you mentioned.

Using a zener+resistor+78xx regulator approach can result in some dimensioning difficulties (the dissipation in the zener can rise to a quite high level).


I also think Resistor + Zener is the best choice - in older TV-sets the tunersetvoltage
was made so.
a 4k7 3Watt Resistor + 13V 1,3W Zenerdiode
+78L05 will work fine

greetings from austria shorty

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