# voltage divider with 150 volt input

Discussion in 'The Projects Forum' started by nimaajbphs, Jul 8, 2011.

1. ### nimaajbphs Thread Starter New Member

Jun 1, 2011
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hello,
I am working on a circuit that needs three different voltages at different parts. I have a 150 volt dc power supply and i need -20V,+20V and +110 V pins to run my circuit. Currently I am using 3 resistors in series with my power supply to produce these voltages. Currently my circuit only needs to draw 15 mA to run. But I need it to be able to draw 300 mA and I don't think that using a resistive voltage devider is the right way of doing it. Trouble is that i don't know how to go about this problem.
Any suggestions or help is much appreciated. Thanks

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2. ### iONic AAC Fanatic!

Nov 16, 2007
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First of all, you can't obtain -20V from a positive source without some other circuitry other than resistors. The ground you drew in between RS4 and RS2 is incorrect. Your current divider will produce 30V across both the 3K resistors (RS4, RS1) and 90V across the 9K resistor (RS3). I know of no Voltage regulators that will accept 150V as input.

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
This is not true if the supply is "floating", or isolated from ground.

If the supply is not grounded, then the ground reference could be applied anywhere one would like to place it.

4. ### iONic AAC Fanatic!

Nov 16, 2007
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I was thinking about that after I posted, but was fast asleep before I could alter it. Thanks.

5. ### CDRIVE Senior Member

Jul 1, 2008
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150V @ 300mA = 45 Watts. Can your supply deliver this?

6. ### nimaajbphs Thread Starter New Member

Jun 1, 2011
14
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Yes the supply is rated for 420 mA at 150 mA
I have found a positive regulator that accepts 150 V as input but I'm not sure how to use it to get the voltages I need
Any suggestions?
Thanks

7. ### kubeek AAC Fanatic!

Sep 20, 2005
4,691
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You mean at 150V right?
Care to post it´s name?

8. ### nimaajbphs Thread Starter New Member

Jun 1, 2011
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Last edited: Jul 8, 2011
9. ### iONic AAC Fanatic!

Nov 16, 2007
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I'm lost. All I see is a Power supply that can output 150V, and for more than \$200. You said in your first post that you have the power supply.

10. ### SgtWookie Expert

Jul 17, 2007
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I think Kubeek meant to ask what's the name and part number of the 150v regulator that was found, not the 150v power supply that you already have.

Jun 1, 2011
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Nov 16, 2007
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13. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Well, after a quick look through the data sheet it appears applicable.

14. ### SgtWookie Expert

Jul 17, 2007
22,183
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There's just that elephant in the room called "power dissipation".

Our OP says he needs 300mA current - although I don't know if that's total or each voltage needs 300mA available.

Just for grins, let's start with the 110v output. And let's say we're going to call the negative terminal of the supply ground.

So, the power dissipation in the regulator would be roughly (150v-110v)*300mA = 12 Watts, which might be do-able with a good-sized heat sink.

(150v-20v)*300mA = 39 Watts.
That is a heck of a lot of heat to get rid of.
For the TO220 package, junction to package is 3°C/Watt, so in order to keep the regulator from overheating at 125°C, you would have to keep the package at 8°C/46.4°F or lower (8°C+3°C*39W=125°C, alternatively 125°C-3°C*39W=8°C).
Also, 130V differential from in to out exceeds the 125v maximum.
It's also at the maximum output current for the junction temp being high; which it'll be.

Last edited: Jul 9, 2011
15. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
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You just verified my reluctance to commit to this circuit. Too much heat and at the extremes of the IC's limits. Maybe if he cools it with liguid nitro, he might make it last for a few months!

16. ### nimaajbphs Thread Starter New Member

Jun 1, 2011
14
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Good point =/
Alright given this information I've decided to go a different route. I will be using a second power supply to provide the -30V and +30V and I will use the 150V power supply to provide the high voltage I need.

On a related note does anyone know of any opamps that operate with up to 150 voltage difference as supply?
using another opamp seems to be the easier way to go. FYI the opamp will be used to drive a sinusoidal signal from 0 to 100 Volts at maximum frequency of 200 Hz and the load will draw roughly 25 mA on average so power dissipation should be managable I think.

Thanks for the responses everyone.

17. ### CDRIVE Senior Member

Jul 1, 2008
2,223
99
There's a job waiting for you in politics. How else do you expect to accumulate a good resume of miss-speaks?

Glad to hear that nimaajbphs has opted for the minimal waste approach though.

18. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
MSKennedy makes high-voltage opamps.
Just for example, here's one datasheet:
http://www.mskennedy.com/client_images/catalog19680/pages/files/130rc.pdf
Main page: http://www.mskennedy.com

Actually, that seems to be their best-suited opamp by supply voltage alone for what you're doing.

It's not going to be cheap.

You'll probably find it much less expensive to use a switching PWM controller to drive a transformer, supplying the sine wave to the reference and using some sort of optocoupler feedback from the secondary side. It'll take a good bit of work to come up with a decent design, though.

19. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
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Perhaps I should offer my resume to Sarah Palin!!!

The word I really wanted was not reluctance or even reluctant, but rather reservation. ...more political spin, I know! Ye Haa, I might have a real chance!