# 50VDC to 24VDC????

Discussion in 'The Projects Forum' started by duckdive117, May 21, 2009.

1. ### duckdive117 Thread Starter New Member

May 21, 2009
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0
I am trying to use a solinoid valve that requires 12-24VDC 150mA. I have a transformer that is running a pump on the unit that puts out 50VDC.
I calculated maybe using a resistor I can up with 27.5 OHM and 61.88 Watts.

Is this correct or did I calculate wrong is there a easyer way of doing it? If this is the best way can someone direct me to a place I can Buy them?

Thanks
Josh Greene

Apr 5, 2008
15,805
2,389
Hello,

I think the calculations are wrong.
The wanted voltage drop is 50 - 24 Volts = 26 Volts.
The wanted current is 150 mA.
The resistor needed is 26 Volts / .15 Ampere = 173.333 Ohms. (R=U/I)
The first nearest E-series value is 180 Ohms.
The power will be .15 * .15 * 180 = 4.05 Watts. (P= I*I*R)
Take a resistor of 180 Ohms 10 Watt and you will be safe.

Greetings,
Bertus

3. ### duckdive117 Thread Starter New Member

May 21, 2009
8
0
I placed a 75 ohm 4 Watt resistor on it and it droped it to 19VDC and the resistor gets pretty hot.

Apr 5, 2008
15,805
2,389
Hello,

If you say that a 75 Ohm resistor drops 19 Volts.
The current is 19 / 75 = 0.253 A
The power in the resistor is 0.253 * 0.253 * 75 = 4.8 Watts.
No wonder the resistor is getting very hot.
The current through the solenoid will also be to high.

Greetings,
Bertus

5. ### duckdive117 Thread Starter New Member

May 21, 2009
8
0
I put on a 75ohm 4watt and it drops to 16VDC not 19 sorry if I put another 75 OHM one on it goes up to 20VDC so your calculations seem correct.

Would a 10 Watt not get as hot as a 4 watt?

Apr 5, 2008
15,805
2,389
Hello,

OK. recalculation with 16 vols drop.
The current through the resistor will be 16 / 75 = 0.213 A
The power in the resistor will be 0.213 * 0.213 * 75 = 3.4 Watts.
The 0.213 A (213 mA) is still to high for your solenoid.
You still have 34 volts accross the solsnoid wich is to much.

Greetings,
Bertus

7. ### duckdive117 Thread Starter New Member

May 21, 2009
8
0
Ok a little more info If I put the 75 OHM resistor on by its self I get 16VDC out of the resistor to the solinoid. If I put two 75 OHM resistors in series I Get 145 OHMs and only 12VDC out of the resistors to the solinoid. If I put them in parrell I get 40 OHMS 20VDC out of the resistors.

So would I get higher voltage with a higher watt resistor??? cause according to the calculation (I found out what I did wrong in my original calculation so I now come up with the same numbers as you do) I would be putting a 180 OHM resistor on, but when the resistance is 145 Im only getting 12VDC??

Apr 5, 2008
15,805
2,389
Hello,

Can you make a drawing of the different situations you descibe? (with voltages)
It is a little bit confusing without a drawing.

Greetings,
Bertus

9. ### duckdive117 Thread Starter New Member

May 21, 2009
8
0
OK I hooked it up all three ways and got actuall reading insted of roundng or abouts.

• ###### OHM.JPG
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Apr 5, 2008
15,805
2,389
Hello,

Are the voltages given the voltage accross the solenoid?
If so the solenoid has a lower resistance as expected.

Greetings,
Bertus

11. ### duckdive117 Thread Starter New Member

May 21, 2009
8
0
no its the voltage out of the resistors

Apr 5, 2008
15,805
2,389
Hello,

How did you measure the voltage?
Is it from the resistor to ground?
With what did you measure it?

Greetings,
Bertus

13. ### duckdive117 Thread Starter New Member

May 21, 2009
8
0
I measured the voltage out of the resistors to ground with a multimeter.

Apr 5, 2008
15,805
2,389
Hello,

There must be something connected from the resistor to ground.
If there is nothing connected from the resistor to ground you will not have a voltage drop.
Is the solenoid connected from the resistor to ground?
If not there is someting wrong with the multimeter.

Greetings,
Bertus

15. ### duckdive117 Thread Starter New Member

May 21, 2009
8
0
I am using a transformer that is 50VDC, i am running the positive wire to the resistor. them I am using the output of the resistor on the positive of the meter and the ground output of the transformer on the negative of the meter.

For some reason you may have to zoom in on the pic for it to show up right.

• ###### transforme.JPG
File size:
21 KB
Views:
20

Apr 5, 2008
15,805
2,389
Hello,

I still can not understand why there is a voltage drop if only the multimeter is connected.
What range did you select on the multimeter?
What type of multimeter is it?
What are the ratings of the transformer?
Is there a capacitor on the DC side?
Can you post a photo of the setup?

Greetings,
Bertus