# Voltage drop switch

Discussion in 'The Projects Forum' started by skunky, May 5, 2006.

1. ### skunky Thread Starter New Member

May 5, 2006
9
0
hi, I am working on a lil project which will automatically shut off when a voltage drop is detected. I know this is just as easy as a low battery indicator but instead of driving a LED I need a relay to turn on a high current device so I use a relay but I can't figure out which transitor to turn on the relay.

Enviroment:

For a high current application >15A
when voltage drop from 14 to 13, power OFF.
otherwise ON.
so 14V ON --> 13V OFF

Here's what I came up but need some suggestion.

a NPN transistor
13V Zdiode
12V relay + 1N4001

I am stuck on which transistor I should use. Can anyone help?

2. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
How much current does the coil of your relay need? If 0.5A or less, try a 2N2222.

3. ### skunky Thread Starter New Member

May 5, 2006
9
0
Thanks, The relay needs about 60mA( 12V, 201 Ω )
I checked the spec of the base voltage of 2N2222 is around 1.3V but I need it to precisely turn off when it's below 13V instead of partially off.

What's the minimum I and V require to activate the transistor?

I did some calculation here:

I(base) = 72mA/ 100 = 0.72mA = 0.00072A

Vcc= 14V
Zdiode = 13V
(14-13)/0.00072 = 1388 Ω (R1)

Is the minimum base current correct? so anything belows 0.72mA would turn off the transitor or partially off?

Thanks

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
I think your circuit would benefit from a base-return resistor. A base-return resistor is a resistor that is connected between the base and ground. A base-return resistor insures that current is pulled out of the base when you need the transistor to turn off.

hgmjr

5. ### skunky Thread Starter New Member

May 5, 2006
9
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Thanks, how big of the resistor should I use ? Power dissipation is an issue so as low as possible for energy saving purpose.

6. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
Perhaps I misunderstand, but won't the relay open if Ic drops below sustaining current for the coil?

You could use a potentiameter on your base to fine-tune for your operating requirements.

Power dissipation shouldn't be much of an issue when the voltage across the potentiameter (or voltage divider, if no potentiameter is chosen) is only a volt or two.

7. ### skunky Thread Starter New Member

May 5, 2006
9
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that 's what I am concerning and a potentiameter sounds like a good idea since no transistors are 100% acurate as their spec shown. Thanks.

Anyone know how big of a base-return resistor I should use ?

8. ### Gorgon Senior Member

Aug 14, 2005
113
0
Hi,
First of all, change your zener diode to 12V, this will give you headroom for the Vbe of 0.6-0.7V of the transistor. Use a 1k pot with the tap connected to the base to trim the release point at 13V.

TOK

9. ### skunky Thread Starter New Member

May 5, 2006
9
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Thanks, 2V should be enough to fine tune the transistor. Should I also use a potentialmeter for base-return resistor ? I've included my new schematic, is this correct ?

10. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
You really should have some hysteresis, to keep the relay from chattering, and you at least need a resistor in series with the pot or with the base, to limit base current when you run the pot to the zero ohms end. Here is a circuit with hysteresis that will switch cleanly and be very temperature stable.

11. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
This is what I had in mind when suggesting a potentiameter:
[attachmentid=1418]

12. ### skunky Thread Starter New Member

May 5, 2006
9
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BIG thanks ! I'll build my prototype see how it goes. Just wondering...is there any chip that does this whole thing ?

Thanks

Jan 10, 2006
613
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I agree with Ron H. You may find that as soon as the load is disconnected the battery voltage may rise enough to switch the transistor back on so the circuit may cycle.
Looking at Ron's diagram, thats pretty much what I would use, although I think I'd be putting a resistor inline with the Transistor base, and I'm pretty sure that if you reduce R2 (1M) you should be able to increase the Hysteresis (differentual).

14. ### skunky Thread Starter New Member

May 5, 2006
9
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thanks again for the help, I've looked over every electronic supplier here but none of them carry 1N5994B , is there any alternative I can use? and the 10k potentiometer i found seems a bit too big, can I use a 10k trimpot instead ?

Thanks

15. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
Try 1N4734 or BZX84B5V6 for the 5.6v zener.

Trim pot would be my first choice. Set for optimum operation and lock in with a wee drop of nail polish.

Dec 2, 2005
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17. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
LM393 has an open collector output. You don't need another base resistor.
And you are right about increasing the hysteresis.