# Help with multi-voltage dc input

Discussion in 'Electronics Resources' started by bytraper, Sep 28, 2010.

1. ### bytraper Thread Starter Active Member

Sep 28, 2010
126
4
Hi everyone,
I have a problem I'm really not sure how to properly solve.

I have a dc 12-48v rail (the voltage will always be either dc 12v, 24v, 36v or 48v) and can switch between them at any time. I have 12v relays using multiple jumpers and resistors at the moment doing the trick but its really just a band aid solution and its painful to change all the time. Each relays draws 100ma (so 400mA total).

Has anyone got any ideas on how to do this simply to make it more automatic while keeping some sort of efficiency?

I'm only a learner with electronics and this may seem simple to some but I'd appreciate any help I can get

2. ### majsyd2010 Member

Aug 29, 2010
61
1
I really didn't understand what you trying to do ... Would you be able to write a bit more on the main principal of your project n relation to power supply ..

3. ### bytraper Thread Starter Active Member

Sep 28, 2010
126
4
The supply voltage is either 12v, 24v, 36v or 48v DC (1-4 batteries)

I want to know how I can make the power to the relay coils always 12 Volts rather than having to set jumpers and resistors every time that I change the voltage at the supply?

4. ### Wendy Moderator

Mar 24, 2008
20,765
2,536
Just ground the X2 top relays, and connect the bottom 2 relays to 12V, for a X4 relay configuration.

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Looking at it I suspect you are leaving key information out, like when do you wat to activate the relays.

Last edited: Sep 29, 2010
5. ### bytraper Thread Starter Active Member

Sep 28, 2010
126
4
Nope, theres no key information left out. Its exactly as I asked.
If i linked the top and bottom for 12v the relay coils would blow when I put 24v through the system.

My question would have been better rephrased

"Whats the most simple and efficient way to drop an input of 12v-48v to ~12v with 400ma to power 4x coil relays".

Cheers and thanks for the help so far!

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Unless you use a switch mode control current regulator then you are stuck with a linear solution - with the attendant wasted heat problem. A simple linear solution (other than using straight switched resistors) would involve a power transistor regulator to control the current supplied over the full range variable DC source. You would have to deal with the transistor power dissipation - around 14.4Watts with the supply at 48V and 400mA total current. But at least you wouldn't have to switch things on the fly as the supply DC value changed.

Cost wise - A 'simple' PWM regulator (=lower losses) isn't out of the question either. What's your budget for this?

7. ### bytraper Thread Starter Active Member

Sep 28, 2010
126
4
I'm hoping to keep it as simple, efficient and cost effective as possible, The heat and wasted power with the linear was daunting and would waste a lot of power when at 48v.. but 14.4 watts if something I can live with.

I can probably even limit the input to 12v-40v

8. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,769
969
You will need to create a power supply for the relays. This power supply must be fixed 12V output with a nominal input range of 12 to 48v. This is far from a simple problem.
Now if you could limit the voltage to 24-48VDC or only 12-24 your problem goes away as there are many relays that can tolerate that. For example I use a 24V relay from Hasco that will operate just fine from 20 to 48V (drop out stated as 18V,max listed at 70V)

9. ### bytraper Thread Starter Active Member

Sep 28, 2010
126
4
Thats a really good idea, i can have 2 versions 12-24 and 36-48 or even 24-48