design a potential divider

Thread Starter

matelot

Joined Apr 15, 2013
34
I hope this has a simple answer.
I would like to drop 24v to 12v and think a potential divider would be the best way.
Just 2 resistors the same size, but what value?
I have tried to use a 7812 as I was under the impression that the motor I wanted to run for a short period of time was only 500mA but it burned the regulator out.
I have used a 12v battery and measured the motor and it goes up close to 1A now it is installed and running under load.
Is there a reason for what ever value the resistors are? I assume I would be ok with 21 watt resistors?
I have found 2 x 330 Ohm resistors, I am not at the site so just fit and test is not practical but I just wondered if there is a reason for resistor value when calculating potential divider design
Thanks.
 

wayneh

Joined Sep 9, 2010
16,399
Resistors won't work. You need a regulator but one that can handle the current.

Resistors can work in a situation where the load current is small (like <10%) compared to the current passing through the divider. This ensures that the load does not greatly influence the voltage of the resistor divider. If the load current is constant and known and "small", it's like a resistor in parallel with one of the resistors in the divider, and voltage can be set with a decent precision. But the usual case is a varying load which causes voltage to vary as well.
 

Thread Starter

matelot

Joined Apr 15, 2013
34
I read your reply, thanks for the quick response, and went straight to an online shop.
I found L78s12cv at 2 amps. I was unaware there were 2 amp 78 series regulators.
I coudn't find anything bigger, does a regulator exist bigger than 2amp?
Of course a divider would be influenced by the load as one of the resistors and not divide equally!!!
 

Thread Starter

matelot

Joined Apr 15, 2013
34
bare, would it have helped much, it burned out in a few secs?
now you have me thinking, i need it to run a motor for about 10 secs say every hour, i ran it back and forth about four times to test it and it stopped, maybe the regulator tripped and protected itself . I havn't checked if it is ok now and maybe one run every hour with a heat sink would be ok. what do you think? i will check it now.
It still produces 12v. I will try it with a heat sink or maybe just get a 78s12 for 99p and use that with a heat sink
 
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tracecom

Joined Apr 16, 2010
3,944
I am concerned that you say it "burned out." The LM78xx series of regulators have internal protection against over current conditions, and normally shut themselves down temporarily.

However, the starting current on a motor can be much higher than the running current, and may have been so high as to heat the LM78xx regulator almost instantly, and cause it to shut down very quickly. If that is the case, heat sinking probably will not avoid the problem. You will need to add power transistors (e.g., 2N3055) with heat sinks to the regulator.
 

tracecom

Joined Apr 16, 2010
3,944
bare, would it have helped much, it burned out in a few secs?
now you have me thinking, i need it to run a motor for about 10 secs say every hour, i ran it back and forth about four times to test it and it stopped, maybe the regulator tripped and protected itself . I havn't checked if it is ok now and maybe one run every hour with a heat sink would be ok. what do you think? i will check it now.
It still produces 12v. I will try it with a heat sink or maybe just get a 78s12 for 99p and use that with a heat sink
That's a different situation. If the regulator worked two or three times before it quit, then adding a heat sink (and maybe a fan) will help.
 

Thread Starter

matelot

Joined Apr 15, 2013
34
I would be interested in adding a power transistor if only for the experience would you mind giving me some lead as to how to wire that?
There is a serious application but I am involved as much for the pleasure of the experience.
I have been googling 2n3055 and it occurs to me I would need a large current 12v supply for the 2n3055 to drive using my 12 v on the gate, I don't have a large current 12v supply. Am I right on this?
I am feeding from a 24v supply at around 5amps would there be another alternative?
The motor runs to one end and then internal switches (I am assume mechanical) stop it, if I then switch a relay with the output 'crossed' it supplies the 12v the other way and the motor runs back to the stops. I am assuming when not running there would be no load and the regulator, though on, would be drawing little current.
 
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