# Using transistor to switch 12v load from a 24v signal

Discussion in 'The Projects Forum' started by Logrod, Jun 28, 2013.

1. ### Logrod Thread Starter New Member

Jun 28, 2013
16
2
Hi I appreciate that my question maybe a bit basic but I'm trying to get back into electronics after a large number of years away, so please be gentle with the replies

I need to collect the electrics of a 12v bike trailer (turn indicators, stop lights etc) to a towing vehicle with a 24v system. I can't just change the bulbs as I still need to use the trailer with a 12V car as well. I could just use relays (and my girlfriend wonders why if I have a solution I need to find another ) but I'd like to prove to myself that I can do it with transistors. I'd like someone to tell me whether this will work and hopefully give me some advice on what transistors to use. The diagram shows the circuit for each of the terminals (some drive 5W bulbs so the resistor values would change). With a 12V 21W bulb I'm assuming I need a transistor with an Ice max of approx 2A, and assuming a gain of 100, I'd need a base current of 0.02A. I didn't originally have R2 and I was assuming that R1 would need to be 24/0.02 = 1200Ω, but I think that would leave me with the base being at a higher voltage than the collector which would I think be an issue.
So I thought of adding R2 and as long as R1 is > R2 then the voltage at the base should be <12V (have I got that the right way around?).
I'm then struggling as to what value R1 should be and as to whether i need a third resistor between the base and the voltage divider?
I'd be grateful for any help/push in the right direction.
Also could someone suggest what transistors I should look at (eg BD 135, BD 437)

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Go to shop and buy yourself a proper mosfet transistor. For example Buz10, buz11 or IRFZ44. Then you no need to worry about beta and the base current.
and simply use R2 = 10K and R2 = 10K.

Last edited: Jun 28, 2013
3. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Most of a BJT will have a beta lower then 100. Also when BJT work as a switch (in saturation region) we need more current.

BJT base-emitter junction behavior just like a diode. So base voltage will be in renege 0.5V..1V depend on the base current and collector current.
See figure 3 on BD433 datasheet
http://www.fairchildsemi.com/ds/BD/BD433.pdf

Also notice that Hfe for Ic = 2A has a typical value 75 and 50 min.
So your base current need to be greater than 2A/50 = 40mA
Let as assume two times larger (80mA). Now we can find R1 value.
R1 = (12V - 0.7V)/80mA = 140Ω and the power rating for the resistor.
P = 80mA^2 * 140Ω = 1W. And R2 you can use 10K resistor.
The power dissipation in the BJT will be equal to
Ptot = Ic * Vce(sat) = 2A *0.5V = 1W so you need mount the BJT on the small heatsink.

Last edited: Jun 28, 2013
Logrod likes this.
4. ### Logrod Thread Starter New Member

Jun 28, 2013
16
2
Thanks Jony, that was very helpful, and you even answered a question that I hadn't got around to asking about how to calculate power dissipated in the transistor!
If I used a mosfet as you suggested in your first post am I right that as both resistors would be 10k current would be so low that I'd be ok using normal .25W resistors?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Yes you can use 1/4W (0.25W) resistors in renege between 1K to 10K

Pros
- no DC gate current is needed. MOSFET is a voltage control device.
- power dissipation in MOSFET is very low thanks to low Rds(on) resistance.
- no heatsink is needed.

Cons
I don't see any disadvantages. Maybe expect the price because mosfet are more expensive.

6. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
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I'd just use a 24V relay.

7. ### sheldons Well-Known Member

Oct 26, 2011
616
101
Heres 2 possible examples-one using a fet to switch a load and the other a transistor with a relay to switch a load -its not set in stone and its your choice....

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8. ### wayneh Expert

Sep 9, 2010
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I basically agree and would do the same, but don't you have to be a bit more concerned about the dirtiness of the voltage supply? The right MOSFET will be fine but I think they can be a bit more sensitive than a BJT. Not an argument for a BJT for this application, I'm just sayin'.

9. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Could you explain what you mean by saying "dirtiness of the voltage supply".
I know that small-signal MOSFET are very extremely ESD sensitive. But I have no problem with ESD in power MOSFET. Also in switching application I have no problem with MOSFET's. In linear power supply (and linear applications) I notice some problem with MOSFET. They are very prone to parasitic oscillation at high frequencies. And also they don't like overload condition. And in the end I decided to use some old KD501 BJT in my PSU instead of a MOSFET.

10. ### wayneh Expert

Sep 9, 2010
12,156
3,063
I believe the OP is using this on a trailer, thus powered by an automotive system. These can have large (60V) transients that need to be designed around. Or, just live with the occasional failure and repair.

11. ### Logrod Thread Starter New Member

Jun 28, 2013
16
2
Thanks all for the feedback. With regard to the "dirtiness of the voltage supply", yes it's powered by an automotive system (albeit one that probably has a cleaner supply than most with shielded cables for reasons I can go into if you want)..

Can I deal with the worst of these just by sticking a capacitor or two between the "signal" (signal in this case is on/off even for the indicators v low frequency) and ground lines?

What sort of characteristics should I look for in the mosfets (I see 20+ items in the spec data sheets) in order to minimise the risk of failure?

12. ### Logrod Thread Starter New Member

Jun 28, 2013
16
2
Wayneh could you explain to me why this thread would be contrary to the T&S please?

13. ### wayneh Expert

Sep 9, 2010
12,156
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Automotive modifications on any kind are not allowed for topics here. I would think trailer lighting falls within that. Maybe a grey area. You've made it this far!

14. ### Relayer New Member

Jan 1, 2013
18
1
Logrod, your circuit would never work. Why?
Where are you obtaining the 12 volts to power the bulb? i.e The 12V rail indicated in your diagram.
Regards,
Relayer

15. ### Relayer New Member

Jan 1, 2013
18
1
Hmm... Seems I can't edit my last post It must expire or I don't have that option.
Here's a schematic that may work, providing your vehicle that runs off 24 volts has two batteries in series.

I hope the above helps.
Regards,
Relayer

16. ### Logrod Thread Starter New Member

Jun 28, 2013
16
2
Relayer,

I have 12V available in the vehicle (in fact it's from the mid tap between the two 12 volt batteries, but I considered just using the 24 V loom and adding either a power regulator or a 24 -> 12 V DC-DC converter) so I'm fairly certain it will work. From your diagram you seem to be suggesting exactly what I proposed originally with the exception of the use of TIP122 darlington transistors or have I misunderstood? What would be the advantage/disadvantage of your solution over either a simple BJT transistor or a Mosfet?

17. ### wayneh Expert

Sep 9, 2010
12,156
3,063
A darlington requires less base current for a given C-E current. A MOSFET requires virtually none. That's it. For this application, everything else is about heat dissipation, ruggedness and so forth. Personally, I would find the right MOSFET. It will have the lowest heat dissipation and probably won't require a heat sink. Did you say what amperage you need to switch? Actually, a standard automotive relay wouldn't be a bad choice either. It uses more current than even a transistor but is purpose-built to switch these sorts of loads under these conditions.

18. ### Logrod Thread Starter New Member

Jun 28, 2013
16
2
Wayneh,

It's a maximum of a 21W 12V bulb (some are smaller wattage), but I think one of them will have two in parallel so approx 4 amps max. I may end up going back to a relay as the easiest solution but I felt it was 'inelegant' and wanted to do it better