Hopefully simple question (BJT switch) 1

[RITelectric]

I 'm trying to design a circuit which routes an input voltage (somewhere between 24 and 100 Volts) by switching means to one of two outputs using power transistors. I want to minimize the design. Can it be done in one stage, and if so, what configuration would be most appropriate?

 

[nbucska]

Use FET-s to switch. Need 2 FET-s and one inverter since
when one FET in on, the other is off, i.e. at least
2 bipolar transistors.

You can use 2 Solid State Switches.
<nbucska@pcperipherals.com>

 

[Lewish]

How much current are you needing to switch?

 

[RITelectric]

No more than 1A peak. Figure 500mA constant operation.

 

[melone]

How much transitor drop can you handle?

 

[RITelectric]

The transistor voltage drop is no big concern for the application I'm working on. I can adjust the surrounding circuit to accommodate for any losses associated with the transistor.

 

[melone]

What happens if both supplys are connected at the same time? What happens if both supplies are disconnected at the same time? How quickly do you have to switch between supplies? How much capacitance is in your circuit? How much leakage can you tolerate? What about reverse polarity protection?

 

[RITelectric]

there is no 'both supplys' There is one supply. There are two sub-circuits. Power from the supply needs to be fed to one sub-circuit at a time, switched by a power transistor (or two, or whatever number of transistors it takes to accomplish the task as simply as possible). Switching time, capacitance, leakage, reverse polarity, are all insignificant to my question. My only question is: is it possible, and if so, how would I connect my 'parts' to the transistor: ie, what connects to the base, emmitter, and collector.

Chances are, it might not be possible the way I'm expecting it to be, so please clear up any flaws in my approach.

 

[IRstuff]

assuming that you're starting with a TTL signal, you'll need something like a 7404 hex inverter, 2 npn transistors, 2 pnp transistors, 4 10K resistors.

2 of the inverters wired in series. each output goes to an RTL inverter. The RTL inverter has base connected to 7404 output with 10K to Vcc, emitter goes to Vss, collector goes to 10K to your Vhi.

Each RTL inverter collector drives a PNP base with 10K to Vss. PNP emitter goes to Vhi, collector goes to load. The PNP stages are negative logic RTL. This will allow your outputs to go within Vsat of the Vhi, so power dissipation will not be an issue.

TTFN

 

[melone]

You might want to think twice about how &quot;insignifigant&quot; those questions are to your problem. If you connect your two circuits together, either directly or indirectly, you might not like the results.

But in answer to your question, OF COURSE it can be done. However, you better start thinking about power dissapation, current, switching time, etc. otherwise, when you hook this circuit up, you might see smoke.

 

[Lewish]

Is there ever a time when you want both loads &quot;on&quot;? How about both &quot;off&quot;?

I happen to like IRFP240 power FETs, but they require a bit of addition stuff in your application. They are cheap, they will handle 200V, 10A and have an on resistance of less than half an ohm.

 

[RITelectric]

Operational concept: As long as the unit is plugged in, there will *always* be power fed to *exactly one* sub circuit. Power diodes are placed at critical points throughout the entire system to ensure proper current flow / isolation.

All other factors are going to be designed around the characteristics of this switching process, whatever those characteristics may be. The process, however, has to be verified first.

My question still stands, how would one connect BJTs to the above described system to act as a routing switch? If anyone wants to describle a similar switch using JFETs or MOSFETs, please feel free to go into detail as to how it would work and what other components would be necessary to make it work.

I'm sorry if I'm frustrating anyone by asking this question over and over again... and I do very much appreciate all the input from everyone. You are all raising good questions, but due to the nature of my application, the only question I need an answer to is how to hook up the transistors.

 

[IRstuff]

Did you have some question about the hookups given in my answer?

TTFN

 

[RITelectric]

IRstuff: Oops! Sorry I didn't address your response earlier - At first I missed a couple points, but after drawing out what you described I see how your answer works, and thank you for your response.

Since Lewish and melone were still asking me questions about my circuit, I got the impression that they have different solutions still in mind... so I got caught up in discussing it with them to get as much input as possible.

 

[cbarn24050]

Hi irstuff, that circuit dosent look like it will work.

 

[IRstuff]

oops, you're right.

The 10K resistor for the PNP should be in series.

I didn't give the values a whole lot of thought, so someone will need to go through and make sure that the base turn-off through 20K is adequate.

TTFN

 

[47gandalf]

You might look at the power switch device from Clare, Inc. (formerly C P Clare). I'm using a CPC1954Y switch in a current design. I think the older number for the part is PG2601 or something, also from Clare.

It has opto isolated input and in my case switches 120 VAC at up to an amp. I think some or all family members can handle DC loads.