short pulse ---> long hi signal

[huttojb]

I have a PIC core (basically a PIC with all the outputs put to a bread board, So I could use this.

The mosFET has a diode already in it so would this protect it?

 

[Nigel Goodwin]

The mosFET has a diode already in it so would this protect it?

No, it's not 'really' a diode, it's just a byproduct of the manufacturing process, add an external diode!.

 

[ljcox]

No, it's not 'really' a diode, it's just a byproduct of the manufacturing process, add an external diode!.

Agreed and besides, even if this diode could handle the 2 Amp from the EPS when the FET is turned off, it is in the wrong direction to protect the FET from the back EMF.

 

[huttojb]

Agreed and besides, even if this diode could handle the 2 Amp from the EPS when the FET is turned off, it is in the wrong direction to protect the FET from the back EMF.

Len

How are you aware these products come from the EPS???

So I diode will need to be fitted . I'll have to put that in my design.

I'm putting a board together now and have managed to drive the FET so this looks good so far, I'll keep you informed in my progress.

Jason

 

[ljcox]

How are you aware these products come from the EPS???

Because I understand inductance.

So I diode will need to be fitted . I'll have to put that in my design.

Yes

I'm putting a board together now and have managed to drive the FET so this looks good so far, I'll keep you informed in my progress.

Thanks, some don't bother.

 

[huttojb]

Hello.

Where do I put this diode, what direction do I put it in and whats the best diode to use???

jason

 

[ljcox]

Hello.

Where do I put this diode, what direction do I put it in and whats the best diode to use???

jason

See the circuit I posted on 17 March

 

[huttojb]

So across the MPS.

Thank you

 

[ljcox]

So across the MPS.

Thank you

You're welcome. Yes, the diode limits the back EMF to about 0.7 ~ 1 Volt and therefore protects the MOSFET.

Choose a diode that can withstand 2 Amp for a brief period, ie. while the energy stored in the MPS inductance is being dissipated.

Let me know if you're struggling to find a suitable one & I'll help.

 

[Sceadwian]

Depending on the switching frequency and rise and fall times it's not uncommon to rely on the MOSFETs internal body diode. Depends on the actual peak voltage created by the coil, and the MOSFET's specs.

 

[huttojb]

The 1N4148 can rate at 4A for 1:mu: S and 1A at 1mS, would this be OK.

 

[ljcox]

The 1N4148 can rate at 4A for 1:mu: S and 1A at 1mS, would this be OK.

I can't calculate the decay time since I don't know the inductance of the MPS.

If it was say 1 Henry and, given it draws 2A at 12 Volt, it must have a resistance of 6 Ohm, therefore the time constant would be 1/6 second so the 1N4148 would curl up and die.

I suggest you look at the data sheet of the 1N4004.

 

[huttojb]

The resistance of the coil is about 5Ohm, so your estimations are about correct. I'm in Czech Republic at the moment so I'll have a look when I get back to the UK on Monday.

Thanks for working it out,

Jason

 

[ljcox]

Jason,
How is it going?

 

[sirilak]

Hi ljcox,

Now, I used 555 mono with 1khz, 4%dutycircle, 1uf.

I would like to turn to 100khz with short pulse.
I had tried to increse higher freq but very sensitive.

Can 555 do up to 100khz or need to change cap?
or better try 74hc14?


regards,
sirilak

 

[ljcox]

If 1 uF gives you 1kHz, then 10 nF will give you about 100 kHz

 

[sirilak]

Thank you for reply.
I put 500k vr, 250k vr and changed cap.
Than increased freq more than 10khz, the signal was missing.


I though my circuit was not good enough. very simple.

.....below 10khz, ok for me

Thank agian.

 

[ljcox]

you search this forum for 555. There have been many posts on this subject.