Testing switch & DC to AC Converter

Thread Starter

jim0000

Joined Oct 28, 2020
23
I am trying to switch on a DC to AC converter, the load for the primary side of the circuit (where the DMM is measuring across) is a transformer (which I have unfortunately not picked out yet due to being unable to produce a working DC to AC converter, and as a note the only concern for frequency and current I have is to make sure components aren't getting too hot, or damaged, also, ideally I would like a transformer on the scale of 5 inches and under which usually means higher frequency, aside from that frequency and current can vary) and I am trying to step up voltage to upwards of around 390VDC which will be delivered to the load which in my case is a cathode to create the electric field. I just want to create an electric field at the load of the secondary side of the circuit to achieve phase transition in Bose-Einstein Condensate by increasing the voltage instead of lowering the temperature. So as long as I see 390VDC at the cathode I am happy. The rectifier portion seems to be working well (tested separately with AC voltage applied) but I am having trouble with the DC to AC converter portion. The switching event works as expected which means I have approximately 12VDC going into my LM555. However when I measure after the DC to AC converter there's a result of <1VAC. Am I doing something wrong here circuit wise or the way I am measuring? Why such the large voltage drop? R1 = 10kohm, R2 = 100kohm, R3 = 100kohm (initially), C1 = C2 = 10nF

Also, it appears I have a pulsed DC instead of AC at the primary of the transformer. Can this be used? Or should it be strictly AC? Do some transformers allow pulsed DC?
switchconv.png
 
Last edited:

KeithWalker

Joined Jul 10, 2017
1,684
What frequency is the 555 running at?
What type is the P-MOSFET on the output?
The resistor from the output of the 555 to the gate of the FET is far too high. The gate to source junction is capacitive (see the datasheet) and will not have time to properly charge and discharge through 100 Kohms unless the frequency is very low. Replace the resistor with a value of 220 ohms.
Regards,
Keith
 

Thread Starter

jim0000

Joined Oct 28, 2020
23
What frequency is the 555 running at?
What type is the P-MOSFET on the output?
The resistor from the output of the 555 to the gate of the FET is far too high. The gate to source junction is capacitive (see the datasheet) and will not have time to properly charge and discharge through 100 Kohms unless the frequency is very low. Replace the resistor with a value of 220 ohms.
Regards,
Keith
Thank you! This is the PMOS https://www.mouser.com/datasheet/2/149/FQP47P06-1009447.pdf ..and the 555 is running at a frequency of about 1.5kHz. I tried adjusting the resistor to 400ohms, would that do the trick? The load is a step up transformer so I'd like it to be able to step up the AC signal made.
 

crutschow

Joined Mar 14, 2008
26,998
The load is a step up transformer so I'd like it to be able to step up the AC signal made.
You cannot couple a pulsed DC into a transformer as it will saturate the core due to the DC component of the signal.

You could instead use a push-pull driver with a capacitor it series with the transformer to block the DC.

Also that circuit does not generate a 50% duty-cycle signal, which you should have to driver the transformer.
 

Thread Starter

jim0000

Joined Oct 28, 2020
23
You cannot couple a pulsed DC into a transformer as it will saturate the core due to the DC component of the signal.

You could instead use a push-pull driver with a capacitor it series with the transformer to block the DC.

Also that circuit does not generate a 50% duty-cycle signal, which you should have to driver the transformer.
I was actually wondering whether it would have that effect. I was hoping it would instead withstand the dc component and then allow the AC component to pass through to the secondary. How would I implement a push-pull driver with a capacitor in series with the transformer? I am not familiar with that. Also, how did you calculate the duty cycle? How can I just the circuit to achieve the 50% duty cycle?
 

RPLaJeunesse

Joined Jul 29, 2018
158
Your circuit generates pulsed DC, not at all the same as AC. AC is Alternating (direction) Current which tends to not happen unless you have alternating (polarity) voltage.
 

Thread Starter

jim0000

Joined Oct 28, 2020
23
You cannot couple a pulsed DC into a transformer as it will saturate the core due to the DC component of the signal.

You could instead use a push-pull driver with a capacitor it series with the transformer to block the DC.

Also that circuit does not generate a 50% duty-cycle signal, which you should have to driver the transformer.
Also, I edited my original post to be more descriptive. Maybe it will help with understanding what I am wanting
 

Thread Starter

jim0000

Joined Oct 28, 2020
23
Your circuit generates pulsed DC, not at all the same as AC. AC is Alternating (direction) Current which tends to not happen unless you have alternating (polarity) voltage.
That's what I am gathering from comments and reading on the internet. I suppose I need to change things. How could I instead create the AC voltage? It seems like the pulsed DC will not work with a transformer
 

RPLaJeunesse

Joined Jul 29, 2018
158
You need to study the design of isolated DC-DC converters. Start with an old GE or RCA transistor manual, if you can find one. Drop off the output rectifying stage and you have a DC to AC inverter. Build a few and you will blow up transistors until you get it right, assuming you have the patience and education to get there (I have an MSEE and 40 years experience, I'm not there yet). Or just buy a converter/inverter that has an enable pin and the output you need.
 

Thread Starter

jim0000

Joined Oct 28, 2020
23
You need to study the design of isolated DC-DC converters. Start with an old GE or RCA transistor manual, if you can find one. Drop off the output rectifying stage and you have a DC to AC inverter. Build a few and you will blow up transistors until you get it right, assuming you have the patience and education to get there (I have an MSEE and 40 years experience, I'm not there yet). Or just buy a converter/inverter that has an enable pin and the output you need.
If you're not there yet then I am far from it. I am trying to find the quickest solutions if there is one. I am not familiar with the converters/inverters with an enable pin. Can that be attached to a breadboard/protoboard/pcb?
 

crutschow

Joined Mar 14, 2008
26,998
but could a pulse transformer work?
Pulse transformers are designed to transmit low power signal pulses, and are not suitable for transferring any significant power.
How would I implement a push-pull driver with a capacitor in series with the transformer?
Look up push-pull driver.
The simplest is an NPN transistor and a PNP transistor connected as emitter followers with there emitters connected together, the NPN collector to V+, and the PNP collector to common.

Adding a capacitor in series from the push-pull output to the transformer is just what it says.
 

Thread Starter

jim0000

Joined Oct 28, 2020
23
Pulse transformers are designed to transmit low power signal pulses, and are not suitable for transferring any significant power.
Look up push-pull driver.
The simplest is an NPN transistor and a PNP transistor connected as emitter followers with there emitters connected together, the NPN collector to V+, and the PNP collector to common.

Adding a capacitor in series from the push-pull output to the transformer is just what it says.
Okay I will have to look it up and do a little research then. Out of curiosity, what kind of transformers would be best to go from around 10VAC to 390VAC? I've looked at so many, and I really am at a loss on which is best suited.
 

KeithWalker

Joined Jul 10, 2017
1,684
Okay I will have to look it up and do a little research then. Out of curiosity, what kind of transformers would be best to go from around 10VAC to 390VAC? I've looked at so many, and I really am at a loss on which is best suited.
That would depend on the operating frequency, how you are going to drive it and the current requirements. You probably will not find one that exactly fits your needs.The simplest way would be to select a suitable core and wind your own.
 

Thread Starter

jim0000

Joined Oct 28, 2020
23
That would depend on the operating frequency, how you are going to drive it and the current requirements. You probably will not find one that exactly fits your needs.The simplest way would be to select a suitable core and wind your own.
What do you mean by 'whats going to drive it?' The operating frequency can vary, the transformer just cant be over like 5 inches. I know smaller ones typically mean higher frequency so other than size requirements frequency is flexible.
 
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