# L298 High Voltage Power supply

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#### Fergie

##### New Member
This is my first post and i have done a lot of research, however im still at the brick wall I hit weeks ago. I have lost count of how many nights ive stayed up until 7:30am the next morning trying to get this sorted. So i thought maybe someone on here could help me. Ive read the data sheets for the L298, browsed the web and had a look in some books for absolutley ages trying to find out what the problem is.

Basically, im currently building a project that is used for detecting and classifying underwater targets on board an AUV. Im using the Proteus ISIS package to simulate my design. I have a 24V DC supply, 7805 5V voltage regulator this allowing the logic inputs to the L298 H-bridge to operate at lower voltage levels. A L298 H-bridge in which the outputs from the h-bridge are connected to the primary windings of the transformer, with a turns ratio of 1:12.5. The outputs of the h-bridge should be 24V and then be 'boosted' (multiplied by 12.5) to give me my high voltage output readings.

Im having difficulty with the outputs of the L298. I have connected my 24V supply to VS and the 5V from the voltage regulator into Vcc of the L298. In1 & In4, In2 & In3 are connected in parallel to reduce the stress on each channel, and so are out1 & 4, out2 and out 3 of the l298. When i apply a logic HIGH (5v from regulator) to the Enable line and In1, 1n4, and a LOW (0V) to in2 and in3, the outputs of the h-bridge show 23.9V on out1,out4 (which are connected together in parallel) and also on out2 and out3 . My thought was that out1,out4 should be putting out +24V and out2,out3 showing -24V. The other problem i am encountering is, on the secondary winding of the transformer, i have placed 2 voltage probes and the voltages im getting are so rediculously small, 1.9x10^-13 .

They should be about +/-300V on the secondary side of the transformer. I have missed out the rectifer stage of the circuit in this description.

If anyone could give me a hand or even some advice. I really have honestly spent so much time on this and everything/test i try just doesnt seem to solve anything. Im not one to give up but this is my last resort. Any advic/tips would be very much appreciated. I can included a diagram later on if needed. Thanks again for taking time to read.

Fergie.

#### Ubergeek63

##### Well-Known Member
losing 1V on the high side AND the low side is typical and known as the saturation voltage on the spec.

What type of transformer and at what frequency are you trying to drive it?

#### Fergie

##### New Member
Im using the simple transformer model in the proteus simulation package. Consists of a small iron core. I'm using a 50Hz signal to drive it, but as far as im aware the 50Hz ac signal is done inside the H-bridge itself? Although I have no way of clarifying this as there are no properties for me to edit in the H-bridge used in proteus. I tried using an AC supply but my spec insists it must be a 24V DC signal powering the H-bridge.

However in my notes it does say the output from the h-bridge should be a 50Hz pseudo-sin wave and the waveform seen on the winding of the transformer should be a square wave. Thats another problem im having in Proteus is when i go to simulate my waveforms my computer crashes. I have windows vista and have tried it on XP, windows '98 and it does the same

Sorry about the late reply ive been busy with coursework from university.

##### Banned
Something is very wrong with your circuit then, are you using a legal copy of Proteus?

#### Fergie

##### New Member
Yeah i am, i have told my supervisor that my graphs are not running properly and hes not really to helpfull to be honest. So ive been working on the microcontroller part of my project until I can think of something new to try. I really have tried almost everything and just cant figure out whats going wrong. But without my simulation I cannot get results nor can i actually build my circuit which sucks. I have only 2 weeks to do it and i really cant see it being finished on time. bad times

#### Ubergeek63

##### Well-Known Member
Im using the simple transformer model in the proteus simulation package. Consists of a small iron core. I'm using a 50Hz signal to drive it, but as far as im aware the 50Hz ac signal is done inside the H-bridge itself? Although I have no way of clarifying this as there are no properties for me to edit in the H-bridge used in proteus. I tried using an AC supply but my spec insists it must be a 24V DC signal powering the H-bridge.
what ever gave you that idea? not only do you have to supply two square waves to the L298, you have to make sure there is dead time between them! the L298 is nothing more than a dumb bridge with level shifters, nothing more!

Dan

#### Fergie

##### New Member
Thanks Dan, This may sound like a stupid question but how can i do that? I have the In1,In4 inputs wired to my 5V Vcc for the logic HIGH and the enable line also. I know my microcontroller is used to control the h-bridge to produce a sin wave which is fed into the primary winding of the transformer, but i cant select my micro-controller from the library in proteus. And the guys that done this project last year somehow managed to get their simulation results in the crocodile physics simulator. I construct the same circuit they did using the same values for components etc and my waveforms were nothing what they had. Could i try using a different microcontroller within proteus and try setting it up to produce this or ?

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#### dougy83

##### Well-Known Member
Why are in1, in4 connected to 5v? This will give a constant ~24V at out1, out4.

As Dan stated, you should be be supplying 2 square waves to the L298; one wave is the opposite polarity of the other. So if the 2 waves are called A & B, connect wave A to in1 & in4 and connect wave B to in2 & in3.

#### Fergie

##### New Member
I connected an alternating source to the primary side and im am finally getting 169.0V on the other side so im getting there, the spec only requires +-150V so its now achieved, cheers for the heads up guys. I have been trying to sort this out for ages, its well appreciated.

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#### Ubergeek63

##### Well-Known Member
I connected an alternating source to the primary side and im am finally getting 169.0V on the other side so im getting there, the spec only requires +-150V so its now achieved, cheers for the heads up guys. I have been trying to sort this out for ages, its well appreciated.
You're quite welcome glad to help

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