# Amplifying 25uA to 450mA

Discussion in 'General Electronics Chat' started by ATM, Mar 24, 2010.

1. ### ATM Thread Starter Member

Oct 8, 2009
31
0
Hey,

I need to amplify the 25uA coming from a pin of a PIC16F88 microcontroller to around 450mA so I can operate a solenoid valve. What are my options?

Is it / Would it be possible to use a darlington pair with hFE of around 18000?

2. ### russ_hensel Distinguished Member

Jan 11, 2009
820
47
Why is only 25 uc comming from pic. Currents almost a 1000 times larger are normally available.

3. ### iulian28ti Member

Dec 4, 2009
42
0
Better a Sziklai pair.
If 25uA is nominal, the two bases of the Darlington will drop quite a lot of voltage so your current will be reduced.

4. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
That is 85.1 dB of Current Gain. There are several problems with trying to do this in a single stage. The first is that across a lot of darlington transistors the gain will be represented by a statistical distribution. If 18,000 was the mean value and the standard deviations was 4,000, then over 60% of the transistors would have a gain in the range of 14,000 to 22,000. A random selection from a purchased lot could fall anywhere in that range.

When going for a large amount of gain it is better to do it in multiple stages. That way no one stage will have a sensitive dependence on transistor parameters. In addition it will be easier to maintain stability and avoid oscillation.

If you only need to make one of these, you can of course search for the single part that meets your needs. In my opinion this would be a short sighted approach.

5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
ATM was mistaken about the PIC output pin current. The actual per-pin source/sink limits are closer to +/-20mA, not uA.

The easiest solution is to just use a logic-level N-channel power MOSFET.
IRLD014 and IRLD024 MOSFETs can sink 1.7A or 2.5A (respectively), have a built-in body diode, come in a 4-pin DIP package, and are stocked by Mouser and Digikey, among other distributors.

Connect the SOURCE terminal to ground, gate to the microcontroller I/O pin using a 270 Ohm resistor, and sink the current from your solenoid valve via the MOSFET drain. Connect the other side of the solenoid to your +V supply.